OH stretch overtone spectroscopy and transition dipole alignment of HOD

Photoacoustic spectroscopy in room-temperature H2O/HOD/D2O cells has been used to probe the 3 nu(OH) and 4 nu(OH) overtone bands in HOD via excitation with a single-mode (160 MHz) injection-seeded optical parametric oscillator (OPO) and pulsed dye laser, respectively. Transitions corresponding to HOD are isolated by comparison between H2O/HOD/D2O and pure H2O overtone spectra and analyzed with a Watson asymmetric top Hamiltonian. Least-squares fits yield the vibrational dependence of A(nu), B-nu, and C-nu rotational constants as well as a Birge-Sponer analysis of the vibrational overtone series for up to nu(OH) = 4. Relative intensities of a-type and b-type bands are reported for 3 nu(OH) and 4 nu(OH) which indicate that the transition dipole moment vector tilts away from the OH bond with increasing nu(OH) excitation. This observation is qualitatively inconsistent with simple local-mode, bond-dipole model predictions of the vibrational transition moments. However, the data are in excellent agreement with full 3D quantum variational calculations based on the H2O potential of Polyansky er al. [J. Chem. Phys. 105, 6490 (1996)] and dipole moment surface of Gabriel et al. [J. Chem. Phys. 99, 897 (1993)]. These results highlight the relative importance of electrical versus mechanical anharmonicity effects in preparing highly excited vibrational states of HOD via laser overtone pumping. (C) 1998 American Institute of Physics.