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MARIO E. FAJARDO, AFRL/MNME, Energetic Materials Branch, Ordnance Division, U.S. Air Force Research Lab, 2306 Perimeter Rd., Eglin AFB, FL 32542-5910. mario.fajardo@eglin.af.mil.
Analysis of the rovibrational infrared (IR) absorption spectra of water (H2O, D2O) molecules isolated in solid parahydrogen (pH2) reveals their existence as very slightly hindered rotors, with rotational constants reduced by only 2-5% from their gas phase values. Clustering of residual orthohydrogen (oH2) molecules with water monomers results in the appearance of several new IR absorption features. For Type B water monomer bands (e.g.
1,
2) most of the new features appear near the vibrational origin, and were originally interpreted as indicating the presence of "non-rotating" water molecules. However, for Type A bands (e.g.
3,
2+
3, 2
2+
3,
1+
3) very little IR activity is observed near the vibrational origin, refuting this appealingly simple explanation. Here we propose a new interpretation which assumes a semi-rigid C2v structure for the ground state of the oH2-water complexes, with the oH2 acting as a proton donor to the water oxygen atom. In this picture, the oH2-water complex spectra can be understood as parallel and perpendicular bands of an asymmetric top near the prolate symmetric top limit. Thus, the features bunching near the Type B vibrational band origins arise from the
K = 0 selection rule for parallel bands, while the more widely separated features in the Type A bands arise from the
K = \pm1 selection rule for perpendicular bands.