Laser initiated explosions and chemiluminescence

Explosions can be induced in mixtures of SF6with appropriate fuels, by a single pulse of a free running CO2laserapprox 1.5 mus [full width at half maximum (FWHM)]. A typical threshold for initiation of reaction was somewhat above 4 J for a 1:1 mixture of SF6and SiH4, at a total pressure of 10 torr, with the unfocused beam illuminating 8 cm²of cell area. High levels of luminosity were generated covering both the near ultraviolet, visible, and infrared. The spectral and temporal characteristics of the following fuels and fuel-oxidizer combinations (with SF6present) are summarized: the most prominent emitter from SiH4is S*2, B³Sigma-_{u}; Sn(CH3)4+ N2O [SnO*; SnF*] ; Pb(CH3)4+ N2O [PbO*; PbF*; PbS*] ; Bi(CH3)3+ N2O [BiF*]; B2H6with and without N2O [BO*; BO*2; S*2]. Several atomic lines were also recorded. Estimates of the energy density deposited by the laser pulse suggest that initiation of rapid reaction was primarily due to attack of superexcited SF6by highly reducing radical species.     

Stable discharges in an HF laser with large electrode seperation

The characteristics of a fast discharge HF chemical laser are described. A Blumlein pulse forming line is used with a resistive cathode (50 Ω . cm resistivity germanium). Uniform arc-free discharges have been obtained in gas mixtures of SF6/H2/Ar, SF6/C2H6/Ar, and SF6/C3H8/Ar with electrode separations up to 5.2 cm.     

A laser augmented reaction: SF6+ SiH4→S*2+ SiF4+ HF + H2· retention of isotopic selectivity during detonation

A single unfocused pulse of a free running CO2laser, area ∼ 8 cm², initiates an explosive reaction between SF6and SiH4. This occurs at a minimum energy of 4 J [full width at half maximum (FWHM)sim 1.5 /mus] of which about one half is absorbed in an 8 cm long cell; total pressure 12 torr; 0.65 <p(SiH4)/p(SF6) < 1.8. The spectral and temporal distributions of the emitted chemiluminescence depend sensitively on the fuel to oxidizer ratio, and on the pulse energy; we investigated the range 4 → 20 J. The principal emission is due to S2(B^{3}Sigma-_{u} rightarrow X^{3}Sigma-_{g}). Transitionsupsilon' (0-4) rightarrow upsilon" (2-15)were recorded. In the³Sigma-_{u}state, vibrational temperatures range from 3000-13000 K. The luminosity peaks sharply at (SiH4)/(SF6) = 1.0 ± 0.05. On each side of the maximum of the emission versus composition curve [at (SiH4)/(SF6) ≈ 0.95 and 1.22, for a 12 J pulse] the residual SF6(0.2-0.5 percent of initial amount) is enriched in³⁴SF6; the observed fractionation factors at these two compositions are 8 ± 2. The separation between the two sharply peaked optimum compositions appears to increase with increasing pulse energy. Preliminary results with other fuels suggest that the concurrent absorption of CO2laser radiation by the fuel, as well as a highly exothermic reaction, are pre-requisite for fine tuning of composition, injected power, and total pressure for optimum isotope fractionation.     

Subnanosecond-jitter laser-triggered switching at moderate repetition rates

A 50-kV laser-triggered spark gap operated at repetition rates to 50 pps is described. Jitter of delay time as low as ± 0.1 ns was demonstrated. Dependence of delay and jitter was investigated as a function of gas mixture, pressure, laser power, and electrode material. Subnanosecond jitter was obtained with as little as 0.17 mJ in a 6-ns giant pulse from a Nd⁺³in a YAG laser. The dielectric gases used included a high proportion of Ar mixed with N2or SF6.     

HF and DF laser action by a pulsed microwave discharge

A pulsed electrodeless microwave discharge withE-field enhancement by a tapered waveguide has been employed to produce HF and DF laser radiation from premixtutes of SF6+ H6and SF6+ D2. The 720 patm (690 patm) HF (DF) laser had an average multiline output power of 6 mW (1 mW) for an efficiency of 0.1 percent (0.02 percent) while operating at 1 kHz (450 Hz) pulse-repetition frequency (PRF). The HF (DF) laser was operated at reduced output power and efficiency up to 4.2 kHz (2.5 kHz) PRF. Reduced HF laser efficiency was obtained when C3H8was substituted for H2.     

Intense electron-beam initiation of a high-energy hydrogen fluoride (HF) laser

A superradiant hydrogen fluoride (HF) laser is described in which an intense electron beam, propagating in a SF6and C2H6mixture, is used to initiate chemical reactions that produce vibrationally excited HF molecules. The variation of laser energy as a function of SF6pressure for fixed composition and as a function of composition at a fixed SF6pressure has been investigated. These experimental parametric curves are discussed in terms of the relevant physics. The maximum laser energy of 228 J in a 55- ns full-width-at-half-maximum (FWHM) pulse was obtained with a mixture of400-torr SF6and 40-torr C2H6. Measured efficiency for converting deposited beam energy to laser energy propagating in the forward direction was as large as 8 percent.     

