Microwave ionization of highly excited hydrogen atoms

Within a 1-dimensional model we calculate quantum mechanically the probability to ionize a highly excited hydrogen atom by a monochromatic microwave field. Based on a detailed analysis of the ionization process we developed a computational scheme as well as a simple physical framework which are presented and discussed. Our calculations are in good agreement with the experimental results. We show that the experimentally measured ionization thresholds are due to a sharp transition between two localization regimes and that recently measured structures below the classical chaos border are due to unresolved clusters of Floquet pseudo crossings. We propose an experimental method by which one could measure the distance and distribution of crossing Floquet eigenvalues.     

Collisional ionization of highly excited neon atoms by nonpolar hydrocarbons

When benzene, acetylene and ethylene were allowed to collide with Ne Rydberg atoms (17?n?40), significant Ne⁺ ions were observed. Their cross sections were estimated, with reference to the ionization by H₂O, to be as large as 10^?14–10^?15 cm². However, no Ne⁺ signals were observed when ethane, methane and N₂ were used as targets. Theoretical estimates of the cross sections for ionization by interaction of the quadrupole moment and polarizability of the first three molecules are several orders of magnitude smaller than the experimental cross sections given above. A similarity of this phenomenon to the scattering of a thermal electron by benzene etc., observed by Christophorou et al., is suggested.     

Competitive ionization processes of anthracene excited with a femtosecond pulse in the multi-photon ionization regime

To clarify the ionization mechanism of large molecules under multi-photon ionization conditions, photo-electron spectroscopic studies on anthracene have been performed with electron imaging technique. Electron kinetic energy distributions below a few eV reveal that three kinds of ionization channels coexist, viz., vertical ionization, ionization from Rydberg states, and thermionic hot electron emission. Their relative yield is determined by the characteristic of the laser pulse. The duration in particular influences the ratio between the first two processes, while for higher intensities the last process dominates. Our results provide strong evidence that internal conversion plays an important role for the ionization of the molecule.     

The Lamb shift of excited S-levels in hydrogen and deuterium atoms

A specific combination of s-state Lamb shift ΔE _L(1s _1/2)-n ³ΔE _L(ns _1/2) is considered. Its value is calculated both in the hydrogen and deuterium atoms for n up to 12. The result includes all correction which can contribute more than 1 kHz, particularly the one-loop corrections for both the self energy and the vacuum polarization, and the two-loop contribution. Nuclear finite-size corrections for the isotopic difference of the combination are also evaluated.     

Quasiresonance charge exchange of hydrogen isotopes mesic atoms in excited states

Processes of charge exchange of hydrogen isotopes mesic atoms in excited states at low collision energies 10^?2?E?1 eV are studied. The cross sections calculated depend on energy like ～E ^?1 and are of an order of the atomic cross sections (～10^?16 cm²). It is shown that the high rates (～10¹² s^?1) of charge exchange and thermalization of mesic atoms in excited states at the liquid hydrogen density are comparable with the rates of cascade transitions in mesic atoms.     

Pulse radiolysis studies of the quenching processes of Xe* excited atoms

The time resolved fluorescence of Xe ₂ ^* excimers in pure xenon and Xe-M (M=H₂,N₂,N₂O,CO₂) mixtures has been observed. The formation rate constant of Xe ₂ ^* , k₂=(1.1±0.1)·10^–30 cm⁶/s and the lifetime of the excimer precursors, ₀(100 ± 40) ns were evaluated. The quenching rate coefficients of Xe(6p) states by M have been found to be in the range of (0.5–1.3)·10^–9 cm³/s. The basic parameters and operating characteristics of the newly constructed pulse radiolysis set up based on SINUS-5 electron accelerator are also presented.     

Excitation transfer in collisions of highly excited alkali atoms. Cesium-cesium collisions

Possible mechanisms of the resonance and near-resonance excitation transfer in collisions of Cs^**(8²P) with Cs(6²S) are discussed. Estimated cross section for the intramultiplet mixing is in close agreement with experimental results.     

Temperature dependence of the quasiclassical reactivity for vibrationally excited hydrogen molecules colliding with hydrogen atoms

A discussion of reactants vibrational energy and temperature dependence of reactive rate constants for the hydrogen atom hydrogen molecule reaction is presented for a matrix of values calculated at 0 < v <10 and temperatures in the range from 300 K to 4000 K. A parametrization ofthe results is attempted. A comparison with rate constant values obtained from an approximate quantum treatment is also reported.     

Collisional ionization of alkali atoms by vibrationally excited N2

Chemiionization of alkali atoms by active nitrogen is studied in a crossed beam apparatus. Vibrationally excited N₂ in the electronic ground state is responsible for the ionization rather than electronically excited N₂ in the A ³∑_u⁺ state. The ionization cross section is of the order 10² A². The experimental data is consistent with the distribution of the vibrational levels of N₂ (X¹ ∑_g⁺) predicted by Bray or Caledonia and Center.     

