TEA Laser Research Articles

Frequency‐dependent branching ratio in the infrared multiphoton photochemistry of diethyl carbonates

AbstractUnimolecular decomposition of 0·1 Torr of diethyl carbonate, diethyl‐d5 carbonate and diethyl‐d10 carbonate induced by irradiation with a CO2 TEA laser was investigated. In all cases only ethylene, ethanol and CO2 were obtained. In the case of diethyl‐d5 carbonate, both ethylene‐d4 and ethylene‐d0 were observed together with the corresponding isotopic variants of ethanol. The chemical branching ratio was determined as a function of irradiation frequency and laser fluence by analysis of the ethylene‐d0/ethylene‐d4 ratio. RRKM theory applied to the branching ratio data requires very different average energies for two sets of fluence and frequency conditions that give identical total yields.

13C-selective IRMPD of CBr2F2/Cl2 and CCl2F2/Br2 systems

The CO2 TEA laser irradiation of CBr2F2 in the presence of Cl2 yielded 13C-enriched CBrClF2 and 13C-enriched CCl2F2 under selected experimental conditions. As the photolysis proceeded, the 13C concentration of CBrClF2 decreased gradually and that of CCl2F2 increased up to 90% or higher. These results can be explained by the mechanism involving the secondary 13C-selective IRMPD of the primary product CBrClF2. On the other hand, the carbon-containing product for a CCl2F2/Br2 system was only CBrClF2; the further IRMPD of which probably regenerated CBrClF2 in the presence of Br2. The decomposition probabilities of 12C- and 13C-containing molecules in both systems were measured as functions of laser line, laser fluence, and reactant pressures.

Dynamics of the SiF 4-sensitized decomposition of PH 3 by a pulsed CO 2 tea laser

The dynamics of the SiF 4-sensitized decomposition of PH 3 in an IR laser photon field were examined within the framework of a model which takes into account three effects: energy absorption, energetics of the process and shock wave expansion. The absorption of energy causes heating of the gas mixture initially present in the absorption volume and subsequent unimolecular decomposition of PH 3 molecules. Since the decomposition process is endothermic it brings about a gradual decrease in the temperature and thus quenches the process. The primary absorption creates a shock wave causing expansion of hot molecules into the cool surrounding volume. The latter process brings about cooling of the hot gas and also quenches the decomposition process. Taking into account the above-mentioned effects, the temperature profiles ( T vs. time and geometry of the reaction zone) were evaluated on the basis of statistical thermodynamics, assuming that, at any stage, the system attains thermal equilibrium. Yields of decomposition were then derived using k uni values calculated by the Rice-Ramsperger-Kassel-Marcus method. This approach was used to examine the effects of irradiation time (number of pulses), fluence, pressure, initial composition and presence of foreign gases (helium, nitrogen) on the yield of PH 3 decomposition. The derived dependences were compared with the experimental decomposition yields reported previously.

Infrared laser photochemistry of silane-chloromethane mixtures

By use of a pulsed CO{sub 2} TEA laser at 944.19 cm{sup {minus}1} and fluences in the range of 0.49-0.71 J/cm{sup 2}, the infrared photochemistry of SiH{sub 4}-CH{sub 3}Cl mixtures has been studied in a pressure range of 50-100 Torr and over a temperature range of 295-428 K. The gaseous products observed are H{sub 2}, CH{sub 4}, Si{sub 2}H{sub 6}, and SiH{sub 3}Cl, with trace amounts of Si{sub 3}H{sub 8} and perhaps CH{sub 3}SiH{sub 2}Cl. As is usual in silane decomposition, a brown solid product containing silicon, hydrogen, and, under some conditions, chlorine was also produced. The photochemical conversion is best described by initial decomposition of SiH{sub 4} to SiH{sub 2} and H{sub 2} followed by competition of SiH{sub 4} and CH{sub 3}Cl for SiH{sub 2} molecules. The production of CH{sub 4} is believed to occur via the decomposition of highly energized CH{sub 3}SiH{sub 2}Cl* (formed by SiH{sub 2} insertion into the C-Cl bond of CH{sub 3}Cl), yielding CH{sub 4} and SiHCl as products. SiH{sub 3}Cl is then formed by the secondary reaction of SiHCl with SiH{sub 4}.

