Double-resonance studies of rotational autoionization of H2

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Abstract
Two‐color resonantly enhanced multiphoton ionization combined with photoelectron spectroscopy detection was used to study rotational autoionization of np Rydberg states of H2 near the first ionization threshold. In this two‐step experiment, one laser was used to excite a two‐photon transition to the E,F 1Σ+g, v’=E0, J’=0–4 levels, and a second laser was used to probe single photon transitions to the rotationally autoionized np Rydberg series coverging to the X 2Σ+g, v+=0, N̄+=1–6 rotational levels of the ion. Assignments were made for a large number of Rydberg states converging to v+=0 and for several interlopers converging to v+=1 and 2. Of the five dipole allowed Rydberg series converging to v+=0 excited from each intermediate J’ level (J’>2), two are allowed to rotationally autoionize in a coupling scheme that assumes ejection of pure p waves in the ionization process and singlet coupling of the spins of the ion core and the outgoing electron. Members of these Rydberg series have large half‐widths, and the v dependence of the half‐widths is in good agreement with simple quantum defect theory predictions. Ionization via the change of four quanta of rotational energy was found to be significantly slower than ionization via the change of two quanta of rotational energy. Ionization was also observed for the series that are forbidden to rotationally autoionize according to this simple coupling scheme, and it is shown that both the inclusion of f waves in the autoionization process and singlet–triplet mixing may be required to explain these observations.