Knowledge Bank

Spectra of $(CH3)3X-C\equiv C-H$ are being studied in the region of the acetylenic stretching fundamental using the technique of optothermal detection of the absorption spectrum of a collimated molecular beam, using an F-center laser for excitation. Species with $X=C$ and Si have already been studied. The t-butyl compound shows Lorentzian lineshapes with a full width of $=700$ MHz, indicating that excitation in the C-H oscillator decays exponentially with a time constant of 500 psec. Substitution of Si for the C substantially raises the density of states. The spectrum, however, shows narrower lines that are partially structured. This structure is at least partially due to unresolved K and internal rotation splittings. We presently attribute the reduced rate in intramolecular relaxation to the heavier mas of Si which reduces kinetic coupling of modes that involve Si motion. In order to test this conjecture we plan to take the spectrum of the X=Sn compound in the near future.