Theoretical study of the reaction of H atoms with vibrationally highly excited HF molecules

Vibrationally highly excited molecules react extremely fast with atoms and probably with radicals. The phenomenon can be utilized for selectively enhancing the rate of reactions of specific bonds. On the basis of quasiclassical trajectory calculations, the paper analyzes mechanistic details of a prototype reaction, H + HF(v). At vibrational quantum numbers v above 2, the reaction exhibits capture-type behavior, that is, the reactive cross section diverges as the relative translational energy of the partners decreases, both for the abstraction and for the exchange channel. The mechanism of the reaction for both channels is different at low and at high translational energy. At low vibrational energy, the reaction is activated, which is switched to capture-type at high excitation. The reason is an attractive potential that acts on the attacking H atom when the HF molecule is stretched. In contrast to the 6-SEC potential surface of Mielke et al., the switch cannot be observed on the Stark-Werner potential surface, due to a small artificial barrier at high H-HF separation, preventing the reactants from obeying the attractive potential and also proving the importance of the latter. The exchange reaction can be observed even when the total energy available for the partners is below the exchange barrier, because at low translational energies the product F atom of a successful abstraction step can re-abstract that H atom from the intermediate product H2 molecule that was originally the attacker.     

Improved study of the ionization of hydrogen atoms in thermal energy collisions with metastable He(23 S) atoms

The first electron spectrometric study of the ionizing reaction of metastable He(2³ S ₁) atoms with ground state hydrogen atoms has been carried out with sufficiently high resolution to partially resolve the rotational structure due to formation of rovibrationally excited HeH⁺ (v, J) ions at two different beam source temperatures (300 K and 90 K). The electron energy spectrum has been reproduced in model quantum calculations, using a new large scale ab initio calculation of the He(2³ S)+H(1² S)²Σ-potential. The imaginary part has been adjusted to yield a satisfactory fit to the measured spectrum. The collision energy dependence of the associative ionization electron spectra and of the total and partial ionization cross sections is discussed in some detail. No significant signs for limitations of the used local complex potential method, indicated by results of an earlier study of the He(2³ S)+H(1² S) system, have been found in the present work, in which the calculations were carried out with an improved and corrected program.     

Diabatic representation for the excited states of the Na2 molecule: application to the associative ionization reaction between two excited sodium atoms

A two-electron model potential method is proposed to compute diabatic electronic excited states for Na₂. The configuration space is first divided into two subspaces corresponding to singly and doubly excited configurations respectively. Next this partition is modified to ensure a correct dissociation limit for the ground state. The matrix element of the electronic Hamiltonian between the two subspaces can be extrapolated along a Rydberg series up to the ionization continuum. The first order M.Q.D.T. treatment of Giusti (1980) is then used to estimate the cross-sections for the reaction Na(3p)+Na(3p)→Na ₂ ⁺ +e ^?, considering various symmetries of the intermediate Na₂ molecule. A marked selectivity in favour of the³Σ _u ⁺ symmetry is found and the estimated cross-section σ ～ 5 Ų for a collision energy of 0.05 eV is in satisfactory agreement with the experimental results.     

Time-dependent photoionization of azulene: competition between ionization and relaxation in highly excited states

Pump-probe photoionization has been used to map the relaxation processes taking place from highly vibrationally excited levels of the S(2) state of azulene, populated directly or via internal conversion from the S(4) state. Photoelectron spectra obtained by 1+2(') two-color time-resolved photoelectron imaging are invariant (apart from in intensity) to the pump-probe time delay and to the pump wavelength. This reveals a photoionization process which is driven by an unstable electronic state (e.g., doubly excited state) lying below the ionization potential. This state is postulated to be populated by a probe transition from S(2) and to rapidly relax via an Auger-like process onto highly vibrationally excited Rydberg states. This accounts for the time invariance of the photoelectron spectrum. The intensity of the photoelectron spectrum is proportional to the population in S(2). An exponential energy gap law is used to describe the internal conversion rate from S(2) to S(0). The vibronic coupling strength is found to be larger than 60+/-5 microeV.     

Rotational effect in the ionization of a highly excited atom by collision with a polar molecule

If the collisional ionization is chiefly due to energy transfer from the polar-molecule rotation to an electron in a high Rydberg state of the atom, then theory predicts that the cross section averaged over a thermal distribution of rotational states should show step-like structure as a function of the energy of the Rydberg state. This structure has been experimentally detected, and it can be considered as direct evidence of the rotational effect in the collisional ionization.     

Adiabatic photodissociation of acenaphthylene dimers in the excited singlet state

Fluorescence spectroscopic studies of acenaphthylene dimers in saturated hydrocarbon solution have revealed that the dimer (A₂) photodissociates to     

Excitation transfer in collisions of highly excited alkali atoms. I. Cesium-noble gases collisions

The mechanisms of intramultiplet mixing and intermultiplet transitions in a highly excited cesium atom, induced by collisions with inert gas atoms, are discussed. It is argued that the crossing of the excited ²Π-molecular state by the repulsive ²Σ-state provides the main channel for nonelastic events. Estimated cross sections are in disagreement with available experimental data.     

One- and two-photon spectroscopy of highly excited states of alkali-metal atoms on helium nanodroplets

Alkali-metal atoms captured on the surface of superfluid helium droplets are excited to high energies (≈3?eV) by means of pulsed lasers, and their laser-induced-fluorescence spectra are recorded. We report on the one-photon excitation of the (n+1)p←ns transition of K, Rb, and Cs (n=4, 5, and 6, respectively) and on the two-photon one-color excitation of the 5d←5s transition of Rb. Gated-photon-counting measurements are consistent with the relaxation rates of the bare atoms, hence consistent with the reasonable expectation that atoms quickly desorb from the droplet and droplet-induced relaxation need not be invoked.