Laser Isotope Separation by Multiple Photon Absorption

This invention relates to methods of isotope separation and enrichment and more particularly to methods of isotope enrichment wherein molecular species containing a particular isotope are selectively excited by infrared laser light sufficiently to undergo chemical reactions whereas unexcited molecules of the species do not.

It is known in the art that lasers may be used to selectively induce chemical reactions which effectively result in isotope separation or enrichment. See, e.g., N. V. Karlov, "Laser-Induced Chemical Reactions," Applied Optics, vol. 13, p. 301 (1974). A preferred approach has been two-step irradiation by two lasers of differing wavelengths. The first laser, which is usually indicated to be an infrared laser, is used to selectively excite those molecules of a compound containing a particular isotope of an element. The second laser, which is usually indicated to be an ultraviolet laser, is used to further excite these molecules sufficiently for them to undergo a chemical reaction. As a practical matter, it is difficult to make the second laser as selective in its action as is the first laser so that it tends to excite all molecules of the compound containing the element of interest. Accordingly, for isotope separation or enrichment, a rather severe constraint placed on this second laser is that it be of a wavelength such that the combined excitation produced by it and the first laser is sufficient to induce chemical reaction, but the excitation produced by it alone will not induce chemical reaction. Heretofore the art has not taught that the first infrared laser, even if sufficiently intense, permits any more freedom of choice of the photon energy of the second laser, than the photon energy of the infrared laser. It certainly has not taught that if the infrared laser is sufficiently powerful it may readily eliminate any need for the second laser at all.

It is known that through use of single-photon absorption leading to photodissociation, selective or preferential laser-induced chemical reaction useful for isotope separation or enrichment may be achieved. The art teaches that for this single-photon technique, lasers operating in the visible or the ultraviolet are required.

A major disadvantage of the use of visible or ultraviolet lasers in the single photon or two-steps process is that the cross sections for direct, single photon absorption of light in these regions of the spectrum are quite small. In addition, it is quite difficult to achieve high laser power operation with any efficiency in these regions of the spectrum.

Our invention avoids or substantially overcomes the foregoing disadvantages of the prior art laser isotope separation methods. In its broad aspect, our invention encompasses a method of obtaining elemental values enriched in a particular isotope of an element which comprises the following steps. A fluid is obtained which comprises a first molecular species contained two or more isotopes of the element. This first molecular species has, in the presence of sufficiently intense laser light, a density of vibrational levels sufficient to allow multiple photon absorption to higher vibrational levels from a first lower vibrational level by absorption of the same energy as can excite this from molecular species to this first lower vibrational level. This fluid is irradiated with infrared laser light of a frequency which selectively excites to a first lower vibrational level only those molecules of this first molecular species containing a particular isotope of the element. This infrared laser light has an intensity sufficient to produce multiple photon absorption from the first lower vibrational level to a plurality of higher vibrational levels. The resulting multiple photon absorption produces a sufficiently energetic vibrational state in those molecules containing the particular isotope that they undergo chemical reaction. Alternatively, the resulting multiple photon absorption produces a sufficiently energetic vibrational state in those molecules containing the particular isotope that on irradiation by laser light of a second frequency they undergo chemical reaction. For isotope separation, the products of this chemical reaction are then separated from the unreacted molecules of the first molecular species.

The fluid may be either liquid or gaseous but is preferably a gas. The first molecular species may be diatomic or polyatomic but is preferably a polyatomic of four or more atoms. The chemical reaction may be either reaction of the highly excited molecules of the first molecular species with a second molecular species, or dissociation of the excited molecules of the first molecular species, or reaction of at least one of the dissociation products of the excited first molecular species with a second molecular species. The second molecular species may be admixed with the first molecular species and the mixture irradiated. In this case, the second molecular species does not strongly adsorb the laser light and is substantially nonreactive with the unexcited first molecular species but highly reactive with either the highly excited molecules of the first molecular species or with at least one of the dissociation products of the excited molecules of the first molecular species.

In accordance with the method of the invention, BCl.sub.3 is highly enriched in .sup.10 B by irradiating a mixture of natural BCl.sub.3 and hydrogen gas with an intense beam of pulsed CO.sub.2 laser radiation. Likewise, SF.sub.6 is highly enriched in .sup.34 S by irradiating a mixture of natural SF.sub.6 and hydrogen gas with an intense beam of pulsed CO.sub.2 laser radiation.

FIG. 1 is a schematic representation of selective singlephoton photodissociation in accordance with the teaching of the prior art.

FIG. 2 is a schematic representation of a two-step process for producing selective chemical reaction in accordance with the teaching of the prior art.

FIG. 3 is a schematic representation of multiple photon absorption resulting in selective chemical reaction in accordance with the method of the invention.

FIG. 4 is a schematic representation of a two-step process for producing selective chemical reaction in accordance with the method of the invention.

