Vibrational cooling after photo isomerisation: First application of a novel intramolecular thermometer

Femtosecond VIS-pump-IR-probe experiments (time resolution: 300fs,
detectable absorption change: 5.10(-5), frequency range:
1000-1800cm(-1)) were applied to the ultrafast photo isomerisation
of azobenzene and the protonated Schiff base of retinal. The energy
of the pump photon heats the vibrational system of the molecule,
leading to pronounced anharmonic frequency shifts which are
dominated by the anharmonic coupling between the investigated high
frequency modes to hot low frequency modes. A model caking into
account this intramolecular anharmonicity and assuming a well
defined intramolecular temperature gives a good fit of the
experimental data.