Statistical intramolecular vibrational relaxation and its hindrance: The fundamentals of (CH3)3C–C≡CH and (CH3)3Si–C≡CH

We have measured the fundamental acetylenic C–H stretch near 3.0 μm of (CH3)3C–C≡CH and (CH3)3Si–C≡CH using an optothermal, molecular beam spectrometer. We find that the individual R(J) lines of the hydrocarbon are Lorentzian with a FWHM of 800 MHz indicating statistical intramolecular vibrational relaxation (IVR) with a 400 ps lifetime. The R(J) lines of the silicon compound are clearly asymmetric and, in addition, show a FWHM of about 150 MHz indicating a much longer (>2 ns) lifetime. The increase in IVR lifetime in the larger density of states molecule may be due to reduced kinetic coupling resulting from the heavier Si atom.