The effect of the dipole force and its fluctuation on the motion of Li atoms in an intense, one-dimensional, near-resonant standing light wave has been investigated, both theoretically and experimentally. The duration of the interaction of the atoms with the standing wave was varied from several tens of spontaneous emission lifetimes to several hundred. For a standing wave frequency blue-detuned from resonance, diffusive heating can dominate the time-averaged dissipative dipole force so that there is no steady-state momentum distribution. However, for sufficiently large blue detunings the rate of diffusion is so slow that the resulting distribution approaches a quasi-steady state. For red detunings the diffusion is balanced with the force and a true steady-state is achieved. A Monte Carlo method based on the density matrix equations in the dressed-state representation is applied to simulate the atomic motion. The dynamics of atom channeling is studied. One of the important applications of this investigation is adiabatic laser cooling of atoms. Lithium atoms channeled in the nodes of an intense standing wave radiation field are cooled to near the recoil limit, a single photon's momentum $\hbar{k}$, by adiabatically reducing the radiation intensity. The final momentum distribution has a narrow component with a root mean squared momentum of 2 $\hbar{k}$ in one-dimension. The data are compared with the results obtained by the Monte Carlo method. This process may be useful for cooling and increasing the phase-space density of atoms confined in a magnetic trap.

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.

A method is described which utilizes the difference in vapor pressure between C$\sb{60}$ and heavier fullerenes to produce C$\sb{60}$ powder with a purity of 99.97%. Using the material from this process allows for the growth of high purity polycrystalline C$\sb{60}$ thin films. These films are characterized using Raman spectroscopy and temperature dependent photoluminescence. The temperature dependence of the photoluminescence spectra indicates that both intermolecular and intramolecular processes are involved in the radiative recombination of the excited states. A model is proposed to describe the temperature dependence of the photoluminescence. A possible interpretation of this model is that there is a barrier to the formation of self trapped excitons.

Alamethicin is a transmembrane ion channel at low concentration, and a lytic agent of cell membrane at high concentration. It is a small size polypeptide (20 amino residues), and contains a large section of amphiphilic $\alpha$-helix, which is an often-encountered secondary structure motif in membrane active peptides and proteins. Its membrane-active functions, typical secondary structure, and relatively small size made this peptide be an ideal model for studying the interaction of proteins with membranes. This thesis provides two novel methods to obtain structural information of such a peptide-lipid system. By the method of oriented circular dichroism (OCD), we are able to determine the orientation of the alamethicin with respect to the membrane bilayer. Depending on the alamethicin concentration and the water content in the membrane, alamethicin either perpendicularly inserts into the bilayer or binds parallel to the membrane surface. By the method of lamellar x-ray diffraction, we found the membrane bilayer thickness reduced by increasing concentration of alamethicin and decreasing relative humidity. From these two mutually complementary studies we constructed a consistent picture for the interaction between alamethicin and the membrane. When alamethicin concentration is low, the peptide molecules adsorb near the area of lipid head group, which effectively expands the average cross sectional area of the lipid molecules and makes the hydrocarbon chains more disordered, so that the lipid bilayer becomes thinner than pure lipid membrane. When alamethicin concentration reaches the critical point, where membrane structure becomes favorable for the insertion of alamethicin, alamethicin undergoes a transition from the surface state to the insertion state. The insertion of alamethicin would introduce much water into membrane, so the transition would happen only when relative humidity is high. Our alamethicin-lipid interaction model explains the spontaneous insertion of alamethicin at high aqueous concentration causing the lysis of membrane. It also suggests a possible gating mechanism for alamethicin ion channels.

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.

The rare gas alkali ionic molecules are established as a new class of ionic excimers which emit in the vacuum ultraviolet (VUV) regime of the electromagnetic spectrum. Temporal and spectral characteristics of these species with (XeRb)$\sp{+}$ at 164 nm and (XeCs)$\sp{+}$ at 160 nm have been investigated by soft x-ray excitation in a laser-produced plasma and by high energy pulsed electron beam pumping in this work. Soft x-ray pumping of XeF(B $\to$ X) and (XeRb)$\sp{+}$ yielded the first observation of excimer molecules formed by reactive kinetics in a laser-produced plasma. The spectrum of (XeRb)$\sp{+}$ was observed. The spectral structure could be assigned to 3 dipole allowed transitions originating from the 0$\sp{+}$(II), 1(II), 1(I) upper states ending in the 0$\sp{+}$(I) ground state. A kinetic study of electron beam pumped mixtures of Ar/Xe/Rb and Ar/Xe/Cs was performed. The observed temporal decays of (XeRb)$\sp{+}$ and (XeCs)$\sp{+}$ were analyzed. The results suggest that electron deactivation is surprisingly not a dominant quenching process for rare gas alkali ions. The radiative lifetimes are 150 $\pm$ 50 ns for (XeCs)$\sp+$ and 250 $\pm$ 50 ns for (XeRb)$\sp+$. In addition several quenching rate constants were extracted from the experimental results. These rate constants and lifetimes were incorporated into a kinetic model for these species. This kinetic model reproduced the experimental observations well providing the electron quenching rate coefficient is kept to a maximum of 5 $\times$ 10$\sp{-9}$ cm$\sp3$s$\sp{-1}$. Estimates for the upper state densities were computed using this model to be $\sim$4 $\times$ 10$\sp{15}$ cm$\sp{-3}$ for (XeCs)$\sp+$ and 2 $\times$ 10$\sp{15}$ cm$\sp{-3}$ for (XeRb)$\sp+$. Including absorption due to the photoionization of the alkali atoms, the net gain coefficients are computed to be on the order of 10$\sp{-2}$ cm$\sp{-1}$. Therefore the rare gas alkali ionic excimers appear to be a promising class of candidates as storage media for VUV lasers.

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.

The distribution of light in a tissue is determined by it's optical properties. Several techniques are available for determining a tissues optical properties, and models are available for predicting the distribution of light within a tissue of known optical properties. The accuracies of these models were compared. The optical properties of neonatal skin were determined in the visible region from 450-750 nm. The reduced scattering coefficient, $\mu\sb{\rm s}$(1-g), increases directly with gestational maturity of the infant. The increase in the reduced scattering coefficient with gestational maturity is due to the accompanying increase in size and density of the collagen fibers. In neonatal skin, the optical density perceived by reflection, the depth probed by photons escaping from the surface, and their pathlength in the tissue are dependent on wavelength and on collection geometry. The penetration of visible light into neonatal skin is strongly dependent on wavelength and on gestational age.

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.