Preparation and characterization of long-lived molecular Rydberg states: Application to HD (original) (raw)
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Spectroscopic Observation and Characterization of H + H − Heavy Rydberg States †
The Journal of Physical Chemistry A, 2009
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 + Hion-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 + Hdissociation 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 .
Chemical Physics Letters, 1999
Mass-analyzed threshold ionization experiments have enabled mapping of the n-dependent Rydberg state survival probability for a series of molecules. Utilizing vacuum ultraviolet photons, one-photon Rydberg manifold spectra of Ar, HCl, N , C H and O were produced, and the prospects of photoinduced Rydberg ionization experiments examined. It was 2 6 6 2 found that the widths of Rydberg manifolds for the molecules studied are quite different. Hydrogen chloride and nitrogen have the narrowest manifold width, followed by benzene, and then oxygen. These varying widths are most strongly Ž. correlated with the angular momentum i.e., quantum defect of the initially prepared Rydberg orbital.
Dynamics of Very High Molecular Rydberg States: The Intramolecular Processes
1994
Classical trajectory computations are used to document and examine the purely intramolecular decay dynamics of very high Rydberg states of an isolated cold molecule. The Hamiltonian is that of an anisotropic ionic core about which the Rydberg electron revolves. The equations of motion are integrated using action angle variables in order to ensure numerical stability for many orbits of the electron. Examination of individual trajectories verifies that both "up" and "down" intramolecular processes are possible. In these, the electron escapes from the detection window by a gain or loss of enough energy. Either process occurs in a diffusive like fashion of many smaller steps, except for a very small fraction of prompt processes. The results for ensembles of trajectories are examined in terms of power spectra of the different modes of motion and in terms of the decay kinetics.
The Journal of Chemical Physics, 1996
The intensity anomalies in the spin-orbit and rotational branching ratios in the zero kinetic energy pulsed-field ionization ͑ZEKE-PFI͒ spectra via the F 1 ⌬ 2 , D 1 ⌸ 1 , and f 3 ⌬ 2 Rydberg states of HCl have been studied. In general, the branching ratios are observed to depend on three parameters employed in the pulsed field ionization experiment: ͑i͒ the delay time between excitation and ionization; ͑ii͒ the magnitude of the bias electric field; and ͑iii͒ the magnitude of the applied pulsed electric field. The results can be rationalized on the basis of the increasing number of autoionization decay channels that become available to the high-n Rydberg states as each ionization threshold is surpassed. The delay dependence of the ZEKE-PFI spectra via the F 1 ⌬ 2 state has been analyzed in more detail by ab initio calculations. These calculations show that the observed spin-orbit branching ratios can be reproduced thereby giving evidence for a nonexponential decay of the high-n Rydberg states ͑nϷ100͒.
The Journal of Chemical Physics, 1995
The effect of an electrical field on the dynamics and decay kinetics of a high Rydberg electron coupled to a core is discussed with special reference to simulations using classical dynamics and to experiment. The emphasis is on the evolution of the system within the range of Rydberg states that can be detected by delayed pulsed ionization spectroscopy ͑which is nϾ90 for both the experiment and the computations͒. The Hamiltonian used in the computations is that of a diatomic ionic core about which the electron revolves. The primary coupling is due to the anisotropic part of the potential which can induce energy and angular momentum exchange between the orbital motion of the electron and the rotation of the ion. The role of the field is to modulate this coupling due to the oscillation of the orbital angular momentum l of the electron. In the region of interest, this oscillation reduces the frequency with which the electron gets near to the core and thereby slows down the decay caused by the coupling to the core. In the kinetic decay curves this is seen as a stretching of the time axis. For lower Rydberg states, where the oscillation of l is slower, the precession of the orbit, due to the central but not Coulombic part of the potential of the core, prevents the oscillation of l and the decay is not slowed down. Examination of individual trajectories demonstrates that the stretching of the time axis due to the oscillatory motion of the electron angular momentum in the presence of the field is as expected on the basis of theoretical considerations. The relation of this time stretch to the concept of the dilution effect is discussed, with special reference to the coherence width of our laser and to other details of the excitation process. A limit on the principal quantum number below which the time stretch effect will be absent is demonstrated by the computations. The trajectories show both up and down processes in which the electron escapes from the detection window by either a gain or a loss of enough energy. Either process occurs in a diffusive like fashion of many smaller steps, except for a fraction of trajectories where prompt ionization occurs. The results for ensembles of trajectories are examined in terms of the decay kinetics. It is found that after a short induction period, which can be identified with the sampling time of the available phase space, the kinetics of the decay depend only on the initial energy of the electron and on the magnitude of the field, but not on the other details of the excitation process. The computed kinetics of the up and down channels are shown to represent competing decay modes. A possible intramolecular mechanism for long time stability based on the sojourn in intermediate Rydberg states is discussed. The available experimental evidence does not suffice to rule out nor to substantiate this mechanism, and additional tests are proposed. The theoretical expectations are discussed in relation to observed time resolved decay kinetics of high Rydberg states of BBC ͑bisbenzenechromium͒ and of DABCO ͑1,4-diazabicyclo͓2.2.2͔octane͒. The experimental setup allows for the imposition of a weak ͑0.1-1.5 V/cm͒ electrical field in the excitation region. The role of the amplitude of the time delayed field, used to detect the surviving Rydberg states by ionization, is also examined. The observed decay kinetics are as previously reported for cold aromatic molecules: Most of the decay is on the sub-s time scale with a minor ͑ϳ10%͒ longer time component. The decay rate of the faster component increases with the magnitude of the field. Many features in such an experiment, including the absolute time scales, are similar to those found in the classical trajectory computations, suggesting that the Hamiltonian used correctly describes the physics of the faster decay kinetics of the high Rydberg states.