ChemInform Abstract: The SiF4-Sensitized Decomposition of GeH4 by a Pulsed CO2 TEA Laser

(P40 line; 1027.4 cm-1) is studied in the fluence range 0.2-0.9 J/cm2 and over a total pressure range of 1-70 torr.

Infrared laser induced decomposition of hexafluorobenzene and some monosubstituted derivatives. Intermediacy of the pentafluorophenyl radical

The infrared multiphoton laser induced reactions of hexafluorobenzene and related pentafluoro analogues (pentafluorobenzene, pentafluorochlorobenzene, pentafluorobromobenzene, and pentafluoroiodobenzene) have been investigated using a CO 2 TEA laser. The study was carried out in order to define the decomposition products and to attempt to clarify their mode of formation. Thus, the products (relative yield, %) of the irradiation of C 6F 6 (1027.3 cm −1; 0.73.J/cm 2; 10 pulses; 25% decomposition) were C 2F 4(64), C 6F 5CF 3(28), C 2F 6(7), CF 4(1) and that for C 6F 5H (949.4 cm −1; 0.80 J/cm 2; 10 pulses; 25% decomposition) was C 2F 4 and C 6F 5CF 3. Increasing the number of pulses in the reaction with C 6F 6 decreased the amount of C 2F 4 and increased the amount of C 6F 5CF 3 and C 2F 6 indicating secondary and tertiary reactions. Addition of halogen (X 2, X = Cl, Br) to these reactions caused different products to be formed. Thus, the irradiation of a C 6F 6/Cl 2 mixture (7.4/7 Torr; 1027.3 cm −1; 0.7 J/cm 2; 35 pulses; 35% reaction) afforded C 6F 5Cl(46); CF 3Cl(24) and CF 2Cl 2(30). Irradiation of C 6F 5H/X 2 mixtures afforded mainly C 6F 5X + HX. For example, C 6F 5H/Br 2 (10/40 Torr; 949.4 cm −1; 0.93 J/cm 2; 10 pulses; 10% reaction) gave C 6F 5Br and HBr exclusively. Irradiation of C 6F 5-X (X = Cl, Br, I) (977.2 cm −1; ca. 0.74 J/cm 2; 200 pulses, 39–74% reaction) gave C 6F 6 and a minor amount of decafluorobiphenyl [(C 6F 5) 2], a radical combination product of the pentafluorophenyl radical (C 5F 5·). Increasing the fluence in these reactions gave similar produts in most cases but in some instances increased the amount of C 2F 4 formed. The reactions and product distribution of the hydrogen substituted derivative (C 6F 5H) was examined in the presence of Br 2 as a function of laser fluence and halogen concentration. It was found that the threshold for C 6F 5H decomposition was higher for the reaction involving Br 2 (as compared with the reaction involving Cl 2 or neat C 6F 5H). The presence of Br 2 also decreased the amount of C 6F 5H that was decomposed, indicating a quenching process. The decomposition path with the lowest activation energy for these molecules is thought to be C 6F 5X→C 6F 5· + X· and was accessible using a laser pulse with a fluence as low as 0.7 J/cm 2. Using a higher laser fluence ( ca. 1.2 J/cm 2) diand triatomic radicals were defined by spectroscopic identification of the ·CF and :CF 2 species. These reactions are discussed in light of the formation of the C 6F 5· radical during a primary, larser induced, process. Subsequent decomposition to smaller fragments, combination with other radicals or scavenging by added reagents also takes place depending on the reaction conditions.

A maintenance-free TEA nitrogen laser for analytical spectroscopy.

A reliable, microcomputer-controlled nitrogen laser was developed, as a pumping source of a tunable dye laser for industrial analytical spectrometry. Long-term maintenance-free operation was attained by the use of internal gas circulation, automatic ga exchange, and output stabilization. TEA laser action with a gas mixture of He/N2 made the gas exchange easy. The output power was 400 kW, and the maximum repetition rate was 60 Hz.

