Glen P. Perram

Collisional broadening and shift of the rubidium D1 and D2 lines (52S12 → 52P12, 52P32) by rare gases, H2, D2, N2, CH4 and CF4

Abstract The collision induced broadening and line shifts for the Rb D1 ( 5 2 P 1 2 -5 2 S 1 2 ) and D2 ( 5 2 P 3 2 -5 2 S 1 2 ) transitions at low buffer gas pressure (0–300 torr) and T = 394 K were obtained from high resolution laser absorption spectra. The shifts, δ, and broadening, γL for H2, D2, N2, CH4 and CF4 are in MHz per torr δ(5 2 P 1 2 ) = −2.17, −2.22, −7.41, −7.92, −5.41; δ(5 2 P 3 2 ) = −3.83, −4.09, −5.79, −7.0, −5.73; γ L (5 2 P 1 2 ) = 17.3, 14.1, 16.3, 29.1, 18.7; γ L (5 2 P 3 2 ) = 26.4, 20.6, 18.3, 26.2, 17.3 , respectively. Similar results are reported for rare gas collision partners. These values have been further interpreted using the impact approximation for collisional broadening to obtain the parameters for the Leonard Jones interaction potentials. The rates for pressure broadening are clearly correlated with polarizability of the collision partner.

Remote Identification and Quantification of Industrial Smokestack Effluents via Imaging Fourier-Transform Spectroscopy

Industrial smokestack plume emissions were remotely measured with a midwave infrared (1800-3000 cm(-1)) imaging Fourier-transform spectrometer operating at moderate spatial (128 × 64 with 19.4 × 19.4 cm(2) per pixel) and high spectral (0.25 cm(-1)) resolution over a 20 min period. Strong emissions from CO(2), H(2)O, SO(2), NO, HCl, and CO were observed. A single-layer plume radiative transfer model was used to estimate temperature T and effluent column densities q(i) for each pixels spectrum immediately above the smokestack exit. Across the stack, temperature was uniform with T = 396.3 ± 1.3 K (mean ± stdev), and each q(i) varied in accordance with the plume path length defined by its cylindrical geometry. Estimated CO(2) and SO(2) volume fractions of 8.6 ± 0.4% and 380 ± 23 ppm(v), respectively, compared favorably with in situ measurements of 9.40 ± 0.03% and 383 ± 2 ppm(v). Total in situ NO(x) concentration (NO + NO(2)) was reported at 120 ± 1 ppm(v). While NO(2) was not spectrally detected, NO was remotely observed with a concentration of 104 ± 7 ppm(v). Concentration estimates for the unmonitored species CO, HCl, and H(2)O were 14.4 ± 0.3 ppm(v), 88 ± 1 ppm(v), and 4.7 ± 0.1%, respectively.

Infrared signatures from bomb detonations

Abstract Remote observations of the temporal and spectral characteristics of the infrared emissions from bomb detonations have been correlated with explosion conditions. A Fourier transform interferometer was used to record spectra in the 1.6–20 μm range at spectral resolutions of 4–16 cm −1 and temporal resolutions of 0.047–0.123 s. Field observations of 56 detonation events included a set of aircraft delivered ordinance and a series of static ground detonations for a variety of bomb sizes, types and environmental conditions. The emission is well represented by a gray body with continuously decreasing temperature and characteristic decay times of 1–4 s, providing only limited variability with detonation conditions. However, the fireball size times the emissivity as a function of time can be determined from the spectra without imaging and provides a more sensitive signature. The degree of temporal overlap as a function of frequency for a pair of detonation events provides a very sensitive discriminator for explosion conditions. The temporal overlap decreases with increasing emission frequency for all the observed events, indicating more information content at higher frequencies.

Methodology for comparing worldwide performance of diverse weight-constrained high energy laser systems

The Air Force Institute of Technologys Center for Directed Energy has developed a software model, the High Energy Laser End-to-End Operational Simulation (HELEEOS), under the sponsorship of the High Energy Laser Joint Technology Office (JTO), to facilitate worldwide comparisons across a broad range of expected engagement scenarios of expected performance of a diverse range of weight-constrained high energy laser system types. HELEEOS has been designed to meet JTOs goals of supporting a broad range of analyses applicable to the operational requirements of all the military services, constraining weapon effectiveness through accurate engineering performance assessments allowing its use as an investment strategy tool, and the establishment of trust among military leaders. HELEEOS is anchored to respected wave optics codes and all significant degradation effects, including thermal blooming and optical turbulence, are represented in the model. The model features operationally oriented performance metrics, e.g. dwell time required to achieve a prescribed probability of kill and effective range. Key features of HELEEOS include estimation of the level of uncertainty in the calculated Pk and generation of interactive nomographs to allow the user to further explore a desired parameter space. Worldwide analyses are enabled at five wavelengths via recently available databases capturing climatological, seasonal, diurnal, and geographical spatial-temporal variability in atmospheric parameters including molecular and aerosol absorption and scattering profiles and optical turbulence strength. Examples are provided of the impact of uncertainty in weight-power relationships, coupled with operating condition variability, on results of performance comparisons between chemical and solid state lasers.