Observations of Molecular Rydberg State Decay for n = 10-200
Time-resolved ZEKE (zero electron kinetic energy) spectroscopy has enabled measurements of the rates of decay of molecular Rydberg states close to (within a few cm-l of) the ionization threshold. The ion yield as a function of photon energy in this region cannot be understood without knowing both the ionization potential and the principal quantum numbers, n, of the states involved. Unfortunately, these two unknowns are linked by the equation hv = IP-R / (n-a)*; an independent method must be used to measure the ionization potential, IP. We have found two molecules appropriate for these measurements, bis(benzene)chromium (BBC) and diazacyclooctane (DABCO), both of which have very long Rydberg series with resolved transitions up to n = 35 (BBC) and n = 70 (DABCO). Highly accurate values of the ionization potential (10.5 cm-I) were obtained by extrapolating these series. The line widths are shown to decrease with increasing n with a power of n of 2.5 1 0.3. These are the first measurements of the n dependence of the nonradiative rates over a large range of n for polyatomic molecules in Rydberg states. However, one cannot simply extrapolate the lifetimes (as determined from the widths) to the region near the ionization threshold where the n = 100-200 states are being excited. There the lifetimes as determined directly by ZEKE spectroscopy are longer by several orders of magnitude than the values obtained by extrapolation.
The Journal of Chemical Physics, 1996
The results of rotationally resolved resonance enhanced multiphoton ionization photoelectron spectroscopy and zero kinetic energy-pulsed field ionization studies on HBr via various rotational levels of the F 1 ⌬ 2 and f 3 ⌬ 2 Rydberg states are reported. These studies lead to an accurate determination of the lowest ionization threshold as 94 098.9Ϯ1 cm Ϫ1. Observed rotational and spin-orbit branching ratios are compared to the results of ab initio calculations. The differences between theory and experiment highlight the dominant role of rotational and spin-orbit interactions for the dynamic properties of the high-n Rydberg states involved in the pulsed field ionization process.
Comptes Rendus Physique, 2004
The ungerade ns + nd Rydberg states of C 2 H 2 converging to the ground state of the C 2 H + 2 cation have been investigated in the energy range 74 000-88 000 cm −1 by (3 + 1)-multiphoton ionisation (REMPI) and by VUV absorption spectroscopy at the Super-ACO synchrotron radiation facility. Both methods have allowed the selective analysis of the Rydberg transitions with rotational resolution. Mulliken's semi-united atom model, in which predissociation has been taken into account, was used to understand the relative three-photon intensities among the different electronic transitions within the same Rydberg supercomplex. Lifetimes have been evaluated and illustrate very different behaviours towards predissociation for the observed Rydberg states. To cite this article: S. Boyé et al., C. R. Physique 5 (2004). 2004 Académie des sciences. Published by Elsevier SAS. All rights reserved.
Decay dynamics of high Rydberg states in atoms and molecules
Decay dynamics of high Rydberg states above the first ionization limit have been studied. The decay rate to the ionization continuum, due to spin orbit coupling for atoms (Ar), or coupling to the dense manifold of available rovibronic states in poly-atomic molecules, was measured. This decay is non-exponential and can be influenced by external fields. The stabilization of very high (ZEKE) states is demonstrated and possible mechanisms accounting for these very long lived stated are discussed.