PRL 119, 203203 (2017)

PHYSICAL REVIEW LETTERS

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Different Time Scales in the Dissociation Dynamics of Core-Excited CF4
by Two Internal Clocks
H. Iwayama,1 C. Léonard,2 F. Le Quéré,2 S. Carniato,3 R. Guillemin,3 M. Simon,3 M. N. Piancastelli,3,4 and E. Shigemasa1
1

UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
Laboratoire Modélisation et Simulation Multi Echelle UMR 8208 CNRS, Université Paris-Est Marne-la-Vallée,
F-77454 Marne-la-Vallée, France
3
Sorbonne Universités, UPMC Univ Paris 6, CNRS, UMR 7614, Laboratoire de Chimie Physique-Matière et Rayonnement,
F-75005 Paris, France
4
Department of Physics and Astronomy, Uppsala University, SE-75120 Uppsala, Sweden(Received 17 February 2017; revised manuscript received 13 June 2017; published 15 November 2017)2

Fragmentation processes following C 1s → lowest unoccupied molecular orbital core excitations in CF4
have been analyzed on the ground of the angular distribution of the CFþ
3 emitted fragments by means ofAuger electron-photoion coincidences. Different time scales have been enlightened, which correspond toeither ultrafast fragmentation, on the few-femtosecond scale, where the molecule has no time to rotate andthe fragments are emitted according to the maintained orientation of the core-excited species, ordissociation after resonant Auger decay, where the molecule still keeps some memory of the excitation
process before reassuming random orientation. Potential energy surfaces of the ground, core-excited, andfinal states have been calculated at the ab initio level, which show the dissociative nature of the neutral
excited state, leading to ultrafast dissociation, as well as the also dissociative nature of some of the final
ionic states reached after resonant Auger decay, yielding the same fragments on a much longer time scale.
DOI: 10.1103/PhysRevLett.119.203203

Following core excitation, the relaxation pathway of amolecular system might involve complex fragmentation
patterns, depending upon the nature (bound or dissociative)of the core-excited state and/or of the final states reachedduring the relaxation process. In particular, in manysystems elongation and breaking of chemical bonds canoccur during the lifetime of the excited state: in this case,the dissociation is defined as ultrafast, since typical lifetimes of core-excited states in molecules containing lightatoms are of the order of few femtoseconds (see, e.g.,
Refs. [1–10]). Molecular fragmentation can also occur in a
final state reached after resonant Auger decay, if such a
final state is unstable. In this case, the time scale for the
breaking of the chemical bond is much longer.
The competition between electronic decay and fragmentation in molecules was reported already in the literature(see, e.g., the seminal papers [11–13] on HCl, DCl, Cl2 , and
HF). Ultrafast phenomena can be identified by means of
their clear signature in the resonant Auger spectra, which
exhibit two types of well-separated lines: together with thespectral features of the molecular final states, extra peaks
related to fragments are present, and can be assigned on the
ground of their dispersion law [4–8,11–13]. This is thebasis of the so-called core-hole clock method, which allows
characterizing ultrafast phenomena with an “internal
clock,” the core-hole lifetime, rather than by the use of atime-resolved source [1–3].
While ultrafast dissociation is possible during the corehole lifetime and can be detected in the resonant Auger
0031-9007=17=119(20)=203203(5)spectra, dissociation on a longer time scale, and, namely,
after a final state is reached which can in turn be
dissociative, does not yield a clear signature in the decay
spectra, because the fragment spectral patterns cannot be
detected.
A direct comparison between pathways leading to thesame fragmentation products, but of a different nature, and,in particular, with very different time scales, has not yet
been attempted, which is the heart of the present work.
Here we examine a prototypical system for fast relaxation dynamics, and, namely, CF4 . This molecule is known
to undergo ultrafast fragmentation yielding the excitedC F3 species after C 1s core excitation to the LUMO
(lowest unoccupied molecular orbital). We have alreadyreported such fragmentation pattern [14]. Resonant Augerion coincidences for this system have been published [15].
However, the relaxation process of CF4 is rather complex,
because fragmentation is possible not only due to thedissociative nature of the intermediate core-excited state,
but also because some of the ionic final states reached after
resonant Auger decay are in turn dissociative.
A crucial point is that although the produced charged
fragment CFþ
3 is the same in both cases, these two
dissociation patterns are very different, due to their different
time scale. In particular, the angular distribution of theejected fragment reflects the time scale of the bondbreaking: if the bond rupture occurs during the core-hole
lifetime, the molecules are still oriented along the directionof the electric field vector, and the fragments are ejected in a

