Continued improvement of a single-mode frequency-stabilized 1.083 $\mu$m LNA laser developed at this laboratory has resulted in greater frequency stability and higher output power. Use of this laser with the Rice Flowing Helium Afterglow apparatus has resulted in considerable insight into Penning ionization reaction processes. Also, in the present work, a modified Coherent Inc. CR899-21 Ti:Sapphire laser is being used as a source of 1.083 $\mu$m radiation. A simple technique using a confocal Fabry-Perot etalon and stabilized helium-neon laser is used to limit long-term drift in the laser output frequency to $\leq$1 MHz per day. Measurements using a beam of He(2$\sp3$S) atoms show that the laser can provide stable He(2$\sp3$S) optical pumping resulting in polarizations approaching 100%. The laser is also being employed in studies of He(2$\sp3$P) collisions at low temperatures using laser-induced fluorescence and line-shape analysis. (Abstract shortened with permission of author.)

Potassium atoms in selected high-lying Rydberg states (n $<$ 1200) are used as a tool to probe threshold behavior in electron-molecule collisions. Collisions with non-polar electron attaching molecules such as CCl$\sb4$ are dominated by electron capture in a binary interaction between the Rydberg electron and target molecule allowing the study of electron attachment at electron energies of only a few $\mu$eV. Analysis of the data shows the cross section for electron capture is consistent with the Wigner threshold law for an inelastic s-wave process. Collisions with polar molecules can lead to Rydberg atom destruction through transfer of molecular rotational energy to the Rydberg electron. Rydberg atom data obtained with polar targets are not consistent with scattering from a static dipole. However, the data are consistent with a threshold law that takes into account a rotationally-averaged induced dipole interaction that can possibly support bound or virtual states.

Many interesting physical phenomena have been observed in electron-stimulated desorption studies of the cyanide molecule on alkali halide and alkali metal surfaces. We have performed a theoretical investigation of the nature of the cyanide-surface bond and of the desorption process using an ab initio density functional theoretic method of calculating electronic structure in the local density and Born-Oppenheimer approximations. We compare our results with experiment, and offer an explanation for an anomalous non-Boltzmann, temperature-independent rotational distribution experimentally observed in cyanide desorbed from these surfaces. As a verification of the method, we have performed extensive calculations on different bare alkali halide and alkali metal clusters and compared them with experimental results.

The use of spin-labelling to study the dynamics of He (2 $\sp3$S) metastable atom ionization of atoms and simple molecules is described. Briefly, He (2 $\sp3$S) atoms created by a microwave discharge are spin-polarized in a flowing afterglow via optical pumping with circularly-polarized 1.08 $\mu$m (2 $\sp3$S) $\Rightarrow$ (2 $\sp3$P) radiation from an LNA laser. A target gas is injected into the flowtube, and electrons created in Penning ionization reactions diffuse through a differentially-pumped aperture and are energy- and spin-analyzed with a hemispherical energy analyzer in series with a retarding-potential Mott polarimeter. Data are reported for Ar, CO$\sb2$, CO, H$\sb2$O, O$\sb2$, NO, NO$\sb2$, SO$\sb2$, and Cl$\sb2$ target gases. The generally accepted model of Penning ionization, the so-called exchange model, suggests that a target electron of appropriate spin tunnels to fill the He 1s hole with simultaneous ejection of the He 2s electron, which, for polarized He (2 $\sp3$S) atoms, produces fully polarized electrons. It is found that fully polarized electrons are ejected in reactions with closed-shell, negative electron affinity targets such as Ar and CO$\sb2$, independent of the positive ion final state, in agreement with the exchange model. Substantially lower polarization is measured for open-shell targets, such as O$\sb2$ and NO, and for targets with large electron affinity, such as NO$\sb2$ and SO$\sb2$. A strongly attractive entrance channel potential is possible in these cases. The exact depolarization mechanism is unclear, but is probably related to the formation of the ionic quasi-molecule. For some targets (Cl$\sb2$, for instance), excitation transfer with subsequent autoionization of the core-excited Rydberg target state created occurs, and these reactions also produce electrons with reduced polarization.

