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.     

The influence of the optical stark shift on the two-photon excitation of the molecular hydrogen E, F1Σ+gstate

The first demonstration of the use of the optical Stark effect to secure resonance for a two-quantum process in the ultraviolet is reported. Following excitation of molecular hydrogen by 193 nm radiation, intense stimulated emission on both the Lyman and Werner bands is observed. The Werner (2-5)Q(1)transition at 117.5 nm exhibits a conversion efficiency of ∼0.2 percent. It is found that electron collisions with excited molecules provide the most probable mechanism for excitation of theC^{1}Pi_{u}levels with a rate constant estimated to bek_{e} sim 7 times 10^{-6}cm³/s.     

Absorption in the near-ultraviolet wing of the Kr2F*(410 nm) band

An intracavity laser technique has been used to study the absorption of electron-beam pumped Ne/Kr/F2gas mixtures (196 and 300 K) in the "blue wing" of the Kr2F emission continuum. The experiments were conducted at 358 nm using theupsilon' = 0 rightarrow upsilon" = 1transition of the N2(C rightarrow B) laser. Comparing the results with the predictions of a computer model, the species primarily responsible for absorption have been identified as Ne+2, Kr+2, and Kr2F*. The photoabsorption cross sections for Ne+2and Kr2F (Kr+2F-) at 358 nm have been estimated to be8.1 cdot 10^{-19}and5.4 cdot 10^{-18}cm², respectively. The Kr2F* absorption cross section is roughly 20 percent of that reported for Kr+2at the same wavelength. The fluorescence efficiency of Kr2F* ine-beam excited 94.93 percent Ne/5 percent Kr/0.07 percent F2(P_{total} = 4000torr) gas mixtures has been found to be a factor of 2.8 higher than that of the N2(C rightarrow B) band in Ar/5 percent N2mixtures. Also, the rate constant for quenching of Kr2F* by F2was measured to be(4.1 pm 0.5) cdot 10^{-10}cm³. s^-1at 300 K and(3.0 pm 0.5) cdot 10^{-10}cm². s^-1at 196 K.     

Pulsed discharge initiated chemical lasers -- Part II: HF laser emission from NF3and N2F4systems

A transverse, multiple-arc pulsed discharge has produced laser emission in NF3and N2F4mixed with H2, CH4, C2H6, HCl, HBr, and natural gas.P_{10}, P_{21}, P_{32}, and P43HF transitions were observed. The peak powers measured ranged from ∼8.5 to 25 kW with typical pulsewidths ofsim0.3 mus.     

Near single-line operation of a free-burning CS2/O2/N2O flame laser with a nondispersive optical cavity

The CS2/O2/N2O flame laser has been operated for the first time under conditions in which the spectral output is nearly single line. This transition is theP_{10-9}(17) of CO at 5.4265 μm, the same transition which was observed to oscillate in single-line fashion by Hirose et al. in an electrically initiated CO chemical laser. It is suggested that the unique behavior of this line may be due to its close proximity to aPbranch transition in an adjacent band, namely theP_{9-8}(23) line, such that the gain profiles of the two lines overlap. Calculations suggest that at the conditions of these experiments, the separation of the line centers for this pair is about 0.3 Å or less. TheP_{10-9}(17) transition was also found to be totally absent under certain conditions of high multiline power, particulary at low O2and N2O flows. This may be due to absorption by a high-bandRbranch transition at 5.4266 μm, namely theR_{15-16}(32) line.     

Pulsed N2O molecular laser studies

The afterglow pulse-gain technique was used to measure the relaxation of the 00°1 upper laser level in N2O from 300 to 700°K. The rate constantk_{N_{2}O}(torr^-1s^-1) varies with temperature aslog10k_{N_{2}O} = 4.53 - 10.9T^{-1/3}. Measurements of the relaxation time at 300°K agree with the fluorescence technique results of Yardley [1]. Gain studies were also made with gas mixtures in a nonflowing system. Single-pulsed N2O + CO + He mixtures at 300°K showed considerable peak gain. However, subsequent pulses of these mixtures show that the gain is reduced because of the chemical reaction forming CO2. Measurements of this transformation from one molecular laser mixture to another are discussed.     

On the inversion mechanism of the molecular hydrogen vacuum ultraviolet laser

The resonances in the 11-14-eV region produced in thee-H2interaction are suggested as mechanisms to invert the population between the upper laser statesC^{1}pi_{u}andB^{1}Sigma+_{u}and the ground stateX^{1}Sigma+_{g}of the hydrogen molecule.     

