Laser chemistry

Quack M 1993 Molecular quantum dynamics from high resolution spectroscopy and laser chemistry J. Mol. Struct. 292 171-95... 

Quack M 1992 Time dependent intramolecular quantum dynamics from high resolution spectroscopy and laser chemistry Time Dependent Quantum Molecular Dynamics Experiment and Theory. Proc. NATO ARW 019/92 (NATO ASI Ser. Vol 299) ed J Broeckhove and L Lathouwers (New York Plenum) pp 293-310... 

Quack M 1981 Faraday Discuss. Chem. Soc. 71 309-11, 325-6, 359-64 (Discussion contributions on flexible transition states and vibrationally adiabatic models statistical models in laser chemistry and spectroscopy normal, local, and global vibrational states)... 

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. 

B2.5.351 after multiphoton excitation via the CF stretching vibration at 1070 cm. More than 17 photons are needed to break the C-I bond, a typical value in IR laser chemistry. Contributions from direct absorption (i) are insignificant, so that the process almost exclusively follows the quasi-resonant mechanism (iii), which can be treated by generalized first-order kinetics. As an example, figure B2.5.15 illustrates the fonnation of I atoms (upper trace) during excitation with the pulse sequence of a mode-coupled CO2 laser (lower trace). In addition to the mtensity, /, the fluence, F, of radiation is a very important parameter in IR laser chemistry (and more generally in nuiltiphoton excitation) ... 

Figure B2.5.14. The IR laser chemistry of CF I excited up to the dissociation energy with about 17 quanta of a CO2 laser, The dissociation is detected by uncertainty limited cw absorption (hv ), see figures...

Quack M 1989 Infrared laser chemistry and the dynamics of molecular multiphoton excitation Infrared Rhys. 29 441-66... 

Quack M 1995 IR laser chemistry Infrared Rhys. Technol. 36 365-80... 

Quack M and Thdne H J 1987 Absolute and relative rate coefficients in the IR-laser chemistry of bichromophoric fluorobutanes tests for inter- and intra-molecular selectivity Chem. Rhys. Lett. 135 487-94... 

This leads to the possibiUty of state-selective chemistry (101). An excited molecule may undergo chemical reactions different from those if it were not excited. It maybe possible to drive chemical reactions selectively by excitation of reaction channels that are not normally available. Thus one long-term goal of laser chemistry has been to influence the course of chemical reactions so as to yield new products unattainable by conventional methods, or to change the relative yields of the products. 

This reaction has been carried out with a carbon dioxide laser line tuned to the wavelength of 10.61 p.m, which corresponds to the spacing of the lowest few states of the SF ladder. The laser is a high power TEA laser with pulse duration around 100 ns, so that there is no time for energy transfer by coUisions. This example shows the potential for breakup of individual molecules by a tuned laser. As with other laser chemistry, there is interest in driving the dissociation reaction in selected directions, to produce breakup in specific controllable reaction channels. 

Flam, F. (1994). Laser Chemistry The Dght Choice. Science 266 (14 October), 215-17. 

One of tbe central properties of lasers is the ability to furnish large numbers of photons at very specific energies. This ability has caused many investigators to hope that laser chemistry might be possible, that is that the energy from the laser might be deposited in molecules in very specific ways in order to initiate very selective, interesting, or remunerative chemistry. 

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 number of different ways in which laser chemistry might work can be imagined. 

Bond- or Mode-selective Laser Chemistry. Suppose we wish to break a specific bond in a molecule or cause a molecule to rearrange in a specific way, and the desired transformation is not the one which will occur if the molecule is simply heated (i.e., it is not the weakest coordinate in the molecule). Can we, by selectively exciting with a laser the bond or motion in question, cause the desired transformation to occur in greater than thermal yield ... 

A number of points are clear. First, in all cases the major expense of laser photons is the hardware, not the energy (even at Austin prices). Secondly, the intrinsically greater efficiency of the lower-energy lasers, especially the economic attractiveness of the CO2 laser, is evident. One can easily understand why laser chemistry schemes based upon multiphoton infrared absorption attract so much effort. Thirdly, on a per-unit-time basis the ion laser is more than twice as expensive to operate than even the rather exotic excimer laser. This is because of the inherent energetic inefficiency of the rare-gas plasma as a gain medium and because of the extrinsic, and hideous, expense of ion laser plasma tubes (and their poor reliability). 

To summarize the state of technology for the chemist wishing to practice laser chemistry the laser devices exist with the capability one would like, but they are expensive. We may expect that cheaper pulsed laser systems based upon excimer, Nd YAG, N2, alexandrite, etc. may be in the offing in the near future. This has already begun to happen with a new generation of N2 pumped dye lasers from two manufacturers. No such prospects presently exist for c.w. lasers in the visible and ultraviolet, but one may hope that the ion laser will be radically improved or supplanted soon. For chemical applications which can use infrared excitation, satisfactory devices presently exist and the price is right. 

The following case study contains examples of several topics discussed in previous sections, including some aspects of laser technology, laser spectroscopy and laser chemistry. A variety of lasers and laser techniques are applied in a straightforward manner to the problem of ascertaining structural and dynamical information on an excited electronic state of wide chemical interest. This information is obtained rather simply, illustrating the potential of laser techniques in the resolution of problems in solution chemistry. 

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