1D05
Trial to Observe Wavepacket Motions near the Potential “Funnel” inStilbene Photoisomerization by Pump-dump-probe Spectroscopy(Molecular Spectroscopy Laboratory, RIKEN)
Zhengrong Wei, Satoshi Takeuchi, Tahei Tahara[Introduction]
In ultrafast reactions, it is considered that the excited-state molecule relaxes rapidly to groundstate through a potential “funnel”, where the excited- and ground-state potential energy
surfaces (PES) get close to each other. This potential region is expected to provide anefficient channel for rapid relaxation and to play a key role in ultrafast reactions. However,
our knowledge is very limited on how the molecular structure (or nuclear wavepacket)
evolvesinthisimportantpotential

region.

Therefore, it is highly desirable to obtain
quantitative information about the wavepacket
motion near the funnel region.

With thisfundamental interest in mind, we tried to observe
the wavepacket motion approaching the funnel
region in a model system, photoisomerization ofcis-stilbene,using

spectroscopy.

pump-IR

dump-probe
Fig. 1 Schematic illustration of the potentialfunnel region in cis-stilbene photoisomerization.

[Experiment]
The output of a Ti: sapphire amplifier was divided into three beams for generating the pump,dump and probe pulses, respectively. The pump pulse at 266 nm was produced through 3rdharmonic generation of the 800 nm fundamental pulse. The dump pulse at various infrared
wavelengths was generated by a commercial OPA system pumped by the 800 nm pulse. The
white continuum probe pulse was obtained by focusing the fundamental pulse on a sapphireplate.

[Results and Discussion]
As shown in Figure 1, it is suggested that the photogenerated S1 state of cis-stilbene initially
undergoes a twisting deformation and forms the relaxed S1 state, corresponding to a shallow

S1 state, which exhibits a strong Sn←S1absorption at 640 nm, is a precursor of theisomerization and is converted to the S0 state(trans and cis) in a picosecond through the funnelregion.

Absorbance Change/mOD

local minimum of the S1 potential. This relaxed30-12520

Dump Wavelength:1.5 µm (6500 cm )Dump Time: 0.5 ps
Dump off
Dump on151050500550600650700

Wavelength/nm

To investigate the wavepacket motion

Fig. 2 The transient absorption spectrum of cis-that moves toward the funnel region, westilbene with/without dump pulse (pump-probe

introduced an infrared dump pulse at 0.5 ps after

delay time: 1 ps; pump-dump delay time: 0.5 ps).

photoexcition and monitored the transient absorption by the probe at 1 ps. As shown in figure
2, the Sn←S1 transient absorption at 640 nm is decreased significantly by the dump pulse at
1.5 μm (6500 cm-1). This result strongly indicates that the S1 population is depleted by thedump pulse, implying that the S1 population is resonantly driven back to the S0 state by thestimulated emission dumping. We evaluated the0.08

following dump efficiency,
Abs (dump  on) ,Abs (dump  off )

0.00
0.04

(1)

and plotted it against the pump-dump delay. Asshows in figure 3 (b), the dump efficiency shows

Dump efficiency

dump efficiency  1 (a)
16 fs

0.04

0.00
0.06
0.03
0.00
0.04
0.02

evaluated by an analysis taking account of the0.02

instrumental response (FWHM=133 fs). The rise

0.00
-0.5

time that the wavepacket needs to reach the0.00
0.03
0.01

-1(b)
11 fs

0.02

almost instaneous rise as short as 11 fs, which wastime of the dump efficiency corresponds to the

Dump: 8500cm

Dump: 6500cm

-1(c)
13 fs

Dump: 4500cm

-1(d)
17 fs

Dump: 3500cm

-1(e)
76 fs

-1

Dump: 2470cm
0.0

0.5

1.0

1.5

Pump-dump Delay Time/ps

Fig. 3 Dump efficiencies as a function of pumpdump delay time for various dump photon energy.

potential region, where the S1-S0 energy difference
matches the 6500-cm-1 dump photon energy. To monitor the wavepacket motion at different
PES regions, we carried out experiments with changing the dump photon energy, as shown infigure 3. It was found that the rise is almost identical in the range of 10-20 fs for the dump
photon energy from 8500 cm-1 to 3500 cm-1, while it is substantially longer (76 fs) for the2470 cm-1 dump pulse (we note that the temporal response of the experiment does not change
significantly with change of the dumping wavelengths).
The pump-dump-probe result clearly indicates the wavepacket motion toward the PES
region, where the S0 and S1 potentials get as close to as 2470 cm-1, and provides its
quantitative characterization near the funnel region.