Faraday Discussions
Cite this: DOI: 10.1039/d1fd00015b

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Spiers Memorial Lecture: Introduction toultrafast spectroscopy and imaging ofphotochemical reactions
Toshinori Suzuki

Received 9th February 2021, Accepted 16th March 2021
DOI: 10.1039/d1fd00015b

A brief overview is presented on ultrafast spectroscopy and imaging of photochemical
reactions by highlighting several experimental studies reported in the last ﬁve years. A
particular focus is placed on new experiments performed using high-order harmonic
generation, X-ray free electron lasers, and relativistic electron beams. Exploration of
fundamental

chemical

reactiondynamics

using

these

advanced

experimental

methodologies is in an early stage, and exciting new research opportunities await in this
rapidly expanding and advancing research ﬁeld. At the same time, there is noexperimental methodology that provides all aspects of the electronic and structuraldynamics in a single experiment, and investigations using diﬀerent methodologies withvarious perspectives need to be considered in a comprehensive manner.

I. Introduction
Chemists have unravelled the atomistic details of invisible chemical reaction
mechanisms using imagination and logical thinking based on carefully designed
control experiments. It is a longstanding dream of chemists and students to beable to watch chemical reactions directly and understand their mechanisms. The
most useful instrument for watching invisibly small objects is a microscope, andthe technology of microscopy has advanced dramatically. Electron microscopy,
which utilizes the ultrashort wavelength of matter waves, has provided us withextremely high spatial resolution that is unachievable using conventional optical
microscopy. In 2020, two research groups reported that the resolution of singleparticle cryo-electron microscopy1–3 has reached a remarkable spatial resolutionof 1.2 Å.4,5 Cryo-electron microscopy is a revolutionary tool for structural biology,
as it enables structural determination of biomolecules and their assemblies
without crystallization. Previously, X-ray crystallography has been most widely
employed for structural determination of biological samples; however, anobstacle was that high-quality crystalline samples, which are required for thisDepartment of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan. E-mail:suzuki@kuchem.kyoto-u.ac.jp
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method, are very diﬃcult to prepare for important samples such as membraneproteins. The 2017 Nobel Prize in Chemistry was awarded to three researchers
who pioneered cryo-electron microscopy. Scanning tunnelling microscopy6 andatomic force microscopy7 were developed in the 1980s, and they have enabled us
to “touch” a molecule adsorbed on a solid surface with a thin metallic needle andto determine its molecular structure. Furthermore, recently developed tipenhanced Raman spectromicroscopy further utilizes the metallic needle as bothan antenna and a focusing lens for radiation to perform spatial mapping of the
spontaneous Raman intensity with atomic resolution. When the metal tip is
carefully placed just above a molecule within a vertical distance of 2 A, the radiation eld is strongly localized in an extremely narrow spatial region and theRaman spectrum of the molecule becomes dependent on the tip position. Fig. 1
shows the two-dimensional maps of individual Raman band intensity measured
by scanning the tip over a Co(II)–tetraphenyl porphyrin molecule adsorbed on an
ultracold copper surface. These two-dimensional patterns of the spontaneous
Raman intensity are reminiscent of the normal modes of molecular vibration
corresponding to these Raman frequencies; more precisely, the patterns reect
the modulation amplitude of a local electronic wavefunction caused by vibrational motion.8,9 The advances and performance of these microscopy techniques
are truly impressive. On the other hand, current microscopy techniques do nothave suﬃcient temporal resolution to track ultrafast structural changes duringchemical reactions that occur on a femtosecond time scale. The desire for realtime observations of chemical reactions has led to continuous eﬀorts todevelop more advanced ultrafast spectroscopy and imaging methods for studying
photoinduced molecular processes.
Over the last half-century, the shortest temporal duration of a laser pulse has
been reduced by more than nine orders of magnitude, from nanoseconds toTip-enhanced Raman Spectromicroscopy (TERS): (left) schematic of TERS ofa Co(II)–tetraphenyl porphyrin molecule on Cu(100) at 6 K. The silver tip acts as a focusinglens with a Gaussian waist and as a radiating antenna. (Right) top: experimental images
recorded by monitoring the Raman scattering for the indicated vibrational frequencies.
The images are overlaid with the molecular frameworks. Middle: theoretical simulations.
Bottom: computed normal-mode vibrations. Adapted from ref. 8.
Fig. 1

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Quantum beats: (a) ﬂuorescence intensity decay proﬁles after coherent excitationof multiple vibronic bands of jet-cooled anthracene. Diﬀerent emission bands weremonitored for the two cases. The illustration shows coupling of two classical oscillators or
two quantum states. (b) Charge migration in a superposition state of the D0 and D1 cationicstates of iodoacetylene. The illustration shows periodic movement of an electron holebetween the I atom and the acetylenic part. Adapted from ref. 15 and 16.
Fig. 2

attoseconds.10–12 (Electron pulse durations have also been reduced to attoseconds.13,14) In 1984, Felker and Zewail observed vibrational quantum beats
created by coherent excitation of multiple vibronic states of S1 anthracene with 15
ps pulses (Fig. 2(a)).15 The characteristic oscillatory features of the uorescencesignals indicate that two oscillators interact with each other and exchange theirvibrational energies. In 2015, Kraus et al. demonstrated coherent superposition of
the ground (D0) and the rst excited (D1) states of iodoacetylene cations separated
by 2.23 eV using strong eld ionization, and observed electronic quantum beatswith a period of 1.85 fs, during which nuclear motion is essentially frozen.16 This
electronic quantum beat corresponds to periodic motion of an electron hole inthe cation; the electron hole is initially localized at the iodine atom at 0 fs, and it
migrates into the acetylenic part and then returns to the iodine atom aer 1.85 fs.
At the forefront of metrology, attosecond spectroscopy is paving a new avenue forstudying electron dynamics in matter, such as charge migration, photoionization
delay, and electron current in a molecule.11,17–19 On the other hand, the time scale
for atomic motion in chemical reactions is from femtoseconds to picoseconds, as
determined by the atomic masses and the forces acting on the atoms; the shortest
vibrational periods for chemical bonds such as C–H and O–H are approximately
10 fs. Therefore, important targets of research such as internal conversion,
photoisomerization, ring-opening, charge transfer, and solvation are in the
femtosecond time range.20,21 From a technical viewpoint, it is not at all trivial togenerate deep ultraviolet (UV) pulses with temporal durations much less than 10
fs,22 so ultrafast spectroscopy in the deep UV region requires further innovation.
There are many more challenging problems and room for further development.
High-order harmonic generation (HHG) and X-ray free electron lasers (XFELs)
provided signicant inuences in ultrafast spectroscopy in the extreme UV and XThis journal is © The Royal Society of Chemistry 2021

