Analysis of the nu(8)+nu(9) band of HNO3, line positions and intensities, and resonances involving the upsilon(6)=upsilon(7)=1 dark state

Using a high-resolution (R = 0.0025 cm(-1)) Fourier transform spectrum of n itric acid recorded at room temperature in the 1100-1240 cm(-1) region, it has been possible to perform a more extended analysis of the nu(8) + nu(9) band of HNO3 centered at 1205.7075 cm(-1). As in a recent analysis of this band [W. F. Wang, P. P. Ong, T. L. Tan, E. C. Looi, and H. H. Tee, J. Mel. Spectrosc. 183, 407-413 (1997)], the Hamiltonian used for the line position s calculation takes into account, for the upper state, the Delta K = +/-2 a nharmonic resonance linking the rotational levels of the nu(8) = nu(9) = 1 "bright" vibrational state and those of the "dark" nu(6) = nu(7) = 1 vibrat ional state. More than 4800 lines were assigned in the nu(8) + nu(9) band, which involve significantly higher rotational quantum numbers than in previ ous works. On the other hand, and surprisingly as compared to previous stud ies, the nu(8) + nu(9) band appears to be a hybrid band. In fact, nonneglig ible B-type transitions could be clearly identified among the much stronger A-type lines. Accordingly, a set of individual line intensifies were measu red for Lines of both types and were introduced in a least-squares fit to g et the A- and B-type components of the transition moment operator. Finally, a synthetic spectrum of the 8.3-mu m region of HNO3 has been generated, us ing for the line positions and line intensities the Hamiltonian constants a nd the expansion of the transition moment operator which were determined in this work. in this way, the B-type and the A-type components of the nu(8) + nu(9) band appear to contribute for about 1/4 and 3/4, respectively, to t he total band intensity. (C) 1999 Academic Press.