Photochemistry laser

Let us first consider laser-induced chemical reactions. The excitation energy of one or several reactands which initiates and drives the chemical 

2) On a medium time scale (typically in the ns to ys range, depending on the pressure in the sample) the reaction takes place before the excitation energy of a molecule has been transferred by collisions to other molecules. The excited molecules may react with a larger probability than the ground state molecules. 

3) On a still larger time scale (ys to cw operation) the excitation energy is uniformly distributed among all molecules of the sample, resulting in an in- 

For the first two time regions mode-locked or pulsed lasers must be used. In most experiments performed so far, pulsed CO2 lasers or chemical lasers were utilized for vibrational excitation of reactant molecules. As a specific example of a laser-enhanced bimolecular reaction we consider the reaction 

Another example is the synthesis of SF5NF2 by CO2 laser pulses of lO s duration [14.8]. While the standard synthesis requires relatively high pressures of the commencing product and heating to 425 K in a sealed reac- 

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Laser Photochemistry. Photochemical appHcations of lasers generally employ tunable lasers which can be tuned to a specific absorption resonance of an atom or molecule (see Photochemical technology). Examples include the tunable dye laser in the ultraviolet, visible, and near-infrared portions of the spectmm the titanium-doped sapphire, Tfsapphire, laser in the visible and near infrared optical parametric oscillators in the visible and infrared and Line-tunable carbon dioxide lasers, which can be tuned with a wavelength-selective element to any of a large number of closely spaced lines in the infrared near 10 ]lni. 

Another area of research ia laser photochemistry is the dissociation of molecular species by absorption of many photons (105). The dissociation energy of many molecules is around 4.8 x 10 J (3 eV). If one uses an iafrared laser with a photon energy around 1.6 x 10 ° J (0.1 eV), about 30 photons would have to be absorbed to produce dissociation (Eig. 17). The curve shows the molecular binding energy for a polyatomic molecule as a function of interatomic distance. The horizontal lines iadicate bound excited states of the molecule. These are the vibrational states of the molecule. Eor... 

The examples given above represent only a few of the many demonstrated photochemical appHcations of lasers. To summarize the situation regarding laser photochemistry as of the early 1990s, it is an extremely versatile tool for research and diagnosis, providing information about reaction kinetics and the dynamics of chemical reactions. It remains difficult, however, to identify specific processes of practical economic importance in which lasers have been appHed in chemical processing. The widespread use of laser technology for chemical synthesis and the selective control of chemical reactions remains to be realized in the future. 

The practical questions concerning laser chemistry may be tersely stated. (1) Will it work If so, (2) is it interesting as opposed to being a trivial extension of known, non-laser photochemistry (3) Is it a practical tool for real chemists as opposed to full-time laser technologists And, (4) are the goods worth the price charged - is it economically worth the effort To make a long story short, the answers are (1) yes, in many but not all cases (2) yes (3) almost and (4) yes, in some cases, but the product had better be valuable. 

A. P. Baronavskl, Ultraviolet Laser Photochemistry, in "Lasers as Reactants and Probes in Chemistry" (W. J. Jackson and A. Harvey, Eds.), Howard University Press, Washington, D.C., 1984. 

Keywords elimination, trapping, radicals, isomerization, laser photochemistry... 

This expression leads to 36 35 = 0.059 at 494 nm. Wintgens et al. used Aberchrome-540 as an actinometer for studies of one- and two-laser photochemistry of short-lived reaction intermediates. 

The development of new methods in laser photochemistry and spectroscopy will be stimulated and better understanding of problems related to excited-state reactions of large molecules, as well as to the structure and function of proteins and biomembranes, will be achieved. 

These compounds are volatile in vacuo, U(OCF2CF3)6 remarkably boiling at 25 °C at 10 mmHg pressure. U(OMe)6, which sublimes at 30 °C at 10 mmHg pressure, was investigated as a candidate for IR laser photochemistry leading to uranium isotopic enrichment. 

In an interesting application of high Intensity laser photochemistry. 2-methylbenzophenone has been converted to the enthrone (208) the reaction proceeds via the E-enol (209) which Is produced along with the shorter lived... 

E.E.Nikitin, Vibrational relaxation of diatomic molecules on potentially reactive atoms, in Gas-dynamics lasers and laser photochemistry. Moscow, University Press, 1978, p.l36... 

Adam, W., Kliem, U., Peters, E. M., Peters, K., Von Schnering, H. G. Preparative vis-laser photochemistry. Qinghaosu-type 1,2,4-trioxanes by molecular oxygen trapping of Patemo-Buchi triplet 1,4-diradicals derived from the bicyclic enol lactones D1,6- and D1,10-2-oxabicyclo[4.4.0]decen-2-one and p-benzoquinone. J. Prakt. Chem. 1988, 330, 391-405. 

Since that time, laser photochemistry has become a popular subject and with it have come the laser photochemists looking for a photon target. Obviously, the first laser photons would be aimed toward isotope separations which required the narrow band-widths which the laser so uniquely provided but spin-off targets have since included the separations of reactor fuel components in reprocessing and/or waste isolation systems. Although much has been promised from the application of lasers to the reprocessing of nuclear fuels, there has been very little evidence that would... 

Scheme 2.5. Two-color two-laser photochemistry of 1- and 2-NpCH2-OBP involving intramolecular triplet ENT, selective excitation of Np(T i)C1I2-OBP to NpCH2-OBP(T ), and cleavage of the C—O bond from NpCH2-OBP(T ). Dotted square shows the excitation energy delocalization.

Hatanaka, K., Tsuboi, Y., Fukumura, H. and Masuhara, H. (2002) Nanosecond and femtosecond laser photochemistry and ablation dynamics of neat liquid benzenes. J. Phys. Chem. B, 106, 3049-3060. 

Experiments on the laser photochemistry of formaldehyde51 indicate that the probable mechanism involves the excitation of the ground state S0 to a ro-vibrational state of the first excited singlet state S, ... 

Wilson, R.M., Walsh, T.F., and Gee, S.K., 1980, Laser photochemistry the wavelength dependent oxidative photodegradation of vitamin K analogs, Tetrahedron Lett. 21, 3459-3463. 

Further literature descriptions of laser initiated photodissociation of UF5 in the gas phase are few. Letokhov and Moore have outlined various general aspects and problems dealing with UF5 laser photochemistry and isotope separation (23). In... 

This article is concerned with the developments in instrumentation and techniques in photochemistry and spectroscopy during the period July 1980— June 1981. Such a wide ranging topic is impossible to review at all critically, nor is it feasible to consider every publication concerning photochemical instrumentation. Consequently, many reports concerned merely with the application of established techniques have been omitted. In this respect, it should be noted that the relative brevity of some sections (for example plasma sources, u.v.-visible spectroscopy) in no way reflects the use or application of these techniques, but merely their advanced state of development. Further it is apparent that, during the past decade, a swing away from developments in instrumentation for conventional photochemistry in favour of spectroscopy and laser photochemistry has occurred. This has been reflected in the following discussion. The author would like to thank Dr. Mike West for several helpful discussions during the preparation of this manuscript. 

The photoactivation of organometallic catalysts and the infrared laser photochemistry in low-temperature matrices have been the subject of recent reviews. 

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