Intermittent scanning for continuous-wave quantum cascade lasers is proposed along with a custom-built laser driver optimized for such operation. This approach lowers the overall heat dissipation of the laser by dropping its drive current to zero between individual scans and holding a longer pause between scans. This allows packaging cw-QCLs in TO–3 housings with built-in collimating optics, thus reducing cost and footprint of the device. The fully integrated, largely analog, yet flexible laser driver eliminates the need for any external electronics for current modulation, lowers the demands on power supply performance, and allows shaping of the tuning current in a wide range. Optimized ramp shape selection leads to large and nearly linear frequency tuning (>1.5 cm−1). Experimental characterization of the proposed scheme with a QCL emitting at 7.7 μm gave a frequency stability of 3.2×10−5 cm−1 for the laser emission, while a temperature dependence of 2.3×10−4 cm−1/K was observed when the driver electronics was exposed to sudden temperature changes. We show that these characteristics make the driver suitable for high precision trace gas measurements by analyzing methane absorption lines in the respective spectral region.

This article summarizes our recent experimental investigations of high-temperature kinetics of Si-and Cl-containing precursor molecules relevant to chemical vapor synthesis and ceramic processing. Reaction systems using SiCl 4 and SiHCl 3 highly diluted in argon, which were studied in a shock tube using the combination of thermal pyrolysis and laser flash photolysis methods, are described. In situ concentrations of the atomic species Si, Cl, and H were measured simultaneously using the atomic resonance absorption spectroscopy. The measured properties were sensitive to a limited number of elementary reactions, which could be analyzed in terms of rate coefficients.

The kinetics of the thermal decomposition of tin tetrachloride has been studied experimentally and theoretically. Ab initio MO calculations showed that SnCl 4 finally decomposed into Sn( 3 P) and four chlorine atoms through four subsequent Sn-Cl bond dissociation channels. Two sets of kinetic experiments were performed using a shock tube equipped with atomic resonance absorption spectroscopy (ARAS). Chlorine atoms were first measured over the temperature range of 1250-1700 K and the total density range of 1.7 × 10 18 to 8.9 × 10 18 molecules cm -3 . The rate coefficient for the initial reaction step, SnCl 4 (+M) f SnCl 3 ( 2 A 1 ) + Cl (+M) (eq 1a), was found to be in the falloff region fairly close to the low-pressure limit under the present conditions. The second-order rate coefficient based on the Cl-atom measurements was determined to be k 1a 2nd ) 10 -5.37(0.62 exp [-(285 ( 18) kJ mol -1 /RT] cm 3 molecule -1 s -1 (error limits at the 2 standard deviation level). The second group of experiments was carried out by detecting tin atoms over the temperature range of 2250-2950 K and at a total density of 3.2 × 10 18 molecules cm -3 . The second-order rate coefficients for the subsequent reaction steps: SnCl 2 ( 1 A 1 ) (+M) f SnCl( 2 Π) + Cl (+M) (eq 3a) and SnCl( 2 Π) (+M) f Sn( 3 P) + Cl (+M) (eq 4a) were obtained to be k 3a 2nd ) 10 -8. 36(0.86 exp[-(310 ( 42) kJ mol -1 /RT] cm 3 molecule -1 s -1 and k 4a 2nd ) 10 -9.50(0.78 exp [-(265 ( 40) kJ mol -1 /RT] cm 3 molecule -1 s -1 , respectively. The Rice-Ramsperger-Kassel-Marcus (RRKM) calculations including variational transition state theory were also applied for reactions 1a and 3a. Structural parameters and vibrational frequencies of the reactants and transition states required for the RRKM calculations were obtained from the ab initio MO calculations. Energy barriers of the reactions, E 0 's, which are the most sensitive parameters in the calculations, were adjusted until the RRKM rate coefficients matched the observed ones. These fittings yielded E 0,1a ) 326 kJ mol -1 for reaction 1a and E 0,3a ) 368 kJ mol -1 for reaction 3a, in good agreement with the Sn-Cl bond dissociation energies of SnCl 4 and SnCl 2 , demonstrating that the experimental data for k 1a and k 3a were theoretically reasonable and acceptable.

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Laser flash photolysis of SiCl4 at 193 nm was studied behind reflected shock waves at temperatures 1000 K ≤ T ≤ 2050 K and pressures between 1.4 and 1.8 bar. The atomic resonance absorption spectroscopy (ARAS) was applied for time-resolved measurements of Si- and Cl-atom concentrations in gas mixtures containing 10−100 ppm SiCl4 highly diluted in argon. The atoms formed during photolysis of SiCl4 were analyzed in terms of yields, defined as the fractions of the maximum atom concentration to the initial SiCl4 concentration behind the shock wave. At temperatures 1000 ≤ T ≤ 1510 K, Cl atoms were detected immediately after the laser flash. The Cl yield was found to be temperature dependent in the range between 0.1 and 2% of the initial SiCl4 concentration. Si atoms first occurred at temperatures T > 1750 K, when SiCl4 already has started to decompose. In contrast to Cl, the Si yield obtained correlates with both the temperature and the delay time between the shock-induced gas heating and the laser pulse. This effect was attributed to the photolysis of SiCl2, which is formed during the thermal decomposition of SiCl4. A reaction model including the thermal dissociation of SiCl4 and the photodissociation of SiCl2 is suggested and discussed, in which also reactions of Cl and Si atoms with SiCl4 are considered to describe the measured concentration profiles.