Double-well states of ungerade symmetry in H2: first observation and comparison with Ab initio calculations

Photolysis Production and Spectroscopic Investigation of the Highest Vibrational States in H2 (X1Σg+ v = 13, 14)

Rovibrational quantum states in the X1Σg+ electronic ground state of H2 are prepared in the v = 13 vibrational level up to its highest bound rotational level J = 7, and in the highest bound vibrational level v = 14 (for J = 1) by two-photon photolysis of H2S. These states are laser-excited in a subsequent two-photon scheme into F1Σg+ outer well states, where the assignment of the highest (v,J) states is derived from a comparison of experimentally known levels in F1Σg+, combined with ab initio calculations of X1Σg+ levels. The assignments are further verified by excitation of F1Σg+ population into autoionizing continuum resonances, which are compared with multichannel quantum defect calculations. Precision spectroscopic measurements of the F-X intervals form a test for the ab initio calculations of ground state levels at high vibrational quantum numbers and large internuclear separations, for which agreement is found.

Spectroscopic observation and characterization of H(+)H(-) heavy Rydberg states

A series of discrete resonances was observed in the spectrum of H2, 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 lambda = 94-96 nm in the first step, and tunable ultraviolet radiation in the range lambda = 310-350 nm in the second step. The resonances, detected via H+ and H2+ 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 H2+ ions, as well as into atomic fragments, both H(n = 2) and H(n = 3). Lifetimes are found to scale as n3.

Observation of a Rydberg series in H+H-: a heavy Bohr atom

We report on the realization of a heavy "Bohr atom," through the spectroscopic observation of a Rydberg series of bound quantum states at principal quantum numbers n=140 to 230. The system is made heavy by replacing an electron inside a hydrogen atom by a composite H- particle, thus forming a H+H- Coulombically bound system obeying the physical laws of a generalized atom with appropriate mass scaling.

Multiphoton ion pair spectroscopy (MPIPS) with ultrashort laser pulses for the H2 molecule

Time-dependent Schrödinger equation, TDSE, simulations have been performed in order to prepare and study via MPIPS the evolution of vibrational wave packets on the ion pair electronic state potentials B''B1Sigma(u)(+) and Hh1Sigma(g)(+) of the H2 molecule. Using ab initio potential surfaces and transition moments, we present two- and three-photon excitation schemes with ultrashort pulses (tau <or= 10 fs) to prepare coherent superpositions of the two ion pair H(+)H(-) and H(-)H(+) states from the doorway B1Sigma(u)(+) state, which result from the strong radiative coupling between these two electronic states. The simulations are used to estimate the time evolution and recursion times of vibrational wave packets at large internuclear distances, usually not accessible by single-photon spectroscopy. Conditions for the localization of the ion pair states are proposed.

Study of the B″B¯Σu+1 state of H2: Transition probabilities from the ground state, dissociative widths, and Fano parameters

The transition probabilities, the dissociation widths, and the associated Fano parameters for rovibronic lines with J"=0,...,3 of the absorption bands of the B"B 1Sigmau+ state out of the X 1Sigmag+ v"=0 ground state were measured over the complete vibrational progression, showing clearly that only the inner-well state with B" 4psigma character can absorb vuv light and predissociate efficiently. The absolute values of these transition probablities, predissociation widths, and Fano parameters were found to agree well with ab initio calculations if one takes into account that the calculations neglect nonadiabatic couplings.

Decay dynamics of the long-range H¯Σg+1 state of D2 and H2: Experiment and theory

We present accurate experimental measurements of the lifetimes of rovibrational levels of the long-range H1Sigmag+ state for both D2 and H2, obtained directly from the observation of the time-dependent decay of the fluorescence from these excited levels. These results improve upon and extend those of Reinhold et al. [J. Chem. Phys. 112, 10754 (2000)]. Several decay pathways are open to these levels including fluorescence, predissociation, and autoionization. We present theoretical results for each of these processes, each calculated using the simplest but still appropriate level of theory. In particular, the theoretical calculations provide a quantitative explanation of the dramatic vibrational dependence of the observed lifetimes, the isotope dependence of the lifetimes for levels well localized within the H potential well and therefore not subject to significant tunneling, and an insight into the role of enhanced tunneling in autoionization. In these calculations each of the rovibrational levels of the H state is treated individually, without having to engage in a global coupled-state calculation.

Enhancement and control of H2 dissociative ionization by femtosecond VUV laser pulses

We report ab initio calculations of H2 ionization by VUV/fs 10(12) W/cm2 laser pulses including correlation and all electronic and vibrational degrees of freedom (DOF). Inclusion of the nuclear DOF leads to a substantial increase of resonance enhanced multiphoton ionization. By varying pulse duration, it is possible to control the ratio of dissociative to nondissociative ionization as well as the final H+(2) vibrational distribution. For pulses longer than 10 fs and proportional to omega>0.46 a.u., dissociative ionization entirely dominates, which is a very unusual situation in photoionization studies.