Laser-excited States Research Articles

Effect of population redistribution through Na ∗-Ar and Na ∗-Na ∗ collisions on saturation curves in a vapour cell

We report measurements of saturation curves and saturation parameters for Na pumped to the 5S or 4D states by two-photon absorption or broadband radiation from a flash lamp pumped dye laser. The experiments were carried out in a vapour cell with Na densities of 10 12 cm -3 and Ar pressures ranging from 0 to 100 Torr. In general, saturation curves for the laser-excited states were similar to those for lower levels. On the other hand, levels lying above the primarily excited states exhibited abnormal, quadratic saturation curves, which were also observed for the 5S state with 4D excitation and for the 3D state with both 5S and 4D excitation. The normal saturation curves are interpreted on the basis of population redistribution caused by radiative transitions or by inelastic collisions with Ar atoms. The quadratic behaviour is ascribed to energy pooling collisions, an interpretation which is supported by the results of time-resolved fluorescence intensity measurements, showing a secondary fluorescence peak after the termination of the laser pulse. The latter peak could be interpreted on the basis of l-mixing collisions of Na “Rydberg atoms” with Ar. The contribution from ionic channels could be neglected under our experimental conditions.

ELECTRON-ATOM-PHOTON INTERACTIONS IN A LASER FIELD

We are currently studying the scattering of low and intermediate energy electrons by sodium atoms in a laser field. The experimental method is discussed, with particular attention to atomic after resonant photon absorption. Total scattering cross sections by 3 2 ~ sodium atoms are presented. In this paper we will discuss the experiments being conducted in the New York University Atomic Beams Laboratory, which involve the scattering of low and intermediate energ-j electrons by alkali atoms in ground and laser-excited states. The principal goals of this research are, 1) to effect as a comparison of ab-initio calculations for simple few-body systems with experiment as possible, 2) to reduce the collision experiment to a determination of separate observables, e.g., direct and exchange scattering amplitudes, selected angular momentum orientations, relative phases of scattering matrix elements, etc., using state selection and analysis, 3) to obtain reliable absolute cross sections for these, without requiring normalization to theory or other experiments, 4) to achieve sufficient redundancy in these state-sensitive cross section measurements to overdetermine scattering amplitudes, permitting detailed scrutiny of timeindependent collision theory, 5) to use the laser as a tool in state-preparation and analysis for both fine and hyperfine states, as well as short-lived electronically excited states, 6) to look for direct intense-field effects on electron-atom collisions, and finally, 7) to study the atom-laser interaction itself. From the point of view of this work, therefore, electron-atom scattering in the presence of laser interactions can be considered to be a natural extension of field-free scattering, which makes it possible to perform better collision experiments, but which do not add any intrinsically new physical processes to the two-body scattering problem. In 6) above, this of course changes; however generally speaking very strong fields are required. On the other hand, even when the laser field is not strong, it is necessary to understand the laser-atom interaction in order to obtain absolute excited-state cross sections. This last point will be discussed more fully later. We will touch on all six items noted above in the present paper, concentrating of course on items 6, 7 and we will attempt to show how they relate to one another. We consider one-electron atoms, such as the alkalis or atomic hydrogen, and refer the reader to the literature for discussing of collision theory and details of previous work1. For electron scattering by alkali atoms, close coupling calculations have been extremely successful in predicting behavior of ground-state Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985123 C1-242 JOURNAL DE PHYSIQUE csllision cross sections. This is in part due, of course, to the relatively simple nature of the interaction, in which electron-correlation only plays a role through (cooperative) core polarization effects. In addition, because of the large (and spherical) core shielding, close-coupling convergence is extremely rapid, particularly with respect to optically-connected electric dipole-type matrix elements. Accordingly these systems can serve as stringent test cases for comparison of theory and experiment. The experimental work described here has been performed using variations of the atomic beam recoil technique2, wherein observation of the scattering is made on the heavy partner, i.e. the atom, rather than the electron. The method possesses the intrinsic advantages that (1) it is possible to directly observe change in the atomic magnetic quantum number, caused either by spin-exchange or by angular momentum projection changes; (2) absolute values of cross sections can be obtained, without need for knowledge of the atomic beam density; ( 3 ) laser-excited atoms can be spatially separated, using photon-recoil, from unexcited atoms enabling determination of absolute excited-state cross sections. The work discussed herein was performed on a new atomic beams apparatus, which is described briefly below. Mutually orthogonal rectangular electron, atom and laser beams intersect in an interaction volume; the (ground-state) atom beam is velocityand hyperfine-state-selected by a tunable hexapole electromagnet. It is recoil-scattered by both electron and laser beams. After travelling through a field-free drift regionu 3 m long its spatial dispersion is analyzed by a surface ionization detector of square cross sectional area, capable of displacement in two dimensions. Fig. 1 shows a schematic view of the experimental setup. SINGLE-MODE RING DYE

