Experimental and ab initio studies of the HDO absorption spectrum in the 13165-13500 cm-1 spectral region

The HDO absorption spectrum was recorded in the 13 165-13 500 cm(-1) spectral region by intracavity laser absorption spectroscopy. The spectrum (615 lines), dominated by the 2 nu(2) + 3 nu(3) and nu(1) + 3 nu(3) bands, was assigned and modeled leading to the derivation of 196 accurate energy levels of the (103) and (023) vibrational states. Finally, 150 of these levels were reproduced by an effective Hamiltonian involving two vibrational dark states interacting with the (023) and (103) bright states. The rms deviation achieved by variation of 28 parameters is 0.05 cm(-1), compared to an averaged experimental uncertainty of 0.007 cm(-1), indicating the limit of validity of the effective Hamiltonian approach for HDO at high-vibrational excitation. The predictions of previous ab initio calculations of the HDO spectrum (H. Partridge and D. Schwenke, J. Chern. Phys. 106, 4618-4639 (1997)) were extensively used in the assignment process. The particular spectral region under consideration was used to test and discuss the improvements of new ab initio calculations recently performed on the basis of the same potential energy surface but with an improved dipole-moment surface. The improvements concern both the energy levels and the line intensities. In particular, the strong hybrid character of the nu(1) + 3 nu(3) band is very well accounted for by the new ab initio calculations. (C) 2000 Academic Press.