High resolution photoionization study of ion-pair formation in H2, HD, and D2

A new photoionization mass spectrometer which is significantly superior to previous instruments in both wavelength resolution and ion intensity is described. It has been used in the study of photon-induced ion-pair formation at 780 K in para-H2, ordinary H2 and D2 (the ortho-para equilibrium mixtures), and HD in the wavelength region from 718 to 700 A, at a wavelength resolution (FWHM) of 0.035 A for para-H2, 0.07 A for ordinary H2 and D2, and 0.15 A for HD. The threshold for formation of ion pairs from para-H2 is used with the accurately known values of the dissociation energy of H2 and the ionization potential of atomic hydrogen to find a value of the electron affinity of the hydrogen atom of 0.754 eV in excellent agreement with the theoretical value of 0.75421 eV calculated by Pekeris. The observed thresholds for ion-pair formation in D2 and HD are also in excellent agreement with the calculated positions. For para-H2 the region within 0.5 A of the threshold has a continuumlike appearance contains window resonances which are interpreted as predissociating members of a Rydberg series converging to H2+(v=9, N=2). The continuum results from closely spaced predissociating Rydberg states converging to H2+(v=9 N=0). A number of other tentative Rydberg state assignments were made for para-H2 based on the results of an analysis using multichannel quantum defect theory. For HD, the H-/D- ratio is approx. 2 just above threshold and appears to decrease with decreasing wavelength; however, this is due, at least in part, to discrimination by the detection system against the more energetic H- ions. Some conclusions and inferences regarding the mechanism for predissociation in H2 and HD are discussed, particularly the possible importance of the 4fsigma 1Σ+/sub u/ state