CO2-laser

With M = He, experimeuts were carried out between 255 K aud 273 K with a few millibar NO2 at total pressures between 300 mbar aud 200 bar. Temperature jumps on the order of 1 K were effected by pulsed irradiation (< 1 pS) with a CO2 laser at 9.2- 9.6pm aud with SiF or perfluorocyclobutaue as primary IR absorbers (< 1 mbar). Under these conditions, the dissociation of N2O4 occurs within the irradiated volume on a time scale of a few hundred microseconds. NO2 aud N2O4 were monitored simultaneously by recording the time-dependent UV absorption signal at 420 run aud 253 run, respectively. The recombination rate constant can be obtained from the effective first-order relaxation time, A derivation analogous to (equation (B2.5.9). equation (B2.5.10). equation (B2.5.11) and equation (B2.5.12)) yield... 

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

The record m the number of absorbed photons (about 500 photons of a CO2 laser) was reached with the CgQ molecule [77]. This case proved an exception in that the primary reaction was ionization. The IR multiphoton excitation is the starting pomt for a new gas-phase photochemistry, IR laser chemistry, which encompasses numerous chemical processes. 

B2.5.351 after multiphoton excitation via the CF stretching vibration at 1070 cm. More than 17 photons are needed to break the C-I bond, a typical value in IR laser chemistry. Contributions from direct absorption (i) are insignificant, so that the process almost exclusively follows the quasi-resonant mechanism (iii), which can be treated by generalized first-order kinetics. As an example, figure B2.5.15 illustrates the fonnation of I atoms (upper trace) during excitation with the pulse sequence of a mode-coupled CO2 laser (lower trace). In addition to the mtensity, /, the fluence, F, of radiation is a very important parameter in IR laser chemistry (and more generally in nuiltiphoton excitation) ... 

Figure B2.5.14. The IR laser chemistry of CF I excited up to the dissociation energy with about 17 quanta of a CO2 laser, The dissociation is detected by uncertainty limited cw absorption (hv ), see figures...

Figure B2.5.15. Iodine atom fonnation in the IR laser ehemistry of CF I (exeitation at 1074.65 em probe on the F = 4 —> F = 3 hyperfme stnieture transition, see figure B2.5.12.) (a) The absorbanee as a fiinetion of time (effeetive absorption eross seetion frill eiirve, left ordinate) shows elear steps at eaeh maximum of the mode loeked CO2 laser pulse sequenee (intensity, broken eurve, right ordinate), (b) The Ifaetion Fp of dissoeiating moleeules as a frinetion of fluenee F.

Figure B2.5.17. (a) Time-dependent intensity / and redueed fluenee F/Fq for a single-mode CO2 laser pulse used in the IR laser photolysis of CF I. Fq is the total fluenee of the laser pulse, (b) VIS-REMPI iodine atom signals obtained with CO2 laser pulses of different fluenee (after [113]).

Apart from the obvious property of defining pulses within short time intervals, the pulsed laser radiation used in reaetion kineties studies ean have additional partieular properties (i) high mtensity, (ii) high monoehromatieity, and (iii) eoherenee. Depending on the type of laser, these properties may be more or less pronouneed. For instanee, the pulsed CO2 lasers used in IR laser ehemistry easily reaeh intensities between... 

Although this limit is not always reaehed. The same is true for the eoherenee of the radiation. Eaeh of these properties ean be exploited for partieular ehemieal applieations. The monoeliromatieity ean be used to initiate a ehemieal reaetion of partieular moleeules in a mixture. The laser isotope separation of and in nafriral abimdanee exploits the isotope shift of moleeular vibrational frequeneies. At 10-50 em, the eorresponding shift of IR absorption wavenumbers is large eompared to the speetral width of the CO2 laser... 

