The temperature dependence of conversion of $\rm He(2\sp3S\sb1)$ metastable atoms to $\rm He\sb2(a\sp3\Sigma\sbsp{u}{+})$ metastable molecules in the three-body reaction $\rm He(2\sp3S\sb1)+ 2He(1\sp1S\sb0)\to He\sb2(a\sp3\Sigma\sbsp{u}{+}) + He(1\sp1S\sb0)$ has been investigated over the temperature range 65K-700K. This reaction is thermally activated as a consequence of a long range repulsive barrier in the $\rm He(2\sp3S\sb1)-He(1\sp1S\sb0)$ interaction potential. The data reveal that there are two reaction channels with distinctly different activation energies. The temperature dependence of the measured rate coefficient k$\sb{\rm s}($T) is accurately described by $\rm k\sb{s}(T) = \lbrack 87.4\ T\ exp({-750}/T) + 4.1 T\ exp({-200}/T)\rbrack \times 10\sp{-37}\ cm\sp{-6}sec\sp{-1}$. The first activation energy, 750 $\pm$ 70K (63 $\pm$ 6meV), is equal to the known $\rm He(2\sp3S\sb1)$-$\rm He(1\sp1S\sb0)$ repulsive barrier height. The second activation energy is 17 $\pm$ 2 meV. The temperature dependences of the rate constants for collision-induced mixing among $\rm He(2\sp3P\sb{J,m\sb{J}})$ levels, and for conversion of He(2$\sp3$P) atoms to $\rm He\sb2(b\sp3\Pi\sb{g})$ molecules in the three body reaction $\rm He(2\sp3P) + 2He(1\sp1S)\to He\sb2(b\sp3\Pi\sb{g}) + He(1\sp1S)$ have been investigated over the range 1.4$\sim$300K. The measured thermally-averaged cross section for $\rm He(2\sp3P\sb{J,m\sb{J}})$ mixing in collisions with ground state helium atoms are described by the function $\rm\sigma\sb{pm}(T) = (4.4 + 20.6/T\sp{1/3})\times 10\sp{-15}cm\sp2,$ and can be understood in terms of Langevin theory. The measured rate coefficients for the three body reaction exhibit a strong inverse temperature dependence, $\rm k\sb{p}(T) = (0.04 + 2.18/T)\times 10\sp{-30}\ cm\sp6{\cdot}s\sp{-1},$ which suggests that, unlike conversion of $\rm He(2\sp3S\sb1)$ to $\rm He\sb2(a\sp3\Sigma\sbsp{u}{+}),$ there is no activation energy required for this reaction. A magneto-optical trap for helium 2$\sp3$S metastable atoms has been designed and constructed, utilizing superconducting magnet gradient coils and a Ti:Sapphire ring laser for pumping the helium 2$\sp3$S-2$\sp3$P transition. He(2$\sp3$S) atoms are produced by a weak discharge in helium gas at temperature 1.3K. The discharge products flow through an orifice into the trap cell, where the He(2$\sp3$S) atoms are trapped and ground state helium atoms are rapidly cryopumped by zeolite pellets that cover most of the cell bottom. Preliminary experimental results suggest that $\sim$10$\sp6$ atoms are trapped, with a trap lifetime of about 0.2 sec limited by He(2$\sp3$S) - He(2$\sp3$P) Penning reactions. Ultimately, it is estimated that a substantial number of atoms can be trapped and cooled for much longer times in a near-perfect vacuum. Measurements of decay times of the trapped atoms should yield rates for $\rm \sp{4,3}He(2\sp3S)$ - $\sp{4,3}$He(2$\sp3$S) and resonantly-enhanced He(2$\sp3$S) - He(2$\sp3$P) Penning reactions in the ultra-cold quantum regime, and perhaps the He(2$\sp3$S) natural lifetime.

The spectra of the high-lying vibrational levels of the $A\sp1\Sigma\sbsp{u}{+}$ and $1\sp3\Sigma\sbsp{g}{+}$ singly excited states of both $\sp6\rm Li\sb2$ and $\sp7\rm Li\sb2$ are obtained via photoassociation of colliding ultracold atoms confined in a magneto-optical trap. The least bound state of the $a\sp3\Sigma\sbsp{u}{+}$ ground state potential, obtained by two-photon photoassociative spectroscopy, is also presented for both $\sp7\rm Li\sb2$ and $\sp6\rm Li\sb2$. The vibrational levels have resolved hyperfine structure, whose relative energy splittings and transition strengths are accurately modeled. The photoassociative spectra are used to precisely determine both the singlet and triplet s-wave scattering lengths for both isotopes, which are important parameters in systems designed to observe quantum degenerate effects and Bose-Einstein condensation.

