Dissipative wave packet dynamics of the intramolecular hydrogen bond in o-phthalic acid monomethylester

We investigate the infrared laser-driven ultrafast dynamics of the carboxy-deuterated title compound in the condensed phase using a system bath approach. The three-dimensional relevant system comprises the OD stretching and bending motion as well as a low-frequency out-of-plane twisting vibration modulating the hydrogen-bond strength. The dominant relaxation and dephasing channels are identified giving rise to three terms which contribute to the system bath interaction: (i) A linear system-solvent coupling leading to the relaxation of the low-frequency mode. Here, a classical molecular dynamics simulation is performed to obtain the spectral density and thus the relevant relaxation time scale for this mode. (ii) A coupling which is quadratic in the system coordinates and responsible for pure dephasing. (iii) A fourth-order coupling which involves the release of vibrational energy into the environment by means of the simultaneous excitation of two intramolecular bath vibrations and a solvent mode. Using a quantum master equation approach it is demonstrated that this model is in accord with the results of recent infrared pump-probe and four-wave mixing experiments. The dynamics is discussed in terms of a diabatic representation defined with respect to the high-frequency stretching and bending modes. The behaviour of the low-frequency mode can be characterized as a dissipative wave packet motion in the diabatic ground state which contains contributions from bath-induced coherence transfer out of the initially excited diabatic state.