Laser assisted collisions

Collision-induced electronic spectra have many features in common with rovibrotranslational induced absorption. In this Section, we take a look at the electronic spectra. We start with a historical note on the famous forbidden oxygen absorption bands in the infrared, visible and ultraviolet. We proceed with a brief study of the common features, as well as of the differences, of electronic and rovibrotranslational induced absorption. Recent work is here considered much of which was stimulated by the advent of the laser - hence the name laser-assisted collisions. The enormous available laser powers stimulated new research on laser-controlled, reactive collisions and interactions of supermolecules with intense radiation fields. In conclusion, we attempt a simple classification of various types of electronic collision-induced spectra. 

The advent of the laser has stimulated new research in collisional physics. The term laser-assisted collision was coined to describe the various research activities that have evolved. These studies are concerned with electronic transitions in supermolecules and will be briefly considered here. Similarities between laser-assisted collisions (LAC) and collision-induced absorption (CIA) exist, both in the types of phenomena considered and in the calculations of the spectra [208]. 

A more general term for laser-assisted collision is photon-assisted collision (PAC), since it is not always necessary that lasers provide the light. Both photon-assisted collisions and collision-induced absorption may be... 

Table 7.1. Comparison of laser-assisted collisions (LAC) and collision-induced absorption (CIA) [208],...

P. R. Berman. Three-state model for laser-assisted collisions. In J. Szudy, ed., Spectral Line Shapes 5, Ossolineum, Warsaw, 1989. 

In the experiments with Rydberg atoms it is very difficult to observe radiatively assisted collisions with cross sections more than a factor of 10 smaller than the resonant collision cross sections, so the deviations from Eq. (15.29) are not apparent. However, in other contexts, such as laser assisted collisions, this limitation does not apply, and it is interesting to consider how the above description passes over into the weak field regime, in which Jm(KEmv//oj) is small. If we restrict the integration in Eq. (15.27) to the large r region of space, in which the approximations we have used are valid, we can rewrite Eq. (15.27) as... 

In conclusion, it appears that relativity can affect significantly the dynamics of laser- assisted collisions at intensities achieved by several laser sources currently operated worldwide, [13]. 

Moreover, the absorption spectra or the emission during the dissociation [03, (Imre et al. 1982), Nal (Foth et al. 1982)] give direct information on the intermediate states of reaction, and on the dynamics, not only at the turning point as for the laser-assisted collisions. 

The study of reactive excited states of van der Waals complexes is the link between the laser-assisted collision and the photodissociation approach it brings the collisional problem into a much simpler photodissociation problem. Here, a cold complex which has a defined geometry is formed between the collision partners and optically excited to trigger the reactive process. This creates the photodissociation of a molecule with very weakly bound ground state. The van der Waals spectroscopy has already allowed the accurate determination of the interatomic potential [Na-Ar (Smalley et al. 1977 Tellinghuisen et al. 1979), HgAr (Breckenridge et al. 1985, 1994 Fuke et al. 1984)]. More complex collisional... 

It can be pointed out that the van der Waals technique is the equivalent of the excitation of the collision pair or the laser-assisted collision, and we will discuss the differences and similarities between the excitation of the complex and the collisional process. 

Comparison between collisions with the metastable Xe atom, laser-assisted collision and van der Waals excitation has been performed by Richman et al. (1993) on the Xe + BrCCl3 system. Excitation of xenon with BeCCl3 leads to two possible channels formation of XeBr or XeCl. The relative proportion of each product is highly sensitive to the entrance channel and to the experimental conditions, as listed below. 

As shown by Fig. 8 we can see collisions in which stimulated emission of up to three microwave photons occurs using very low microwave powers. It is interesting to note that laser assisted collisions typically require optical intensities of MW/cm2 to be visible at all [Falcone 1977], In Fig. 8 it appears that the one photon assisted collision disappears only to reappear at higher microwave fields. This unexpected feature becomes more apparent if we plot the resonance signal vs the microwave field, as shown in Fig. 9. Also shown by the lines in Fig. 9 are the results of a model describing collisions in which from zero to three photons are emitted. 

That lasers have played a key role as promoters and as probes of chemical reactions is well known and extensively documented.1,4,7,62-72 In many of these applications the laser is employed as an intense, nearly monochromatic, light source whose characteristics ensure species selectivity, a well-characterized spectroscopy, and adequate intensity for multiphoton processes. Some possible applications, notably laser-assisted collisions 73,74 and transition-state spectroscopy,75,76 are yet in their infancy, but the extant studies already suggest considerable promise for influencing and probing chemical reactions. 

The effect of non-resonant laser fields upon the cross-section for simple chemical reactions ( laser-assisted collisions ) is currently of theoretical interest, and... 

Spectroscopy of the transition state and laser-assisted collisions... 

Laser-assisted collisions are most readily demonstrated experimentally when reaction is only possible on the electronically excited state. The harpoon reactions... 

---