Potassium Rydberg atoms are used to investigate the lifetimes, on a picosecond timescale, of transient negative ions formed during dissociative electron attachment to CCl$\sb4$ and CF$\sb3$I. The transient negative ions are formed during Rydberg electron transfer reactions of the type$${\rm K}(n{\rm p})+AB\to{\rm K}\sp{+}+AB\sp{-*}\to{\rm K}\sp{+}+A\sp{-}+B$$For low values of principal quantum number $n\ (n\ \sbsp{\sim}{<}\ 20)$ the electrostatic interaction between the ${\rm K}\sp{+}-AB\sp{-*}$ and ${\rm K}\sp{+}+A\sp{-}$ ion pairs perturbs the ion trajectories. As a consequence, the final angular and velocity distributions of the ${\rm K}\sp{+}+A\sp{-}$ ions are dependent on the lifetime of the $AB\sp{-*}$ intermediate ion. The $AB\sp{-*}$ lifetime is determined by comparing product ion distributions, determined using position sensitive detection techniques, with the results of a Monte Carlo simulation that models the kinematics of the reaction. The data show that the lifetime of the excited $\rm CF\sb3\Gamma\sp{*}$ intermediate formed by Rydberg electron capture is short, $\sbsp{\sim}{<}2$ ps, and that the lifetime of the CCl$\sbsp{4}{-*}$ intermediate is $7.5\pm2.5$ ps.

High power femtosecond VUV/XUV pulses covering new spectral regions have been generated through nonlinear interaction of a femtosecond KrF laser with xenon and argon. Conversion efficiencies as high as 10$\sp{-3}$ have been observed. A six-wave mixing process and several four-wave mixing processes under intense light field have been experimentally studied. The temporal duration of the femtosecond VUV pulse has been experimentally measured for the first time based on the defocusing caused by a femtosecond laser produced plasma.

A system to control and stabilize the output of visible laser diodes was developed and used to measure the velocity distribution of lithium atoms in a laser cooling experiment. Circuitry was designed and built for controlling the diode temperature and current, and optical feedback from a grating was used to further tune the laser and to narrow its lineshape. In the experiment, atoms from a thermal lithium beam were slowed to near zero velocity using a multi-frequency relay chirp technique.

We have obtained high-lying vibrational spectra for the $\rm 1\sp3\Sigma\sbsp{g}{+}$ and $\rm 1\sp1\Sigma\sbsp{u}{+}$ excited states of both $\rm \sp7Li\sb2$ and $\rm \sp6Li\sb2$ via photoassociation of colliding ultracold lithium atoms confined in a magneto-optical trap. Photoassociative spectroscopy of ultracold atoms is a powerful tool for probing molecular states near the dissociation energy that are inaccessible by other methods. For an ultracold lithium vapor, the distribution of unbound ground state energies is so small that precise free-bound spectroscopy is possible. Molecular hyperfine structure is resolved. We calculate accurate dissociation energies for both the $\rm 1\sp3\Sigma\sbsp{g}{+}$ and $\rm 1\sp1\Sigma\sbsp{u}{+}$ excited states of Li$\sb2$. A model potential is constructed from a combination of experimental and theoretical information, and used to assign vibrational quantum numbers. We observe rotational structure and assign rotational quantum numbers.

Negative ion formation via electron transfer in thermal energy collisions between K(nd) Rydberg atoms and simple polyatomic molecules is studied at low-to-intermediate values of principal quantum number, n (n $<$ 40). At these values of n, the Rydberg electron can no longer be considered simply as a free electron of equivalent energy, because effects associated with the reduced size of the Rydberg atom and the proximity of the atom's charged core become important. We have observed and investigated several novel phenomena, associated with both dissociative and non-dissociative electron transfer. For example, marked n-dependences have been observed in the measured rate constants for free ion production by Rydberg electron attachment. These are due to the rapidly decreasing size of the Rydberg atom which results in atomic opacity, and to the increasing post-attachment electrostatic attraction between the product positive and negative ions. In the case of dissociative Rydberg electron transfer to simple halogenated hydrocarbons, XY, K(nd) + XY $\to$ K$\sp+$ + (XY$\sp-)$* $\to$ K$\sp+$ + X$\sp-$ + Y where XY is CF$\sb3$I, CF$\sb2$Br$\sb2$, CF$\sb3$Br, CH$\sb2$Br$\sb2$, CCI$\sb4$, CFCl$\sb3$, or CHCl$\sb3$, angular asymmetries were discovered in the velocity distributions of the product negative ions. Analysis of these data provides valuable insight into the translational energy release that accompanies dissociation of the transient intermediate molecular negative ions, and their lifetimes. Measurements of the spatial distributions of the product K$\sp+$ ions provide additional information on the dissociative attachment process. A new reaction channel was discovered in the case of non-dissociative Rydberg electron transfer to CS$\sb2$. This channel, which results in the formation of long-lived CS$\sb2\sp-$ ions that undergo rapid electric-field-induced electron detachment in fields of only a few kilovolts per centimeter, is discussed together with a possible theoretical model of this surprising phenomenon.