Electronic- and phonon-terminated laser emission from Ho3+in BaY2F8

Infrared spontaneous and stimulated emission from Ho³⁺in BaY2F8is reported. In addition to the familiar⁵I_{7} rightarrow⁵I8transition at 2 μ,⁵F_{5} rightarrow⁵I5emission at 2.4μ and⁵I_{6} rightarrow⁵I7emission at 2.9μ are discussed. There are several unusual features of the 2-μ laser emission. At room temperature, phonon-terminated laser emission is observed at 2.171 μ. At 77 K a complex CW laser output is observed in a wavelength interval lying on the shoulder of a fluorescence line. The complex output is attributed to oscillation in transverse modes of the resonator. Oscillation is not observed in the strongest emission line, despite a large terminal state splitting of 310 cm^-1. These results are explained on the basis of a theory developed earlier for transition metal ion lasers. The validity of the model is supported by demonstrating the tunability through loss modulation predicted by theory. The observation of these effects is made possible by the very low internal scattering loss in the crystals. The⁵F_{5} rightarrow⁵I5laser lines near 2.4 μ represent relatively low gain transitions with pulse durations limited by accumulation in a longer lived terminal state. The dynamics of laser emission indicate the possible absence of thermal equilibrium in the excited state. For the 2.9-μ transition the bottleneck posed by a longer lived terminal state may be eliminated by the addition of Eu³⁺or Pr³⁺, but laser emission could not be obtained.     

Kinetic model of photolytic O2(1Δ) generation

Kinetic analysis of photolysis of O3:O2:He mixtures by UV light has been studied. Dependence of O2(¹Δ) yield on partial pressures, flashlamp intensity, and duration was investigated. Results indicate sufficient O2(¹Δ) yield for operation of a high-power atomic iodine laser on the I*(²P_{1/2})underrightarrow{1.31 mu}m I(²p_{3/2}) transition.     

The photoionization cross sections of the Rg*2[3Σ+u] excimer states for Ne*2, Ar*2, and Kr*2

Based on the recently determined Rydberg series of the³Sigma+_{u}excimer states of Ne*2, Ar*2and Kr*2, the photoionization cross sections of these molecules are calculated using a single-channel quantum defect method. These cross sections are found to differ considerably from those of the asymptotic metastable atomic Rg*(ns^{3}P_{2}) states, but are in good agreement with recently reported experiments at isolated wavelengths. The implications of these results for VUV and XUV lasers are discussed.     

Broad-band gain in optically pumped S2

TheB^{3} Sigma-_{u} rightarrow X^{3}Sigma-_{g}system of S2has many of the characteristics required of a tunable laser: a broad fluorescence spectrum and a Franck-Condon shift between theBandXstates. In this paper, theupsilon' = 4level of theB^{3} Sigma-_{u} rightarrow X^{3}Sigma-_{g}system of S2is pumped by the absorption of the 308 nm XeCl excimer radiation. Rare gas-S2collisions redistribute the population among theupsilon'levels, and the subsequent emission contains intense bands throughout much of the neat UV and visible region of the spectrum. Broad-band gain is reported on two blue-green bands (2,17) and (3,18) populated by rare gas relaxation. Absorption is reported on the near UV bandsupsilon' = 0 rightarrow upsilon" rightarrow 6,7,8where the lower levels are filled by the transient population cascading down theXstate manifold. A fluorescence study is presented which examines the potential advantages and limitations of this system.     

Near-infrared2Δ-2Π nitric oxide laser with predissociated lower state

The spectrum of the light emitted at 10 200 Å by an NO laser is photographed with high resolution. The rotational analysis fully confirms the assignment made earlier to the transitionF^{2}Delta-C^{2}Pi(1, 1) between two electronically excited levels of the NO molecule, both of which are configurationally mixed with valence levels.     

Amplification cross section of the 1.052-µ transition of Nd3+in POCl3- SnCl4-(H2O) system by three different methods

We report the measurement of the amplification cross section σ21of the Nd³⁺ion in the POCl3-SnCl4-(H2O) system by three different methods. The first is based on the simultaneous measurement of the variation of fluorescence andQ-spoiled laser emission. The second is purely spectroscopic. The third is based on the measurement of k12at several temperatures. The two last methods give similar results. The most probable value issigma_{21} = 8.5 times 10^{-20}cm². An explanation for the lowest valuesim 6 times 10^{-20}cm²found with the first method is given.     

