CW CO2 laser

Preparing isotopically enriched carbonyls and related species is not always a trivial problem. We have recently developed (20) a method which looks particularly promising in some cases. CW CO2 laser irradiation of a gas phase mixture containing SF5 as an energy transfer agent can promote thermal chemistry without complications due to wall reactions, e.g. 

Spjut and his associates heated levitated microparticles and measured their temperatures radiometrically. They levitated a microparticle in a bihyperbol-oidal quadrupole, and they irradiated the particle from opposite sides by splitting a cw CO2 laser beam and directing the beams by means of mirrors to illuminate the particle symmetrically (Spjut et al., 1987). The particle temperature was measured by multiple-color pyrometry (Spjut, 1987 Spjut and Bolsaitis, 1987) using narrow-band infrared detectors. The temperature... 

Figure 24. Variation of ionic intensities as a function of time after isolation and thermalization in the FTICR cell of the externally generated cluster ion, (H20)4H at a pressure of CH4 of 2.0 x 10 torr in the FTICR cell and irradiation by (a) 300-K blackbody emission, and (b) 1.8 W cm" CW CO2 laser.

Figure25. Logarithmic plot of vs. CW CO2 laser intensity for the unimolecu-...

The CO2 lasers were also investigated in connection with chaotic behavior, and here we mention the most important papers in the field. The chaotic behavior associated with a transverse mode structure in a cw CO2 laser was observed in 1985 [40]. In the CO2 laser with elastooptically modulated cavity length, a period doubling route to chaos was also found [41]. 

Figure 2. Configuration used for double laser experiment. A pulsed CO. laser (laser 1) was used to desorb and ionize the sample ana a cw CO2 laser (laser 2) was used to dissociate the trapped ions.

Figure 3. Top. Ion/molecule reaction product formed by reaction of cis-dichloro-trans-dihydroxo-bis-2-propanamine platinum (IV) with its 50 eV electron impact fragments. Bottom. Dissociation pattern produced upon irradiation with cw CO2 laser, same experimental conditions and delay times as in the top spectrum. (Reproduced from ref. 18. Copyright 1987 American Chemical Society.)...

Dissociation obtained upon irradiation with cw CO2 laser, same... 

Figure 7. Top. (M-H)- ion of sucrose formed by desorption with pulsed CO2 laser beam. Bottom. Dissociation upon irradiation with the cw CO2 laser. Experimental conditions are same as in the top spectrum.

Figure 9. Top. Negative fragment ion formed from the substituted sulfonamide by laser desorption using pulsed CC>2 laser. Bottom. Dissociation upon irradiation with a cw CO2 laser, all other conditions identical to those in the top spectrum.

SFg-sensitized decomposition of 2-nitropropane using 603 CW CO2 laser. Main process identified as C-N bond rupture to give ultimately propene and nitrous acid Collisionless IRMPE/D of DNj shown to depend 604... 

Halogen exchange between C F, and BCI3 induced by 623 CW CO2 laser excitation. Major products are C FjCl and BCljF... 

The photodissociation rate of the cyanobenzene cation at 568 nm is found to be significantly increased in the presence of CW CO2 laser radiation.Without the laser present, the dissociation is via a sequential two-step mechanism (19), and the... 

CW CO2 laser irradiation. The rate constant for the forward reaction is increased by a factor of >1000 to 2.6 x 10 cm molecule" s" under conditions in which each (CH30H)2H species absorbs an average of 4 photons from the laser. The CW laser-induced MPD of (CH3)2C1 and its deuteriated derivatives and of Cp3l have been reported, and saturation effects in the pulsed MPD of CHjOHF" have been investigated. 

Marking on bare metals is achieved by laser engraving with Nd YAG laser. The pulse or CW CO2 lasers described above cannot mark on bare metal because the beam is reflected off the surface without any absorption. Therefore, metals can only be marked if they are anodized, painted, or coated with an absorbing material. 

The reactions between NO and vibrationally excited O3 have also been studied by Braun and his colleagues, but under somewhat different conditions from those in Gordon and Lin s experiments. In their experiments, the ozone is excited by the square wave modulated output from a cw CO2 laser. Although they also observed enhanced reaction, their conclusions appear to differ in detail from those of Gordon and Lin. They believe that under their conditions, rapid V-V equilibra tion among the O3 modes precedes reaction. Their results can then either be interpreted on the basis that all these modes are equally active, in which case at 300 K the ratio of the enhanced rate constant for (79a) to the thermal rate constant is 4.2, or by assuming that only the v, and V3 stretching modes effectively promote reaction when the ratio of rate constants becomes 16.4. Kurylo era/. prefer the former of these two explanations. 

Novel industrial applications may evolve from continuous wave (CW) CO 2 laser-driven homogeneous pyrolysis of hydrocarbons (e.g. C4Hlo n-heptane % which seems to be advantageous over conventional pyrolysis methods For example, the CW CO2 laser SF -sensitized decomposition of cyclohexane in the gas phase yields mainly ethene and 1,3-butadiene (equation 7). The product distribution is almost invariable with the extent of conversion and different from conventional pyrolysis No acetylene is formed... 

D. L. Spears and C. Freed, "HgCdTe Varactor Photodiode Detection of cw CO2 Laser Beats beyond 60 GHz," Appl. Phys. Lett. 23, 445 (1973). 

Laser pyrolysis processing is carried out within a stainless steel chamber. A port on the top of the chamber provides access for a convergent beam of an industrial cw CO2 laser. Prior to laser irradiation, the processing chamber is evacuated to approx. 10 torr, flushed with nitrogen, and backfilled with nitrogen to approx. 760 torr. The chamber is then continuously fed with ammonia and hdium with a pumping rate of 150 seem and 100 seem for ammoiua and helium respectively. 

The now usual technique of preparing macroscopic quantities of fullerenes is the arc discharge in an inert atmosphere between carbon electrodes, followed by the collection of the soot produced and the removal of its soluble parts. Other methods reported are laser ablation of graphite and of polyimides, incomplete combustion of hydrocarbons in sooting flames, continuous-wave (CW) CO2 laser pyrolysis of benzene-oxygen vapor, and many not listed here [2,111,112]. It would appear that almost any kind of soot contains fullerenes and carbon nanotubes, but without the new methods of analysis, it was impossible to identify them. In some instances, the last step in the preparation involved heating of the soot over a catalytic metal, a procedure that might be called catalytic pyrolysis. 

Keywords— Laser Percussion, CW CO2 Laser, Biological Tissue, Laser-induced Sound, Microphone. 

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