Patent Number: 047624027
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a system making it possible to obtain a selective reaction in photochemical processes on the basis of laser beams incorporating means for distributing said beams. The invention applies to photochemical processes requiring the combined action of several light emissions of different wavelengths in order to obtain a selective reaction, such as an isotopic separation or a photoisomerization. Isotopic separation can e.g. be used for eliminating an isotope which is incompatible with the industrial use of a product, such as in the case of purifying metals or for selecting a useful isotope, e.g. carbon or uranium. By the rearrangement of the atoms of a molecule previously excited by light emissions, the photoisomerization of molecules makes it possible to obtain a molecule having different spectroscopic and chemical properties. To obtain the sought selective reaction, it is possible to proceed in known manner in two stages. The first stage consists of selectively exciting an isotopic or chemical species on the basis of one or more laser radiations. The second stage consists of causing the transformation of the previously excited species by a final laser radiation having an adequate energy. The selective excitation of the species, i.e. the molecule or atom, takes place in known manner by successive passages of the molecule or atom in question to levels having an ever higher energy by absorption of photons, each photon coming from a pulse laser with a particular wavelength. In certain cases, selective excitation can be obtained by absorption of a single photon and therefore by the passage of the molecule or atom in question to a single energy level. The excited species is transformed by irradiating it with a laser beam of a wavelength such that it clears the level corresponding to said transformation. This leads either to the formation of a new molecule or to the ionization of a molecule or atom. In this way, it is possible to distinguish the species formed from other species and separate them. FIG. 1 shows an example of transitions at several levels in U.sup.235 making it possible to bring about its ionization. Thus, to separate the isotope U.sup.235 from uranium vapour, use is made of a selective excitation beam S.sub.1 constituted by two beams S'.sub.1 and S".sub.1 of respective wavelengths .lambda.'.sub.1 and .lambda.".sub.1, which bring the atoms of isotope U.sup.235 to two successive levels 1, 3. A final beam S.sub.2 of wavelength .lambda..sub.2 brings the excited atoms of U.sup.235 into an ionization state 5. The ionization energy of isotope U.sup.235 is equal to 6.12 eV, so that each of the wavelengths .lambda.'.sub.1, .lambda.".sub.1, .lambda..sub.2 is approximately 600 nm. In order to optimize the isotopic separation of U.sup.235, use can be made of a fourth wavelength .lambda..sub.1 " associated with a beam S.sub.1 "' in order to bring the atoms already at an intermediate energy level 7, occupied by a thermal process, to level 1, so as to be ionized following successive irradiations at wavelengths .lambda.".sub.1 and .lambda..sub.2. Throughout the remainder of the text, S.sub.1 will be used for the beam permitting the selective excitation of the species in question, whereby S.sub.1 can contain a single wavelength .lambda..sub.1 or can result from a superimposing of beams S'.sub.1, S".sub.1, . . . S.sub.1.sup.(n) of wavelengths .lambda.'.sub.1, .lambda.".sub.1, . . . .lambda..sub.1.sup.(n), with n being an integer equal to at least 1 and S.sub.2 is the beam of wavelength .lambda..sub.2 permitting the ionization or photodissociation of the previously excited species. The different wavelengths are obtained in known manner from dye lasers, (e.g. rhodamine lasers) excited by other lasers, which can be copper vapour lasers. This gives pulse-type light emissions of a few dozen ns and a repetition frequency of a few kHz. In known manner, beams S.sub.1 and S.sub.2 are transmitted in the same propagation direction into an enclosure containing the substance from which a chemical or isotopic species is to be extracted and which is in the form of a vapour flow. The effective absorption sections of the transitions corresponding to the selective excitation and transformation of the species in question can differ. The effective absorption sections of the transitions corresponding to the selective excitation can be 10 to 100 times greater than that corresponding to the transformation. Moreover, to retain a good selectivity, an excessive power of beam S.sub.1 must not be used, because this would lead to a loss of selectivity resulting e.g. from