Laser jet technique

In compiling the information in this chapter, I have relied heavily on several very comprehensive reviews that have appeared over the past few years [1-7]. In particular, the 1978 review by T irro et al. [1] is extremely thorough in describing the intra- and intermolecular photophysics and chemistry of upper singlet and triplet states. In fact, rather than reproduce the same details here, I direct the reader to this review for a summary of upper state behavior reported prior to 1978. (A description of azulene and thione anomalous fluorescence is included since these systems are the best-known systems that display upper state behavior.) I also direct readers to the reviews by Johnston and Scaiano [2] and Wilson and Schnapp [3] which focus on the chemistry of both upper triplet states and excited reaction intermediates as studied by laser flash photolysis (one- and two-color methods) and laser jet techniques. Also, Johnston s thorough treatment of excited radicals and biradicals [4] and the review of thioketone photophysics and chemistry by Maciejewski and Steer [5] are excellent sources of detailed information. 

Having recognized that upper states can potentially contribute in a non-negligible way to the photochemical behavior of a molecule, efforts have been made over the past 15 years to probe this behavior using a number of methods in particular, two-color (two-laser) flash photolysis and the laser jet technique [9]. 

It should be noted that these two techniques compliment each other. The two-color approach provides spectroscopic and kinetic information about the system but end-product analysis is difficult because of the small amounts of products generated by this method. On the other hand, while the laser jet technique... 

It is notable that the study of 154 involved a combination of laser flash and laser jet techniques which again points to the utility of the latter in obtaining product information about transients that are produced on too small a scale to be studied by laser flash photolysis. 

The recent development of the two-color and laser jet techniques has facilitated the exploration of the chemistry and photophysics of upper excited state and excited reaction intermediates, including neutral radicals. Upper states and excited radicals participate in many of the same reactions as lower states (but often with different efficiency) as well as in many reactions that are not accessible to lower states because of energy considerations. The use of two-color and laser jet techniques in several recent studies is a powerful combination that is just now beginning to be utilized in the study of multiphoton transients. 

Radicals are also further photolyzed to give products. Scheme 5 shows the photolysis of benzil 9 that is conducted by the laser-jet technique in CCl4 giving 12,13, and 14 as photoproducts. Compound 14 is a photochemical product of the benzyl radical 10, which is generated by a two-photon process from 9. The laser-jet reactions of diphenylmethyl radicals in CCI4 and alcohoHc solvents are also reported. 

Scheme 6 is an interesting example of radical photolysis conducted by the laser-jet technique." Radical 16 is generated by a two-photon reaction of 15. When radical 16 absorbs another photon in CHjClj it gives 18, similar to the reaction shown in Scheme 5. However, when 16 is photolyzed in MeOH, the excited radical 16b ionizes to cation 17, which reacts with MeOH to give 19. Similar reactions are also observed with 1-naphthyl" and 4-biphenylmethyl radicals."...

Plasma-jet diamond techniques yield growth rates of about 980 p.m/h (163,164). However, the rate of diamond deposition is still one to two orders of magnitude lower than the HP—HT technology, which is about 10,000 p.m/h (165). Diamond deposition rates of ca 1 p.m/s have been reported usiag laser-assisted techniques (166). This rate is comparable to the HP—HT synthesis. 

The laser jet approach typically involves focusing a CW argon ion laser at a high velocity jet of a solution containing the compound of interest. A light amplication effect is produced by this technique which can result in very high concentrations of multiphoton transients. 

Two groups within NASA, one at Goddard Space Flight Center, under the direction of Bill Heaps, and the other at the Jet Propulsion Lab under Laudenslager and McDermid, have respectively developed balloon-borne LIDAR methods for OH and developed eximer laser systems for extending the LIDAR method to lower detection thresholds. This research has important implications for both stratospheric and tropospheric research. Such laser ranging techniques also have important implications for intercomparison studies, particularly with mm-wave and far infrared emission ex periments. 

