Spin labelling techniques, specifically the use of electron-spin-polarized He(23S) metastable atoms coupled with energy-resolved spin analysis of the ejected electrons, are used to investigate the dynamics of He(23S) deexcitation at solid surfaces. Data for a clean Au(100) surface are presented that show that deexcitation occurs exclusively through resonance ionization followed by Auger neutralization. The electrons involved in Auger neutralization are observed to be correlated in spin and possible reasons for this are discussed. Results obtained at Xe and NO films adsorbed on cooled Au(100) and Cu(100) substrates, respectively, show that He(23S) metastable atom deexcitation is analogous to gas-phase Penning ionization. Detailed differences are apparent that can be attributed to effects associated with the underlying substrate and interactions involving neighboring atoms in the film.

Spin labelling techniques, specifically the use of electron-spin-polarized He(2 S-3) metastable atoms coupled with energy-resolved spin analysis of the ejected electrons, are used to investigate the dynamics of He(2 3S) deexcitation at solid surfaces. Data for a clean Au(100) surface are presented that show that deexcitation occurs exclusively through resonance ionization followed by Auger neutralization. The electrons involved in Auger neutralization are observed to be correlated in spin and possible reasons for this are discussed. Results obtained at Xe and NO films adsorbed on cooled Au(100) and Cu(100) substrates, respectively, show that He(2 3S) metastable atom deexcitation is analogous to gas-phase Penning ionization. Detailed differences are apparent that can be attributed to effects associated with the underlying substrate and interactions involving neighboring atoms in the film.

A single-mode, frequency-stabilized LNA (lanthanum neodymium magnesium hexaluminate) ring laser developed for application in optical pumping and optical manipulation of He(2 3S) metastable atoms is described. This laser routinely provides output powers ≳300 mW at 1.083 μm when pumped with 5 W from an argon-ion laser.

Measurements of ejected electron energy distributions are used in conjunction with electron spin labeling techniques to probe the mechanisms by which He(2(3)S), He(2(1)S), and He(2(3)P) atoms are deexcited at Ar and Xe films adsorbed on a cooled Cu(100) substrate. The data for both surfaces contain features similar to those observed in gas-phase Penning ionization, indicating that ejection results, in part, from Auger deexcitation, i.e., surface Penning ionization. For Xe, however, additional features are observed that can be attributed to resonance ionization of an incident excited atom followed by neutralization of the resulting He' ion through an interaction that involves neighboring Xe atoms in the film. Indeed, the Xe data provide an exceptional example of a surface at which Auger deexcitation and resonance ionization occur in parallel with one another, with a branching ratio that changes significantly as the internal energy of the incident atoms increases. The ejected electron yield from both Ar and Xe films is substantially higher than for clean Cu(100), indicating that such films might form the basis of an efficient thermal-energy helium metastable-atom detector.

Spin-labeling techniques, specifically the use of electron-spin-polarized He(2 3S) metastable atoms coupled with energy-resolved spin analysis of the product electrons, are used to investigate the dynamics of Penning ionization in collisions between He(2 3S) atoms and H2O, SO2, NO, and NO2. The data complement earlier studies of the reaction dynamics based on analysis of the energy distribution of the product electrons and confirm that ionization can occur via a number of different reaction channels. For example, the present results show that in collisions with targets having a positive electron affinity, ionization via the ionic channel is important and that effects due to spin-orbit coupling must be considered. The data also provide evidence that exchange may be important in collisions with open-shell targets.

The rate coefficients for mixing between He(2 P-3(J,MJ)) levels during collisions with ground-state helium atoms and for conversion of He(2 P-3(J)) atoms to He-2(b 3PI(g)) molecules via three-body reactions in helium gas have been investigated over the temperature range 1.6-300 K. The measured rate coefficients for collisionally induced P-state mixing decrease slowly with decreasing temperature, from (1.8 +/- 0.5) X 10(-9) cm3 s-1 at 300 K to (4.5 +/- 0.5) X 10(-10) cm3 s-1 at 4.2 K. The rate coefficients for the production of He-2(b 3PI(g)) molecules via three-body reactions are observed to increase with decreasing temperature and are described by the relation k(p) congruent-to (2.5 + 267T-1) X 10(-32) cm6 s-1. This behavior, which is very different from that noted in earlier studies of the conversion of He(2 S-3(1)) atoms to He-2(a 3SIGMA(u)+) molecules through three-body reactions, suggests that the reaction is not thermally activated.

