Real-Time Probing of Intramolecular Vibrational Energy Redistribution and Intermolecular Vibrational Energy Transfer of Selectively Excited CH₂I₂ Molecules in Solution

Abstract

Competition between intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of excited methylene iodide (CH2I2) in solution has been measured in real time. After excitation of the C−H− stretch overtone and C−H− stretch containing combination bands of CH2I2 between 1.7 and 2.4 μm an increase followed by a decrease in the transient electronic absorption at 400 nm has been monitored. The transient absorption has been attributed to vibrational energy flow from the initially excited degrees of freedom to vibrational states with larger Franck-Condon (FC) factors for the electronic transition (long wavelength wing) and energy loss due to energy transfer to the solvent. A model based upon the dependence of the electronic absorption on the internal energy 〈E〉 of CH2I2 has been used to determine the times for intramolecular vibrational energy redistribution and intermolecular energy transfer to the solvent. In the simplest version of our model the internal energy of the molecule probed by the population of the FC-active modes rises and decays exponentially on a picosecond (ps) time scale, which reflects the initial intramolecular vibrational energy redistribution and the subsequent energy transfer to the solvent. This simple approach was able to accurately describe the measured transient absorption for all solvents and excitation wavelengths. Overall time constants for IVR have been found to be on the order of 9−10 ps, almost independent of the excitation wavelength, the excited modes, and the solvent. In contrast, energy transfer to the solvent takes significantly longer. Overall time constants for VET have been determined in the range between 60 and 120 ps depending on the solvent, the excitation energy, but not on the mode which was initially excited.

Vibrational relaxation of CH2I2 in solution: Excitation level dependence

“…͑Because the s ϩ b combination band for CH 2 I 2 coincides with an absorption in C 6 D 6 but not C 6 H 6 , we use nondeuterated benzene for experiments on that state.͒ For each of the vibrational excitation levels, the strongly interacting solvents accelerate both intramolecular vibrational relaxation and intermolecular energy transfer, consistent with previous qualitative interpretations. 43,44,46 Specifically, the presence of a dipole moment in CDCl 3 does not significantly change the IVR or IET times compared to the nonpolar solvent CCl 4 , but nearest neighbor interactions of solvent molecules with CH 2 I 2 , such as the → C-I * charge transfer in the case of benzene and n O → C-I * for acetone, have a large effect on the energy transfer times. Relaxation times following excitation of the 2 s overtone of CH 2 I 2 in supercritical CO 2 by Sekiguchi et al 61 provide another reference point.…”

“…Although as many as five electronic transitions appear in the ultraviolet absorption spectrum, 52 the lowest energy band dominates at long wavelengths. The temperature-dependent parameters for the absorption are extrapolations from spectra taken over the temperature range of 284 -322 K. Recent 400 nm absorption measurements of CH 2 I 2 heated in shockwaves show that the extrapolation may underestimate the change, 46 but since the vibrational temperature is a phenomenological parameter describing the vibrational energy content in the Franck-Condon active modes, the model adequately characterizes the change in absorption during energy transfer. We model the time evolution of the vibrational temperature with an exponential rise and decay, representing IVR and IET, and use the temperature-dependent absorption spectrum to obtain the resulting signal at each probe wavelength.…”

“…Graener and Laubereau 30 and Bakker et al 34 were the first to study vibrational relaxation of CH 2 I 2 following picosecond excitation of the C-H stretch fundamental vibrations, and their results indicate a state-specific IVR mechanism. Our recent work [43][44][45] suggests that the IVR mechanism changes between the C-H stretch fundamental and the first overtone, and Abel and co-workers 46,47 show that the IVR rate does not scale with the total density of states between the stretch-bend combination region and the stretch overtone. The past studies also show that the IET rate depends on the initial excitation level and that the solvent influences the rates of both IVR and IET.…”

Vibrational relaxation of CH3I in the gas phase and in solution

“…8,24 Pump-probe experiments directly observe energy flow within and out of a molecule by exciting a nonstationary vibrational state with one laser pulse and then probing the evolution of the system as a function of time with a second pulse. 25,26 Infrared absorption, [27][28][29][30][31][32] anti-Stokes Raman scattering, [33][34][35][36][37] and ultraviolet absorption [38][39][40][41][42][43][44][45][46][47][48][49] are the most common methods for probing the vibrational dynamics in the ground electronic state and are generally most useful in the condensed phase where it is possible to obtain a high density of vibrationally excited molecules. In fact, there are relatively few timeresolved studies of IVR in the ground electronic state for gas phase systems.…”

Chemical dynamics of vibrationally excited molecules: Controlling reactions in gases and on surfaces

“…Studying the fate of an initially energized molecule requires time resolution on the order of the characteristic few hundred femtosecond encounter time, and modern ultrafast laser technology puts experiments in that regime. The most extensive studies of vibrational dynamics in liquids follow the historical pattern of first assessing energy transfer (25), and there are recent examples (54)(55)(56)(57)(58)(59)(60)(61)(62)(63)(64)(65)(66)(67)(68)(69) that particularly address the relaxation of COH stretching excitations. The general picture that emerges from these studies is that in relatively weakly interacting solvents, such as chloroform, the vibrational state structure and couplings of the molecule influence the energy flow within the initially excited molecule most strongly.…”