Low energy electron interactions with C$\sb2$Cl$\sb4$ and C$\sb6$F$\sb6$ molecules have been studied by using Rydberg atoms. It is observed that the electron attachment process occurs through various channels. Collisions of Rydberg atoms with C$\sb2$Cl$\sb4$ molecules result in the formation of Cl$\sp-$ ions and C$\sb2$Cl$\sb4\sp-$ ions. The data suggest the presence of the long lived C$\sb2$Cl$\sb4\sp-$ ions and that the C$\sb2$Cl$\sb4\sp-$ ions undergo rapid autodetachment forming an electron and a neutral C$\sb2$Cl$\sb4$ molecule. Similar reactions with the C$\sb6$F$\sb6$ molecules result in the formation of both long lived C$\sb6$F$\sb6\sp-$ ions and short lived C$\sb6$F$\sb6\sp-$ ions, that suffer rapid autodetachment forming an electron and a neutral molecule. The present data are compared with the previous data obtained by Swarm and Threshold Photoelectron Spectroscopy techniques.

Absolute differential cross sections for electron capture and loss by neutral hydrogen atoms incident on various gases are presented. The measurements cover a laboratory angular range of 0.02$\sp\circ$ to 1.77$\sp\circ$ and a laboratory energy range of 2.0 to 5.0 keV. The target gases include H$\sb2$, O$\sb2$, N$\sb2$, Ar and He. Integrated experimental cross sections are compared with total cross sections reported by other investigators.

Dissociative electron attachment reactions of the type K(nd) + AB $\to$ K$\sp+$ + AB$\sp{-*}$ $\to$ K$\sp+$ + A$\sp-$ + B have been investigated for polyatomic molecules BrCN, CF$\sb2$Br$\sb2$, CF$\sb3$Br and Fe(CO)$\sb5$. Rate constants for the formation of various negative ions were measured for a range of principal quantum numbers n. At high n (n $>$ 40), the Rydberg electron can be treated as 'essentially' free electron and the present data are compared with the free electron data obtained by the TPSA technique. At lower n, the free electron model must be modified to take into account post-attachment electrostatic interactions between the product ions and effects associated with the reduced size of the Rydberg atom. The translational energy release in such reactions was measured using the TOF-PSD technique. Comparison with the excess energy of reaction for BrCN and CF$\sb2$Br$\sb2$ shows that the energy released by electron capture is well distributed among the internal modes of the intermediate negative ion prior to dissociation. In contrast, data for CF$\sb3$Br indicate that the electron capture takes place into a repulsive state resulting in the immediate dissociation of the intermediate negative ion before significant redistribution of excess energy can occur.

$C\sb{60}\sp-$ ion formation has been observed in collisions between K(np) Rydberg atoms with principal quantum numbers $\rm35\leq n\leq125,$ which corresponds to electrons with mean energies in the range $\sim$2 meV to $\sim$170 $\mu$eV, and $C\sb{60}$ molecules. This results from Rydberg electron transfer and has permitted study of the low energy electron attachment properties of Buckminsterfullerene, which is known to capture free electrons with energies up to $\sim$12 eV. Present data agree with a previous Rydberg atom study but disagree with theoretical predictions and also with previous free electron-$C\sb{60}$ studies. Results from both theory and the free electron experiments suggest that a potential barrier of $\sim$240 meV exists for electron capture by $C\sb{60},$ preventing electron capture at very low energies. The present data are compared with previous experiments and with theory, and possible explanations for the observed discrepancy, including evidence for an image-charge-bound state of $C\sb{60},$ are discussed.