Spin labeling techniques are being used to investigate the dynamics of metastable atom deexcitation at surfaces. A beam of thermal-energy electron-spin-polarized He(2{sup 3}S) atoms is directed at the surface, and the energy and polarization distributions of the ejected electrons are measured. At clean, high work function metal surfaces He(2{sup 3}S) deexcitation proceeds via resonance ionization of the incident atom followed by Auger neutralization of the resulting He{sup +} ion. The data show that the electrons involved in the neutralization process tend to have antiparallel spins. This spin correlation may be explained in terms of modifications to the local surface electronic structure due to the presence of the He{sup +} ion, and efforts are underway tb model such effects theoretically. Measurements on surfaces comprising several monolayers of Xe frozen on a cooled substrate show features analogous to those observed in gas-phase Penning ionization, suggesting that electron ejection occurs, in part, through Penning ionization of adsorbed atoms. Other features, however, are apparent and can be attributed to interactions with neighboring atoms and to effects introduced by the substrate. These features are being explored further using He(2{sup 1}S) and He(2{sup 3}P) atoms and substrates with different work functions.

A magneto-optical trap for He(2{sup 3}S) metastable atoms has been constructed, utilizing superconducting magnetic gradient coils and a Ti:Sapphire ring laser for pumping the helium 2{sup 3}S-2{sup 3}P transition. The He(2{sup 3}S) atoms are produced by a weak rf discharge in helium gas at a temperature of 1.4K. The discharge products flow through a small orifice into the trap cell, where a fraction of the He(2{sup 3}S) atoms are trapped and ground state helium atoms are rapidly cryopumped by zeolite pellets that cover most of the cell bottom. Preliminary experiments suggest that {approximately}10{sup 6} atoms are trapped in a small volume at {approximately}1 mK, with a trap lifetime of 10-100 msec limited by resonantly-enhanced He(2{sup 3}S)-He(2{sup 3}P) Penning reactions. Ultimately, it is estimated that a substantial number of atoms can be held at ultra-low temperature in near-perfect vacuum, in a dc magnetic trap. Measurements of decay times of the trapped atoms should yield rate coefficients in the quantum regime for He(2{sup 3}S)-He(2{sup 3}P) and He(2{sup 3}S)-He(2{sup 3}S) Penning reactions, and perhaps the He(2{sup 3}S) radiative lifetime.

Spin-labeling techniques, specifically the use of electron-spin-polarized He(2(3)S) metastable atoms coupled with energy-resolved spin analysis of the product electrons, are used to investigate the dynamics of Penning ionization in collisions involving He(2(3)S) atoms. Results obtained using CO2, CO, Cl2, and O2 target gases are presented that illustrate the capabilities of this approach. In particular, the data for Cl2 and O2 confirm that ionization via ionic channels is important and show that exchange and spin-orbit effects must be considered.

Energy distributions of electrons ejected from an atomically clean Cu(100) surface by incident thermal-energy helium 2 3S, 2 1S, and 2 3P atoms are presented, which show that each species is deexcited exclusively through resonance ionization followed by Auger neutralization. These data, when coupled with earlier measurements in this laboratory using electron-spin-polarized He (2 3S) atoms, require that the electrons involved in the Auger neutralization process tend to have antiparallel spins. This work provides direct confirmation of spin correlation in Auger neutralization of ions outside a paramagnetic surface.

A technique for absolute calibration of a Mott polarimeter is described that makes use of electrons of accurately known polarization obtained through surface Penning ionization. This approach provides higher count rates and lower uncertainties than obtained in earlier measurements based on gas-phase Penning ionization.

Electron spin-polarized metastable Helium atoms are being used to probe the properties of various metallic, semiconductor, and insulating surfaces. Spin and energy analysis of electrons ejected as a result of metastable impact provide new insights into the mechanism by which metastable atoms are de-excited at the surface. These experiments also provide information concerning the electronic spin and density of states at the surfaces in question. These studies require a beam of electron spin-polarized 2{sup 3}S metastable atoms. This beam is produced by optical pumping using 1.08 {mu}m 2{sup 3}S-2{sup 3}P radiation. A single-mode Ti:Sapphire laser, developed for this purpose, provides around 300mW at the transition frequency. To maintain stable optical pumping, however, the laser output frequency must be stabilized to <1MHz/day. Here, the authors describe a simple, novel stabilization scheme based on a scanning Fabry-Perot etalon and a stabilized He-Ne laser that has been developed to achieve this goal. The performance of this device will be discussed.