超高速反応する分子の核波束実時間観測と励起状態ポテンシャル曲面のトポロジー
Real-Time Observation of Nuclear Wavepacket Motion ofReacting Molecules and Topology of Potential Energy Surfaces

Molecular Spectroscopy Laboratory, RIKENJAPANTahei TAHARA
ER研究会, Ｊｕｌｙ 9, 2007

The Band Width in Time and Energy

Δt·Δν≥R1 ps

15 cm-1

100 fs150 cm-1

10 fs

1500 cm-1for Gaussian

Observation of Nuclear Wavepacket Motion

oscillatory featureprobe

pump
v=1

v=0

We can observe the wavepacket motion
using ultrashort laser pulses(time-domain vibrational spectroscopy)

Time-domain vs Frequency domain
Heterodyned Impulsive Stimulated Raman of CCl4

Matsuo and Tahara, CPL, 264, 636(1997)

timeTime-domain vs Frequency domain
Heterodyned Impulsive Stimulated Raman of CCl4

Matsuo and Tahara, CPL, 264, 636(1997)

Time-domain vibrational data is equivalent to frequency-domain data.
They are converted to each other by Fourier transformation.

Why Time-Domain Measurement?

Time-domain vibrational spectroscopy
is very powerfulto study excited-state molecules,
especially when
they have only very short lifetimes.
We can get insight about
reactive potential energy surfaces,
which are not simply harmonic!

Energy

Ultrafast reactions are weird special problems?

S1hν

ProductS0Reactant

Nuclear Coordinate

Nuclear Wavepacket Motion of Potential Energy Surface
Photodissociation
Diphenylcyclopropenone

Excited-State Proton Transfer
10-Hydrobenzoquinoline

OOhν

Photoisomerization
cis-Stilbene

NHhνhν
ONH

parallelassistperpendicularObservation of nuclear wavepacket motion
of“reacting” short-lived excited states
Photoisomerization of cis-stilbene
Photo-induced structural change ofbis(2,9-dimethyl-1,10-phenanthroline)copper (I)Photoisomerizationof cis-stilbene

Stilbene:A Fundamental Molecule in Organic PhotochemistryPerformance
NOPA
Tunability
Pulse Duration
Pulse Energy
Rep. Rate

500 – 750 nm
10 – 15 fs
10 μJ
1 kHz

Two Color Pump-Probe Experiments
Pump
250 - 375 nm, 20 fs
Probe
500 - 750 nm, 10 - 15 fs
Time Resolution
30 fs
Sensivitity
0.03 mOD

Apparatus: Tunable Two-Color Pump-Probe Spectrometer based on NOPA

Tunability: Pump 250 - 375 nm

Riedle; Kobayashi; Cerullo & Silvestri

Probe 500 - 750 nm

Time-Resolution of UV-vis Two Color Pump-Probe: 30 - 40 fs (Cross Correlation)

Photoisomerization of Stilbene

Photoisomerization of Stilbeneε (103 M-1 cm-1)

Absorption Spectra of cis-Stilbene

Observation of Pump-Induced Wavepacket Motion of S1 cis-Stilbene

Observation of wavepacket motion
through Sn ← S1 absorption
Creation of wavepacket motion

PumpΔTProbe

Time-Resolved Absorption Signal of cis-Stilbene
pump 315 nm; probe 660 nm; 5x10-3 M

Oscillation (~240 cm-1)
Dephasing is much fasterthan isomerization.

S1 lifetime :1.25 ps

Time-Resolved Absorption of cis-Stilbene
The isomerization rate significantly
depends on the solvent.
The observed wavepacket motion
is not directly correlated tothe reaction coordinate.

Decay of Vibrational Coherence of S1 cis-Stilbene

The wavepacket motion is insensitive to the change of solvent.

Wavepacket Motion in Photoisomerization of cis-Stilbene

Initial Wavepacket Motion

IVR

The observed wavepacket
motion is“perpendicular”
to the reaction coordinate.

