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Picosecond Measurement of Interband Saturation, Intervalence Band Absorption, and Surface Recombination in Germanium

Description: The picosecond optical response of five thin germanium samples was measured following intense optical excitation using two variations of the excitation and probe technique. Seven-picosecond laser pulses of wavelength 1.054 um were used to measure the optical transmission of the samples for a variety of probe delays, excitation fluences, and sample temperatures. These parametric experiments were performed in an effort to determine if carrier cooling, carrier diffusion, or carrier recombination dominates the carrier dynamics immediately following excitation. The studies of a 5.7 um thick sample indicated that Auger recombination does not dominate the carrier dynamics, but that the carriers most likely cool immediately to within a few optical phonons of the lattice temperature. Lattice heating may also occur depending on excitation level. Neither cooling nor diffusion was ruled out as a major contributor to the transient optical response. A numerical analysis indicated that, although diffusion may be minimized in the thinner samples, the importance of surface recombination increases as the sample thickness decreases. The lattice temperature dependence of the optical transmission was found not to be in disagreement with the known temperature dependence of the low-density diffusion coefficient. Finally, new structure was observed in the data which is consistent with an increased intervalence band absorption at the highest excitation levels.
Date: August 1984
Creator: Perryman, Gary Paul

Photoconductivity Investigation of Two-Photon Magneto-Absorption, PACRH, and Deep Levels in n-InSb

Description: A high resolution photoconductivity investigation of two 13 -3 photon magneto-absorption (TPMA) in n-InSb (n - 9 x 10 cm ) has been performed. This is the first time that two-photon absorption in a semiconductor has been studied with cw lasers only. With a stable cw CC>2 laser and a highly sensitive sampling and magnetic field modulation technique, a minimum of 4 2 transitions in the TPMA photoconductivity spectra can be observed. Most of these transitions are a result of the usual spherical approximation TPMA selections rules (An =0, ±2; As = 0 for e ⊥ B and Δn = 0; Δs = 0 for e || B) . However, some transitions, in particular several near the TPMA band edge, are not explained by these rules. The TPMA spectra have been found to depend upon crystallographic orientation. This has not been previously observed. The temperature variation of the fundamental energy gap Eg between 2 and 100° K is also obtained from TPMA experiments.
Date: May 1982
Creator: Goodwin, Mike Watson

Low-Velocity K-Shell Ionization Cross Sections for Protons, Deuterons and Alpha Particles Bombarding Thin Metal Targets

Description: The purpose of this work was to examine the effect of the use the assumption κω2K/ΕCM «1 in calculating K-shell ionization cross sections in the plane wave Born approximation (PWBA) where κω2K is the observed binding energy of the K-shell and ECM is the energy of the incident particle in the center of mass system. Avoiding this assumption produces a threshold for ionization at Ecm = κω2K. Calculations employing the assumption, which leads to the use of approximate limits of integration, do not go to zero for even the .Lowest values of the incident energy.
Date: May 1981
Creator: Rice, Roger Karl

Infrared-Microwave Double Resonance Probing of the Population-Depopulation of Rotational States in the NO₂ and the SO₂ Molecules

Description: A 10.6 ym C02 laser operating a power range S P 200 watts was used to pump some select vibrational transitions in the NO2 molecule while monitoring the rotational transitions (91/9—'100/10), (232f 22 ~~"*242,23> ' (402,38 "393,37) in the (0, 0, 0) vibrational level and the (8q,8—*"^1,7) rotational transition in the (0, 1, 0) vibrational level. These rotational transitions were monitored by microwave probing to determine how the population of states in the rotational manifolds were being altered by the laser. Coincidences between some components of the V3-V2 band of N02 and the C02 infrared laser lines in the 10 um region appeared to be responsible for the strong interaction between the continuous laser beams and the molecular states.
Date: December 1982
Creator: Khoobehi, Bahram

Picosecond Laser-Induced Transient Gratings and Anisotropic State-Filling in Germanium

Description: We present a comparative theoretical study of the transient grating coherent effects in resonant picosecond excitation-probe experiments. Signals in both the probe and conjugate directions are discussed. The effects of recombination, non-radiative scattering and spatial and orientational diffusion are included. The analysis is applied to both a molecular and to a semiconductor model. Signal contributions from concentration and orientational gratings are distinguished and their temporal natures discussed. The theory is used to explain our recent observations in germanium. Included are discussions of picosecond transient grating self-diffraction measurements that can be understood in terms of an orientational grating produced by anisotropic (in k-space) state-filling. Though there have been predictions and indirect experimental evidence for isotropic state-filling in germanium, this is the first direct experimental indication of anisotropic state-filling in a semiconductor.
Date: December 1982
Creator: Boggess, Thomas F. (Thomas Frederick)

Linewidth Parameters, Dipole Moments, and Microwave Spectrum of Nitrogen-Substituted Methyl Cyanide

Description: The shape of collision-broadened microwave absorption lines is reviewed, along with a number of other broadening mechanisms. The Anderson and Murphy-Boggs linewidth theories are reviewed in detail. Several published modifications to these theories are reviewed. Computer programs which numerically evaluate linewidths and lineshifts are presented. Approximations are made to reduce the need for extensive use of the modified Bessel functions, thereby reducing computation time. Only dipole-dipole forces are considered.
Date: August 1981
Creator: Messer, James Keith

A Gauge-Invariant Energy Variational Principle Application to Anisotropic Excitons in High Magnetic Fields

Description: A new method is developed for treating atoms and molecules in a magnetic field in a gauge-invariant way using the Rayleigh-Ritz energy variational principle. The energy operator depends on the vector potential which must be chosen in some gauge. In order to adapt the trial wave function to the gauge of the vector potential, the trial wave function can be multiplied by a phase factor which depends on the spatial coordinates. When the energy expectation value is minimized with respect to the phase function, the equation for charge conservation for stationary states is obtained. This equation can be solved for the phase function, and the solution used in the energy expectation value to obtain a gauge-invariant energy. The method is applicable to all quantum mechanical systems for which the variational principle can be applied. It ensures satisfaction of the charge conservation condition, a gauge-invariant energy, and the best upper bound to the ground-state energy which can be obtained for the form of trial wave function chosen.
Date: December 1983
Creator: Kennedy, Paul K. (Paul Kevin)