Last year we extended the application of the picosecond-jet technique to the study of the dynamics of isolated molecules in various stages of solvation with various solvents (water, alcohol, etc). - The idea was to study this controlled solvation and its dependence on the energy redistribution. Also we wanted to examine the photodissociation of these different solvated species or complexes following selective pumping by the picosecond laser. The systems we studied in some detail are azine-solvent complexes made in the jet with He or Ar as the carrier gas. 

The very first fluorescence spectra of jet-cooled exciplexes indicated the existence of two types of ground-state van der Waals adducts. For instance, the anthracene-dimethylaniline system displayed two types of cluster bands in the fluorescence excitation spectrum broad ( 150 cm ) and structureless, leading to typical ex-ciplex emission, and narrow (1 cm ), leading to resonance-type emission [10, 20]. It was assumed that they are due to different 1 1 adducts, distinguished by different geometries. Recently, laser-based techniques were developed that allow the discrimination of different species. One is hole-burning spectroscopy and another— mass-selected photoionization. 

The benzophenone sensitized photodenitrogenation of 2,3-diazabicyclo[2.2.1]alkanes 1 has been achieved using the laser/liquid jet technique with a CW argon ion laser. 

Other less-common deposition methods include electrospraying in a stable cone-jet mode to generate highly reproducible spots of as little as 50-pL volumes [33] and a laser transfer technique that allows the accurate deposition of picoliter volumes of proteins onto the solid surface [34]. 

Saab et al. [92, 93] developed a doctor-blade spreading technique for preparing the catalyst layers in a faster and highly reproducible fashion. The ink was coated onto laser jet transparency material wifli a doctor blade device driven by an X-Y chart recorder time base. The chart recorder time base is a highly accurate mechanical drive with a broad range of selectable speeds (20-0.05 cm/s), which is useful when compensating for liquids of varying viscosity. Machined aluminum... 

Gas phase spectroscopy of jet-cooled neutral molecules benefited from the development of laser-vaporisation and laser-desorption techniques as well as their coupling with a supersonic expansion. Improvements of spectroscopic procedures involving several lasers, e.g. the IRAJV double resonance spectroscopy, helped to collect information on weakly populated conformers. 

Ishiuchi S, Yamada K, Chakraborty S, Yagi K, Fujii M (2013) Gas-phase spectroscopy and anharmonic vibrational analysis of the 3-residue peptide Z-Pro-Leu-Gly-NH2 by the laser desorption supersonic jet technique. Chem Phys 419 145... 

In addition, inorganic polyhydrosilanes (Si H2 +2, Si H2 ) can be used as precursors for the preparation of amorphous silicon the compounds are easily decomposed by laser irradiation or by treatment at elevated temperatures. If laser irradiation is used, the amorphous silicon can even be structured [97]. Polysilanes can be coated from solution or printed by ink jet techniques and then be converted into silicon for the use in transistors or solar cells [98-101],... 

There are no standards for the comparison of the efficiency between pulse laser photolysis and laser-jet photolysis. However, when the efficiency is compared with parameters — yield of 57/photon and yield of 57/fluence (photons-m -s" ) — the laser-jet photolysis technique showed an increase of both parameters by three orders of magnitude compared with that in the simple XeCl laser photolysis. - A similar trend was also observed when the leaving group X is changed to OPh, SPh, and SePh. ... 

More commonly, the resonant two-photon process in Figure 9.50(c) is employed. This necessitates the use of two lasers, one at a fixed wavenumber Vj and the other at a wavenumber V2 which is tunable. The first photon takes the molecule, which, again, is usually in a supersonic jet, to the zero-point vibrational level of an excited electronic state M. The wavenumber of the second photon is tuned across the M to band system while, in principle, the photoelectrons with zero kinetic energy are detected. In practice, however, this technique cannot easily distinguish between electrons which have zero kinetic energy (zero velocity) and those having almost zero kinetic energy, say about 0.1 meV... 

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