Isotopic Effects in XH3 (C3v) Molecules: The Lowest Vibrational Bands of PH2D Reinvestigated

We have derived, for MXmY3 (C3v symmetry) molecules which satisfy the conditions of a small ratio of atomic masses mH/mX and of equilibrium angles Y–X–Y close to π/2, simple isotopic relations for rotation–vibration parameters αβλ for the case where one light atom Y (=H) is replaced by a heavier one (=D). The usefulness and predictive power of such relations for the assignment and in the fit were tested by analyzing novel high resolution Fourier transform spectra of the PH2D molecule. The region of the three lowest-lying vibrational–rotational bands ν4, ν6, and ν3 was studied. The accurate analysis was made possible with the help of recently obtained (O. N. Ulenikov, H. Bürger, W. Jerzembeck, G. A. Onopenko, E. S. Bekhtereva, and O. L. Petrunina, J. Mol. Struct., in press) precise rotational energies of the ground vibrational state of PH2D. This improvement, in conjunction with better resolution and higher sensitivity, enabled us to assign transitions with higher values of quantum numbers J and Kc and to obtain more accurate rotation–vibration energies of upper states than previously. These energies, fitted with a Watson-type Hamiltonian in A reduction and IIIl representation, lead to a physically meaningful set of spectroscopic parameters which reproduce the experimental energies with a precision close to experimental uncertainty. Agreement between predicted and fitted Coriolis and vibrotational constants is noted; apparent limitations are caused by significant deviation of PH3 and PH2D from the idealized local mode case model.