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ray regions. Exploration of chemical reaction dynamics with these methods is stillin an early stage, and exciting new research opportunities await in this rapidly
expanding and advancing research eld. HHG utilizes an oscillating electric eldof a strong laser pulse to induce tunnel ionization of rare gas atoms, accelerationof emitted photoelectrons, and coherent recombination of the photoelectronsand photoions, and it generates a burst of radiation from the extreme UV to theso X-ray region.23,24 On the other hand, XFELs are large-scale linear accelerators,
with a length of several hundred meters to kilometers, to generate a relativistic
beam of electrons and inject it into an undulator, in which an intense X-ray pulse
is generated by self-amplication of stimulated emission of radiation (SASE).25
The development of XFELs and HHG led to many breakthroughs in experimental
research on photoinduced chemical dynamics. For example, prior to the realization of XFELs, ultrafast X-ray experiments were primarily limited by the pulse
duration of synchrotron radiation, which is of the order of 50–100 ps, except fora few experiments performed using a slicing technique.26–28 However, femtosecond spectroscopy in the hard X-ray region can now be routinely performed
with XFELs.
A photoabsorption spectrum provides some valuable information on the photoinduced dynamical processes, because the spectrum corresponds to the Fouriertransform of the autocorrelation function hJ(0)jJ(t)i of the nuclear wave packet
J(t) in the excited electronic state.29 Photoexcitation with an ultrashort opticalpulse creates a replica of the ground-state vibrational wave function on the excitedstate potential energy surface, and this spatially localized vibrational (nuclear) wave
function (or wave packet) evolves with time according to the potential gradient. If
the nuclear wave packet prepared on the excited state potential energy surface
leaves the Franck–Condon region and never returns, hJ(0)jJ(t)i diminishes rapidly
with time and the corresponding absorption spectrum exhibits a broad bandfeature. On the other hand, the spectrum reveals clear vibrational structures if the
wave packet repeatedly returns at least in part to the Franck–Condon region. Thus,
absorption spectroscopy enables us to decode information concerning the nuclear
wave packet dynamics at the outset of photo-induced dynamics, if the intrinsic
spectral features are not obscured by spectral broadening due to hot bands (transitions from an ensemble of thermally populated levels) and/or inhomogeneous
broadening (due to diﬀerences in the local solvation environment in a solution). On
the other hand, ultrafast pump–probe spectroscopy enables tracking of entire
photochemical reactions from the Franck–Condon region to the nal product andprovides far more information on the electronic and structural dynamics inphotochemical reactions. These two viewpoints are complementary and indispensable for understanding the chemical reaction.
Observation of electronic dynamics is certainly important for understanding
electron transfer and redox reactions.30 Moreover, elucidation of electronic
dynamics is generally important for understanding photochemical reactions,because these reactions oen proceed via non-adiabatic transitions between
diﬀerent electronic states.31–34 The non-adiabatic transition is observed as thebreakdown of the Born–Oppenheimer (or adiabatic) approximation that separates
nuclear and electron motion in the quantum mechanical description of a molecule. When this approximation holds, a chemical reaction is regarded as
quantum wave packet motion of nuclei on a single potential energy surface, forexample, the ground-state surface. However, a photochemical reaction begins at
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one of the excited-state potential energy surfaces reached by photoabsorption,and surface crossings between the excited and ground states are ubiquitous. In
the vicinities of these crossings, the Born–Oppenheimer approximation inevitably
breaks down and non-adiabatic transitions between the potential energy surfaces
occur. These transitions “create” new reaction paths, which are not anticipated in
the framework of the Born–Oppenheimer approximation. As an example,
a photochemical ring-opening reaction of 1,3-cyclohexadiene (CHD) to form 1,3,5hexatriene (HT) is schematically shown in Fig. 3. This reaction is involved in
producing previtamin D in our skin under sunlight and is also one of the bestknown examples of the Woodward–Hoﬀmann rule regarding the chemical reaction pathway dictated by molecular orbital symmetry.35–37 As shown in Fig. 3, thepotential energy surface for the rst excited state of CHD accessed by UV
absorption is not adiabatically correlated to the ground state of HT, so nonadiabatic transitions must be involved in this reaction. The characteristicfeature of this reaction is that the HOMO and LUMO of CHD are correlated with
the LUMO and HOMO of HT, respectively; therefore, the energy ordering of the
HOMO and LUMO is reversed during this reaction. The ground state of HT has
double occupancy of the 11b orbital, and it is correlated with the doubly excitedS**
2 state of CHD. Therefore, the ring-opening reaction from the S1 state of CHD is
induced by non-adiabatic transitions through the S1–S2 and S1–S0 conical intersections of the potential energy surfaces.38 The concept of non-adiabatic transitions is well known for one-dimensional systems, and a simple analytical(Landau–Zener) formula approximating the non-adiabatic transition probability
is described in textbooks on quantum mechanics. On the other hand, the nonadiabatic dynamics in the high-dimensional conguration space of polyatomic
molecules are highly complex, and their understanding requires accuratecomputation of the potential energy hypersurfaces and experimental measurements of various observables regarding electronic dynamics.

Fig. 3 Non-adiabatic dynamics in the photochemistry of CHD: qualitative presentation of
the potential energy curves involved in the ring-opening reaction of CHD. C2 symmetry
notation is presented to clarify the correlation among the molecular orbitals. However, thereaction path preserving C2 symmetry leads to a strongly avoided crossing with a large S1/
S0 energy gap. Therefore, the reaction proceeds through the S1/S0 conical intersection
with non-C2 symmetry. HT has three isomers as indicated.
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Elucidation of the structural dynamics of photoexcited molecules is essential
for understanding their properties and functions. Structural biology elucidates
the physiological functions of biomolecules from their shapes. In organic
chemistry, the beautiful geometrical structures of crown ethers39 and BINAP–
metal complex catalysts40 provide highly useful functions. The cis–trans photoisomerization of retinal provides us with our ability to see. Although transientabsorption spectroscopy and photoelectron spectroscopy are most useful for
elucidating electronic dynamics, it is generally diﬃcult to determine complex
molecular structures with these spectroscopic methods. Ultrafast X-ray/electron
diﬀraction methods (diﬀraction from a disordered sample is termed scattering
but has the same meaning) that directly determine interatomic distances inmolecules play an invaluable role in elucidating structural dynamics in chemical
reactions.41–48 Historically, electron diﬀraction has made a signicant contribution as a methodology for accurate determination of molecular structures in the
ground electronic state. For example, Fig. 4(a) shows the electron diﬀraction
pattern recorded for gaseous CCl4 in the 1950s,49 revealing beautiful interferencesamong the electron waves scattered by carbon and chlorine atoms. The interatomic distances were extracted from this pattern as shown in Fig. 4(b), whichclearly shows two peaks corresponding to the C–Cl and Cl–Cl interatomic
distances. The great advantage of diﬀraction experiments is that interatomic
distances (or their distribution functions) can be determined. Ultrafast X-ray/electron diﬀraction probes the transient structure of a molecule with the same
principle. Elastic X-ray scattering is caused by oscillation of electrons in an X-ray
radiation eld (Thomson scattering), while elastic electron scattering is caused bydeection of incoming electrons by electrostatic interactions with all the electrons and nuclei in a target molecule (Rutherford scattering). Since the scattering
intensity in the former case is about ve orders of magnitude smaller than that in
the latter case, ultrafast X-ray diﬀraction studies of gaseous molecules have beenhighly challenging. This situation has been completely changed by XFELs, which
provide an X-ray photon ux (1012 photons per pulse) that is seven orders ofmagnitude greater than the ux (3.7  104 electrons per pulse) of an electron
beam.50 The signal intensity in ultrafast X-ray diﬀraction measurements has

