Patent Number: 047624027
Section: claims

1. A system making it possible to obtain a selective reaction in photochemical processes on the basis of laser beams comprising: in a sealed enclosure, the substance whereof a species is to be extracted, said substance being in the form of a vapour flow,  laser sources emitting towards said enclosure a beam S.sub.1 permitting a selective excitation of the species to be extracted and a beam S.sub.2 permitting a transformation of said excited species, wherein it comprises means for distributing the beams having:  means for superimposing the beams S'.sub.1, S".sub.1, . . . S.sub.1.sup.(n) with n being an integer at least equal to 1, and for constituting several beams S.sub.1 introduced at several points through the vapour to be treated,  means for introducing into the enclosure the resultant beams S.sub.1 and also beam S.sub.2 so as to make them colinear, whilst distinguishing them by one of their characteristics, said introduction means being periodically distributed on parallel arms defining the propagation directions of said beams in the enclosure. 2. A system according to claim 1, wherein it also comprises quarter-wave plates making it possible to obtain a circular polarization of beams S.sub.1 and S.sub.2, located upstream and downstream of the introduction means. 3. A system according to claim 1, wherein the means for introducing beams S.sub.1 and beam S.sub.2 into the enclosure are constituted by Glan prisms into which said beams are injected with two orthogonal polarizations and in directions which, following their passage into the prisms, can be colinear, each prism being located on an arm at the points where S.sub.1 is reintroduced into the enclosure. 4. A system according to claim 1, wherein it also comprises means for inverting the beams S.sub.1 and S.sub.2 along parallel arms. 5. A system according to claim 1, wherein the means for introducing beams S.sub.1 and beam S.sub.2 into the enclosure, in cavity operation, comprise a Glan prism at each end of an arm corresponding to a propagation direction of beams S.sub.1 and S.sub.2 into the vapour and on each of the arms in the enclosure, each of the beams S.sub.1 and S.sub.2 being introduced with the same polarization into one of the Glan prisms and in directions which, following their passage into said prisms, make it possible to have the same propagation direction, corresponding to an arm, but of the opposite sense. 6. A system according to claim 1, wherein, in cavity operation, it comprises means for reflecting the beams S.sub.1 and S.sub.2 on to themselves, said means comprising plane mirrors associated with Pockels cells, the assemblies constituted by the plane mirrors and Pockels cells being positioned symmetrically at each end of the arms and means for inverting beam S.sub.2 towards other arms, said means comprising a Pockels cell located on each arm. 7. A system according to claim 1, wherein the means for superimposing the beams S.sub.1 ', S".sub.1, . . . , S.sub.1.sup.(n) so as to form beams S.sub.1 incorporate a group of semitransparent plates having a reflection coefficient 0.5, which successively splits the different beams S'.sub.1, S".sub.1, . . . S.sub.1.sup.(n) into two, whilst superimposing the split beams in order to obtain the different beams S.sub.1 used in the system.