On the energy dependence of the hyperfine interaction in excited states of NO2

We report on the hyperfine structure of N=1, J=3/2 levels of 62 NO2 vibronic states in the 16 850-21 500 cm-1 region, as determined via quantum beat spectroscopy. The hyperfine structure of these levels of mixed (A) over tilde B-2(2)/(X) over tilde (2)A(1) electronic character is dominated by the (X) over tilde (2)A(1) Fermi-contact interaction, and a decrease in the hyperfine splittings with increasing energy is revealed when our results are compared with previous studies in the 11 200-13 700 cm(-1) region. This comparison also reveals the loss of a correlation between band intensity and Fermi-contact constant. A detailed comparison of our results with theoretical predictions for the 16 600-18 700 cm(-1) region is presented. We find that vibrational averaging of the (X) over tilde (2)A(1) Fermi-contact interaction is reflected in the hyperfine interaction of bands in this region, and should also be a factor in producing the small hyperfine splittings observed at energies near dissociation threshold. (C) 2001 American Institute of Physics.