Laser Control and Manipulation of Molecules - American Chemical

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Real-Time Spectroscopy of Molecular Vibration Using Sub-5-fs Pulses Takayoshi Kobayashi, Akira Shirakawa, and Takao Fuji Department of Physics, Faculty of Science, University of Tokyo Hongo 7-3-1, Bunkyo-Ku, Tokyo 113-0033, Japan

Transform-limited (TL) visible pulses with as short a duration as 4.7 fs with a 5 μJ pulse energy have been generated for the first time from a novel noncollinear optical parametric amplifier (NOPA). Applications of the sub-5-fs pulse source to the real-time spectroscopy of conjugated polymers, dye molecules, and J-aggregates are described. Several new phenomena, namely mode coupling, dynamic Duschinsky rotation, and dynamic intensity borrowing are found to take place in a polydiacetylene cast film, cresyl violet doped in polymers, and J-aggregates of porphyrin toluene sulfonate in aqueous solution.

Introduction Time-resolved spectroscopy has long been a powerful method of electronic structure in the exited molecules and photochemical intermediates. The time resolution has been improved from the microsecond regime in 50-60's to

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172 femtosecond regime in 90's. The pulsed light from mode-locked lasers has enabled to study ultrafast dynamics and determination of structures of molecules in their electronic excited state and intermediate states in photochemical reaction by the measurement of transient electronic and vibrational spectra. Zewail et al.(l, 2) have succeeded in the observation of the quantum mechanical tunneling of wavepacket in the photodissociation of "quasi" two-atom molecules like Nal. That study triggered the femtochemistry field and transition-state spectroscopy and rapid growth of the field is still in progress. Recently, sub-5-fs visible pulse laser based on noncollinear optical prametric amplifier was constracted by our group.(3) The sub-5-fs pulses is very powerful for pump-probe real-time vibrational spectroscopy, since the oscillation period becomes as short as 1 lfs in the case of 3000 cm" . Information about the phase relation among vibrational modes and the initial phase of oscillation induced by photoexcitation through vibronic coupling can be obtained by sub-5fs real-time spectroscopy described below. These informations cannot be obtained by any conventional stationary or time-resolved Raman spectroscopies. Therefore the applicability of such short pulses to the study of molecular vibrational systems is to be mentioned to show how useful the short pulse is demonstrated in several real systems. 1

A conjugated polymer: mode coupling The ultrafast dynamics of a quasi-one-dimensional conjugated polymer polydiacetylene (=CR-C = C-CR'=) (R and R' are substituents) was studied by pump-probe spectroscopy using visible sub-5-fs pulses(4). The spectrallyresolved differential transmittance change in a thin film of a ladder polymer poly (5,7,17,19-tetracosatetraynylene bis(N-butoxycarbonylmethyl) carbamate) (PDA-4BCMU4A(8)) (5) at various wavelengths is shown in Fig. 1. The signal is dominated by a long-living multimode wave-packet motion including three intense stretching mode signals C - C ( ~ 1220 cm" , 27 fs), C=C (~ 1450 cm" , 23 fs), and C = C ( ~ 2080 cm , 16 fs). Both features of the electronic and molecular dynamics indicate the relaxation from a l ^ - f r e e exciton to a geometrically-relaxed 2*A state within 60-80 femtoseconds via vibronic coupling and internal conversion(6,7). This conclusion can be obtained from extremely weak fluorescence from the exciton state. The quantum yield is estimated to be lower than 10 . The geometrical relaxation is clearly visualized by a real-time frequency analysis using a spectrogram(8). The spectrogram at 1.75 eV shown in Fig. 2 n

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1000 2000 Frequency (cm")

Figure 1. Transient transmittance change of a PDA-4BCMU film at the probe energies of 1.96 and 1.75 eV. The Fourier power spectra of the oscillating components integrated over the time range from 20 fs to 1.5 ps are also shown on the right, (reproducedfrom Ref. 9)

400 600 Delay Time (fs)


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Figure 2. Spectrogram at 1.75 eV. The Fourier amplitude increases from black to white. The window function is a Hanning-type with the FWHM of 150 fs. The center-of-massfrequency(solid curve) and integrated amplitude (dashed curve) of each stretching mode are shown on the right. The thin dashed lines indicate the corresponding Raman frequency positions measured in the stationary Raman spectrum. The bandwidth for the integration is 200 cm' . (reproduced from Ref. 9)


175 evidently exhibits the highly vibronic non-equilibrium state characterized by the instantaneous mode frequency and amplitude modulations. The C - C and C=C stretching frequencies and amplitudes are n out-of-phase modulated with the period of 145 fs corresponding to the 230 cm bending mode of the C-C=C bond. There is a possibility of a kind of artificial interference between two neighboring modes in the spectrogram analysis because of the limited spectral resolution determinced by the finite width of the window function. This effect will be discussed in detail elsewhere.(9) The feature of modulation is well explained by the coupling of the stretching modes via the bending motion in the geometrically-relaxed butatriene-like backbone. This is the first observation of such diabatic molecular motions which modify the vibrational frequencies of modes coupled to each other by the relevant mode.


