Photochemistry with lasers

One example of selective photochemistry with lasers is the gasphase photochemical addition of bromine to olefin molecules induced by the monochromatic light near 6940 A from a pulsed, tunable ruby laser, as studied by Tiffany... 

Measurement and evaluation of photokinetics assume that the reaction solution is spatially homogeneous to be able to take into account the problems with the amount of light absorbed. Homogeneous medium and stirring are prerequisites. However, in photochemistry with lasers or in viscous media the dependence of concentration on the volume element irradiated and observed has to be taken into account. Strategies to consider these problems... 

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. 

Looking ahead, coherent laser pulses covering the complete spectral range of valence bond excitation from the UV to the IR spectral region are becoming available (see, e.g., [119]), and we expect SPODS to increase in importance in coherently controlled photochemistry with applications ranging from reaction control within molecules up to discrimination between different molecules in a mixture and laser-based quantum information technologies. 

The problem of quenching alkali resonance radiation in E-VR energy-transfer collisions with simple molecules is important as a model case for basic processes in photochemistry and serves its own right for a variety of practical applications, such as in laser physics. It has been studied for many years in the past, but only recent progress has led to information of the final internal energy of the molecule. In particular, crossed-beam experiments with laser-excited atoms allow a detailed measurement of energy-transfer spectra. There can be no doubt that the curve-crossing... 

Moser, J. and M. Gratzel. 1982. Photochemistry with colloidal semiconductors. Laser studies of halide oxidation in colloidal dispersions of TiOj and a-FezO,. Helv. Chim. Acta 65, 1436-1444. 

V. APPLICATIONS OF LASER IONIZATION MASS AND PHOTOELECTRON SPECTROSCOPY A. Photochemistry with Intense, Pulsed UV Laser Excitation... 

Long, S.R., J.T. Meek, P.J. Harrington, and J.P. Reilly (1983), Benzaldehyde photochemistry studied with laser ionization mass and photoelectron spectroscopy, J. Chem. Phys., 78, 3341-3343. 

Surface photochemistry can drive a surface chemical reaction in the presence of laser irradiation that would not otherwise occur. The types of excitations that initiate surface photochemistry can be roughly divided into those that occur due to direct excitations of the adsorbates and those that are mediated by the substrate. In a direct excitation, the adsorbed molecules are excited by the laser light, and will directly convert into products, much as they would in the gas phase. In substrate-mediated processes, however, the laser light acts to excite electrons from the substrate, which are often referred to as hot electrons . These hot electrons then interact with the adsorbates to initiate a chemical reaction. 

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... 

Hiroshi Fukumura received his M.Sc and Ph.D. degrees from Tohoku University, Japan. He studied biocompatibility of polymers in the Government Industrial Research Institute of Osaka from 1983 to 1988. He became an assistant professor at Kyoto Institute of Technology in 1988, and then moved to the Department of Applied Physics, Osaka University in 1991, where he worked on the mechanism of laser ablation and laser molecular implantation. Since 1998, he is a professor in the Department of Chemistry at Tohoku University. He received the Award of the Japanese Photochemistry Association in 2000, and the Award for Creative Work from The Chemical Society Japan in 2005. His main research interest is the physical chemistry of organic molecules including polymeric materials studied with various kinds of time-resolved techniques and scanning probe microscopes. 

There may be several reasons for the difference between gas phase and matrix photochemistry, and we outline one possible explanation. Even at 355 nm (XeF laser), a uv photon has more energy (equivalent to 335 kJ mol-1) than is needed to break one M—CO bond (89,90). In a matrix, the isolated Fe(CO)5 molecule is in intimate contact with the matrix material, and any excess energy can be rapidly lost to the matrix. In the gas phase, collisions are the principal pathway for loss of this excess energy. Under the conditions used in the gas phase photolysis, the mean time between collisions was relatively long and the excess energy could not... 

The photochemistry of a representative molecule of this class, C03(CO)gCCH, was investigated in gas phase in our laboratory using laser photolysis followed by MPI detection of the photofragments (41). Figure 5 shows the photofragment mass spectrum of this compound obtained by MPI with photolysis at 450 nm and 337 nm. 

The relative ease with which lasers can produce high concentrations of excited states can be important in initiating multi-molecular photochemistry. It is trivial to produce 0.1 M or greater photon "concentration" in a 1 y volume over a 10 ns period of time. Subsequent multimolecular reactions of excited states or labile photofragments are limited principally by the unimolecular lifetimes involved. 

---