Tetrafluorosilane-sensitized decomposition of germane by a pulsed carbon dioxide TEA laser

The decomposition of GeH{sub 4}, sensitized by multiphoton absorption of unfocused infrared laser radiation by SiF{sub 4}, has been studied in the fluence range 0.2-0.9 J/cm{sup 2} and over a total pressure range of 1-70 Torr. Strong absorption of energy from a pulsed CO{sub 2} laser photon field (P40 line; 1027.4 cm{sup {minus}1}) and subsequent energy-transfer processes cause an increase in the temperature of the system. As a result of these processes, decomposition of GeH{sub 4} occurs leading to the formation of stoichiometric amounts of metallic germanium and molecular hydrogen. The effects of initial composition, pressure, fluence, and presence of foreign gases (He, H{sub 2}) on the yield of GeH{sub 4} decomposition were examined. A reaction model is proposed which assumes that molecules initially present in the absorption volume are heated by the laser pulse and subsequently decompose unimolecularly. Since the decomposition process is exothermic, it prompts a further increase in temperature and thus enhances the decomposition process. Both absorption and decomposition create a shock wave causing expansion of hot gas into cool surrounding volume. The latter process brings about cooling of the hot gas and quenches the decomposition process. Taking into account the above-mentioned effects the temperature profiles (Tmore » versus time and geometry of the reaction zone) were evaluated on the basis of statistical thermodynamics, assuming that, at any stage, the system attains thermal equilibrium. Yields of decomposition were then derived with k{sub uni} values calculated by the RRKM method. The proposed approach accounts satisfactorily for the experimentally observed dependencies, and we believe it can be generally applied to the description of photosensitized processes in the infrared region.« less

Laser-induced electron source in a vacuum diode

Experiments were conducted in which a high-power CO2 TEA laser interacted with metallic cathode in a high-vacuum (10−8 Torr) diode. For power densities lower than 5×107 W/cm2, no current was detected. For power densities in the range of 5×107–5×108 W/cm2, the Cu cathode emitted a maximum current of 40 mA. At a higher power density level, a circuit-limited current of 8 A was detected. The jump of a few orders of magnitude in the current is attributed to breakdown of the diode gap. The experimental results are similar to those of a triggered vacuum gap, and a thorough comparison is presented in this paper. The influence of the pressure in the vacuum chamber on the current magnitude shows the active role that adsorbed gas molecules have in the initial breakdown. When the cathode material was changed from metal to metal oxide, much lower laser power densities were required to reach the breakdown current region.

Ultrasonic generation with a pulsed TEA CO2 laser

The authors have demonstrated that air breakdown produced just above a metal surface by a pulsed CO2 TEA laser, is an efficient source of ultrasound within the metal. The normal force source produces both longitudinal and shear waves in the bulk and Rayleigh surface waves. No damage is produced on the metal surface. Electromagnetic acoustic transducers provide complementary noncontacting detectors.

Improved excitation by double discharge and semiconductive preionisers-electrodes in tea UV and IR lasers

Intense laser operation is reported by a novel design of TEA laser heads employing double discharge and semiconductive preionisers-electrodes. Their performance, beam profile and electrical characteristics are presented and are compared to the characteristics of semiconductor or spark array preionised laser heads. These novel laser heads of minimum complexity are capable of producing N 2 laser pulses up to 12mJ of energy and efficiencies up to 0.11%, or CA 2 laser pulses up to 800 mJ of energy and efficiencies up to 5.6%, at pulse repetition rates up to 100 Hz. To the best of our knowledge, these figures compare favorably with any published figures, obtained from lasers of similar size.

