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

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.