Forbidden rotational Q branch of OPF 3 observed by microwave Fourier transform spectroscopy

Abstract

The pure rotational spectrum that becomes weakly allowed through centrifugal distortion mixing has been investigated between 8 and 26 GHz for phosphoryl trifluoride (OPF 3) in the ground vibronic state using a pulsed microwave Fourier transform spectrometer. The molecule is of particular interest because it is near spherical top. Ten Q-branch series following the (Δ k = ±3) selection rule have been obtained for a lower state K = ∥ k∥ between 5 and 18 with 24 ≤ J ≤ 94. The ( A 1 - A 2) splitting has been observed in OPF 3 for the first time. Splitting measurements have been made for the series k = ±9 ← ±6 and k = ±12 ← ±9. In addition, the allowed dipole transition ( J = 1, K = 0) ← (0, 0) has been measured with the same spectrometer. A total of 319 perpendicular transitions, this one parallel transition, and the 138 lines in the parallel R branch obtained earlier by Smith [ Mol. Phys. 32, 621–645 (1976)] have been treated with a weighted least-squares analysis. Determinations have been made of both rotational constants ( A and B), seven diagonal distortion constants ( D J , D JK , D K , H J , H JK , H KJ , and H K ), and two distortion constants (ϵ and ϵ J ) that characterize the (Δ k = ±3) matrix elements. The data set has been used to investigate the redundancies that exist in the quartic and sextic Hamiltonian for symmetric tops with small values of (A − B) 1 2 (A + B) . Pulsed double resonance methods have been extended to three-level systems for which the transition moment arises only from the centrifugal distortion mixing.