Stimulated emission as a major channel for the depopulation of laser-excited states in condensed media

In condensed media, due to the high molecular densities, stimulated emission may become the dominant channel for depletion of population in laser-excited states. The phenomenon is fairly general. The criteria necessary for such a process, the specific case of vibrational excitation in matrix-isolated CH 3F, and some practical implications are considered.

Stimulated emission as a major channel for the depopulation of laser-excited states in condensed media

In condensed media, due to the high molecular densities, stimulated emission may become the dominant channel for depletion of population in laser-excited states. The phenomenon is fairly general. The criteria necessary for such a process, the specific cases of vibration excitation in matrix-isolated CH 3F and CD 3F, and some practical implications are considered.

Final-state distributions in charge transfer from sodium Rydberg atoms near the matching velocity

Reports measurements of the distribution of final n states that are populated in charge transfer collisions of Na+ ions with Na in highly-excited Rydberg states. Relative cross sections for final principal quantum numbers n'=18-42 are shown for laser-excited Na initial states 24d, 25s, 28d, 29s, 33d and 34s at kinetic energies 550-2000 eV. These results span the velocity-matching region in which projectile and orbital speeds are comparable. At the matching velocity the final-state distribution exhibits a maximum close to the n' that corresponds to the smallest change of internal energy relative to the initial state. The distribution shifts toward lower n' and becomes broader as collision velocity is increased. Initial s and d states yield similar distributions above but not at the matching velocity. The results are consistent with excitation of only mod m1 mod <or=1 sublevels referred to the incident beam direction, but several l' orbital angular momentum values appear to be present. Results for Ne+ and Ar+ incident on Na(24d) are similar to Na+ at equal velocity. The range of orbital speeds in a final-state distribution is approximately proportional to the projectile speed irrespective of initial state. The electronic orbital speed that corresponds to the centre of the distribution is a linear function of the projectile speed and initial orbital speed. The distributions disagree with predictions of the Brinkman-Kramers approximation.

Laser-stimulated adspecies interaction with a semiconductor surface

Abstract : The surface electronic charge distributions of a one-dimensional semiconductor are compared for the ground state and the laser-excited surface states. A charged adspecies interaction potential with these excited surface states is examined for the case of silicon. The use of a laser to enhance desorption or adsorption by this process is discussed. (Author)

Photofragmentation dynamics and reactive collisions of laser-excited electronic states

Tunable laser excitation followed by observation of infrared fluorescence provides a means of readily studying electronically excited photoproducts and their reactions. A number of specific examples involving the production of I*(52P1/2) and Br*(42P1/2) upon molecular photodissociation are considered. The quantum yield of I* production from HgI2 is obtained as a function of wavelength. An inconclusive search was made for unobserved states in IBr which lead to I* product atoms and for states of BrCl which lead to Br*. Photodissociation of CH2I2 in an intense laser field is observed to undergo multiphoton dissociation. Quenching and reactive collisions of Br* and I* with halogens, I2, Br2 and Cl2, and interhalogens, IBr, ICl and BrCl, are investigated. Electronically adiabatic reactive channels are detected for the collisions I*+ Br2→ IBr + Br* and I*+ IBr → I2+ Br*. Vibrationally excited HBr product molecules are observed in 10% of the quenching collisions of Br* with H2S.