Conventional electron diﬀraction experiment: (a) experimental and calculated
electron diﬀraction patterns for CCl4. (b) Radial distribution function (interatomic
distances) extracted from the data shown in (a). The shorter distance corresponds to C–Cl
and the longer one corresponds to Cl–Cl. Adapted from ref. 49.
Fig. 4

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become even greater than that in ultrafast electron diﬀraction (UED) measurements.48 In electron and X-ray diﬀraction experiments, the results are usually
analysed using an independent atom model (IAM), which views a molecule as anarray of spherical atoms and predicts the overall diﬀraction signal expected forthe molecule from the scattering power (atomic form factors) of individual atoms.
The IAM does not consider the formation of chemical bonds, polarization orelectronic excitation. Some recent studies employ quantum chemical calculationsof molecular electronic states for the analysis, and this will be a standard way ofanalysis in the future.
A nuclear wave packet on the excited-state potential energy surface graduallydisperses as it propagates. Moreover, it even spreads over multiple potential energy
surfaces via spin–orbit and non-adiabatic (vibronic) coupling. Internal conversion
to the ground state also leads to branching into diﬀerent chemical structures. Thus,
the probability density distribution for nuclei becomes more and more diverse as
the reaction proceeds, although it remains a quantum mechanical superposition(Schrödinger’s cat) prior to decoherence. A broad nuclear probability density
distribution generally prevents a clear diﬀraction pattern from being obtained,making structural determination more challenging. In these cases, wave packet orclassical trajectory calculations on ab initio potential energy surfaces can be
employed for the analysis of experimental results.

II. Photochemical reactions of isolated
molecules
In retrospect, it was the Faraday Discussion in 1999 where the author presented therst experimental report on time-resolved photoelectron imaging (TRPEI).51,52
Therefore, I will begin with this memorable topic. We attempted real-timeobservation of ultrafast S2 1B2u(p, p*) / S1 1B3u(n, p*) internal conversion inpyrazine. Pyrazine is one of the best-known examples of molecules that undergo
ultrafast internal conversion through the conical intersections of the potential
energy surfaces, and has been studied as a benchmark system by many theorists.31,34 For example, Seel and Domcke presented model calculations for timeresolved photoelectron spectroscopy53,54 of S2 / S1 internal conversion in pyrazine.55 However, the temporal resolution of our experiment in 1999 was only 0.2–
0.3 ps, which was insuﬃcient for real-time observation of S2 / S1 internalconversion which was theoretically predicted to occur in less than 30 fs. As analternative, we observed the S1 / T1 3B3u(n, p*) intersystem crossing, which wasa well-known intermediate coupling case of radiationless molecular transition.52
Subsequently, lamentation four-wave mixing (FFWM) in a noble gas enabled us
to generate ultrashort pulses at 264 and 198 nm and improved the instrumental
time resolution to 22 fs.56 TRPEI using FFWM successfully identied S2 / S1
internal conversion that occurs with a time constant of 22 fs based on the ultrafastchange in photoelectron angular anisotropy.57 We have further shortened the
FFWM wavelength to 133 nm (9.3 eV) in order to observe the entire cascaded
electronic dephasing process from the S2 state of pyrazine, including S1 / T1
intersystem crossing and S1 / S0 internal conversion.58,59
Fig. 5 shows two-dimensional (2D) slices through the 3D velocity distributionsof photoelectrons obtained using 264 nm pump and 133 nm probe pulses. The
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Fig. 5 Time-resolved photoelectron imaging spectroscopy: (left) 1D slice of the potential
energy surfaces of the pyrazine molecule. The potential energy curves along the totallysymmetric 6a mode indicate a conical intersection between S2(p,p*) and S1(n,p*). (Right)
2D slices through the 3D photoelectron scattering distributions. The initial excitation
occurs to the S2 state, ultrafast internal conversion to S1 occurs within 22 fs, and further
relaxation to T1 and S0 occurs. 133 nm photons can induce ionization from all electronic
states. 2D slices through the 3D photoelectron scattering distributions were obtained at (a)1, (b) 13, (c) 25 and (d) 49 fs, and (e) 1, (f) 10, (g) 40 and (h) 80 ps. The photoemission signalsin (a) and (b) are primarily from S2, those in (d) and (e) are from S1, and the photoemission
signal in (h) is from T1 and S0. The directions of the polarization vectors for the 264 and
133 nm pulses are parallel to each other and both are in the vertical direction in the planeof the ﬁgure. Adapted from ref. 59.

pump wavelength is resonant with the S2 ) S0 absorption band near the origin.
The probe photon energy is almost the same as the ionization energy of pyrazine;
therefore, all electronic states involved in cascaded electronic dephasing from theS2 state can be detected using the 133 nm probe pulses. Close examination of theimages between 0 and 49 fs reveals the disappearance of the intermediate velocity
component, indicating the occurrence of internal conversion from S2 to S1. The
photoelectron signal also reveals a vibrational quantum beat in the totallysymmetric (accepting) mode n6a in S1 (not shown here). Further relaxation from S1
to T1 and S0 is seen from the clear variation of the images between 1 and 40 ps.
The formation of S0 is the end of the photophysical process but not of thephotochemical reaction of pyrazine. The highly vibrationally excited pyrazine in
S0 ultimately undergoes three-body dissociation into HCN and acetylene.60 The
dissociation, however, occurs on a sub-millisecond time scale, which is far
beyond the observation time window of TRPEI. Some researchers suggest that the1
Au(n, p*) state is also involved in the dynamics; however, its importance has been
evaluated diﬀerently depending on the computational methods.61–64
Fig. 6(a) is a photoelectron spectrum for the S2 state extracted from thephotoelectron images. There are three strong bands. The red (D1) and green (Dx)bands correspond to photoemission of the electrons indicated by the same colourin the pp* electron conguration shown in Fig. 6(b); these are allowed transitions
for the pp* conguration [(1b2g)2(5b1u)2(1b1g)1(6ag2)(2b3u)1]. On the other hand,the blue (D3) band in Fig. 6(a) is an ionization transition not expected for thisconguration and is ascribed to ionization from a diﬀerent electron congurationof [(1b2g)1(5b1u)2(1b1g)2(6ag)2(1au)1] (Fig. 6(b)). Therefore, the blue band indicatesthe presence of a conguration interaction in the S2 state. The CI coeﬃcients for
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Fig. 6 Inﬂuence of conﬁguration interaction: (a) photoelectron kinetic energy (PKE)
distribution measured for the short-lived S2 state prepared by 264 nm photoexcitationfrom S0 and 133 nm ionization with a short delay time. The observed photoelectron
spectrum is ﬁtted using four bands indicated in diﬀerent colours. (b) Electron conﬁgurations in S1, S2, D0, D1, D3, Dx, and Dy. S2 has two major electron conﬁgurations due to b1g–
b3u and b2g–au electron promotion, which is contrasted with S1 characterized by a single
electron conﬁguration. The photoelectron bands coloured in red, blue and green in (a)arise from the removal of an electron indicated in the same colour from one of the twoelectron conﬁgurations in S2. The PKEs for D1–S2 and Dx–S2 are almost the same as thosefor D0–S1 and Dy–S1. Therefore, the photoelectron images in Fig. 5(a) and (d) look similar
except for the distribution corresponding to the D3–S2 band. Adapted from ref. 59.

