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Physical Review A

Effect of quenching in resonant coherent excitation of hydrogen atoms scattered from LiF surfaces

Andrey V. Lugovskoy, Igor Bray

Published Mar 10, 2003• 0 citations
Publisher

Abstract

In this paper we analyze theoretically the ``signal background'' observed in experimental studies of the radiative decay of hydrogen atoms following resonant coherent excitation (RCE) in grazing collisions with a LiF surface. The radiation intensity is evaluated by treating the RCE within a time-dependent (classical-trajectory) quantal approach, and by assuming that the excited electronic states are quenched whenever the atom-surface distance drops below a certain ``switching distance.'' Associating the mechanism of excited-state quenching with the charge transfer to the LiF conduction band we show that the hereto unexplained signal background can be attributed to the dependence of the switching distance on the incident energy.

DOI
10.1103/physreva.67.032901

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In this paper we analyze theoretically the signal background'' observed in experimental studies of the radiative decay of hydrogen atoms following resonant coherent excitation (RCE) in grazing collisions with a LiF surface. The radiation intensity is evaluated by treating the RCE within a time-dependent (classical-trajectory) quantal approach, and by assuming that the excited electronic states are quenched whenever the atom-surface distance drops below a certain switching distance.'' Associating the mechanism of excited-state quenching with the charge transfer to the LiF conduction band we show that the hereto unexplained signal background can be attributed to the dependence of the switching distance on the incident energy.
The coherence for diffraction effects during grazing scattering of fast hydrogen and helium atoms from a LiF(001) surface with energies up to some keV is investigated via the coincident detection of two-dimensional angular distributions for scattered projectiles with their energy loss. For keV H atoms, we identify electronic excitations of the target surface as the dominant mechanism for decoherence, whereas for He atoms this contribution is small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.
Coherence properties of projectiles, found relevant in ion-atom collisions, are investigated by analyzing the influence of the degree of coherence of the atomic beam on interference patterns produced by grazing-incidence fast-atom diffraction (GIFAD or FAD). The transverse coherence length of the projectiles, which depends on the incidence conditions and the collimating setup, determines the overall characteristics of GIFAD distributions. We show that for atoms scattered from a LiF(001) surface after a given collimation, we can modify the interference signatures of the angular spectra by varying the total impact energy, while keeping the normal energy as a constant. Also, the role played by the geometry of the collimating aperture is analyzed, comparing results for square and circular openings. Furthermore, we study the spot-beam effect, which is due to different focus points of the impinging particles. We show that when a region narrower than a single crystallographic channel is coherently illuminated by the atomic beam, the spot-beam contribution strongly affects the visibility of the interference structures, contributing to the gradual quantum-classical transition of the projectile distributions.
The charge transfer of Rydberg hydrogen atoms at a metal surface is investigated for the first time. The surface ionization of Stark states with various electron density distributions with respect to the surface is examined. Unlike the nonhydrogenic species studied previously, genuine control over the orientation of the electronic wave function in the surface-ionization process is demonstrated. A comparison of the results for a range of collisional velocities for the most redshifted Stark state with principal quantum numbers n=20-36 with the classical over-the-barrier approach shows a good agreement for the onset of the ion signal, but the shallow rise in signal is not accounted for. An excellent fit of the experimental results can be achieved using a simple semiempirical model.
The power spectrum of the radiation emitted by a driven collision-damped two-level system is evaluated. The driving field, which is treated classically, is assumed to oscillate harmonically near the atomic resonance frequency, and its intensity is allowed to assume arbitrary values. The collisions are assumed to be strong, i.e., to instantaneously thermalize the state of the atom. The limiting forms of the power spectrum of the radiated field are discussed for the cases of low and high excitation of the atomic system.
This paper is a theoretical discussion of a "coherence effect" in the excitation of hydrogen atoms to the n=3n=3n=3 state by electron bombardment. By coherence is meant that the density matrix describing the atom after the collision has off-diagonal matrix elements in the field-free basis. The differential cross section for producing Balmer-$\ensuremath{\alpha}$ photons at 90\ifmmode^\circ\else\textdegree\fi{} to the electron beam is calculated as a function of an applied longitudinal electric field. The off-diagonal elements show up as a dependence of the observed light on the directions of the electric field. Theoretical results based on the Born approximation are compared with an experiment at 200 and 500 eV. It is pointed out that this method for measuring relative phases of scattering amplitudes can be generally applied and is not restricted to n=3n=3n=3 excitation of hydrogen.
The cross sections for quenching collisions between excited sodium atoms and N2{\mathrm{N}}_{2}N2​, H2{\mathrm{H}}_{2}H2​, HD, and D2{\mathrm{D}}_{2}D2​ molecules in their electronic ground states have been determined using a delayed-coincidence method. The sodium atoms in mixtures with the gases were excited with 20-nsec pulses of NaD radiation, and the delays between the exciting and fluorescent pulses were measured with a time-to-pulse-height converter. The resulting pulse-height spectra, which were accummulated in a 400-channel kicksorter, yielded the effective lifetimes ${\ensuremath{\tau}}^{\ensuremath{'}}$ of the sodium resonance state, which are shorter than the natural lifetime $\ensuremath{\tau}$ because of the nonradiative decay induced in collisions with the molecules. The cross sections for collisions with N2{\mathrm{N}}_{2}N2​, H2{\mathrm{H}}_{2}H2​, HD, and D2{\mathrm{D}}_{2}D2​, obtained from an analysis of the dependence of ${\ensuremath{\tau}}^{\ensuremath{'}}$ on the gas pressures, are 30.4\ifmmode\pm\else\textpm\fi{}1.2, 16.2\ifmmode\pm\else\textpm\fi{}0.3, 11.6\ifmmode\pm\else\textpm\fi{}0.4, and 9.8\ifmmode\pm\else\textpm\fi{}0.3 A˚2{\mathrm{\AA{}}}^{2}A˚2, respectively.