Wavepacket Dynamics of Hydrogen Bonds in the Electronic Ground State

In summary, the couplings discussed so far transform the hydrogen stretching oscillator into a vibrational multi-level system with a multitude of transition lines. 

The interaction with the fluctuating surrounding leads to an additional broadening of the individual lines by vibrational dephasing [22-25]. Nonlinear vibrational spectroscopy allows one to separate the different couplings in the nonlinear time-resolved response following femtosecond vibrational excitation. In particular, the coherent vibrational dynamics can be isolated from processes of population relaxation [26[ and energy redistribution. 

Low-frequency Wavepacket Dynamics of Hydrogen Bonds in the Electronic Ground State 

The electric field envelope of the femtosecond pump pulse which is short compared to the period of the oscillations in Fig. 15.3 (b) covers a frequency range much broader than the energy spacing of individual levels of the low-frequency mode. In other words, the pump spectrum overlaps with several lines of the vibrational progression depicted in Fig. 15.1 (b). As a result, impulsive dipole excitation from the Vqd = 0 to 1 state creates a nonstationary superposition of the wavefunc-tions of low-frequency levels in the Vqd = 1 tate with a well-defined mutual phase. This quantum-coherent wavepacket oscillates in the Vqd = 1 state with the frequency Q of the low-frequency mode and leads to a modulation of O-H stretching absorption which is measured by the probe pulses. In addition to the wavepacket in the Vqd = 1 state, impulsive Raman excitation within the spectral envelope of 

2358 cm T Pronounced oscillatory signals are observed, (c) Fourier transform of the oscillatory absorbance changes of (b) (lower trace) and resonance Raman spectrum of HBTdisplaying a low-frequency band at 120cm (upper trace), (d) Microsopic elongations connected with a wavepacket motion along the 120 cm in-plane mode. 

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