The CO2 laser is a near-infrared gas laser capable of very high power and with an efficiency of about 20 per cent. CO2 has three normal modes of vibration Vj, the symmetric stretch, V2, the bending vibration, and V3, the antisymmetric stretch, with symmetry species (t+, ti , and (7+, and fundamental vibration wavenumbers of 1354, 673, and 2396 cm, respectively. Figure 9.16 shows some of the vibrational levels, the numbering of which is explained in footnote 4 of Chapter 4 (page 93), which are involved in the laser action. This occurs principally in the 3q22 transition, at about 10.6 pm, but may also be induced in the 3oli transition, at about 9.6 pm. 

The energy input into a CO2 laser is in the form of an electrical discharge through the mixture of gases. The cavity may be sealed, in which case a little water vapour must be added in order to convert back to CO2 any CO which is formed. More commonly, longitudinal or, preferably, transverse gas flow through the cavity is used. The CO2 laser can operate in a CW or pulsed mode, with power up to 1 kW possible in the CW mode. 

One of the first applications of this technique was to the enrichment of and "B isotopes, present as 18.7 and 81.3 per cent, respectively, in natural abundance. Boron trichloride, BCI3, dissociates when irradiated with a pulsed CO2 laser in the 3g vibrational band at 958 cm (vj is an e vibration of the planar, D j, molecule). One of the products of dissociation was detected by reaction with O2 to form BO which then produced chemiluminescence (emission of radiation as a result of energy gained by chemical reaction) in the visible region due to A U — fluorescence. Irradiation in the 3g band of BCls or "BCI3 resulted in °BO or BO chemiluminescence. The fluorescence of °BO is easily resolved from that of "BO. 

Figure 9.32 illustrates the isotopic enrichment of SFe following irradiation with a pulsed CO2 laser in the 3g vibrational band, at 945 cm, of SFe, V3 being a strongly infrared active bending vibration. The natural abundances of the isotopes of sulphur are (95.0 per cent), (4.24 per cent), (0.74 per cent) and (0.017 per cent). The figure shows that depletion of SFg has been achieved to such an extent that equal quantities of SFg and SFa remain. 

Show that the 10.6 pm and 9.6 pm radiation from a CO2 laser is due to transitions allowed by electric dipole selection mles. 

The OF radical has also been detected by CO2 laser magnetic resonance (30). The O—F bond length is 0.135789 nm. 

Nonmilitary infrared apphcations for germanium include CO2 lasers (qv), intmsion alarms, and pohce and border patrol surveillance devices. Germanium is used as a thin-film coating for infrared materials to decrease reflection losses or to provide heavy filtering action below 2 p.m. 

Synthetic fused siUcas with low OH levels have also been made experimentally using a CO2 laser instead of the plasma torch (62,63). The glass rate of this process, however, is at least 10 times slower than the plasma approach. 

Optical Properties. The high refractive index (2.42 at 589.3 nm) and dispersion (0.044) are the basis for the brilliance and fire of a properly cut gemstone. The optical transmission out to 10.6 p.m for Type Ila diamonds makes possible windows for CO2 lasers and for devices such as were in the... 

The carbon black studied here was prepared by a CO2 laser-driven pyrolysis of a mixture of benzene, ethylene, and iron carbonyl[34]. As synthesized, TEM... 

It can also be deposited by the hydrogen reduction of the nickel chelate, Ni(C5HFg02)2 at 250°C.P 1 In addition to thermal processing reviewed above, nickel is deposited by laser C VD from the carbonyl with a krypton or a pulsed CO2 laser.P lP l... 

Another example of the CVD of TiN is deposition by laser activation using a CO2 laser with N2H2 and TiC reactant gases. Deposition temperatures are not mentioned but presumed to be low. The composition of the film is substoichiometric (N/Ti[Pg.285]

The main problem in developing fibers for IR transmission is that silica glass is not transparent in that area of the spectrum. Suitable materials include the selenides and other chalcogenide glasses (particularly for the CO2 laser light source) and the fluorohafnate glasses. In addition, materials, such as zirconium fluo-... 

Submicron pSiC powder by reacting silane and acetylene in a 10-50 W continuous-wave CO2 laser beam.P" l... 

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