the former and latter congurations were determined to be 0.70 and 0.54,
respectively, in a computational study by Hackmeyer and Whitten in 1971,65 and0.69 and 0.45, respectively, by Mignolet et al.64 The photoelectron images (a) and(e) in Fig. 5 are of ionization from S2(p, p*) and S1(n, p*), respectively. When wecompare these two images, the only noticeable diﬀerence is the presence andabsence of the intermediate component discussed above; the velocity components seen in Fig. 5(e) are also present in (a). This is because these two photoelectron signals correspond to photoemission from the 2b3u and 6ag molecularorbitals. These orbitals are occupied in both S2(p, p*) and S1(n, p*) (Fig. 6(b)), so
photoemission from these two orbitals provides very similar photoelectron
kinetic energy distributions for S2(p, p*) and S1(n, p*). Thus, disappearance ofthe blue-colored D3 band in Fig. 6(a) served as an unambiguous marker for theultrafast S2 / S1 internal conversion in TRPEI with 133 nm probe pulses.
Pyrazine remains a benchmark system for studies of non-adiabatic transitions;the sensitivities of other spectroscopic methods, such as uorescence upconversion, transient absorption, two-dimensional electronic, and X-ray absorption spectroscopy (NEXAFS: near edge X-ray absorption ne structure), to nonadiabatic dynamics have been theoretically discussed using pyrazine as an
example.66–69
In addition to FFWM, HHG is highly useful for ultrafast photoelectron spectroscopy with extreme UV radiation.70 A seeded free electron laser is also
employed for ultrafast pump–probe photoelectron spectroscopy in combination
with a synchronized table-top femtosecond laser.71 Since photoelectron spectroscopy is highly vulnerable to space charge eﬀects,72–74 active research andThis journal is © The Royal Society of Chemistry 2021

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development related to HHG light sources driven by high-repetition-rate lasers
are underway in many laboratories, aiming at space-charge-freemeasurements.75–77
In the remaining part of this section, I would like to introduce recent progressin experimental studies on photochemical reactions in the gas phase, by taking
examples mainly for the ring-opening reaction of CHD shown in Fig. 3. The rst
example is time-resolved NEXAFS (near edge X-ray absorption ne structure)
spectroscopy. This is analogous to UV/VIS transient absorption spectroscopy,
which probes valence–valence transitions, but NEXAFS corresponds to electronictransitions from inner-shell to valence orbitals. Since the energies of inner-shellorbitals signicantly change with atomic number (or nuclear charge), so dotheir transition energies, making NEXAFS element-selective.30 Several groups
demonstrated time-resolved NEXAFS spectroscopy with table-top HHG lightsources in the carbon K-edge (284 eV) region.78,79 Attar et al. excited CHD with UVlight to induce the ring-opening reaction of CHD, and they observed electronictransitions from C(1s) to the HOMO and higher orbitals.79 The transition to the
HOMO is also allowed, because UV excitation induces the HOMO(12a)–
LUMO(11b) transition, creating a single hole in the HOMO. The black line in
Fig. 7(A) shows the NEXAFS spectrum calculated for the ground state of CHD,
while the red line is for the Franck–Condon region in the photoexcited state ðS*1 Þ:
Since photoexcited CHD undergoes ultrafast molecular deformation along thering-opening reaction coordinate, the energies of the LUMO and HOMO change
rapidly. Consequently, the HOMO and LUMO are considered degenerate in the
middle of the ring-opening reaction. The experimental results shown in Fig. 7(B)do not clearly reveal an ultrafast energy shi of the HOMO band; however, theobserved spectra exhibit a feature at around 282 eV that is consistent with the
expected coalescence of the HOMO and LUMO bands. Currently, many researchgroups are trying to implement similar so X-ray spectrometers based on HHG,

X-ray absorption spectroscopy (NEXAFS): (A) soft X-ray absorption spectra of CHD
calculated for the ground state (black) and the ﬁrst excited state (red). (B) Transientabsorption spectra of CHD measured experimentally with a pump wavelength of 266 nm.
Adapted from ref. 79.

Fig. 7

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and the performance of these instruments is expected to be further improved interms of the temporal resolution and S/N ratio. The element selectivity provided
by NEXAFS will also be useful for studying solution chemistry. For example, in thecase of an aqueous solution, inner-shell absorption by oxygen atoms occurs
around 530 eV; therefore, element-selective spectroscopy can be performed, forexample for carbon- and nitrogen-containing solutes, in aqueous solutions using
the absorption window of liquid water (“water window”).
While time-resolved NEXAFS spectroscopy probes the time-evolution ofunoccupied valence orbitals using electronic transitions from inner-shell orbitals,
time-resolved photoelectron spectroscopy (TRPES) probes occupied orbitals using
the photoelectric eﬀect. In a forthcoming paper by Karashima et al., they have
employed TRPES using the XUV probe pulses (21.7 eV) generated by HHG to
explore the ring-opening reaction of CHD.80 Since the photon energy of XUVradiation exceeds the ionization energies of all chemical species involved in thereaction, it enables observation of the entire electronic dynamics from theFranck–Condon region of CHD to the nal products of HT and CHD. They
combined the XUV laser with an ultrafast DUV laser based on FFWM to achieve
the pump-probe cross-correlation time. The spectra clearly revealed evolution ofa non-stationary electronic state from the singly excited S*1 state to the doubly
excited S**
1 state within 30 fs, rmly establishing the non-adiabatic reaction
mechanism mediated by the doubly excited state. The assignments of thephotoelectron spectra were fully conrmed by the computed electron binding
energies along the ring-opening reaction path using highly accurate quantum
chemical calculations at the level of XMS-CASPT2 (extended multistate complete
active space second-order perturbation theory). They also performed more thorough simulations based on full-dimensional surface hopping trajectory calculations previously reported by Polyak et al.81 and the photoionization dynamics

