The Forbidden Rotational Spectrum of GeH4 in the Ground Vibronic State

Abstract

The pure rotational spectrum of the tetrahedral molecule GeH4 has been studied with a 10.6 m light-pipe absorption cell and a conventional Michelson interferometer. Fifteen lines have been observed between 50 and 130 cm−1 with a theoretical resolution of 0.5 cm−1. These lines have been identified as arising from forbidden (J → J + 1) transitions for the rotational levels J = 9–23. A detailed discussion is presented of the identification of the absorption mechanism with the distortion electric dipole moment μD produced in the ground vibronic state by centrifugal effects. Particular attention is paid to showing that the rotational transitions within the vibrational state v4 do not contribute to the observed spectrum. The theoretical problem of determining the rotational constant B0 and the scalar distortion constant DS from the data is considered. It is shown that the frequency shifts due to the fourth-rank tensor distortion interaction must be taken into account in the analysis. It has been determined that B0 = (2.696 ± 0.003) cm−1 and DS = (3.3 ± 0.6) × 10−5 cm−1. From estimates of absolute line intensities, it has been found that [Formula: see text].