Gurbax Singh

Laser photofragmentation–fragment detection and pyrolysis–laser-induced fluorescence studies on energetic materials

Trace concentrations of energetic materials such as 2, 4, 6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), and hexahydro-1, 3, 5-trinitro-s-triazine (RDX) are detected by laser photofragmentation-fragment detection (PF-FD) spectrometry. In this technique, a single laser operating near 227 nm photofragments the parent molecule and facilitates the detection of the characteristic NO fragment by means of its A (2)Sigma(+)-X (2)Sigma (0, 0) transitions near 227 nm. Fragment detection is accomplished by resonance-enhanced multiphoton ionization with miniature electrodes and by laser-induced fluorescence (LIF) with a photodetector. Experiments are also conducted in the visible region by use of 453.85-nm radiation for photofragmentation and fragment detection. Sand samples contaminated with PETN and RDX are analyzed by a pyrolysis-LIF technique, which involves pyrolysis of the energetic material with subsequent detection of the pyrolysis products NO and NO(2) by LIF and PF-LIF, respectively, near 227 nm. The application of these techniques to the trace analysis of TNT, PETN, and RDX at ambient pressure in room air is demonstrated with limits of detection (signal-to-noise ratio, 3) in the low parts-in-10(9) to parts-in-10(6) range for a 20-s integration time and 10-120 microJ of laser energy at 226.8 nm and approximately 5 mJ at 453.85 nm. An increase in detection sensitivity is projected with an increase in laser energy and an improved system design. The analytical merits of these techniques are discussed and compared with those of other laser-based techniques.

GaAs-laser-induced population inversion in the ground-state hyperfine levels of Cs 133

This paper describes the achievement of population inversion among the hyperfine levels in the ground state of Cs133by optically pumping these atoms with radiation from a GaAs diode laser. The laser output was used to monitor the populations in the two ground-state hyperfine levels as well as to perform the hyperfine pumping. By varying the injection current, a GaAs laser operated CW at about 77°K was used to scan the 8521-A line of Cs133. The intensity of the resonance scattering from cesium vapor served as an indicator of the populations of the two levels involved. Experiments were performed both with neon-filled and with paraflint-coated cells containing the cesium vapor. It was discovered that the diode laser could easily be tuned by manually adjusting the injection current to match either of the hyperfine components of the D 2 optical transition. Possible future applications, including a restudy of the light shifts, the construction of a cesium maser, and the physics of optical pumping with coherent light are discussed.

Temperature-Induced Rebound in Power MOSFETs

Enhancement-mode n-channel power MOSFETs were investigated for rebound. They received 300 krad(Si) gamma dose under positive gate bias with source and drain grounded. The irradiated transistors were thermally annealed with all terminals shorted or under positive gate bias with drain and source shorted, at temperatures from 60°C to 150°C. Threshold voltage rebound was observed for some transistor types under certain experimental conditions.

Investigations into Trace Detection of Nitrocompounds by One- and Two-Color Laser Photofragmentation/Fragment Detection Spectrometry

Trace concentrations of nitrogen dioxide (NO2), nitromethane (CH3NO2), and 2,4,6-trinitrotoluene (TNT) are detected by both one- and two-color laser photofragmentation/fragment detection (PF/FD) spectrometry using one or two lasers. The PF/FD methods studied are (1) one-laser, one-color photofragmentation of the analyte molecule at 227 or 454 nm with subsequent detection of the characteristic nitric oxide (NO) photofragment by one- or two-photon laser-induced fluorescence using its A2Σ+ –X2π (0,0) transitions near 227 nm; (2) one-laser, two-color PF/FD, where a 355 nm laser beam is used for additional analyte photofragmentation and NO is detected by both one- and two-photon LIF as in the previous case; (3) two-laser, two-color PF/FD, where the pump and probe beams are time delayed; and (4) one-laser, one-color PF/FD at 355 nm, where the 355 nm beam photofragments the target molecule and the prompt emission from electronically excited NO (A2Σ+) is monitored in the range of 200–300 nm. PF/FD excitation and emission spectra are recorded and also simulated with the use of a computer program based on a Boltzmann distribution analysis with transition probabilities, rotational energies, and rovibrational temperatures as input parameters. The effects of laser wavelength, laser pump energy, time delay between pump and probe beams, and analyte concentration on PF/FD signal are investigated and reported. Limits of detection [signal-to-noise (S/N) = 3] for the nitrocompounds range from low ppbv to ppmv for 10 s integration time and laser energies of ∼ 5 mJ and 100 μJ for the pump and probe beams, respectively. These results are presented and compared to other PF/FD methods for nitrocompound monitoring.

Radiation-Induced Interface Traps in Power Mosfets

Methods for estimating radiation-induced interface trap density from the current-voltage (I-V) characteristics of MOSFETs are described and applied to commercially available power MOSFETs. The power MOSFETs show severe degradation on radiation exposure with the effects of positive oxide trapped charge dominating; however, interface trap buildup is significant. The results are compared to experimental measurements available on other technologies.

Laser photofragmentation/fragment detection studies on energetic materials

A sensitive technique based on laser photofragmentation/fragment detection spectrometry is reported for the trace detection of energetic materials such as TNT, PETN, and RDX. A single laser operating near 227 nm is used for both the photofragmentation of the parent molecule and detection of the characteristic NO fragment via its A-X transitions near 227 nm. The fragment detection methods employed are resonance-enhanced multiphoton ionization with miniature electrodes and laser-induced fluorescence with a photodetector. Experiments are also conducted in the visible region using 454-nm radiation for photofragmentation and fragment detection. The application of this technique in the trace analysis of the above- mentioned compounds at ambient pressure is demonstrated with limits of detection in the range of sub- to low parts-per- million for a 20-sec integration time and 20-120 (mu) J of laser energy at 227 nm and approximately 5 mJ at 454 nm. An increase in detecting sensitivity is projected with an increase in laser energy and an improved system design.

Comments on "Hyperfine optical pumping of cesium with a CW GaAs laser"

Reference to the authors work (see ibid., vol.QE-7, p.196 (1971)) in a paper by Picque (see ibid., vol.QE-10, p.892 (1974)) is clarified.