Intramolecular vibrational energy redistribution in HCCCH2X (X = Cl, Br, I) measured by femtosecond pump–probe experiments in a hollow waveguide

Abstract

From the analysis of high resolution overtone spectra it is well established that intramolecular vibrational energy redistribution (IVR) from an initially excited CH-stretching vibration is strongly influenced by its chemical environment. Due to a pronounced Fermi resonance between the CH-stretching and CH-bending vibrations a vibrational energy redistribution on the subpicosecond time scale (∼100 fs) is found for alkyl (sp³) CH-chromophores, whereas this doorway for energy flow is blocked for the acetylenic (sp) CH-stretching vibration because of the much lower CH-bending frequency. From the analysis of the high resolution spectra lifetimes for the initial CH-vibrational excitation of 10–100 ps or longer have been derived. In the present work we have investigated the IVR process for HCCCH₂Br, HCCCH₂Cl, and HCCCH₂I after excitation of the first overtone of the CH-stretching vibration of the CH₂X- and the CCH-group by time resolved femtosecond pump–probe experiments in a hollow waveguide. For HCCCH₂Br and HCCCH₂Cl a clearly different IVR behavior was found for the two different chemical environments. For the excitation of the alkyl CH-chromophore very fast initial relaxation times were found together with a slower relaxation process with τ₂ = 15–40 ps, whereas for the acetylenic CH-stretching vibration a relaxation time τ₃ = 70–200 ps has been determined. For HCCCH₂I also for the excitation of the CCH-group a relatively fast relaxation process with a time constant τ₂ = 6 ps could be identified which might result from a not yet identified strong vibrational coupling between the excited first overtone of the acetylenic CH-stretching vibrations with a combination state including the CI-stretching vibration.