13C-separation by a continuous discharge CO 2 laser q-switched at 10 kHz

Compared to atmospheric pressure pulsed CO 2 (TEA) lasers, continuous-discharge CO 2 lasers can deliver photons at much lower costs. By a Q-switched version of such a laser we demonstrated by multiphoton dissociation of CHClF 2 the production of 1 g of C 2F 4 enriched to 50% 13C. This is a larger quantity of high enrichment than has been produced so far using TEA lasers. The process is already automated to a large extent. Scaling up seems feasible. These results have been achieved, although the process turned out to be much more nonlinear for our pulses (length 200 ns) than for TEA laser pulses. This difference is attributed to intensity effects, caused by direct multiphoton transitions in the excitation ladder. To avoid that selectivity breaks down, the gas has to be exchanged more than once per pulse. The required speed is much less, if ~250 mbar of Ar are added to the working gas.

First-stage enrichment in CO2-laser-induced13C separation by a two-stage IRMPD process: IRMPD of CHClF2/Br2 mixtures

We have been studying the practical CO2-laser-induced13C separation by a two-stage IRMPD process. The IRMPD of natural CHClF2 in the presence of Br2 mainly produced CBr2F2, which was found to be highly enriched with13C. The yield and13C-atom fraction of CBr2F2 were examined as functions of pulse number, laser line, laser fluence, total pressure, and Br2 pressure using a CO2 TEA laser with an output less than 1 J pulse−1 in order to optimize experimental conditions for13C separation. For example, we obtained CBr2F2 at a13C concentration of 55% in the irradiation of the mixture of 100-Torr CHClF2 and 10-Torr Br2 with the laser radiation at a wavenumber of 1045.02 cm−1 and at a fluence of 3.4 J cm−2. The mechanism for the IRMPD is discussed on the basis of observed results. Using 8-J pulses, we were able to obtain 1.9×10−4 g of13C-enriched CBr2F2 (13C-atom fraction, 47%) per pulse under selected conditions. It is possible to produce 90% or higher13C by the second-stage IRMPD of the CBr2F2 in the presence of oxygen.

Dynamics of Vibrational Relaxation of Multiphoton Excited SF6 Molecules inGaseous Mixtures Containing Ozone and Inert Gases (He, Ar, Kr)

SF6 at pressure of 0.6 Torr in gas mixtures with ozone (0 ÷ 5 Torr) and inert gases: He, Ar, Kr (0 ÷ 5 Torr) was vibrationally excited with and IR pulsed radiation of CO2 TEA laser at 10P20 laser line (F= 0.6 J/cm2). The vibrational energy transfer from multiphoton excited (MPE) molecules SF6 to the components of gaseous mixture was probed by UV absorption of ozone at λ = 253.6 nm band. The relaxation process of MPE molecules SF6 was proposed to occur through parallel V‐V′ and V‐T collisional channels. Based on the kinetic model, collisional rate constants were determined from the time evolution of UV absorption data after laser shot. The obtained values of relaxation rate constants are: urn:x-wiley:02786273:media:lc039014:lc039014-math-0001 The observed decay of IR fluorescence from MPE SF6 molecules at λ = 6.3 μm wave number in SF6‐Kr gas mixtures had an exponential time profile with the constant characteristic relaxation time (V‐T). The rate of IR fluorescence cluenching allowed us to find an effective V‐T relaxation rate constants for collisional partners: urn:x-wiley:02786273:media:lc039014:lc039014-math-0002 . urn:x-wiley:02786273:media:lc039014:lc039014-math-0003 .

Isotopically selective CVD of silicon by IRMPD of Si2F6

The IRMPD of Si2F6 by a CO2 TEA laser was applied to isotopically selective CVD of silicon. A white film, probably consisting of polymers of SiF2, was deposited on a metal foil during the irradiation of natural Si2F6 with the laser radiation at 951.19 cm−1 and about 1.5 J cm−2. Upon heating, the film became dark brown, evolving SiF4. The30Si content was found to be as high as about 20%.

Isotope separation of32SF6 and12CF3I by a Q-switched CO2 laser

Pulses with energy of 0.5 to 15 mJ and a repetition rate of 20 kHz from a Q-switched CO2 laser with continuous discharge were employed to dissociate32SF6 and12CF3I at 0.25 mbar. Gas exchange by diffusion is rapid enough to maintain isotopic selectivity. Dissociation yields agree with those measured at slightly raised temperature by a TEA laser. Because photons are cheaper from Q-switched lasers than from TEA lasers, economic consequences can result for isotope separation.