Fig. 8 Molecular deformation along the ring-opening reaction path: molecular structures
calculated along the ring-opening reaction path of CHD using XMS-CASPT2. The interatomic distances are indicated in Ångström. MECI stands for the minimum energy conicalintersection. Adapted from ref. 80.
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Papercalculated along these trajectories. Excellent agreement with the experimental
result was found. Fig. 8 shows the molecular structures at critical nuclear
congurations along the ring-opening reaction path calculated using XMSCASPT2. One can see that CHD at the Franck–Condon point in S*1 has two C]Cdouble bonds as in the ground state. However, in the region between the S2/S1 andS1/S0 conical intersections, all of the carbon–carbon interatomic distances
become essentially the same, except for C5–C6 that is going to dissociate.
Considerably greater structural deformation occurs in the ground state of HT,
which can be probed using electron/X-ray diﬀraction methods as described below.
Now we consider the methods to probe structural dynamics. The rst is UED.
Unlike the conventional electron diﬀraction experiment shown in Fig. 4 using
a continuous electron beam, UED employs a pulsed electron beam, so that it istechnically demanding to obtain a high signal-to-noise ratio and high temporaland spatial resolution while avoiding space-charge eﬀects. While photoelectron
spectroscopy and transient absorption spectroscopy are sensitive to electronic
structures and non-adiabatic transitions, ultrafast electron/X-ray diﬀraction
provides invaluable information on structural dynamics.41–47,82–84 Fig. 9 presents
electron diﬀraction data obtained using a relativistic electron beam with anenergy of 3.7 MeV (corresponding to a wavelength of 0.3 pm). The atomic pair(interatomic distance) distribution function for CHD before photoexcitation
shown in Fig. 9(a) is dominated by two peaks corresponding to the distances of

Ultrafast electron diﬀraction: (a) experimental (red) and simulated (light blue) pair
distribution functions for CHD. The two main peaks are related to R1 and R2 in the inset. (b)Simulated pair distribution functions for cZc-HT (blue), cZt-HT (orange), and tZt-HT(green). An experimental function for a delay time of 0.55 ps is also shown (red). (c)Experimental map of the time-evolving pair distribution function. The solid line indicatesthe centre of mass position in peak area g between 3 and 6 
A. Blue and red correspond tonegative and positive signals, respectively. Adapted from ref. 84.
Fig. 9

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the rst and second nearest neighbour carbon atom pairs. The prominent change
observed aer photoexcitation is the appearance of a signal due to larger carbon–
carbon distances (Fig. 9(b)), which can only be possible for HT; therefore, theenhancement of the g peak, which refers to the signal for the interatomic distance
between 3 and 6 A, is simply due to the formation of the HT structure.84
Comparing the time evolution of the observed g peak with theoretical simulations
revealed that the centre of mass position of the g peak exhibits an oscillatory
feature, as shown by the red line in Fig. 9(c). This oscillation is ascribed totorsional motion around the C–C single bonds newly formed by the ring-opening
reaction, and it corresponds to rotational isomerization among the cZc, cZt, andtZt forms. The instrumental response function of this experiment was 160 fs.
Although it is desirable to further improve the temporal resolution, UED with thecurrent resolution is already highly useful for studying structural dynamics invarious reactions.
Diﬀraction experiments generally do not diﬀerentiate electronic states of thetarget molecule, and IAM analysis of diﬀraction signals does not either. Yang et al.
attempted to eliminate this limitation and investigate both the electronic andFig. 10 Inelastic scattering during ultrafast electron diﬀraction: (a–c) Experimental andsimulated electron diﬀraction signals for pyridine. (a) Experimental percentage diﬀerence(PD) signal, normalized to the 9% excitation ratio. (b) Simulated PD signal using theindependent atom model (elastic component only). (c) Simulated PD total (elastic andinelastic) signal using ab initio electron diﬀraction simulation. In (a) to (c), the red dottedline denotes a momentum transfer s ¼ 1.1 Å1. (d and e) Experimental (d) and simulated (e)temporal evolution of the small-angle PD signal (blue, normalized to the maximum value)and dihedral angle (red) deﬁned in (f). The uncertainty is represented by the shaded
regions, calculated as one standard deviation of a bootstrapped dataset (experiment) or
one standard error of the mean (SEM) of all trajectories (simulation). (f) Deﬁnition of thedihedral angle and illustration of a plausible molecular structure. Simulation results areconvolved with a 150 fs Gaussian kernel to account for the experimental instrumental
response function. Adapted from ref. 44.
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44,85

structural dynamics with a single UED experiment.
They excited pyridine
molecules to the S1(n, p*) state with 265 nm light, and investigated subsequent
conformational changes and internal conversion to S0 using UED with an
instrumental time resolution of 150 fs.44 One of the new features of their experiment was detection of inelastically scattered electrons at small scattering angles.
Their results are shown in Fig. 10(a)–(c). The inelastic scattering signal is seen in
Fig. 10(a) in the region of a small transfer momentum s. The lifetime of this signal
is very short (1.1 ps) and comparable with the calculated S1(n,p*) lifetime (1.3 ps),suggesting that it represents the S1 population. Simulation based on the IAM(Fig. 10(b)) does not reproduce the inelastic scattering signal, while simulation
based on the quantum chemically calculated electronic structure of pyridine(Fig. 10(c)) explains both the inelastic and elastic scattering signals.
An important dynamical feature revealed by UED is an ultrafast structuralchange within 500 fs. Since photoexcited pyridine has a vibrational energy of3000 cm1 in the S1 state, the molecule overcomes a small potential barrier(640 cm1) in the ring-puckering coordinate and lis a carbon or nitrogen atom(these elements cannot be diﬀerentiated experimentally) out of the molecularplane. This out-of-plane deformation triggers internal conversion to S0 at a dihedral angle of 150–160 ; Fig. 10(d) and (e) respectively show the experimental andcomputational results for the S1 population and the dihedral angle that is dened
in Fig. 10(f). The two results are in good agreement. It is noted that many theoretical studies indicated that out-of-plane deformation promotes ultrafast
internal conversion in aromatic molecules such as DNA bases; however, suchdeformation cannot be easily identied with spectroscopic methods. Therefore,
the real-time observation of out-of-plane deformation demonstrated for pyridine
is of interest.86 Ultrafast internal conversion followed by intermolecular vibrational energy transfer in solution or in biological environments prevents photochemical decomposition of a molecule; a low quantum yield for photochemical
decomposition is oen expressed as the photostability of a molecule.87,88 Some
researchers have suggested that photostability was the crucial property of DNA
bases for recording genetic information under strong UV radiation on prebiotic
Earth.86,88,89
Finally, Yang et al. ruled out the previous claim90 that pyridine undergoesa ring-opening reaction within the observation time window.75 It is noted that theisomerization and dissociation barriers for pyridine in the ground state have been
calculated to be 90–100 kcal mol1; therefore, pyridine should undergo isomerization and dissociation if the internal energy is suﬃciently large.91 However,these processes take a long time.
Returning to the ring-opening reaction of CHD, this reaction has also beenstudied using X-ray diﬀraction, and the results elucidated another interesting
aspect of the reaction. The issue here is the occurrence of interconversion
between CHD and HT aer their formation by an ultrafast photochemical reaction. The photoexcited molecules that return to the ground state have suﬃcientinternal energy to surmount the potential barriers, so that CHD–HT interconversion may occur in addition to the rotational isomerization of HT discussed
earlier. Ruddock et al. investigated CHD–HT interconversion by exciting CHD to
the 3p Rydberg state at 200 nm (6.2 eV), rather than to the S1 state excited at
267 nm, and they followed the structural evolution using ultrafast X-ray diﬀraction up to a delay time as long as 1 ns.43 Since the photoexcited Rydberg state of
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CHD undergoes internal conversion to the ground state within several hundred
femtoseconds, the X-ray scattering signals at delay times from picoseconds to 1 ns
are primarily from highly vibrationally excited CHD and HT molecules in S0.
Fig. 11(a) shows the time-dependent X-ray scattering signal in two selected
regions of the transferred momentum q (corresponding to s in electron diﬀraction). The rapid change within 1 ps is attributed to a ring-opening reaction to
produce HT and non-reactive internal conversion to CHD from the Rydberg state,and both processes are complete well within 1 ps. Surprisingly, the diﬀraction