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very directional manner, while if the dissociation takes
place in the final states it takes much longer times, themolecular ions have time to rotate and to reach a random
distribution, and the ejection of the fragments is rather
isotropic. However, we will show in the following discussion that some “memory” of the orientation induced byprimary excitation can remain.
While there is a rather large literature describing ultrafast
dissociation enlightened in resonant Auger spectra, there isno analogous bulk of works on angular distribution offragments.
In a previous paper [16], photoion imaging for CFþ
3 ionsproduced after valence ionization of CF4 was reported, andfound that for some of the photon energies used the
distribution was not isotropic. This finding was related
to the populations of excited states with symmetry lower
than the T d symmetry group of the CF4 ground state.
Similar observations were reported in another paper [17] by
threshold photoelectron-photoion imaging measurements.
However, the time scales and the wave-packet dynamics ofthe processes were not discussed in either work.
We performed resonant Auger electron-ion coincidence
measurements, which allowed us to enlighten the angular
distribution of the fragments, which is very different
according to the time scale of photodissociation.
In particular, fragments emitted during the core-hole
lifetime possess a directional angular distribution, since themolecule is oriented by the primary excitation process with
one of the C─F bonds along the Ē vector, and has no time
to rotate during few femtoseconds.
At variance with that, when dissociation occurs after an
unstable final state is reached following Auger decay, ittakes much longer time, and the molecule can reassume arandom distribution, which can lead to an isotropic distribution of the emitted fragments. However, we were able
to detect some subtle differences between the fragmentation patterns for a specific final state on resonance and off
resonance, hinting at the molecule keeping some “memory”
of the excitation process.
The experiments were performed on the soft x-ray beamline BL6U at UVSOR Synchrotron Facility, Okazaki,
Japan. The radiation from an undulator was monochromatized by a variable-included-angle varied-line spacing
plane-grazing monochromator. The photon energy of
298.4 eV was used for C 1s-to-LUMO core excitations,and the photon energy resolving power was set to 10 000.
The polarization vector was horizontal. To estimate offresonant photoionization contributions, we also conductedmeasurements at photon energy of 296.4 eV. The monochromatized radiation was introduced into a cell withsample gases.
The experimental setup for Auger electron-photoion
coincidence (AEPICO) measurements comprised a double
toroidal electron analyzer and an ion momentum spectrometer, each of which is equipped with time- andweek ending
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FIG. 1. Top: schematic of core excitation followed by ultrafast
dissociation and resonant Auger emission from the fragment.
Bottom: schematics of resonant Auger decay in the molecule
followed by dissociation in the molecular ion.

position-sensitive detectors. Full details of its design andoperation are described elsewhere [18,19].
In Fig. 1 we show a cartoon of the two different
processes leading to the same fragment CFþ
3 , one on thetime scale of a few femtoseconds, the other one on a much
longer one. High-resolution resonant Auger spectra andtheir detailed assignment are reported in Ref. [14]. Here weconcentrate on Auger electron-photoion coincidence measurements with sufficient electron kinetic energy resolution
to identify the various features in the decay spectrum.
In Fig. 2, top, red line, we show the energy position of
the resonant Auger electrons emitted after C 1s → LUMO
excitation on a binding energy scale. In our previous paper[14], we were able to identify the spectral features as relatedþto the intact molecular ion CFþ
4 or to the CF3 fragment on
the ground of their dispersion law. The filled red spectrum
shows coincidence measurements of resonant Auger electrons and CFþ
3 ions.
The electron kinetic energies of peak labeled area A in
the figure correspond to the resonant Auger decay of theC F3 fragment originated from ultrafast dissociation, while
the electron energy of the peak labeled area B is related to a−1
molecular CFþ
4 final state, namely, the 3t2 one [14].
þThis is the first indication that CF3 is produced in two
different photodissociation channels.
In Fig. 2, bottom, we report the angular distribution of
the CFþ
3 fragments in coincidence with electrons of the area
A (left panel), and of the area B (right panel). The directionof the Ē vector is vertical in both cases.
In both lower and upper panels the contribution of direct
photoemission has been eliminated by subtracting spectra
taken off resonance. Such contribution does not exist forarea A, which appears only on resonance. For area B, theline shape is very different on and off resonance (seeRef. [14]). In particular, it is distorted by the nuclear motion
due to the wave packet evolving along the potential energy

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Kinetic energy of Auger electrons [eV]Area A{Counts{Area B

Binding energy of Auger final states [eV]

Y [mm]

Area A

Area BE

X [mm]

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PHYSICAL REVIEW LETTERS

PRL 119, 203203 (2017)

X [mm]

FIG. 2. Top, red line: resonant Auger spectrum after C 1s →LUMO excitation; solid red area: coincidences between theresonant Auger electrons and the CFþ
3 ion. Bottom: angular
distribution of the CFþ
3 fragments in coincidence with theelectrons emitted in the resonant Auger decay of the fragment(left panel) and in a molecular resonant Auger decay (right panel).
The direction of the Ē vector is indicated.

