Vibrational cooling after ultrafast photoisomerization of azobenzene measured by femtosecond infrared spectroscopy

The vibrational cooling of azobenzene after photoisomerization is investigated by time resolved IR spectroscopy with femtosecond time resolution. Transient difference spectra were obtained in a frequency range where phenyl ring modes and the central N=N-stretching mode absorbs. The experimental data are discussed in terms of a simple theoretical model which was derived in order to account for the off-diagonal anharmonicity between the investigated high-frequency modes and the bath of the remaining low-frequency modes in a polyatomic molecule. It is shown that these off-diagonal anharmonic constants dominate the observed transient absorbance changes while the anharmonicity of the high-frequency modes themselves (diagonal anharmonicity) causes only minor effects. Based on the transient IR spectra, the energy flow in the azobenzene molecule can be described as follows: After an initial ultrafast intramolecular energy redistribution process, the decay of the related intramolecular temperature occurs via intermolecular energy transfer to the solvent on a time scale of ca. 20 ps. (C) 1997 American Institute of Physics.