Fig. 11 Ultrafast X-ray diﬀraction. (a) Experimental percentage diﬀerence scattering signal

after 200 nm photoexcitation of CHD measured as a function of the pump–probe timedelay. Plotted are averages over two momentum transfer vector (q) ranges (dots) and the
kinetic ﬁts (lines). q is given by 2k0 sin q where k0 is the incident wave vector and q is thescattering angle. (b) Relative populations of the three components on a logarithmic
timescale: electronically excited CHD*, the vibrationally excited ground state CHDhot, andthe vibrationally excited ground state HThot. The background colours indicate the diﬀerent
time regimes of the kinetics: blue is dominated by the decay of CHD*, orange is dominatedby CHDhot, and green indicates the equilibration to 67% HThot and 33% CHDhot. (c)Percentage diﬀerence scattering pattern for hot HT. Experimental data points are indicated with purple dots, and the scattering pattern calculated using molecular dynamics
simulation is indicated by a black solid line. (d) Histogram of the three torsional angles in
HT used for simulating the scattering pattern shown in (c). Adapted from ref. 43 and 92.
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intensities in the two diﬀerent regions continuously change, which points toa slow isomerization reaction in S0. Ruddock et al. analysed the time-dependent
X-ray scattering distribution, P(q,t), as an incoherent sum of the scattering signals
from diﬀerent chemical structures, as expressed by the following equation,
P(q,t) ¼ [CHD*(t)]SCHD*(q) + [CHDhot(t)]SCHDhot(q) + [HThot(t)]SHThot(q)where [CHD*(t)], [CHDhot(t)], and [HThot(t)] are the time-dependent populationsof CHD in the Rydberg state, vibrationally excited CHD produced by internalconversion to the ground state, and the HT product in the ground state, respectively. SCHD*(q), SCHDhot(q), and SHThot(q) are the associated time-independent
scattering patterns. By least squares tting of time-dependent diﬀraction
signals, they extracted the individual population and scattering patterns for thethree species. The time-dependent populations shown in Fig. 11(b) indicate thatthe reaction starting from the Rydberg state predominantly populates the groundstate of CHD rather than HT, while isomerization from the former to the latterultimately reverses their populations. SHThot(q) determined by the least squares
tting of the time-dependent diﬀraction pattern is shown as purple dots in
Fig. 11(c). Ruddock et al. compared the extracted diﬀraction patterns with thosecomputationally predicted using nuclear trajectory calculations on the groundstate potential energy surface and the IAM (black solid line),92 and they foundgood agreement between them. The distribution of F1, F2 and F3 in hot HT
calculated using molecular dynamics simulations is shown in Fig. 11(d),92 which
reveals a quite broad probability density distribution, as anticipated for highly
vibrationally excited molecules. This work is perhaps one of a few studies that
followed slow isomerization processes on a sub-picosecond to nanosecond timescale under collision-free conditions. This study also demonstrated that X-ray
diﬀraction can capture a minute change in the electron density distribution in
CHD upon single electron excitation to a Rydberg orbital.

III. Photochemical reactions in crystals andsolutions
It is of great interest to study in situ the photoinduced dynamics of molecules thatplay a key role in biological processes. However, one of the obvious problems isthat if a photochemical reaction occurs irreversibly, the same sample cannot be
employed for repeat measurements, and the sample needs to be replaced for eachsubsequent measurement. Serial femtosecond crystallography93 overcomes thisdiﬃculty by introducing a stream of minute crystals for X-ray diﬀraction
measurements. An intense X-ray pulse from an XFEL is diﬀracted before thesample breaks apart (diﬀraction-before-destruction principle), and a fresh crystalline sample is delivered for each measurement. Therefore, this method enables
studies on irreversible reactions and avoids the inuence of radiation damage.94 It
might be thought that a crystalline environment is too rigid for a photochemical
reaction to occur; however, crystals of biological macromolecules contain suﬃcient internal space and are exible enough for structural changes to occur upon
photoexcitation.
Photoreactive yellow protein (PYP) is a blue-light photoreceptor for negative
phototaxis of the purple phototrophic bacterium Halorhodospira halophila. The
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function of PYP is expressed through trans–cis photoisomerization of its chromophore, p-coumaric acid (pCA), which triggers conformational change of theprotein on a millisecond time scale. In an experiment with PYP, Pande et al.
excited a microcrystal with 450 nm light pulses (140 fs) and measured the
diﬀraction of 9 keV X-ray pulses (40 fs). They found that pCA undergoes trans–cis
photoisomerization at around 590 fs.95 The pCA molecule is covalently bonded to
Cys69 in the side chain of PYP, and the phenolate oxygen (O40 ) of pCA is hydrogenbonded to Glu46 and Tyr42. Prior to photoexcitation, the torsional angle (ftail) for
the C2]C3 double bond is 172 (yellow lled circle in the plot shown in Fig. 12).
100–400 fs aer photoexcitation, ftail becomes 136 , and the hydrogen bondlength between pCA–O40 and Glu46 increases from 2.5 to 3.4 A, which enables themolecule to undergo structural change more freely. In 400–1200 fs, ftail diminishes to 53 , and it ultimately becomes 35 at 3 ps (see Fig. 12). The hydrogenbond length between pCA–O40 and Glu46 also returns to 2.94 
A at 3 ps. Although
the X-ray diﬀraction study did not diﬀerentiate the electronic states before andaer the reaction, Pande et al. speculated that the system reaches the S1/S0 conicalintersection at around 590 fs (the red and green background colours in Fig. 12
indicate the excited and ground states, respectively). This speculation is inaccordance with the results of transient absorption spectroscopy, which indicatedthat the population decay of the excited state can be expressed using a multiexponential function with a dominant lifetime component of 0.5–0.6 ps (50%).96,97
Compared with X-ray diﬀraction measurements of crystals, similar experiments for solutions are highly challenging, because solute molecules are
distributed with unequal intermolecular distances and random orientations.
Moreover, the large excess of solvent molecules also creates diﬀraction signals.
The diﬀraction signal from a solution can be categorized into three components:

