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The relationship between quantum-state populations and ion signals in (2+1) resonance-enhanced multiphoton ionization (REMPI) detection of H2, HD, and D2 via the E, F 1Σ+g (v′E=0, J′=J″)–X 1Σ+g (v″,J″) transition is determined by calibration against a thermal effusive source. Correction factors are obtained for 102 rovibrational levels for v″=0, 1, and 2 and J″ ranging from 0 to 17. Within a given v″, rotational correction factors are nearly unity except for the highest J″ levels. The vibrational correction factors vary with v″; (2+1) REMPI detection is 2–3 times more sensitive to v″=1 and 2 than to v″=0. Experimental correction factors are compared with those derived from a theoretical calculation of the two-photon transition moments by Huo et al. [J. Chem. Phys. 95, xxxx (1991)]. In general, the agreement is excellent, which suggests that theoretical correction factors may be used when experimental ones are unavailable.

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A theoretical study is reported for the dependence of the E, F 1Σ+g–X 1Σ+g (vE=0, vX) Q-branch two-photon transitions on the initial vibrational and rotational quantum numbers vX and J for H2, HD, and D2. The magnitude of the two-photon transition moment ‖Mfo‖ increases with vX between 0 and 2, then decreases rapidly at higher vX. This behavior is ascribed to (constructive and destructive) interference effects in the two-photon transition, which vary with the nodal behavior of the initial-state vibrational wave function. The J dependence of ‖Mfo‖ is most strongly affected by the changes in vibrational wave functions caused by centrifugal distortion of the vibrational potentials. At low to intermediate J, the Franck–Condon overlap improves, resulting in a slow, monotonic increase of ‖Mfo‖ with J. At higher J, resonant tunneling between the inner and outer wells of the E, F state introduces sharp variations in the ‖Mfo‖ structure. Near the top of the barrier, nonresonant tunneling in the E-state vibrational wave function causes a general decrease in ‖Mfo‖ with J.

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