A photopreionized atmospheric pressure HF laser

An atmospheric pressure HF laser employing sliding-spark preionization and pulsed, transverse excitation has been successfully operated. Uniform, stable glow discharges measuring 3 × 3 cm in cross section have been produced in flowing mixtures of SF6C3H8, and He gases, giving laser output energies of 425 mJ on a total of 12 vibrational-rotational transitions.     

Infrared pulse transmission in SF6-He mixtures

The transmission ofQ-switched CO2laser pulses through SF6-He mixtures is studied experimentally and compared with predictions generated by computer calculations based on a four-state model for SF6. The results show that for saturating fields the factor determining the rate of absorption is the rotational energy transfer process that feeds the levels interacting with the laser. This conclusion has implications for models of passiveQswitching in molecular lasers and infrared-infrared double resonance.     

Efficient V-V energy transfer in a multiphoton excited mixture of SF6and CF4

Spectral and temporal measurements of infrared fluorescence (IRF) in SF6, CF4, and mixtures of SF6and CF4gases excited by a pulsed CO2laser are reported. Using the 944.2 cm^-1[P(20)] laser line for excitation and measuring the IRF spectra between 700 and 1500 cm^-1, a strong red shifted IRF peak of the ν3mode in pure SF6was found. No IRF was observed under these conditions in pure CF4. In a 13:10 mixture of SF6and CF4, two strong IRF peaks of the ν3modes, shifted to the red relative to the room-temperature fundamentals at 948 cm^-1and 1283.2 cm^-1, were observed. The peaks are almost equal in height and since both molecules have almost the same ν3band intensity, the present result suggests that efficient intermolecularV-Venergy transfer occurs from excited SF6to cold CF4molecules. The temporal behavior of the IRF signals under high excitation (langlenrangle approx 6) in pure SF6exhibits two relaxation times, one of bulk cooling withtau_{1} approx 1ms and the other of aV-Tnature withPtau_{2} approx 20 mus . torr. In the mixture, an additional relaxation of the intermolecularV-Venergy transfer process is observed withPtau_{3} approx 10 mus . torr. The red shift dependence of the IRF peaks on the degree of excitation was also measured and used to examine population distributions, specific IRF transitions, and anharmonicity parameters.     

Experimental investigation of the atomic fluorine laser

The atomic fluorine laser, as produced by a fast pulsed discharge in He-NF3mixtures, is investigated experimentally. Total collisional quenching rates of He 2³S on NF3, BF3, CBrF3, CCIF3, CHF3, CF4, C2F6, CO2, SF6, and N2are reported. Upper limits for collisional quenching rates of He 2¹S, 2³P, and 2¹P atoms on NF3are determined. We conclude that energy transfer from excited neutral He to NF3is not the primary pumping mechanism in the He-NF3atomic fluorine laser.     

Excited-state absorption of SF6at 936.8 cm-1

An effect that is opposite to saturable absorption in the infrared was observed when a CO2laser beam was passed through SF6. An increase of absorption occurred when the radiant flux of the laser was increased to 10 W/cm². The effect was observed for the P(28) line of the00deg1-10deg0band of the CO2laser, and was attributed to absorption from excited vibrational states of SF6.     

Nonlinear-optical device characteristics of gaseous saturable absorbers

Measurements have been made of the third-order nonlinear-optical susceptibilities of gaseous infrared saturable absorbers. These coefficients are relevant to mode locking of infrared lasers such as CO2. The measurements are in good agreement with a theory based on a four-level model of the absorber. Earlier mode-locking observations are interpreted in light of the new data. The value of the transition moment and some other parameters of SF6are deduced from the measurements. For SF6atp = 1torr, the third-order nonlinear coefficient is in the range1-10 times 10^{-23}C.m/V³, when interacting waves differ in frequency by less than the molecular collision frequency. We find that the average dipole transition moment for SF6near theP(16)orP(20)lines of CO2is approximately 0.1-0.2 Debye. The nonlinear coefficient of CH3F is typically three orders of magnitude smaller than that of SF6.     

Br, C, Cl, S, and Si laser action using a pulsed microwave discharge

Low power Br, C, Cl, S, and Si infrared laser action has been obtained using 2.45 GHz pulsed microwave discharges of molecular source gases and helium. The atomic laser molecular source gases were Br2, CO2, CO, CF4, Cl2, CH3Cl, CH2Cl2, CCl4, SF6, CS2, and SiH4.     

