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) spectr al region by intracavity laser absorption spectroscopy. The spectrum (615 l ines), dominated by the 2 nu(2) + 3 nu(3) and nu(1) + 3 nu(3) bands, was as signed and modeled leading to the derivation of 196 accurate energy levels of the (103) and (023) vibrational states. Finally, 150 of these levels wer e reproduced by an effective Hamiltonian involving two vibrational dark sta tes interacting with the (023) and (103) bright states. The rms deviation a chieved by variation of 28 parameters is 0.05 cm(-1), compared to an averag ed experimental uncertainty of 0.007 cm(-1), indicating the limit of validi ty of the effective Hamiltonian approach for HDO at high-vibrational excita tion. The predictions of previous ab initio calculations of the HDO spectru m (H. Partridge and D. Schwenke, J. Chern. Phys. 106, 4618-4639 (1997)) wer e extensively used in the assignment process. The particular spectral regio n under consideration was used to test and discuss the improvements of new ab initio calculations recently performed on the basis of the same potentia l energy surface but with an improved dipole-moment surface. The improvemen ts 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 accoun ted for by the new ab initio calculations. (C) 2000 Academic Press.