Observations of the high vibrational levels of the B′′B̄ 1Σu+ state of H2

Abstract

Double-resonance laser spectroscopy via the EF (1)Σg (+),v(')=6,J(')=0-2 state was used to probe the high vibrational levels of the B('')B̄ (1)Σu (+) state of molecular hydrogen. Resonantly enhanced multiphoton ionization spectra were recorded by detecting ion production as a function of energy using a time of flight mass spectrometer. New measurements of energies for the v = 51-66 levels for the B('')B̄ state of H2 are reported, which, taken with previous results, span the v = 46-69 vibrational levels. Results for energy levels are compared to theoretical close-coupled calculations [L. Wolniewicz, T. Orlikowski, and G. Staszewska, J. Mol. Spectrosc. 238, 118-126 (2006)]. The average difference between the 84 measured energies and calculated energies is -3.8 cm(-1) with a standard deviation of 5.3 cm(-1). This level of agreement showcases the success of the theoretical calculations in accounting for the strong rovibronic mixing of the (1)Σu (+) and (1)Πu (+) states. Due to the ion-pair character of the outer well, the observed energies of the vibrational levels below the third dissociation limit smoothly connect with previously observed energies of ion-pair states above this limit. The results provide an opportunity for testing a heavy Rydberg multi-channel quantum defect analysis of the high vibrational states below the third dissociation limit.

Continuity of heavy Rydberg behaviour in the ungerade ion-pair states of H 2

“…Fig. 2 includes corresponding quantum defects for the v 0 ¼ 0-16 levels as well as those for the theoretical term energies of the v 0 ¼ 43, 45, 47-49, 54 and 55 vibrational levels [15], which were not observed by Chartrand et al [11]. The quantum defect plot shown in Fig.…”

“…Ion-pair vibronic states in H 2 were first observed in a timedomain experiment and named Heavy Rydberg (HR) states by Reinhold and Ubachs [7]. They were also identified in the frequency domain above the n ¼ 3 dissociation threshold by Ubachs and coworkers [8,9] in the gerade manifold and later in the ungerade manifold by McCormack and co-workers [10,11]. Measured energies of vibrational levels below the (1s; 2l) dissociation threshold belonging to the lowest ungerade excited state, the B 1 R þ u state, were recently treated with the HR model, and HR behaviour was found to be continuous from v 0 ¼ 0 up to nearly the (1s; 2l) dissociation limit [12].…”

“…[11]. The third region to the left of the arrow (E b ¼ 3300-6000 cm À1 ) comprises the resonances observed above the (1s; 3l) threshold [10,11] and are listed in Table 1.…”

“…If, as we propose, the diabatic picture is the better first-order description of the spectrum, the probe stage of the excitation scheme used by Ekey and McCormack [10] and Chartrand et al [11] (Fig. 1), but the transition remains a strong one because of the low principal quantum number of the dominant n ¼ 2 configuration, r 1 u 2s 1 , in the Franck-Condon range of 4-6 a 0 (2-3 Å) of the B 00 B 1 R þ u state.…”

“…Here, we extend the HR analysis to lower vibrational levels of the outer well of the B 00 B 1 R þ u state and to ungerade resonances [10,11] observed above the ð1s; 3lÞ dissociation limit (see Fig. 1).…”