FIG. 5 shows the infrared absorption spectrum of SF.sub.6 at 300 K and matching CO.sub.2 lines.

FIG. 6 shows the enrichment factor .beta. for .sup.34 S as a function of the number of CO.sub.2 P(20) laser irradiation events parametric in gas pressure.

FIG. 7 shows the enrichment factor .beta. for .sup.34 S after 2000 shots of CO.sub.2 P(20) radiation plotted as a function of the reciprocal of the total gas pressure p.

FIG. 8 shows the infrared absorption spectra of BCl.sub.3 at 300 K and matching CO.sub.2 lines.

For every molecule, there is a minimum energy state called the ground state. Above this ground state are additional discrete energy states or levels. A molecule in the ground state or excited to a particular energy state may be excited to a higher energy state or level by absorption of radiation of the proper frequency. This change in energy state or level is called a transition. The probability of any transition occurring is given precisely by the Einstein relation:

where p is the transition probability, B is the Einstein coefficient and I(.nu.) is the radiation intensity. In accordance with quantum theory, molecular spectra show that the energy states of a molecule correspond to (a) different electronic orbits, (b) different internuclear distances and forces (c) rotations of some elements of the molecule with respect to other elements of the molecule, or (d) particular combinations of these. The optical spectrum of energy absorbed when the molecule is excited from one energy state to another may thus consist of (a) a vibrational spectrum, (b) a rotational spectrum, or (c) a compound spectrum, e.g., a vibrational-rotational spectrum or a vibrational-electronic spectrum. The combination of energy states is frequently called a manifold. Thus, for example, the vibrational spectrum results from the vibrational manifold.

For a polyatomic molecule no pure electronic absorption spectrum is possible. The reason for this is that in a given electronic state a polyatomic molecule must occupy a particular vibrational level. Each of the possible levels may be called a vibrational-electronic state or, as it has come to be known in the art, a vibronic state. Transitions from one vibronic state to a higher vibronic state result in a compound vibrational-electronic absorption spectrum.

In molecules containing different isotopes, the classical vibration freqencies differ according to ##EQU1## where .nu. is the vibration frequency, and .mu. is the reduced mass of the molecule. The reduced isotopic mass difference .DELTA..nu. is nonzero only if the isotopic atoms are in motion during the vibration. The resultant difference in frequency, .DELTA..nu., is called the isotope shift. The isotope shift causes the absorption spectrum of molecules containing one isotopic species of an element to be somewhat shifted from that of molecules containing another isotopic species of that element. It is readily shown by quantum mechanical arguments that a similar isotopic shift in frequency occurs with vibronic transitions. If the shift in the absorption spectrum is such that narrow nonoverlapping absorption lines exist for molecules containing different isotopic species, then light of the proper frequency will be absorbed only by and excite only molecules containing a particular isotopic species.

Laser isotope separation is possible because the frequency purity of laser light is sufficient to be resonantly absorbed by one isotopic species without affecting other nonresonant isotopes. That is to say, the laser lines are sufficiently narrow to effect absorption by an optical absorption state of one isotope only. This absorption by only one isotope is termed selective excitation.

As shown in FIGS. 1 and 2, the prior art teaches that this selective excitation may be used in either single step or two step processes for isotope separation by laser-induced chemical reaction. In the one-step process shown in FIG. 1, a single photon from a laser operating in the ultraviolet or the visible is sufficiently energetic to produce photodissociation in any molecule which it selectively excites. In the two-step process shown in FIG. 2, laser light of a first frequency selectively excites molecules containing a particular isotope and laser light of a second frequency then imparts sufficient additional excitation to the molecules excited by the light of the first frequency sufficient to cause only them to undergo chemical reaction. Although FIGS. 1 and 2 shows only vibrational excitation, the same considerations apply where electronic states are concerned.

The method of the present invention is predicated on the discovery that infrared laser radiation of sufficient intensity which is of a frequency which selectively excites only molecules containing a particular isotope may readily produce multiple photon absorption in molecules having a sufficient density of vibrational levels or states. In accordance with the invention, the multiple photon absorption may be used to produce (a) an excited vibrational state in the excited molecules at which chemical reaction occurs as shown in FIG. 3, or (b) an excited vibrational state at which a more advantageous frequency of a second laser may be used to produce selective chemical reaction in accorance with the two step process shown in FIG. 4. By chemical reaction is meant either dissociation of the highly excited molecules or reaction with another chemical species.

Multiple photon absorption results in a final state of the system possessing more energy than that of a single incident photon. The phrase "multiple photon absorption" denotes both sequential absorption of n photons where the system passes through real intermediate states and n-photon processes where n photons are absorbed simultaneously and the system may only pass through virtual intermediate states.