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A dye molecule: Duschinsky rotation In order to clarify the mechanism of molecular vibrational mode coupling discovered in the conjugated polymers(4), we studied a simpler system. Cresyl violet (CV) was selected, since the Raman spectrum in literature(lO), is shown to have only one intense Raman signal. Figure 3(a) shows the normalized transient differential transmittance ( AT(t)/T ) as a function of delay time / of the probe pulse after the pump pulse of C V doped in poly(vinyl alcohol) (PVA) against the pump-probe delay time at various wavelengths. Bleaching is observed throughout the probe wavelength region. The observed signal intensity does not decrease detectably within the probed delay time range of 1 ps because the lifetime of the excited singlet state, S is reported to be as long as 3.2±0.1 ns(ll). The oscillations observed at negative and near-zero delays are mainly due to pump-perturbed free-induction decay and coherent coupling(12,13) and also partially due to the interference between the probe and scattered pump. Special care was taken to reduce the artificial interference effect not to deteriorate the signal pattern resulting in the loss of information near zero delay time. It was performed by slightly increasing the angle between the pump and probe up to the level of a few degrees. The oscillation with a 57-fs period can be clearly seen for all the probe wavelengths between 600 and 640 nm. The oscillation period is corresponding to the ringbreathing mode and the period is corresponding to the frequency of 590 cm" observed in the Raman spectrum of CV(10). The Fourier transformation of the trace shown in Fig. 3(b) has a peak at this frequency. These vibrational l?

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(a)

(b)

Figure 3. (a) Time dependence of the transmittance change of cresyl violet doped in polyvinyl alcohol). The intensities are normalized at their positive or negative peaks, (b) Fourier power spectrum of time dependence of oscillating components of A7" integrated over the delay time longer than 200fs. The intensities are normalized at their peaks, (reproducedfrom Ref. 18)


177 signals are mostly due to the wave-packet motion of the excited state. In Ref. (14), the motion of the ground-state wave-packet is much more restricted than that of the excited state, because it is initially weakly displaced from the groundstate minimum and has a small initial momentum created by 6-fs-pulse excitation. In the present experiment the excitation pulse is shorter than 5 fs. Then the motion of the ground-state wave-packet is even more limited; and hence it can be concluded to be negligible and the vibrational structure can be assigned to the wave-packet motion on the excited-state potential surface. In order to investigate the evolution of the instantaneous frequency of the ring-breathing mode, we analyze the transient differential transmittance using the spectrograms shown in Fig. 4. The amplitude and frequency of the peak of the spectrogram probed at 600 nm are shown in the upper part of Fig. 5. It clearly shows that the amplitude and frequency of the ring-breathing mode are modulated. The modulation period and corresponding frequency are estimated to be about 370 fs and 91 cm" , respectively, by Fourier transformation of the timedependent amplitude and instantaneous frequency. The modulations of the amplitude and phase are out of phase by about K . The modulation depths of the amplitude and frequency are about 13 and 4%, respectively. The same features are also observed at a probe wavelength of 620 nm. Such time dependent modulations of the ring-breathing mode parameters and their phase relationship can never be obtained by frequency-domain spectroscopy, and it will be discussed in detail in this paper. These modulations indicate that the ringbreathing mode is coupled to another vibrational mode. At other probe wavelengths, the transmittance change due to the ring-breathing mode is so weak that these modulations of such a small transmittance is difficult to be detected by the spectrograms. The power spectra of amplitude and phase modulations have two symmetric side bands on higher and lower sides of the center frequency. Therefore, it is impossible to discriminate the two cases only from Raman spectrum: (a) The two modes are coupled to each other through another mode with co , and (b) the two modes are independent of each other. In the real-time experiment, each of the depth and phase of the modulations can be determined. From the experimental data shown in Fig. 5, the parameters of side bands can be obtained to be m =0.18 and

Laser Control and Manipulation of Molecules - American Chemical

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