RYDBERG STATE REACTIONS OF ATOMIC AND MOLECULAR-HYDROGEN

Various aspects of the atomic and molecular Rydberg state reactions that result in H-3(+) formation by associative ionization [H*(n = 2) + H-2 --> H-3(+) + e] and by chemiionization [H-2* + H-2 --> H-3(+) + H + e] are presented. For associative ionization, the n = 2 Rydberg state of atomic hydrogen was prepared both by direct photodissociation of H-2 and D-2 into the continuum above the first dissociation limit and by predissociation of H-2 Rydberg states. The most dramatic result of the associative ionization study was the observation of oscillations in the ionization cross section for the reaction involving atoms formed by direct photodissociation. These oscillations arise as a result of a quantum interference effect between two dissociation paths leading to the same final state. The experimental data are presented here with new calculations of this effect by M. Glass-Maujean. For chemiionization, the np sigma and np pi Rydberg states of molecular hydrogen were prepared by photoabsorption of a liquid-nitrogen-temperature sample of similar to 90-95% pure para-H-2; these sample conditions enabled the study of the ion yield of the chemiionization reaction for principal quantum numbers in the range 3-17. The ion yield for n = 3 was zero; the yields for n greater than or equal to 4 increased by many orders of magnitude approximately as n(6) over a rather small range of n and quickly approached that for the analogous ion-molecule reaction. Both the n dependence of the ion yield and the value of n for which the ion yield becomes constant are qualitatively explained by a two-step mechanism for chemiionization involving an ion-molecule reaction of the Rydberg state core (with the Rydberg electron acting as a spectator) followed by autoionization of H-3* to form H-3(+). At least three factors combine to determine the onset of the high-n regime. These are (1) the relative rates of radiative decay of H-2* and collision of H-2*, (2) shielding of the H-3+ ion core by the Rydberg electron at low principal quantum numbers, and (3) the value of the principal quantum number at which rapid decay of the H-3* intermediate by rotational autoionization becomes energetically allowed.