Competing predissociation and preionization in the photoabsorption of H2 above the ionization threshold

Abstract

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 ionic vibrational basis functions to vanish at a boundary R0, after which we average the differential oscillator strength over a range R0+dR. The relevant averaging range dR depends on the density of discrete vibrational levels in the vicinity of selected preionization resonances. 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-H2 (v″=0, J″=0) between the photon wavelengths 782.0 and 786.0 Å. This spectral region exhibits extensive resonance structures due to preionization and predissociation. 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)].