Pressure broadening of the D1 and D2 lines in diode pumped alkali lasers

The absolute absorption and stimulated emission cross-sections, including the effects of hyperfine splitting and pressure broadening at low to moderate pressures are computed and compared with experimental results. The comparison is excellent and requires no fit parameters. An analysis of the degree to which the lineshape can be approximated by a single Lorentzian profile is provided as a function of background pressure.

Phenomenological fireball model for remote identification of high-explosives

Many aspects of detonation phenomena have been well studied over the last century. However, the transient infrared and visible emissions from detonation fireballs have been poorly understood, and this has hampered attempts to remotely identify explosives via combustion signatures. Recently, time-resolved infrared spectra (1800-7000 cm-1, 4cm-1 resolution, 8 Hz) were collected from the detonation of uncased charges of TNT and several kinds of improvised explosive devices in four weight classes (10, 50, 100, and 1000 kg). A simple model for fireball emissions has been developed which accurately describes the observed spectra in terms of the fireball size, temperature, gaseous byproduct concentrations, and grey particulate absorption coefficient. The model affords high-fidelity dimensionality reduction and provides physical features which can be used to distinguish the uncased explosives.

In situ creation of nanoparticles from YBCO by pulsed laser deposition

Abstract Nanoparticles created by the laser ablation of YBCO are reported. The experimental procedure entailed pulsed laser deposition (PLD) of YBCO at a high background pressure of 5 Torr O2. The sizes of the nanoparticles range from ∼3 to 5 nm and are typical of the depositions made using laser energies of 50 mJ per pulse. Optical emission spectroscopy was used to characterize the PLD plume. Under nanoparticle deposition conditions, the visible plume emission is very weak. Even so, the distribution among electronically excited states cannot be described by a single temperature. The plume remains collisionally dynamic even at high pressures and low laser energies and retains considerable excitation when nanoparticles are formed.

Role of excited state photoionization in the 852.1 nm Cs laser pumped by Cs-Ar photoassociation

Photoionization of Cs (6p 2P3/2) atoms during the operation of a Cs D2 line (852.1 nm: 6p 2P3/2→6s 2S1/2) laser, pumped by free→free transitions of thermal Cs-Ar ground state pairs, has been investigated experimentally and computationally. Photoexcitation of Cs vapor/Ar mixtures through the blue satellite of the D2 transition (peaking at 836.7 nm) selectively populates the 2P3/2 upper laser level by the dissociation of the CsAr excited complex. Comparison of laser output energy data, for instantaneous pump powers up to 3 MW, with the predictions of a numerical model sets an upper bound of 8 × 10−26 cm4 W−1 on the Cs (6p 2P3/2) two photon ionization cross-section at 836.7 nm which corresponds to a single photon cross-section of 2.4 × 10−19 cm2 for a peak pump intensity of 3 MW cm−2.

Shock front dynamics in the pulsed laser deposition of YBa2Cu3O7−x

The pulsed laser deposition of YBa2Cu3O7−x targets by excimer laser at fluences of 4–10 J cm−2 in low pressure oxygen backgrounds yields emissive plumes with kinetic energies of 50–200 eV, driving the formation of a shock front with Mach numbers of M = 10–50. The propagation of the shock front is independent of atomic species and adequately characterized by the Sedov–Taylor shock model if the dimensionality of the plume is allowed to deviate from ideal spherical expansion. The ideal efficiency of energy conversion from laser pulse to shock expansion is nearly unity at 1 Torr, but decreases rapidly at lower pressures, where the plume expands beyond the laser footprint during ablation. The low oxygen background pressures, 100–1000 mTorr, typically employed for the production of superconducting films is sufficient for the generation of a strong shock front with shock thickness of 5 mm to less than 0.4 mm, but too low to develop three-dimensional flow. Indeed, dimensionality of the expansion ranges from n = 0.8 to 2.4 over the background oxygen pressure range of 25–1000 mTorr. Shock strength is proportional to the Mach number and inversely dependent on pressure, indicating a thickness limited to approximately the collision mean free path.

SPIN-ORBIT RELAXATION KINETICS OF BR(4 2P1/2)

Pulsed and steady‐state photolysis techniques have been employed to measure the rate coefficients for collisional deactivation of the spin–orbit excited state of atomic bromine, Br(4 2P1/2). Pulsed lifetime studies for quenching by Br2 and CO2 yielded absolute rate coefficients at room temperature of kBr2=1.2±0.1×10−12 and kCO2=1.5±0.3×10−11 cm3/molecule s. The rate coefficients for quenching by rare gases, N2, O2, NO, NO2, N2O, CO, CO2, COS, SO2, SF6, CF4, CH4, H2S, H2, D2, HBr, HCl, and HI, relative to that for Br2 were determined in a steady‐state photolysis experiment. Correlation of the deactivation probabilities with energy defect for the case of electronic‐to‐vibrational energy transfer is demonstrated.