However, by a closer look, we notice that the distribution
even for area B is not completely isotropic. This finding
is confirmed by the angular distributions of the fragments(β values), which are 1:1  0:1 for area A and 0:4  0:1 forarea B, obtained by fitting the angular distributions with thestandard β formula [16].
While the fragment angular distribution can immediately
give a qualitative description on the dissociation taking
place during core-hole lifetime or later, a deeper assessmenton the different time scales is needed, which can be derived
from suitable theoretical work.
In order to describe the potential energy surfaces of all
ground, intermediate and final states involved, and toobtain a quantitative evaluation of the dynamics of thedifferent fragmentation processes, ab initio calculations
were performed. The technical details are described inRef. [20]. The resulting curves have been plotted along theC─F bond distance in Fig. 3 with the definition of the
dissociation limits at large distances. On the bottom, weshow the electronic ground state potential curve of CF4 . On
the middle, the lower CFþ
4 final states are displayed. The
black curves correspond to the 3t−1
2 final state, for which
we have shown in a previous publication that we canobserve resonant enhancement. The three degenerate electronic components in T d symmetry split into one 2A1 andone 2E states in Cs symmetry when the C─F bond length is
stretched. These two states present barrier to dissociation
302

*

C F4

300

1

C*F3(A2A1)+F( 2P)

1

C*F3(X2A1)+F(2P)

A1−d1

298

9 fs

296
294
292
26
24
Energy (eV)surfaces. Therefore, we assume that a possible interference
between direct and resonant contributions is negligible.
We can immediately visualize that in case of ultrafast
fragmentation (left panel) the angular distribution of theCFþ
3 fragments is very asymmetric and peaked along thedirection of the Ē vector, while in the case of CFþ
3 yield
after resonant Auger decay reaching the 3t−1
final
state
the
2
angular distribution of the fragments is much moreisotropic.
Such large difference is due to the very different timescales of the two fragmentation processes. The molecule isoriented by the primary excitation with the Ē vector along
the direction of one of the C─F bonds. If the CFþ
3 fragment
is produced during the lifetime of the core-excited state,
which is of about a few fs, the molecule has no time torotate. If the dissociation takes place after one of themolecular final states is reached following resonant Auger
decay in the intact molecule, in principle the process takes amuch longer time, the molecule has time to rotate, can“forget” the direction of the excitation, and the C─F bondbreaking does not occur preferentially along the directionof the Ē vector, but it is rather isotropic.

B1

A1−d2

B2

A

2

E (3t 2−1)

22

2

20

CF4+

CF3+(1,3E)+F( 2P)

A1(3t 2−1)

CF3+(1,3A2)+F( 2P)

18
CF3+(1A’1)+F( 2P)

16
14
8

CF3(2A1)+F( 2P)

6
4
2

CF4
1

X A1

0
1.2 1.4 1.6 1.8 2
R (Å)

2.2 2.4

FIG. 3. Potential curves along the R ¼ C─F bond distance ofthe ground, intermediate and final states in CF4 . The time step
between the different wave-packet plots is 3 fs. Grayed potentials
are not directly involved in the studied process. The first twoelectronic states of C F4 have been quasidiabatized. All otherelectronic states are adiabatic. The shaded area represents the
decay probability in the fragment (A process) (see text).

