Infrared–infrared double resonance investigation of the N=2 and N=3 stretching manifolds of silane and the local mode model

Abstract

We report on our IR–IR double resonance experiment study of the excited stretching state of silane involving two or three vibrational quanta, and on the interpretation of our results by using the local mode model. The DR spectra were recorded by means of two separately tunable IR beams, one of which was used to pump selected lines inside the υ3 or 2υ3 IR bands, and the other (the probe) to observe transitions from the pumped vibration–rotation states. We characterized 7 out of the 14 vibrational states belonging to these manifolds for the first time, and the A1+F2 diad localized previously at 6362 cm−1 has been studied with much higher accuracy than before. Only the two highest states of the N=3 manifold are still unknown. The vibrational energies are very well reproduced by the local mode model (≂1 cm−1 rms deviation), and the harmonic interbond couplings have been determined more precisely than before. The rotational structures of the excited states are also well accounted for by Champion’s theory of vibration–rotation coupling in Td, adapted to the description of stretching vibrations in terms of local modes. More high resolution (0.01 cm−1 or better) data are needed however to fully assess this point.