Progress in understanding the infrared spectrum of the H2O-O2 dimer

Spectra of the weakly bound H2O-O2 dimer are studied in the region of the H2O ν2 band using a tunable quantum cascade laser to probe a pulsed supersonic slit jet expansion. These are the first gas-phase infrared spectra of H2O-O2 and among only a few such results for O2-containing complexes. Almost 100 infrared lines are assigned based on the ground state combination differences from the microwave spectrum of H2O-O2. These lines belong to a main fundamental band, plus four combination bands lying 2 to 5 cm-1 above the fundamental. All correspond to the ortho-H2O (I = 1) nuclear spin species. Interpretation of the observed rotational levels is discussed. The original microwave analysis conflicts with the infrared results but can be corrected by changing the sign of a term or, better still, by using a published theory for weakly bound open shell complexes. The combination bands suggest that analogous ground state bands should be observable in the millimeter wave range. Many infrared transitions remain unassigned, including another extensive band apparently centered at 1603 cm-1, and some of these are probably due to the para-H2O spin species (I = 0). Splittings due to the unpaired O2 electron spin (S = 1), due to large amplitude tunneling motions, and due to a-axis rotational motion all have similar magnitudes for H2O-O2, so the resulting energy levels will be heavily mixed and not amenable to simple modeling. Accurate theoretical predictions of these effects should be possible for obtaining an enhanced understanding of the observed spectra.