Infrared Spectrum of Co-O2, a 'New' Weakly-Bound Complex

Abstract

Only a few weakly-bound complexes containing the O 2 molecule have been characterized by high-resolution spectroscopy, notably N 2 O-O 2 [1] and HF-O 2 [2]. This neglect is no doubt due in part to the complications added by the oxygen unpaired electron spin. Here we report an extensive infrared spectrum of CO-O 2 , as observed in the CO fundamental band region (∼2150 cm −1 ) using a tunable quantum cascade laser to probe a pulsed supersonic jet expansion. The derived energy level pattern consists of 'stacks' characterized by K, the projection of the total angular momentum on the intermolecular axis. Five such stacks are observed in the ground vibrational state, and ten in the excited state, v(CO) = 1. They are divided into two groups, with no observed transitions between groups, and we believe these groups correlate with the two lowest rotational states of O 2 , namely (N , J) = (1, 0) and (1, 2). In many ways, the spectrum and energy levels are similar to those of CO-N 2 [3], and we use the same approach for analysis, simply fitting each stack with its own origin, B-value, and distortion constants. The rotational constant of the lowest stack in the ground state (with K = 0) implies an effective intermolecular separation of 3.82 Å, but this should be interpreted with caution since it ignores possible effects of electron spin.