VIBRATIONAL COOLING AFTER ULTRAFAST PHOTOISOMERIZATION OF AZOBENZENE MEASURED BY FEMTOSECOND INFRARED-SPECTROSCOPY

The vibrational cooling of azobenzene after photoisomerization is inve stigated by time resolved IR spectroscopy with femtosecond time resolu tion. Transient difference spectra were obtained in a frequency range where phenyl ring modes and the central N=N-stretching mode absorbs. T he experimental data are discussed in terms of a simple theoretical mo del which was derived in order to account for the off-diagonal anharmo nicity between the investigated high-frequency modes and the bath of t he remaining low-frequency modes in a polyatomic molecule. It is shown that these off-diagonal anharmonic constants dominate the observed tr ansient absorbance changes while the anharmonicity of the high-frequen cy 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 intr amolecular energy redistribution process, the decay of the related int ramolecular temperature occurs via intermolecular energy transfer to t he solvent on a time scale of ca. 20 ps. (C) 1997 American Institute o f Physics.