Intramolecular vibrational dynamics in free polyatomic molecules with C═O chromophore bond excited by resonant femtosecond IR laser radiation.

ABSTRACT

In nine polyatomic molecules, we have studied the intramolecular redistribution of vibrational energy from chromophore C═O group excited by a resonant femtosecond IR laser radiation at a wavelength of ∼5 µm. All experiments have been performed in the gas phase using the IR-IR pump-probe technique in combination with the spectral analysis of the probe radiation. For molecules with one C═O end group, characteristic times of intramolecular vibrational redistribution (IVR) lie in the range between 2.4 and 20 ps and correlate with the density of four-frequency Fermi resonances. The IVR times in metal carbonyl molecules are anomalously long, being ∼1.0 ns for Fe(CO)5 and ∼1.5 ns for Cr(CO)6. In the CH3(C═O)OC2H5 and H2CCH(C═O)OC2H5 molecules, it has been observed that there are two characteristic IVR times, which differ by an order of magnitude from each other; this was interpreted in terms of the developed model of "accumulating states". For the ICF2COF molecule, it has been revealed that the IVR time decreases with increasing level of the vibrational excitation of the C═O bond of the molecule.