Spectroscopic Observation and Characterization of H⁺H⁻ Heavy Rydberg States

Abstract

A series of discrete resonances was observed in the spectrum of H 2 , which can be unambiguously assigned to bound quantum states in the 1/R Coulombic potential of the H + H -ion-pair system. Two-step laser excitation was performed, using tunable extreme ultraviolet radiation at λ ) 94-96 nm in the first step, and tunable ultraviolet radiation in the range λ ) 310-350 nm in the second step. The resonances, detected via H + and H 2 + ions produced in the decay process, follow a sequence of principal quantum numbers (n ) 140-230) associated with a Rydberg formula in which the Rydberg constant is mass scaled. The series converges upon the ionic H + H -dissociation threshold. This limit can be calculated without further assumptions from known ionization and dissociation energies in the hydrogen system and the electronegativity of the hydrogen atom. A possible excitation mechanism is discussed in terms of a complex resonance. Detailed measurements are performed to unravel and quantify the decay of the heavy Rydberg states into molecular H 2 + ions, as well as into atomic fragments, both H(n ) 2) and H(n ) 3). Lifetimes are found to scale as n 3 .

Spectroscopic observation of bound ungerade ion-pair states in molecular hydrogen

“…These studies found the spectra dominated by relatively short-range electronic molecular states. It was only recently that Vieitez et al observed spectral features corresponding to bound ion-pair states by using a laser-based xuv light source in a two-photon excitation scheme [19,20]. Consistent with the earlier investigations, the ion-pair resonances were observed in the photoexcitation yield of H coupling with short-range electronic molecular states.…”

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

“…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].…”

Heavy Rydberg states: large amplitude vibrations

“…1 The vibrational levels of ionically bound states in diatomic molecules are known to form Rydberg series 2,3 and such vibronic states are now sometimes referred to as 'heavy Rydberg' states. [4][5][6][7][8] Heavy Rydberg (HR) states are analogous of electronic Rydberg states but with the Rydberg electron replaced by a negative ion. Thus both the center of mass and the reduced mass of HR states differ signicantly from those of electronic Rydberg states.…”