3P021
Nonadiabatic rotational excitaton of benzene
by double pulse intense laser fields(IMS) !Daeyul Baek, Hirokazu Hasegawa, Yasuhiro Ohshima
1. Introduction
Recently, Meijer et al. [1] pointed out that it may be possible to control a rotational state
distribution via NAREX [2] processes using double pulse excitation. However, these experimental
studies on NAREX processes have been limited to only the diatomic molecule. In the present study,
we investigate the rotational excitation process of benzene molecules by varying the time delay
between two intense femtosecond pulses. The rotational state distributions after the double pulses
excitation are probed by a (1+1) resonance enhanced multiphoton ionization technique.
The state distribution depends on the time delay of double pulse and shows characteristic beatpatterns. The analysis of the beat signals reveals that the rotational excitation processes are different toeach other for initial states with different J and/or K.

2. Experiment
The fundamental output from a femtosecond Ti:sapphire laser system (820 nm, 1~2 mJ/pulse, 1kHz) is divided into double pulses of identical energies with a variable time delay !. These pumppulses are collinearly focused onto jet-cooled benzene molecules in a molecular beam to induce
nonadiabatic rotational excitation. A linear chirp is applied to stretch the pulse duration (35 fs fortransform limited) to 160 fs by adjusting the compressor grating. Here, a high pressure pulsed valve
with a stagnation pressure of 90 atm is incorporated for an efficient rotational cooling of molecules.
After ~100 ns of the interaction with the pump pulse, the probe pulse from a nanosecond frequencydoubled dye laser (258 nm, 10 "J/pulse, #E = 0.05 cm-1) ionizes benzene molecules by 1+1 resonanceenhanced multiphoton ionization via the S1 ˥ S0 610 transition. The generated benzene cations aredetected by a time-of-flight mass spectrometer (TOF-MS). The polarization direction of the pump andthe probe pulses is set to be parallel to each other, and along with the ion-extraction field in the TOFMS. The probe pulse was tightly focused with an f = 170 mm lens, while the pump pulse was loosely
focused by an f = 300 mm lens and its focal position was slightly shifted from the probed region by~10 mm to maximize the laser beam overlap.

The state distribution depends on the time delayof double pulse and shows characteristic beat pattern.
In order to investigate the NAREX dynamics of thebenzene molecule, we measured the dependence ofthe rotational state distribution on the delay time forthe states of JK = 00, 20, 11, 21, 31, 22, 32, 42, 33, and 43.
The example of this dynamics is shown in Figs. 1 and2. Fig. 1 shows the dependence of the benzene ionyield on the delay for J = 2, 3, 4 with K = 2. We
consider the following model to explain the observed
beat signals. The rotational wave packet generated
after the interaction with the first pulse of the double
pulse pair at t = 0 can be written as the follow:

Benzene ion yield (a.u.)

3. Results and Discussions
J=4J=3

J=204080
120
Double pulse delay (ps)160

Fig. 1. The experimental (bule line) andsimulated (red line) delay dependence of therotational state population for J = 2, 3, 4 withK = 2.i)'(t ) & $ AJ i J e J i J e% i( JK i t(1)JK i M i

JHere, (JK & EJK / !, AJ J and ) J J are the angular frequency, amplitude and phase of each JK i M i statein the rotational wave packet. If the second pump pulse is irradiated onto the molecule at t =!, thewave packet represented in Eq. (1) is modified as [3],%i( t(2)'(t;! ) & $ BJ J (! )eJKi M iiiiiJKii

Jwhere, BJ J (! ) is the transition amplitude from J i K i M i to JK i M i by the interaction with the double
2pulse. The rotational state population, as observed beat signals in Fig. 1, is proportional to B J i J (! ) .
The explicit form is as follows:2(3)B J J (! ) & $ A J2 J ' A J2 ' J + 2 $ A J J ' A J ' J A J J " A J " J cos( # ( J ' J "! + # JJ" JJ ' )iiiiiiJ '* J "J'where #( J ' J " & ( J 'K % ( J "K and #JJ"JJ ' & ) J J " + ) J "J % ) J J ' % ) J ' J . The information on the phase andamplitudes of the rotational wave packet can be obtained from the Fourier-transformed spectrum of Eq.
(3). The peaks of the FT spectrum appear at #( J 'J " with the intensity of 2 AJ J ' AJ ' J AJ J " AJ "J and withthe phase #JJ"JJ ' .
Accordingly, the components of the rotational eigenstates included in the wave packet and thecoupling strength between rotational eigenstates can be obtained by the observed beat signals. Fig.
2(a) displays the Fourier transformed signal of the observed beat signals for J = 2, 3, 4. As expected,
the peaks appear at the positions corresponding to the rotational energy differences. The peakassignments are also shown in Fig. 2(b). Our experimental result supports the theoretical prediction
about the rotational excitation process.
iiiiiiii(b)
42

630

520

4

123

62

20

10(a)

J=20 5 10 15 20 25 30 35 40 45 50 55 60 65 70
Energy difference (hcB)

Energy (hcB)

FTsignal (arb units)J=3

Angular momentum J

J=4Fig. 2. (a) Fourier-transformed signal of the experimental (bule line) and simulated (red line) delay
dependent behavior of the rotational state population for J = 2, 3, 4 with K = 2, as shown in Fig. 1. (b)Rotational-energy diagram of each probed state (,).

4. Reference[1] A.S. Meijer, Y. Zhang, D.H. Parker, W.J. van der Zande, A. Gijsbertsen, and M.J.J. Vrakking,Phys. Rev. A., 76, 023411 (2007).
[2] H. Hasegawa and Y. Ohshima, Phys. Rev. A., 74, 061401(R) (2006).
[3] H. Hasegawa and Y. Ohshima, Phys. Rev. Lett., 101, 053002 (2008).
[4] H. Hasegawa and Y. Ohshima, Chem. Phys. Lett., 454, 148 (2008).