broadening through saturation, or to transitions with several photons. Following interaction of the beams S.sub.1 and S.sub.2 with the species in question, beam S.sub.1 is very attenuated compared with beam S.sub.2. Thus, the simultaneous presence of these two beams cannot be maintained throughout their passage in the enclosure. As a result of this interaction beam S.sub.2 is not very well used, its energy being wasted in the final part of the path where beam S.sub.1 is highly attenuated. Thus, the prior art means do not make it possible to optimize the use of these beams. SUMMARY OF THE INVENTION The object of the present invention is to obviate this disadvantage. This is achieved through the use of an apparatus making it possible to introduce the selective excitation beam S.sub.1 through the vapour to be treated at several points, whereby said beam can result from a superimposing of beams S'.sub.1, S".sub.1, . . . S.sub.1.sup.(n) of wavelength .lambda.'.sub.1, .lambda.".sub.1 . . . .lambda..sub.1.sup.(n) with n being an integer of at least 1, whereas beam S.sub.2 is only introduced once at the inlet of the apparatus. More specifically the present invention relates to a system making it possible to obtain a selective reaction in photochemical processes from laser beams comprising: in a sealed enclosure, the substance from which it is wished to extract an isotopic or chemical species, said substance being in the form of a vapour flow, laser sources emitting towards said enclosure a beam S.sub.1 permitting a selected excitation of the species to be extracted and a beam S.sub.2 permitting a transformation of said excited species, wherein said system also 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 equal to at least 1, for constituting several beams S.sub.1 introduced at several points through the vapour to be treated, means for introducing into the enclosure the resulting beams S.sub.1 and the beam S.sub.2 so as to make them colinear, whilst still distinguishing them by one of their characteristics, such as a different polarization or an opposite proagation direction, said introduction means being periodically distributed on parallel arms defining propagation directions of said beams in the enclosure, so as to optimize the use of the light energies of the different beams. According to a constructional variant of the system according to the invention, the latter also comprises quarterwave plates making it possible to obtain a circular polarization of beams S.sub.1 and S.sub.2, when it is advantageous to have circularly polarized light beams to interact with the vapour, said means being located upstream and downstream of the introduction means. According to another embodiment of the system, the means for introducing beams S.sub.1 and S.sub.2 into the enclosure comprise Glan prisms, into which said beams are injected with two orthogonal polarizations and in directions such that after their passage in said prisms they are colinear. Each prism is located on an arm at the points where S.sub.1 is reintroduced into the enclosure. According to another embodiment of the system, the latter comprises for inverting beams S.sub.1 and S.sub.2 along parallel arms. According to an embodiment of the system corresponding to cavity operation, the means for introducing beams S.sub.1 and beam S.sub.2 into the enclosure comprise a Glan prism at each end of an arm corresponding to one propagation direction of beams S.sub.1 and S.sub.2 into the vapour and on each of the arms formed in the enclosure. Each of the beams S.sub.1 and S.sub.2 is introduced with the same polarization into one of the Glan prisms at each end of an arm and in directions which, following their passage in said prisms, enable them to have the same propagation direction, but the opposite sense. According to another embodiment of the system corresponding to cavity operation, the latter comprises means for reflecting beams S.sub.1 and S.sub.2 back on to themselves, said means incorporating plane mirrors associated with Pockels cells, each plane mirror - Pockels cell assembly being located at each end of the arms and means for inverting beam S.sub.2 towards other arms, said means incorporating a Pockels cell located on each arm. According to an embodiment of the system, the means for superimposing beams S'.sub.1, S".sub.1, . . . ,S.sub.1.sup.(n) so as to constitute beams S.sub.1 comprise a group of semitransparent plates having a reflection coefficient 0.5 which successively divide into two the different beams S'.sub.1, S".sub.1 . . . , S.sub.1.sup.(n), whilst superimposing the divided beams to obtain the different beams S.sub.1 used in the system.