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

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Abstract
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 78 °K in para-H2, ordinary H2 and D2 (the ortho-para equilibrium mixtures), and HD in the wavelength region from 718 to 700 Å, at a wavelength resolution (FWHM) of 0.035 Å for para-H2, 0.07 Å for ordinary H2 and D2, and 0.15 Å for HD. The threshold for formation of ion pairs from para-H2 occurs at 715.753+0.046−0.008 Å (17.3223+0.0002−0.0011 eV) and this value together with the accurately known values of the dissociation energy of H2 and the ionization potential of atomic hydrogen yields a value of the electron affinity of the hydrogen atom of 0.7542 −0.0004 +0.0013 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 Å of the threshold has a continuumlike appearance contains window resonances which are interpreted as predissociating members of a Rydberg series converging to H+2(v=9, N=2). The continuum results from closely spaced predissociating Rydberg states converging to H+2(v=9 N=0) which were not resolved in the present experiment. 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. No Rydberg state identifications were attempted for the other systems due to the greatly increased complexity of their spectra. For HD which can dissociate to either H++D− or H−+D+, the H−/D− ratio is ∼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 4fσ 1Σ+u state.