The applications of high resolution laser techniques to crossed-beam experiments which are examined in
this paper concern: (a) the reactivity of atomic/molecular excited states and the influence of light
polarization and (b) the measurement of absolute and differential cross sections by use of the laserinduced
fluorescence technique performed in a coherent saturation regime.Examples are provided by the crossed-beam study of the Cs(7P) + H2→CsH + H photochemical
reaction, where two C.W. single-mode tunable lasers are used to excite Cs atoms and to probe CsH
product molecules. It is shown how the fine and hyperfine structures of the Cs atom are involved in the
reaction mechanism (harpooning). The measurement of the total cross section (≈4.10−16cm2) gives an
indication of the overall efficiency of the reactive process (Cs atoms in the 7P state) with respect to the
quenching ones (Cs atoms in 6S, 6P, 5D and 7S states). Finally, the differential cross section shows a
marked forward “peaking” of CsH products which narrows with collision energy but which is rather
independent of the product rotation.