In the present work very-high-n Rydberg-atoms $(n \sim 100 - n \sim 400)$ are used to probe electron-molecule collisions at ultra-low electron energies. Based on the essentially-free-electron model, for sufficiently high n, Rydberg atom-molecule collisions can be described in terms of a binary interaction between the target molecule and the essentially-free Rydberg electron. Since the average kinetic energy of the Rydberg electron is ultra-low $\rm({\sim}85\mu eV - {\sim}1.4meV),$ analysis of the very-high-n Rydberg-atom collision data can provide information on electron-molecule scattering at electron energies corresponding to electron temperatures of only $\sim$1$\sp\circ$K, which are far below those accessible using any alternate approach. Rate constants for destruction of very-high-n Rydberg atoms in collisions with various target molecules have been measured. This study focuses on Rydberg electron transfer to an electron-attaching molecule which results in negative ion formation via the reactions $$\rm K({\it np\/}) + XY \to K\sp+ + (XY)\sp{-*}\ or\ \to K\sp+ + X\sp- + Y\eqno(1)$$and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization $$\rm K({\it np\/}) + XY({\it J\/}) \to K\sp+ + {\it e}\sp- + XY({\it J\/}-1)\eqno(2)$$ The n dependence of the rate constants for Rydberg-atom destruction depends on the reactions involved. In reaction (1) the rate constant is independent of n whereas in reaction (2) it increases nearly linearly with n. When both reactions are possible, the measured n dependence can be explained in terms of contributions from each process. The essentially-free-electron model suggests that the n dependence of the rate constants for Rydberg-atom destruction reflects the energy dependence of the cross sections of the corresponding free electron-molecule collision processes which are $$\eqalignno{&e\sp- + \rm XY \to (XY)\sp{-*}\ or\ \to X\sp- + Y&(1\sp\prime)\cr &e\sp- + \rm XY({\it J\/}) \to {\it e}\sp- + XY({\it J\/}-1)&(2\sp\prime)\cr}$$Analysis of the data therefore provides the behavior of cross sections for these processes at ultra-low electron energies.

In the first use of the Raman effect to detect sulfur atoms, transient CARS spectroscopy has revealed novel relaxation effects in sulfur formed through multiple photon excitation of carbon disulfide vapor. Ground term $\sp3P$ sulfur was monitored through its 396 cm$\sp{-1}$ and 573 cm$\sp{-1}$ fine structure transitions ($\sp3P\sb1\leftrightarrow\ \sp3P\sb2$ and $\sp3P\sb0\leftrightarrow\ \sp3P\sb2$) at various delays after the photolysis laser pulse. It was found that quenching of $\sp1D\sb2$ sulfur to the $\sp3P$ term by collision with the rare gases argon, krypton, and xenon gives clear kinetic and spectral evidence of a population inversion between the $\sp3P\sb0$ and $\sp3P\sb2$ fine-structure levels. Kinetic data indicate no such inversion between the $\sp3P\sb1$ and $\sp3P\sb2$ levels. Extensive modeling of the kinetic data taken at the $\sp3P\sb0\leftrightarrow\ \sp3P\sb2$ transition was performed to obtain the branching ratio into the $\sp3P\sb0$ level for the three rate gases studied. Although kinetic models including all possible processes in this system contain too many unknown parameters to be useful, a simple three parameter model gives reasonable fits to the data. This model yields branching ratios of 0.83, 0.75, and 0.73 for the fractional formation of sulfur's $\sp3P\sb0$ level through quenching from $\sp1D\sb2$ by Ar, Kr, and Xe, respectively. The results of MCSCF-CI calculations of the relevant low-lying Ar-S potential curves suggest that quenching proceeds through a single energetically accessible intersection between molecular terms, which, when correlation and coupling rules are considered, leads adiabatically to the $\sp3P\sb0$ product level that is experimentally observed to dominate. Although calculated Xe-S and Kr-S potential energy curves are not available, comparison with the similar rare gas oxide system suggests that a general explanation for the selective quenching mechanism may involve differences in the spin-orbit coupling strengths between molecular terms at the two crossings that adiabatically correlate with the $\sp3P\sb0$ and $\sp3P\sb2$ levels.