CH 2 I 2 fundamental vibrational relaxation in solution studied by transient electronic absorption spectroscopy

Abstract

Wavelength dependent, transient, electronic absorption spectroscopy of methylene iodide (CH2I2) in CCl4, CDCl3, C6D6, and (CD3)2CO following excitation of the fundamental C–H stretching vibration reveals the time scales of intramolecular vibrational energy redistribution and energy transfer to the solvent. In contrast to the case for overtone excitation, state-specific relaxation to one or a few states that are coupled by low order interactions with the initially prepared state dominates the intramolecular vibrational energy redistribution. This mechanism is consistent with previous infrared pump–probe measurements of CH2I2 fundamental relaxation as are the measured relaxation time scales. We also find a previously unobserved relaxation pathway through weakly-coupled states that have several quanta of excitation in the Franck–Condon active modes, primarily C–I stretch and bend. Although this statistical component is a minor channel in the relaxation, it is the only contribution to the signal at the longest probe wavelengths in CCl4 and CDCl3. Time scales for both intramolecular energy redistribution and intermolecular relaxation to the solvent depend strongly on the strength of interaction with the solvent.