Isomerization Coordinate

Pump-Induced Wavepacket Motion in S1 cis-Stilbene

S1 potential

A curvature of S1 potentialthat determines the wavepacket
motion observed

Dumping time ~ 200 fs:
Time window for the wavepacket
motion to monitor the S1 potential

Fast dumping indicates highanharmonicity of the potential

Observation of Raman-Induced Wavepacket Motion
in S1 cis-Stilbene
Creation of wavepacket motion

Observation of wavepacket motion
through Sn ← S1 absorption

Probe 1

Probe 2

ΔTtdPump
Generation of S1 population

Time-domain Raman
Measurements

Observation of Raman-Induced Wavepacket Motion
in S1 cis-Stilbene
pump 267 nm, 170 fs; probe1 & 2 620 nm, 11 fs
0.02 mol dm-3 in hexadecaneprobe1ΔA (a.u.)probe1probe1Sn← S1 abs
pump – probe2

ΔΔA (mO.D.)

ΔT = 2.0 ps
ΔT = 1.2 ps

ΔT = 0.3 psMutlidimensional S1 Potential of S1 cis-Stilbene

The coordinate of the observed nuclear
wavepacket motion, Qi, is anharmonicallycoupled with another coordinate, Qs.

∂ ⎛ ∂ 2V ⎞ ∂ kQi⎟=⎜2 ⎟⎜∂QS ⎝ ∂Qi ⎠ ∂QSis significant.

The vibrational coordinate ofthe wavepacket motion observed

The nuclear coordinate
close to the reaction coordinatePhotoinduced Structural Chage(Jahn-Teller Distortion)of Cu complex

Bis(2,9-dimethyl-1,10-phenanthroline)copper (I):A Fundamental Molecule in Inorganic PhotochemistryBis(2,9-dimethyl-1,10-phenanthroline)copper (I):
Flattening distortion takes place.

DD2d2[Cu(I)(dmphen)2]+degeneraged☺ Promising as the photocatalyst
and photosensitizer in solar
energy conversion☺ Candidate for molecular switch☺ Real-time observation ofJahn-Teller Distortion

dxz ,dyzπ∗
MLCTexcitation

dxydz2dx2-y2
Metal Orbital

Ligand Orbital

Femtosecond Time-Resolved Fluorescence Spectra

Iwamura, Takeuchi, Tahara, JACS, 129, 5248 (2007).

isoemissive point

0.3 ~ 1 ps

Fluorescence Spectral Change Corresponding to Flattening Distortion

A gradual spectral shift is expected.

S1hνS0Q
0Fluorescence Spectral Change Corresponding to Flattening Distortion

The data suggests “a hollow” on the S1 potential
at the perpendicular configuration.
isoemissive point

S1S0Q0A Realistic Potential Curve of the Jahn-Teller Distortion

1B (S )
2245 fs
Flattening Distortion
660 fs1A (S )21D2d(1B1)

D2

7.4 psT1

Hollow ?

hνIfIfthere
thereisisaaboundboundstate,state,
weweshould
shouldobserve
observevibrations.
vibrations.

S090°

dihedral angle

Ultrafast Pump-Probe Signal of [Cu(dmphen)2]+
pump 550 nm; probe 650 nm; 7x10-2 M in dichloromethane

1B
21A
2

Ultrafast Dynamics of Photo-Induced Flattening of [Cu(dmphen)2]+

D2dD2

660 fsS1 (1A2)

Flattening distortion (b1)

A hollow on the S1 potential

S090°

θWhat do I expect for femtosecond X-ray?

Direct Information on Structural Change
Pulse width > 100 fs (or 1 ps )Structural Information of Short-Lived Transients
Pulse width < 100 fs (or 1 ps )Direct Observation on Coherent Motion

Seeing what we could not see before
creates new science.

Collaborators
Molecular Spectroscopy Laboratory, RIKEN
Satoshi TAKEUCHI
Shoichi YAMAGUCHI
Kunihiko ISHIIHaruko HOSOI
Kentaro SEKIGUCHI
Munetaka IWAMURA
Hidekazu WATANABE
Takumi NAKAMURA
Satoshi NIHONYANAGI
Sobhan SEN
Pratik SEN
Prof. Sanford RUHMAN(Hebrew Univ.)
Akiko ZUSHI

Members, April 2007