Ultraviolet photoionization in CO2 TEA lasers

The factors affecting spark ultraviolet (UV) emission and transmission, photoionization, and photoelectron loss mechanisms in CO2 TEA laser gas mixtures have been investigated and compared with the results of other workers. We found two different sources of UV radiation: N2 and an unidentified emitter, postulated to be associated with ionized electrode material; C (from CO and CO2) appeared to have little effect. UV emission was directly proportional to the amount of stored electrical energy in the spark-discharge circuit and to the cube of the peak current produced by the discharge of this energy. The uniformity of preionization produced by two kinds of spark-free space and notched surface guided was examined and the free-space spark was found to be superior. Photoionization was found to be due to the presence of low ionization potential (IP) alkenes in unseeded and fresh gas mixtures and also to NO and NO2 in sealed devices—these were formed as a result of spark plasma reactions. The nature of the photoionization process in NO and C3H6 (an alkene) was investigated; both were ionized in a single-step process. The absorption coefficient for ionizing radiation in CO2 was measured in a gas mixture containing 9% N2 and a N2 free mixture and values of 1.3 and 1.9 cm−1 atm−1 were obtained. It is thought that this may be due to changes in the spark UV emission spectrum when N2 is present. The loss of photoelectrons by third body attachment to NO, NO2, and O2 was monitored and only the attachment to O2 was found to be significant. However, the oxides of N can play a dominant role in the subsequent negative ion reactions on a millisecond timescale.

The application to sealed CO2TEA lasers of platinum catalysts bonded to aluminium-containing ferritic steel

A durable platinum catalyst, bonded to an aluminium-containing ferritic steel, Fecralloy, has been applied in a long-lived sealed CO2 TEA laser. Here it recombines gaseous O2 and CO, excess of which limits laser life. An experimental determination has been made of the rates at which Fecralloy-Pt catalysts recombine O2 and CO at low concentrations in the basic laser gas mixtures of CO2, N2 and He at a total pressure of one atmosphere. The O2 and CO removal rates are shown to increase with power into the catalyst but to be relatively independent of both reaction time and partial pressures of O2, CO and CO2. Fecralloy-Pt is active at much lower temperatures than a hot platinum wire. Hence, because of its much reduced power requirements and greater strength, it has considerable advantages in compact, portable, sealed CO2 TEA lasers.

TEA laser excited by a self-sustained discharge and emitting due to infrared transitions in Xe I, Kr I, Ar I, and Ne I

Fairly intense output pulses due to infrared transitions in Xe I, Kr I, Ar I, and Ne I rare gases were generated in a simple TEA laser excited by a self-sustained discharge. Characteristics of the energy, spectral, and time parameters of this laser were studied. A specific output energy of 70 mJ liter?1 atm?1 was obtained for Xe I. It was found that the addition of Ar to a binary Ne?He mixture resulted in lasing at the 1.15 ?m wavelength in a TEA laser.

Narrow bandwidth emission from a mirrorless, far infrared, /sup 13/CH/sub 3/F laser

Frequency tuning and linewidth measurements are reported for a pulsed, mirrorless, kilowatt-power-level, far-infrared (FIR) /sup 13/CH/sub 3/F laser operating at 245 GHz. The pump laser is an etalon tunable, single-mode CO/sub 2/ TEA laser. The FIR frequency spectrum was measured with 2.5-MHz resolution on individual 100-ns laser output pulses using harmonic mixing techniques. The linewidth of the amplified spontaneous emission was found to be surprisingly narrow, about 15 MHz. Frequency tuning of the FIR laser, as the pump laser frequency is tuned, is nonlinear, possibly due to inhomogeneous broadening of the gain by the K-level substructure of the rotational states. These results indicate that heterodyne receivers capable of single-shot frequency measurements can be important tools for investigating the properties of Raman FIR lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>