Femtosecond Intramolecular Dynamics after Near-IR Excitation of CH_3I, C_2H_5I, CF_3CHFI, and C_7H_8 Molecules in the Gas Phase and in Solution

摘要

The rapid flow of vibrational energy within a molecule is central for the control and as well for the theory of unimolecular reactions. It defines the lifetime of vibrationally excited states and thus the time during which a specific vibrational excitation can control the outcome of chemical reactions. Times for intramolecular vibrational energy redistribution (IVR) can be deduced either indirectly from time-independent high resolution infrared(IR) spectra or measured directly in kinetic pump-probe experiments. We have applied delayed ultravi-olet(UV) absorption spectroscopy with a time resolution of 150 fs to measure intramolecular vibrational energy redistribution after near-IR excitation of the CH-stretching vibration around 5900 cm~(-1) in CF_3CHFI, CH_3I, C_2H_5I, and C_7H_8. Intramolecular relaxation times τ_(IV R) between 3 and 7 ps have been found in the gas phase. For CH_3I an additional short time of 250 fs has been measured. In the liquid phase IVR is followed by a fast collisional energy transfer of the excitation energy to the solvent molecules. Assuming a two step kinetic mechanism intermolecular relaxation times τ_(transfer) between 10 and 30 ps have been determined.