Effect of the analytical form of the dipole-moment function on the rotational intensity distributions in the high-overtone vibrational bands of carbon monoxide

Abstract

For modelling the line intensities of the low-overtone bands (Δv≤3 in CO), any potential-energy and dipole-moment functions (PEFs and DMFs) that describe the available data can be used. This is not true for the high-overtone bands. The Born-Oppenheimer molecular functions as functions of the interatomic separation r must obey some restrictions in the complex plane of r. In particular, the PEF and DMF must have common singularities in the complex plane and the DMF should not rapidly increase upon displacement from the real axis. Two model DMFs are considered for CO, a fully regular function over the entire complex plane and an irregular one with two branch points corresponding to complex intersections of the ground-state PEF with the excited-states ones. The predictions of the 7-0 band made with the regular DMF turn out to be unstable with respect to small variations in the database, which is associated with very rapid variations of this DMF. On the contrary, the predictions of the irregular one are stable and therefore are considered to be more reliable.