Laser exciplex

Molecular Interaction. The examples of gas lasers described above involve the formation of chemical compounds in their excited states, produced by reaction between positive and negative ions. However, molecules can also interact in a formally nonbonding sense to give complexes of very short lifetimes, as when atoms or molecules collide with each other. If these sticky collisions take place with one of the molecules in an electronically excited state and the other in its ground state, then an excited-state complex (an exciplex) is formed, in which energy can be transferred from the excited-state molecule to the ground-state molecule. The process is illustrated in Figure 18.12. 

If a triplet-state molecule (A ) meets a singlet-state molecule (B ), a short-lived complex can be formed (an exciplex). In the latter, the molecules exchange energy, returning to its singlet state (A ) and B raised to its triplet state (B ). If the new triplet state is relatively long-lived, it can serve to produce the population inversion needed for lasing, as in the He/Ne laser. 

An Xc2 excimer laser has been made to operate in this way, but of much greater importance are the noble gas halide lasers. These halides also have repulsive ground states and bound excited states they are examples of exciplexes. An exciplex is a complex consisting, in a diatomic molecule, of two different atoms, which is stable in an excited electronic state but dissociates readily in the ground state. In spite of this clear distinction between an excimer and an exciplex it is now common for all such lasers to be called excimer lasers. 

The study of short lived excited states is limited by the low concentra- lions in which they are created on excitation with normal light sources. The use of high intensity sources such as flash lamps with suitable flashing rates and laser sources have been helpful in this respect. Triplet-triplet absorption, absorption by excited singlet state to higher singlet state and Absorption by exciplexes (Section 6.6.1) can be effectively observed by sequential biphotonic processes. 

These lasers are also called—incorrectly— excimer lasers. It will be clear that they could be called exciplex lasers. The active material is a gas mixture which contains a halogen (F2 or Cl2 in most cases) and a rare gas such as Kr, Ar or Xe. These cannot form any stable compounds in their ground states, but excited state species do exist and can fluoresce. These excited state species e.g. KrF) are formed through the recombination of ions, for instance... 

The active material in a dye laser is a liquid solution of a dye in a solvent. The fluorescence of the dye is excited by a pump light, which can be a flash of white light (seldom used nowadays) or a UV laser pulse from an Nd/YAG or exciplex laser. Figure 7.19 shows the principle of such a dye laser. The cavity consists of a diffraction grating, G, which plays the role of a totally reflecting mirror, and of a partially reflecting mirror which will let the laser... 

Exciplexes are defined as molecular complexes which are stable under electronic excitation30. The picosecond (or femtosecond) laser photolysis methods are suitable for investigating the very rapid photo-induced processes related to CT complexes. 

Photocycloaddition and photoaddition can be utilized for new carbon-carbon and carbon-heteroatom bond formation under mild conditions from synthetic viewpoints. In last three decades, a large number of these photoreactions between electron-donating and electron-accepting molecules have been appeared and discussed in the literature, reviews, and books [1-10]. In these photoreactions, a variety of reactive intermediates such as excimers, exciplexes, triplexes, radical ion pairs, and free-radical ions have been postulated and some of them have been detected as transient species to understand the reaction mechanism. Most of reactive species in solution have been already characterized by laser flash photolysis techniques, but still the prediction for the photochemical process is hard to visualize. In preparative organic photochemistry, the dilemma that the transient species including emission are hardly observed in the reaction system giving high chemical yields remains in most cases [11,12]. 

As an example, scheme i) gives a new transient Raman spectrum in which all the observed vibrational bands have the same rise time and the same enhancement profile. In scheme ii) all the new bands should have the same rise time but the relative band intensity of the new spectrum should change upon changing the probe laser frequency (if B and C have different optical absorption profiles.). Scheme iii) predicts changes in the relative intensity of the new bands with both the laser probe frequency as well as the time of delay between the photolysis and probe laser pulses. The difference between scheme iii) and iv) is that in iii) the bands of C and D could have different rise and decay times while in iv) they all should have similar rise times. Schemes iii) and v) are similar except that A in iii) disappears permanently upon laser exposure while in v) A regains its concentration and no permanent photochemical damage takes place. In scheme vi) the rise time of the vibrational bands of the (AB) transient (an excimer or an exciplex) should depend on the concentration of B. 

Recent technical developments in laser Raman spectroscopy have made it possible to measure the Raman spectra of short-lived transient species, such as electronically excited molecules, radicals and exciplexes, which have lifetimes on the order of nano- (10-9) and pico- (10-12) seconds. These shortlived species may be generated by electron pulse radiolysis, photo-excitation and rapid mixing. However, the application of electron pulse radiolysis is limited in its adaptability and selectivity, while rapid mixing is limited by mixing rates, normally to a resolution on the order of milliseconds. Thus, photoexcitation is most widely used. 

Excimer laser A source of pulsed coherent radiation obtained from an exciplex. The proper name should be exciplex laser. Typical lasing species are noble gas hahdes (XeCl, KrF, etc.) emitting in the UV domain. 

When primary and secondary co-arylalkylamines are excited, exciplex fluorescence is not observed, but fluorescence quenching does take place (Shizuka et al., 1979). Presumably chemical reaction competes effectively with exciplex formation. However, by means of picosecond laser flash photolysis, the time-resolved absorption spectra of the species produced on reaction of pyrene with diphenylamine have been obtained (Okada et al., 1980b). It was shown that the reaction leads to neutral radicals via an exciplex. Although the formation of neutral radicals in such systems had been previously identified (Okada et al, 1976a) the role of exciplexes had been purely speculative. 

E, F Sg) state of H2 following two-photon excitation with an ArF exciplex laser. 

The relatively recent development of a new class of chemical laser based on the formation of noble gas-halide exciplexes and producing coherent radiation at a number of different u.v. wavelengths has been quickly adopted by both kineticists and spectroscopists. This Section brings together a few studies which have appeared in the past year dealing with the properties (i.e., kinetics, photophysics, etc.) of complexes important in the noble gas-htilide systems. The numerous articles that have appeared giving details of performance characteristics of such lasers (and their improvement) are deemed to be beyond the scope of this Report and are not included. However, the proceedings of two recent conferences on lasers have been published in which much of this information can be found. 

The laser-initiated polymerization of styrene with maleic anhydride has been said to occur through either a singlet or triplet excimer (the authors surely mean exciplex ) of the anhydride as shown in Scheme 4. This study provides an interesting comparison between laser- and u.v.-initiated polymerization, and Table 2 shows clearly that the former is a very energy-efficient system. In the laser-initiated polymerization of a thiol-ene system, oxygen inhibition was not a significant problem and similar conclusions were reached on the energy efficiency of lasers. On a related note, the efficiency of the well-known benzo-phenone-triethylamine complex is apparently enhanced if carboxylic acids are added to the system. ... 

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