COMPETING PREDISSOCIATION AND PREIONIZATION IN THE PHOTOABSORPTION OFH-2 ABOVE THE IONIZATION THRESHOLD

We present a straightforward multichannel quantum defect theory (MQDT) procedure to calculate the broadening of preionization resonances due to competing predissociation in the ionization continuum of molecular hydrogen. We discretize the dissociative continuum by forcing the ion ic vibrational basis functions to vanish at a boundary R(0), after whi ch we average the differential oscillator strength over a range R(0)+d R. The relevant averaging range dR depends on the density of discrete vibrational levels in the vicinity of selected preionization resonance s. This approach yields directly the full spectral linewidth including the effect of predissociation. We apply the method to regions of the photoabsorption spectrum of para-H-2 (v ''=0, J ''=0) between the phot on wavelengths 782.0 and 786.0 Angstrom. This spectral region exhibits extensive resonance structures due to preionization and predissociati on. We obtain semiquantitative agreement with linewidths and branching ratios for ionization and dissociation obtained from experiment, and the MQDT/R-matrix method of Jungen [Phys. Rev. Lett. 53, 2394 (1984)]. (C) 1995 American Institute of Physics.