Serial femtosecond crystallography: structure of p-coumaric acid (pCA) andmeasured torsional angle ftail for the C]C double bond. The pink region corresponds tothe twisted trans form on the excited state potential energy surface (PES), and the lightgreen region corresponds to the cis form on the ground state PES. Experimentally
determined ftail values are indicated with solid spheres. The vertical dashed line indicatesa transition time of about 590 fs. Insets: structures of PYPfast (pink), PYPslow, and PYP3ps(light green), and the dark-state structure PYPref is indicated with a yellow sphere. The
diﬀerence electron density is shown in the insets in red (3s) and blue (3s). Adapted fromref. 95 and 98.
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a solute-only component, a solute–solvent cross component, and a solvent-only
component. The solute–solvent cross component depends on the interatomic
distances between the solute and solvent molecules. The solvent-only component
depends on the temperature and density of the solvent molecules, which are

Ultrafast X-ray liquidography (diﬀraction by a liquid sample): (a) standard procedure for data analysis. For details see ref. 99. The discrepancy between the experimental
and simulation results is minimized using a global ﬁtting method considering the data for
all time delays simultaneously. Boxes in red and blue frames represent solute-related(solute-only and solute–solvent) and solvent-only components, respectively. Boxes withyellow background are involved in iterative adjustment. (b) Nuclear motion of Au atoms in0
the T 1 excited state represented by the multidimensional nuclear coordinates RAB versus
RBC versus q. The projection onto the RAB–RBC plane is shown at the bottom. The equi0
librium distances for RAB and RBC in T 1 and S0 are indicated by the red dashed lines. The
colours of the dots represent the time delays shown on the colour scale. (c) Transient
structures at representative time delays are illustrated, in which Au atoms at each timedelay are represented by yellow dots, and those in the FC region are represented by grey
dots. The black solid lines in (c) indicate covalent bonds. The changes in interatomic
distance and angle are indicated by red and blue arrows, respectively. The ligands areomitted for simplicity. Structural changes are exaggerated for clarity. Adapted from ref. 99
and 100.
Fig. 13

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altered by energy transfer from photoexcited solute molecules or photoexcitationof the solvent itself. Analysis of the highly complex diﬀraction signal froma solution sample is only possible with extensive computational (quantum
chemistry and molecular dynamics) simulations as depicted in Fig. 13(a).99
Nonetheless, with regard to the photochemistry of the dicyanoaurate trimer,[Au(CN)2]3, the analysis was drastically simplied. What was highly favourable
for this experiment was that the scattering signal from the Au atoms was so strongthat the scattering from the C and N atoms and the solute–solvent crosscomponent were negligible. Only the solvent-only component, which is due toheating by intermolecular energy transfer from the solute, was considered. The
dicyanoaurate trimer is weakly bound in the electronic ground state by aurophilicity, which originates from relativistic eﬀects in the electronic structure of Au
atoms and provides a bonding strength comparable with that for hydrogenbonds. Electron promotion from the anti-bonding s*(dz2–dz2) to the bondings(px–px) molecular orbitals triggers formation of covalent bonds in the excitedstate. Kim et al. photoexcited [Au(CN)2]3 in an aqueous solution to the S1 statewith 267 nm pulses and followed the subsequent structural transformation using
X-ray diﬀraction with 12.7 (or 15) keV pulses. Although the dimer concentration
was two times higher than that of the trimer, the extinction coeﬃcient for the
dimer was 27 times smaller than that for the trimer, so the contribution of the
dimer was considered negligible. The FWHM of instrumental response function
was 170–320 fs, depending on the facility (PAL or SACLA). Fig. 13(b) shows thebond lengths and bond angles between the three Au atoms in the trimer
determined as a function of time.100 It can be seen that the rst Au–Au bond isformed within 60 fs, and the second bond is formed in about 300 fs. Duringformation of the covalent bonds, the conguration of the three Au atoms
changes from a bent to a linear structure (Fig. 13(c)). These remarkable
experimental results highlight the high potential of ultrafast X-ray diﬀraction
experiments on solutions made possible using XFELs. One note is that thisreaction has also been investigated using optical spectroscopic methods, such astransient absorption,101 emission,102 and stimulated Raman scattering,103 andthere are some controversies regarding the assignment of the electronic states
and the reaction times. These points are expected to be claried in the near
future.
In transient absorption spectroscopy of solutions, a comprehensive analysis isoen performed on electronic spectra in the visible/UV region and vibrational
spectra in the infrared region. Similarly, X-ray absorption and infrared spectroscopies provide complementary information on the electronic and structuraldynamics. Ferrioxalate [Fe(III)(C2O4)3]3 is known to undergo a rapid photochemical reaction under sunlight and induces decomposition of contaminants inwater. The reaction is also employed as a chemical actinometer. The reaction isoen expressed as,
2[Fe(III)(C2O4)3]3 + hn / 2[Fe(II)(C2O4)2]2 + 2CO2 + C2O42
However, the elementary reaction mechanism was controversial.104–108 Ogi et al.,studied this reaction using transient absorption spectroscopy in the hard X-ray
region using an XFEL. They excited a 0.1 M aqueous solution of ferrioxalate
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with 268 nm pulses and observed the absorption spectrum at the K-edge of iron(7.1 keV) with a time resolution of 200 fs.109 Fig. 14(a) compares the X-ray
absorption spectra of the solution before (black, 3 ps) and aer (blue, 7 ps)
UV irradiation, and the spectrum of the photoproducts (red) extracted using
spectral analysis. The absorption edge for the product was 6 eV lower than that for
the reactant, indicating that the Fe atoms were reduced. The continuous
absorption in the energy region above the absorption edge exhibits oscillatory
features, which are due to quantum mechanical interference of outgoing photoelectron waves scattered by oxygen atoms adjacent to Fe. From these features, theiron–oxygen bond lengths in the product were estimated to be 2.04 
A. Fig. 14(b)shows time proles measured at diﬀerent X-ray photon energies, which indicate
a decrease in the parent signal and immediate appearance of the red-shiedTransient X-ray and infrared absorption spectroscopy: (a) reconstructed spectrumof the photoproduct (red) generated from the static spectrum of [Fe(III)(C2O4)3]3 (S ¼ 5/2)
(black) and the spectrum measured at t ¼ 7 ps (blue). (b) Time proﬁles of the X-ray ﬂuorescence intensities excited at diﬀerent X-ray photon energies and a global ﬁt of the
exponential decay curves (solid lines) measured as a function of time delay after irradiation
by 268 nm pulses. (c) Schematic reaction mechanism for ferrioxalate, and experimental(upper graphs, symbols) and theoretical infrared spectra (lower graphs) for the ferrioxalate
parent, the isomers of the ferrous carbon dioxide primary complex, and the ﬁnal ferrous
dioxalate product together with a free CO2 radical anion. Adapted from ref. 109 and 112.
Fig. 14

