Infrared spectra of CO2-doped hydrogen clusters, (H2)N-CO2.

Clusters of para-H2 and/or ortho-H2 containing a single carbon dioxide molecule are studied by high resolution infrared spectroscopy in the 2300 cm-1 region of the CO2 ν3 fundamental band. The (H2)N-CO2 clusters are formed in a pulsed supersonic jet expansion from a cooled nozzle and probed using a rapid scan tunable diode laser. Simple symmetric rotor type spectra are observed with little or no resolved K-structure, and prominent Q-branch features for ortho-H2 but not para-H2. Observed rotational constants and vibrational shifts are reported for ortho-H2 up to N = 7 and para-H2 up to N = 15, with the N > 7 assignments only made possible with the help of theoretical simulations. The para-H2 cluster with N = 12 shows clear evidence for superfluid effects, in good agreement with theory. The presence of larger clusters with N > 15 is evident in the spectra, but specific assignments are not possible. Mixed para- + ortho-H2 cluster transitions are well predicted by linear interpolation between corresponding pure cluster line positions. [ABSTRACT FROM AUTHOR]