Non-linear deviations of Young's modulus (modulus defects) have been observed in many polycrystalline metals at elevated temperatures ($>$0.7 $T\sb m$). As the quasi-harmonic theory of interatomic crystalline forces predicts a linear temperature dependence of elastic constants up to $T\sb m$, the observed modulus defect is thought to result from grain boundary effects. Comparison of polycrystalline modulus defects with single-crystal elastic constants at elevated temperatures should indicate if grain boundaries are involved in the modulus defect phenomenon. Young's modulus measurements were made from 300 $\leq T \leq$ 1275 K on high purity polycrystalline gold wires using the thin-line ultrasonic pulse-echo method. The adiabatic elastic constants $c\sbsp{\rm ij}{\rm S}$ ($c\sb{11}$, $c\sb{12}$, $c\sb{44}$) were measured on two, high purity, gold single-crystals: (100) and (110) orientations. A modulus defect was observed for each of the $c\sbsp{\rm ij}{\rm S}$ at elevated temperatures. The $c\sbsp{\rm ij}{\rm S}$ were averaged according to Hill's arithmatic and geometric methods and the results compared to the measured polycrystalline Young's modulus. The modulus defect of the polycrystalline specimen was essentially identical to the averaged $c\sbsp{\rm ij}{\rm S}$ at elevated temperatures, indicating that the large modulus defect of gold is not due to grain boundary effects. A modified quasi-harmonic model is proposed to account for the non-linear temperature dependence of the gold $c\sbsp{\rm ij}{\rm S}$ at elevated temperatures. The temperature dependence of the strictly harmonic terms due to thermal expansion are approximated using a Born-Mayer interatomic potential function and the temperature dependence of the atomic vibrational frequencies are included in the model by use of Gruneisen's approximation. The model suggested provides satisfactory agreement with measured $c\sbsp{\rm ij}{\rm S}$ at elevated temperatures.

Measurements of the trap loss rate in ultra-cold magneto-optically trapped $\sp6$Li and $\sp7$Li are presented and compared. Clear evidence is presented for two different trap loss mechanisms involving inelastic collisions between one ground state atom and one excited state atom. The fine-structure changing (FSC) mechanism, in which the excited state atom changes fine-structure level during a collision, is seen to dominate the rate at low trap laser intensities. However, as the intensity is increased, the trap becomes sufficiently deep to be able to contain the products of FSC collisions and this mechanism no longer contributes to trap loss. It is a unique aspect of Li that the energy imparted to an atom in a FSC collision is in a range that may be recaptured with experimentally attainable trap parameters. When the products of all FSC collisions remain trapped, only the radiative escape (RE) trap loss mechanism, due to the emission of a less energetic photon than the initial excitation photon, contributes to the trap loss rate. At small detunings, the rate of the RE trap loss mechanism is seen to be over two orders-of-magnitude smaller than the FSC rate. The FSC trap loss rate for $\sp6$Li and $\sp7$Li were found to be largest at smaller detunings and of comparable magnitude in $\sp6$Li and $\sp7$Li. The RE trap loss rate in $\sp6$Li was seen to be roughly four times the RE trap loss rate in $\sp7$Li. To understand better the dynamics of trap loss and magneto-optical traps in general, a sophisticated model of magneto-optical trap kinetics has been developed. This model has demonstrated that most critical factor determining the maximum velocity an atom may have and yet remained trapped is the initial atom's frame detuning, $\Delta$ - k v, where $\Delta$ is the trap laser detuning, k is the propagation vector of the trap beam most nearly anti-parallel to v, the atom's velocity. All else being equal, minimizing the magnitude of this quantity maximizes the velocity that a trap will retain. More detailed results of this model are also presented.

We have successfully trapped $\sp7$Li in a vapor cell magneto-optical trap (MOT). The trap is studied in detail and the parameters are measured from the experiment. The trapping mechanism has been understood in terms of the radiation force and a simple one dimensional model is used in interpreting and predicting the experimental results. The cold-cold atom collision rate coefficient is measured in a beam MOT. The two collision mechanisms, fine structure changing and radiative escape, are discussed in a molecular picture. One of the channels, the fine structure changing channel, can be turned on and off by the experimental conditions, namely the laser intensity in this experiment. The rate coefficient for each mechanism has been determined from the experiment.