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towards the third limit due to avoided crossing with higher
excited states. The lower-lying valence states do not show a
relevant effect on resonance and are plotted in gray. The top
part of Fig. 3 corresponds to the two lowest electronic statesof the core-excited C F4 species.
In order to estimate the C─F bond elongation during theAuger process, wave-packet propagation was performed.
The first step of the experimental process consists in thevertical excitation of the vibrational ground wave packet of
the X1A electronic ground state of CF4 to C F4 . The
electronic ground state X1A1 of C F4 is interacting witha second 1A1 electronic state. As a consequence, an avoided
crossing occurs in the Franck-Condon region. We performed a quasidiabatization of both electronic components
in order to provide potential energy surfaces suitable for thequantum dynamic step. The resulting diabatized potential
energy curves, 1A1 -d1 and 1A1 -d2 , are depicted in Fig. 3.
The 1A1 -d1 state is bound and dissociates towards thesecond asymptote and 1A1 -d2 is purely dissociative, anddissociate towards the first asymptote. They cross at
RC−F ¼ 1.35 Å, close to the equilibrium position of thebound state.
The wave packet was then set on the bound 1A1 -d1 stateand propagated. Because of the diabatic coupling term,part of the wave packet goes in the dissociative channel
whose population increases to 0.9 after 10 fs. Figure 3
shows schematic snapshots of the wave packet every 3 fs
and it is clear from this propagation rate that a significant
part of the C F4 molecules can extend their dissociative
CF bond to reach the C F3 þ F before Auger decay since
the C1s core-hole lifetime of CF4 has been evaluated at
8.5 fs [28].
From this observation we can derive an interpretation of
the angular distribution of the CFþ
3 fragments of Fig. 2.
As mentioned above, the area A angular distribution
suggests an Auger decay from the C F3 fragment.
Experiments have also shown that it corresponds to decays
from the C F3 fragment, produced by the fraction ofmolecules which undergo C─F bond breaking within the
core-hole lifetime of 8.5 fs [14]. Figure 3 shows that, after
18 fs, more than 20% of the molecules not yet decayed haveone of their C─F bond distance increased by 1 Å. We canconsider this position, labeled A in Fig. 3, as an asymptotic
value in the wave-packet evolution, and leading to the
1 0CFþ3 ð A1 Þ þ F dissociation channel. However, during thecore-hole lifetime (8.5 fs) a relevant fraction of excitedmolecules undergo C─F bond breaking, or at least a large
elongation of one C─F bond, and subsequent Auger decay
in the C F3 fragment, accounting for the peak A in the
decay spectrum. The shaded area in Fig. 3 indicates thatsuch decays can take place in a continuous way down totimes much shorter than the marked asymptotic value. This
process is very fast compared to the estimated rotational
period of tetrahedral CF4 which can be evaluated at 300 fs
from Boltzmann rotational distribution at 300 K.

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In Fig. 3, since the potentials involved here follow aparallel evolution along C─F and are already quite flat at
RC─F ¼ 2.4 Å, the transition energy should be close to theasymptotic energy difference of 277 eV. However, ourmodel considers a fixed tetrahedral geometry for the CF3
fragment. Taking into account the deformation of CF3 fromtetrahedral to planar during dissociation, increases theasymptotic transition energy by 2.6 eV, and we expect
the exact value of transition energies to be very close to the
280 eV, maximum of area A in Fig. 2. The vibrational
excitation by kinetic energy reconversion of CFþ
3 can play arole in the transition energy and was suggested experimentally [14] to be at the origin of the broad area A peakof Fig. 2.
The area B angular distribution may result from two
different processes labeled B1 and B2 on Fig. 3, involvingþ
the 3t−1
2 electronic components of CF4 . Although the final
states are the same, we can distinguish between “faster” and“slower” Auger decay processes.
The B1 process corresponds to a fast (<9 fs) Auger
decay from the C F4 region to 3t−1
2 in tetrahedral geometry.
This final state splits into 2E and 2A1 electronic components. The potential wells in these states each supports six
vibrational states which may eventually dissociate via
tunneling but with lifetimes computed from a Prony
analysis of wave-packet autocorrelation functions [29],far greater than the rotational period of the molecule.
Another dissociation pathway for CFþ
4 is fluorescence tothe lower dissociative electronic states which is also a slow
process. This fluorescence has been observed in anotherexperiment [30]. The B1 state-to-state transition energy is275.5 eV which is in good agreement with the peak
position of area B in Fig. 2. On the overall, this B1
transition gives rise to an angular distribution of fragmentsthat should be isotropic since the CFþ
3 fragment is createdhundreds of femtoseconds after the Auger decay.
In order to explain the slight anisotropy observed in area
B of Fig. 2, a dissociative process faster than the rotation
period must be introduced for the same range of transitionenergies. This B2 process occurs if the Auger decay target
1;3
state is CFþ
3 ð EÞ þ F, when the wave packet reaches thedissociative area for RC─F > 1.9 Å or possibly RC─F as
small as 1.4 Å when the kinetic energy of the fragments istaken into account. The B2 arrow in Fig. 3 shows the statesconcerned in the electronic transition but effective transitions will cover a large energy interval, depending on thetime at which the decay occurs. The equilibrium geometriesof these CFþ
3 electronic states are not planar and no angular
geometry rearrangement of this fragment has to be considered. The asymptotic electronic transition energy isapproximately 272 eV in agreement with area B peak limit
in Fig. 2 and Ref. [14] analysis about the dissociation limitþof the 3t−1
2 electronic components of CF4 .
In conclusion, the fragment angular distribution
patterns measured by Auger electron-photoion coincidence

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measurements are very informative of the nature ofintermediate and final states. Combined with theoretical
calculations, such patterns can yield information on thenature of the dissociation processes and on the time scales
concerned. In particular, we are able to identify threedifferent processes labeled A, B1, and B2, leading to thesame fragment, but of different nature, and with very
different time scales, and therefore different angular
distributions.
The authors are grateful to the staff of the UVSOR
Synchrotron Facility for their support and stable operationof the storage ring during the course of the present
experiment.

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