Pulse-probe measurements in the v3band of SF6at low temperature and low pressure

To improve understanding of multiple-photon absorption by polyatomic molecules, we have performed a series of experiments wherein a pulsed CO2laser irradiated 0.02-0.08 torr samples of SF6at 145 K. A CW probe laser monitored the time response of the induced absorption or transmission at many CO2laser lines that are in or near the ν3absorption band of SF6. The experiments covered a 40-fold fluence range and probe times out to 4 ms. We conclude that the absorbed laser radiation produces a nonthermal vibrational-energy distribution and that intermolecular vibrational-energy transfer is important at early times in redistributing the absorbed energy. We also discuss the influence of other processes on the induced spectrum.     

Collisionless self-focusing of CO210 µm P(20) laser light in SF6

We develop a simple intuitive picture of the vibration-rotation structure of the SF6molecule such that the molecular susceptibility responsible for self-focusing can be calculated. We treat the propagation dynamics by generalizing the standard steady-state Gaussian propagation equations to include the important effect of absorption in the wings of the spatial profile. By calibrating the model to absorption data at CO210 μmP(2)we find good agreement with beam waist data at the same wavelength. Absorption in the wings is dominant at low laser fluences, and the real part of the susceptibility is responsible for the defocusing-to-focusing turnover in the beam waist near 100 mJ/cm², consistent with the interpretation of Nowak and Ham [6].     

Saturation energy density and line profile of the atomic iodine laser transition at high pressure

The saturation energy density of an iodine laser amplifier is proportional to the homogeneous medium linewidth and inversely proportional to the Einstein coefficient. For good energy extraction high values of the saturation energy density are needed, thus also requiring large values of the medium linewidth, especially when the Einstein coefficient increases with pressure, as inferred from fluorescence measurements recently reported in the literature. In order to check this conclusion, we measured the saturation energy density in the pressure range between 0.5 and 4 bar for the buffer gases Ar, SF6, and Xe and also the medium linewidth at a pressure of 2 bar for Ar, SF6, and Xe. It is found that, at least in this pressure range, the Einstein coefficient is constant and does not change with pressure. This is in contrast to the results published in the paper mentioned earlier. From our experimental data an optimum pressure range for high-power iodine laser amplifiers of large extraction efficiency is also inferred.     

Investigations of the properties of SF6as an arc quenching medium

The results of experiments on stationary and nonstationary arcs in SF6are summarized. High temperature gas properties, like the electrical and thermal conductivity as well as the theoretically predicted plasma demixing effects, are determined by electrical and spectroscopic measurements. Investigations on interrupted dc arcs give insight into the energy transport mechanism of the arcs. The transient temperature behavior of gas blast interrupted arcs is measured. Finally, the application of the investigations to circuit breaker arcs is discussed. The following principal results have been found: From the lower time constant of blown N2arcs in comparison to SF6arcs above 10 000 K it follows that the good quenching properties of SF6must be due to processes taking place below 10 000 K. This agrees also with the time-constant measurements in the interrupted cascade arc: at lower temperature--below approximately 8000 K-- the conductance decay in N2is very much slower than in SF6. Further, these measurements revealed that the steep descent in SF6is not caused by electron attachment but is due to energy transport mechanisms. Experimental work about the plasma properties on steady-state arcs, including demixing effects, is in good agreement with theory. Finally the ac experiments show that the temperature profile of the blown ac arcs has a nearly rectangular shape. Variations in current affect mainly the diameter of the arc whereas the temperature variations are fairly small.     

C-MOS/SIS—Using selective SF6etching of {1102} sapphire

A process for selectively etching holes in {1102} sapphire using SF6in H2is described. SiO2, Si3N4, and combinations thereof are studied as possible etchant masks. Refilling the holes with epitaxial silicon produces an SIS (silicon-in-sapphire) wafer wherein the silicon islands are imbedded into the sapphire substrate. The electrical characteristics of C-MOS/SIS transistors are similar to those of conventionally processed SOS devices.     

Behavior of first- and second-positive emission in the N2/SF6laser

Experimental results for the behavior of the emission of the discharge-excited first- and second-positive bands of N2and flow-tube measurements for the deactivation of theA^{3}Sigmamin{u}max{+}andB^{3}Pi_{g}levels of N2by SF6are presented. The results of both these experiments are used to explain the operating mechanisms of the N2/SF6laser.     

Efficient operation of a low-impedance Blumlein discharge initiated HF/DF chemical laser

A study is described on the efficient operation of an HF/DF chemical laser initiated by a low-impedance Blumlein discharge. Some combinations of 0.33 and 0.66-Ω flat-plate Blumlein lines with simple discharge chambers of various active volumes were investigated. For an SF6/H2mixture, an HF laser gave a maximum efficiency of 6.3 percent, and 5 J/1 was extracted. Substitution of D2for H2gave a DF laser output energy as high as 80 percent of the HF chemical laser output.