Laser-induced chemical reaction

The basic principle of laser-induced chemical reactions is schematically illustrated in Fig. 10.7. The reaction is initiated by one- or multiphoton excitation of one or more of the reactants. The excitation can be performed before the reactants collide (Fig. 10.7a) or during the collision (Fig. 10.7b and Sect. 8.6). 

For the selective enhancement of the wanted reaction channel by laser excitation of the reactants, the time span At between photon absorption and completion of the reaction is of fundamental importance. The excitation energy n hco (n = 1,2.) pumped by photon absorption into a selected excited molecular level may be redistributed into other levels by unwanted relaxation processes before the system ends in the wanted reaction channel. It can, for instance, be radiated by spontaneous emission, or it may be redistributed by intramolecular radiationless transitions due to vibrational or spin-orbit couplings onto many other nearly degenerate molecular levels. However, these levels may not lead to the wanted reaction channel. At higher pressures collision-induced intra- or intermolecular energy transfer may also play an important role in enhancing or suppressing a specific reaction channel. 

We distinguish between three different time regimes  

For the first time range, ultrashort laser pulses generated by mode-locked lasers in the femto- to picosecond range (Chap. 6) are needed, whereas for the second class of experiments pulsed lasers in the nanosecond to microsecond range (Q-switched CO2 lasers, excimers, or dye lasers) can be employed. Most experiments performed until now have used pulsed CO2 lasers, chemical lasers, or excimer lasers. For femtosecond pulses Kerr-lens mode-locked Tiisapphire lasers with subsequent amplifier stages are available (see Sect. 6.1). 

Let us consider some specific examples the first example is the laser-induced bimolecular reaction 

For the selective enhancement of the wanted reaction channel by laser excitation of the reactants, the time span Ar between photon absorption and completion of the reaction is of fundamental importance. The excitation energy ri hco n =. ..) pumped by photon absorption into a selected excited 

For the first time range ultrashort laser pulses, generated by mode-locked lasers (Chap. 11), are needed, whereas for the second class of experiments in the nanosecond to microsecond range pulsed lasers (Q-switched CO2 lasers. 

The second example is the spatially and temporally resolved observation of the explosion of an O2/O3 mixture in a cylindrical cell, initiated by a TEA CO2 laser [15.14]. The progress of the reaction is monitored through the decrease of the O3 concentration which is detected by a time-resolved measurement of the UV absorption in the Hartley continuum of O3. If the UV probe beam is split into several spatially separated beams with separate detectors (Fig. 15.5) the spatial progression of the explosion front in time can be monitored. 

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A. D. Bandrauk, E.-W. S. Sedik, and C. F. Matta, Effect of absolute laser phase on reaction paths in laser-induced chemical reactions, J. Chem. Phys. 121, 7764 (2004). 

The concept of saturated laser fluorescence appears attractive in that the fluorescence intensity is directly related to the particular species concentration and becomes roughly independent of the laser intensity at saturation. Such a mode has been invoked already to monitor absolutely flame concentrations of Na a-4), OH (5), C2 (6,7), CH (7,8), CN (8), and MgO (4). However, during a recent study of the behavior of Na and Li in flames (9-11), we have observed evidence for laser induced chemical reactions under saturated conditions which has significant implications for the quantitative exactness of such measurements. 

Summary. In saturated laser fluorescence studies of sodium and lithium in a series of fuel rich H2/02/N2 flames there is evidence for the involvement of laser induced chemical reactions with H20 and H2. Although their reactive probabilities have been shown to be small relative to the corresponding physical quenching interactions they are still sufficient to establish significant... 

By using a commercially available TEA COj laser, the above laser-induced chemical reaction can be made to generate gram quantities of product per hour, which in real laser time could translate into kilograms of product per second [1842a]. 

Fig. 10.7 Schematic diagram of laser-induced chemical reactions with state-selective detection of the reaction products (a) by excitation of the reactants (b) by excitation of the collision pair (ABC)...

C.D. Cantrell, S.M. Freund, J.L. Lyman, Laser induced chemical reactions and isotope separation, in Laser Handbook, vol. 3, ed. by M.L. Stitch (North-HoUand, Amsterdam, 1979) R.N. Zare, Laser separation of isotopes. Sci. Am. 236, 86 (1977) ... 

Laser-stimulated versus laser-induced chemical reactions... 

The study of laser-induced chemical reactions in clusters is normally carried out in a molecular beam environment. One of the great advantages of... 

In order to obtain laser-induced chemical reactions, the substrate, which can be moved, is placed at the focal point of the laser beam in a chemical chamber filled with a reactant gas (Fig. lb). The laser beam passes through a fixed optical system and is focused by means of a lens. 

FIG. 1 Simplified schema for (a) the reactor used for UV/ ozone treatment of polymers (b) the reactor used in order to obtain laser-induced chemical reactions (c) ion or electron source (d) an idealized low pressure discharge (e) the corona discharge. 

Shortly after the primary, elementary mechani.sm of colli-.sionle.ss multiphoton excitation had been proven in molecular beam experiments, " a general theory of thi.s proce.s.s was developed, which formed the ba.sis for program package.s that may be considered to be the starting point of computational laser chemistry . Figure I summarize.s a typical example for the CO -laser induced chemical reaction ... 

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

A. E. Orel and W. H. Miller, Infrared laser induced chemical reactions, Chem. Phys. Lett. 57 362 (1978). 

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