Knowledge Bank

The mid-infrared (1500 - 3800 $\backslash wn$) absorption spectrum of gaseous nitric oxide has been studied at low temperature (99 K) with a long absorption path (160 m) in order to observe weak combination, difference, and overtone bands of the NO dimer. Ten new bands were assigned with greater or lesser certainty. Combined with previous results, they lead to a set of 12 secure and 7 tentative vibrational term values for (NO)$2$, which almost double our knowledge of NO dimer vibrational states. The strongest non-fundamental bands in this region, after the $\nu 1$ (symmetric N-O stretch) + $\nu 5$ (asymmetric N-O stretch) overtone, involve combinations of $\nu 5$ with $\nu 3$ (intermolecular stretch). A new value of 155.5 $\backslash wn$ was obtained for $\nu 4$, the elusive infrared-inactive out-of-plane fundamental. Excitation of $\nu 5$ results in increased frequencies for the intermolecular modes $\nu 2$, $\nu 3$, and $\nu 4$, consistent with the known strengthening of the intermolecular bond.