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absorption spectrum of the photoproduct. From these results, they concluded
that the product was formed within 140 fs aer photoexcitation. Based on the
spectral redshi and Fe–O distance predicted by time-dependent density functional theory calculations, it was concluded that ligand-to-metal charge transfer(LMCT) induces CO2 release from one of the ligands to produce½ðCO$2 ÞFeðIIÞðC2 O4 Þ23 : The LMCT reduces the charge of the oxalate ligand from
2 to 1, which destabilizes the ligand and induces dissociation of a neutral CO2
molecule. The remaining CO2 ligand is only weakly coordinated to Fe, so the
product is expected to dissociate into [Fe(II)(C2O4)2]2 and a CO2 radical anion.
However, the yield and reaction time of this secondary dissociation process could
not be determined using X-ray absorption spectroscopy with a limited observation
time window in limited beam time. Later, transient infrared absorption spectroscopy performed by Mangiante et al. and Straub et al. unambiguouslyconrmed production of neutral CO2 within the response time (< several hundred
femtoseconds) of the infrared spectrometer,110–112 in agreement with the X-ray
experiment. Moreover, ejection of CO2 radical anions in a nanosecond timescale was conrmed. Fig. 14(c) shows the signature of CO2 radical anions (theblue-coloured peak at 1665 cm1) observed aer 1 ns. Infrared spectroscopy
showed that 35% of the photoexcited ferrioxalate undergoes non-reactive internalconversion to the ground state and 65% undergoes dissociation to produce
neutral CO2.111,112

IV. Summary
In this paper, I presented a brief overview of some recent experimental studies inthis eld as an introductory lecture for the Faraday Discussion on “time-resolved
imaging of photo-induced dynamics”. Chemical reaction dynamics involve bothelectronic and structural dynamics, and there is no experimental methodologythat can reveal all aspects of these dynamics in a single experiment. Diﬀerent
experimental methodologies provide complementary information, so theircomprehensive analysis is invaluable. It is also highly desirable to compare
experimental results with theoretical and computational simulations, such as
high-precision quantum chemical calculations and molecular dynamics simulations, for interpretation of the experimental results. The computational methods
are now at a suﬃcient level of accuracy to provide (semi-)quantitative replicationand prediction of experimental results. Computational demand will further
increase in order to deal with more complex systems, in which machine learningor the use of assistive intelligence will be sought.
The experimental methods to track the reaction from start to nish are inherently “universal” in the detection of molecules, and they are inevitably sensitive to
unexcited reactant molecules in the ground state. For example, XUV probe pulses
ionize the reactants to produce photoelectrons, and electron/X-ray pulses arescattered by the reactants to produce diﬀraction signals, even in the absence ofpump pulses. If only 1% of reactant molecules are photoexcited by pump pulses,
99% of the signal caused by probe pulses arises from unexcited reactant molecules.
Experiments are even more challenging for solutions, as the solvent also produces
a signal. Thus, the common technical challenge in these experiments is extractionof a small signal of interest. High stability and a high repetition rate for pulsed
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performing successful measurements. The Linac Coherent Light Source (LCLS) is
being upgraded to LCLS-II, and the repetition rate will increase by four orders ofmagnitude from the current 120 Hz to 1 MHz. The Extreme Light Infrastructure(ELI) is a similarly large laser project launched in Europe, and it will oﬀer user
facilities for high-eld physics and ultrafast laser science.113 ELI-ALPS (ELI attosecond light pulse source) has already started oﬀering light sources producing XUVattosecond pulses.113 The achievement of higher repetition rates is a common goal
for technical development of all quantum beam sources including these large-scale
facilities and table-top systems.
Besides such large-scale facilities, HHG has had a great impact on manylaboratory experiments. Thanks to advances in laser technology and industrial
manufacturing, compact turn-key Ti:sapphire laser systems are now commercially available, and they deliver pulses with durations of 30 fs and energy greaterthan 5 mJ at a 1 kHz repetition rate. These lasers enable straightforward implementation of HHG-based XUV light sources in the laboratory. However, attosecond light sources using carrier envelope phase control and so X-ray
generation using near IR drivers require expertise and/or much investment at thispoint. Optical parametric chirped pulse ampliers are employed in a limited
number of advanced laboratories. These technologies, however, will become moreaccessible for many researchers in the future. Ti:sapphire driving lasers aregradually changing to alternatives such as Yb-based lasers with low thermal
eﬀects, and the capability to achieve high repetition rates up to the megahertz
range. Such new laser systems will certainly improve the performance of laser
spectroscopy and expand opportunities for exploring new aspects of the photophysical and photochemical dynamics of molecules.
Advanced facilities certainly oﬀer exciting research opportunities. However, it
should also be stressed to young people that not all great scientic discoveries are
made at large advanced facilities. Dr Donna Strickland received the Nobel Prize inPhysics in 2018 for chirped pulse amplication, which she studied in the laboratory
during graduate school. Young people certainly have chances for great discoveries
in laboratory work every day. I remember a famous joke that one of my professors
told me many years ago, when I was a graduate student. This story is sometimes
called “the streetlight eﬀect”. The story goes like this. A policeman sees a man
searching for something under a streetlight and asks what he has lost. The mansays he lost his key (or it could be a coin). They both look for it under the streetlightbut they cannot nd it. Then, the policeman asks if the man is sure he lost it here,
and the man replies, no, he lost it elsewhere. Then, the policeman asks why he issearching here, and the man replies, “this is where the light is.” The intention of theprofessor who introduced me to this story was perhaps to convey the stance that
researchers should not take. Young students and researchers have great opportunities in research driven by their curiosity and courage to search in dark places.
The invited and contributed papers of this Faraday Discussion present cuttingedge research in this exciting research eld and will certainly be inspirational tothe participants at the meeting and many other readers of these papers and therecords of the discussion.

Conﬂicts of interest
There are no conicts to declare.
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Acknowledgements
I gratefully appreciate the eﬀorts of the organizers to make this Faraday Discussion
possible as an internet conference at such a diﬃcult time for mankind during the
COVID-19 pandemic. I also express my gratitude to Professor Andrew Orr-Ewing
and Dr Daisuke Koyama for measuring (unpublished) transient infraredabsorption spectra for the photochemical reaction of the ferrioxalate complex.
Financial support by JSPS KAKENHI (Grant Number 15H05753) and the Mitsubishi Foundation is appreciated.

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