Studies of the inversion and lower level relaxation in pulsed TE CO2discharges

It has generally been supposed that the rise and fall times of the gain in pulsed CO2laser discharges reflect the effective lifetimes of the lower and upper laser levels. Consequently, the ratio of the upper to lower level populationsN_{u}/N_{l}should change during the evolution of the gain, and should be a relatively large number at the gain peak. The problems associated with the determination of this ratio using conventional direct gain measurements are discussed, and a new in-cavity technique is described that has sufficient precision to determine the ratioN_{u}/N_{l}as a function of time. The results obtained unambiguously demonstrate that the lower laser level empties rapidly during the rise time of the gain. This conclusion is further reinforced by some measurements concerning the time variation of the gain in the hot band (01^{1}1 rightarrow 11^{1}0).     

A TE duolaser providing simultaneous lasing on CO2and N2O transitions

Simultaneous laser action on CO2and N2O transitions has been achieved in a helical resistor-pin-type transversely excited (TE) laser. The laser emits typically a power of 10-100 kW in a 700-ns-long pulse which consists basically of two overlapping pulses, the peaks of which do not coincide, and which can be associated with the CO2and N2O emission, respectively. The conditions of optimal operation are described and the effects of some parameter changes are indicated.     

Theoretical analysis of combustion-driven 16µm CO2gas dynamic lasers

A numerical analysis is presented with a combustion-driven 16 μm CO2gas dynamic laser operating on liquid fuel and liquid oxidizer constituted of benzene (C6H6) and nitrous oxide (N2O), respectively. Adopting a sharp-edged nozzle with an area ratio of 100, optimization of equivalence ratio φ, reservoir pressure P0, and nozzle throat heighth*was performed by considering conditions of no steam condensation and exhausting the combustion gas to the atmosphere. The analysis showed that the maximum value ofG_{16} = 0.48m^-1for small-signal gain andE_{16}^{max} = 9mJ/(l . atm . pulse) for available specific energy would be attainable with theP(15)line of the (02⁰0)-(01¹0) transition.     

Low-frequency noise in Cr—SiO2—N-Si tunnel diodes

Low-frequency noise of Cr-SiO2-n-Si tunnel diodes with about 30-Å-thick oxides is investigated as function of bias, frequency, and temperature. Measurements of1/fnoise are explained by a theory employing the two step tunneling model of Sah. Electrons from the Si conduction band are trapped by states at the Si-SiO2interface and then tunnel into bound states of the oxide located close to the interface. The oxide states of density N00can be represented by a frequency dependent parallel admittance exhibiting frequency-dependent thermal noise that modulates the dc currentItunneling through the oxide barrier. This generates flicker noise at the device terminals proportional toI^{2}N_{00}and inversely proportional to frequencyfand tunneling areaA. The valueA = 5. 10-3A0, determined by fitting theoretical and experimental curves at low frequency, is only a small fraction of the gate area A0, since tunneling preferentially occurs through the thinnest parts of the oxide. The currentIalso exhibits full shot noise at high frequency and low current. Qualitative agreement between theoretical and measured noise is found over 9 decades. Measurements at low temperature show additional noise of generation-recombination centers at larger frequencies and currents.     

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.     

Pressure dependency of the NF3-H2transverse-discharge pulse-initiated HF chemical laser

The pressure dependency of the performance of the NF3-H2chemical-laser system has been evaluated. The laser energies at various chemical compositions and initiation discharge energies are presented as functions of Pressure. Overall efficiency, laser pulsewidth, and effects of additive gases are also presented. For all compositions the laser energy maximized at a pressureP_{max}which was observed to be dependent on the pulsewidth of the initiation discharge. At pressures belowP_{max}the laser energy was proportional to the partial pressure of NF3or H2. No change in slope of the laser energy versus pressure curve was observed in going from low-pressure nonexploding regimes to high-pressure exploding regimes, implying that the reactions causing the explosion did not contribute to the lasing. Lasing usually occurred in two peaks, the first containingHF(V=2) rightarrow HF(V=1)lines and the second containingHF(V=3) rightarrow HF(V=2)andHF(V=1) rightarrow HF(V=0)lines. These data indicate that lasing is due to the reaction sequence: 1)NF_{3} + e^{-} = cdot NF_{2} + Fcdot + e^{-}; 2) Fcdot + H_{2} = HF+(V) + Hcdot; and 3)HF+(V) + hnu = HF+(V-1) + 2hnu.     

Photoexcitation and photodissociation lasers--Part I: Nitric oxide laser emissions resulting from C(2π) → A(2Σ+) and D(2Σ+) → A(2Σ+) transitions

-1-μm laser emission was detected when NO or NO2was flash-photolyzed, with or without dilution, in the vacuum UV above 165 nm. The emission was identified as theC(²π)rightarrow A(²Sigma+)