We report a study of the autoionizing np Rydberg states near the lowest ionization threshold of ${\mathrm{H}}_2$. Using resonant two-photon excitation, intermediate states in specific rotovibrational levels in the double well, E,F ${}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ states are prepared. Then, a second, tunable laser is used to photoionize via excitation of the np Rydberg states. Because of the stepwise laser excitation scheme employed in our experiment the photoionization occurs from states with vibrational wave functions very similar to those of the ${\mathrm{H}}_2^{+}$ core. As a consequence, the autoionizing states appear as nearly symmetric resonances, rather than the highly asymmetric Beutler-Fano profiles observed from the direct photoexcitation from the ground state of ${\mathrm{H}}_2$. Our experiments show that the J=1 np states are broader than the J=3 np states converging to the same limit, suggesting that the two states autoionize into the ɛp and ɛf continuum, respectively. We compare our observations with a theoretical analysis using a multichannel quantum defect theory. The J=1 states reveal the profound effect caused by the perturbation of the autoionizing Rydberg series converging to the lowest vibrational and rotational state of ${\mathrm{H}}_2^{+}$ by low-n states converging to higher vibrational states of the ${\mathrm{H}}_2$-ion core.

• Received 28 November 1988

DOI:https://doi.org/10.1103/PhysRevA.39.3979