In the vibrational manifold, the spacing between vibrational levels changes as transistion to higher vibrational levels occurs. Accordingly, although it was understood that isotopically selective excitation could readily be made to occur between the ground state and a lower vibrational state by absorption of an infrared photon of the proper energy, no consideration had heretofore been given to the possibility of additional absorptions, of photons of this same energy, thereby transporting the molecule up the vibrational manifold to chemical reaction. The reason for this is that because of the change in the spacing of the vibrational levels, there was thought to be little likelihood that transitions to higher excited states would match the energy of these photons capable of producing the initial transition from the ground state.

The exact mechanism by this multiple photon absorption occurs is complex and not yet fully understood. However, it is known that due to anharmonic affects and the increased number of atoms in polyatomic molecules, the density of vibrational states is greatly increased so that the probability of matching the infrared photon energy with transitions between excited states is greatly increased. In addition, when the infrared laser radiation is sufficiently intense, i.e., has a sufficiently high power density, the electric field associated with this power density serves to further increase the probability of matching transitions between excited states through the process of optical frequency Stark broadening. Further enhancement of the multiple photon absorption is through the mechanism of resonantly enhanced multiphoton transitions in which there is no detectable lifetime of the molecule in the intermediate states. Finally the selection rules for multiple quantum absorptions are relaxed at high vibrational energies in some molecules.

To minimize collisional scrambling effects, it is essential that the molecular species being irradiated be present in a dilute fluid. This fluid may be in principle be a liquid of the molecular species or a solution of the molecular species in an appropriate solvent; however, it is preferred that the molecular species be present as a gas.

In one embodiment of the invention, multiple photon absorption of infrared photons results in an excited state at which the selectively excited molecules undergo chemical reaction with a second chemical species present in admixture with these molecules. In another embodiment, the multiple photon absorption results in a dissociation of the selectively excited molecules. In a third embodiment, the multiple photon absorption results in dissociation of the selectively excited molecules with at least one of the dissociation products undergoing rapid chemical reaction with a second chemical species present in the irradiated fluid. It will be readily understood by one of ordinary skill in the art that if a second chemical species is admixed with the first molecular species for the purpose of reacting with either highly excited molecules or dissociation products of the first molecular species, the second chemical species must not absorb the infrared laser radiation to any substantial degree and must be substantially inert to unexcited molecules of the first molecular species but highly reactive with highly excited molecules of this species or with at least one of the dissociation products of this species.

The method of the invention may be practiced with any infrared laser of sufficient power that has a frequency which permits selective excitation of a molecular species containing a particular isotope desired to be acted upon. The CO.sub.2 laser is particularly well suited for use with the method of this invention is that there are CO.sub.2 lines which coincide with spectral features of the volatile halides of many elements. Examples of such halides include SF.sub.6, SiF.sub.4, CH.sub.3 F, BCl.sub.3, NF.sub.3, and CF.sub.2 Cl.sub.2. Because hydrogen reacts rapidly with atomic halogens, CO.sub.2 laser irradiation of admixtures of such volatile halides with hydrogen is a particularly advantageous way to produce selective chemical reaction in the method of the invention.

Mixtures of SF.sub.6 and H.sub.2 with a constant SF.sub.6 mole fraction of 0.1 were irradiated with focused CO.sub.2 laser radiation. The CO.sub.2 laser was a conventional double discharge design delivering a multimode output of 1 to 2 joules in a 200 ns gain switched pulse. The radiation was focused with f/1.5 Ge and BaF.sub.2 lenses into sample cells containing the SF.sub.6 /H.sub.2 mixture at total pressures varied from 1 to 10 torr. The peak laser power at focus was estimated at .gtorsim. 6 .times. 10.sup.9 watts/cm.sup.2. The sample cells were brass right circular cylinders 7.62 cm long with an i.d. of 2.54 cm fitted with KCl windows giving a total gas volume of 38.6 cc. The sample cell length and lens f number are dictated by the damage threshold of the KCl windows.

Following irradiation by a specified number of laser pulses the residual SF.sub.6 was analyzed by scanning mass peaks 127 and 129 with a quadrupole mass spectrometer. These masses correspond to the .sup.32 S and .sup.34 S isotopes in the dominant ion SF.sub.5.sup.+ produced in the mass spectrometer. Analysis of the spectrum is simplified by the presence of only one stable F isotope and a separation of two mass units between the desired peaks thus eliminating possible confusion due to H atom attachment to the ions of interest. Scans of interrupted SF.sub.6 /H.sub.2 mixtures yielded a normal ratio of .sup.34 S/.sup.32 S .tbd. R.sub.o = 0.044 .+-. 0.002.

Table I lists the enrichment factor .beta.=R/R.sub.o of the unreacted SF.sub.6 as a function of the number of irradiation events, gas pressure, fraction reacted and laser wavelength. The quantity R is defined as the measured .sup.34 S/.sup. 32 S ratio following irradiation. FIG. 5 shows the infrared absorption spectrum of SF.sub.6 at 300 K and the position of various CO.sub.2 laser lines relative to that spectrum. In FIG. 5, the solid curve is the prediction of a band contour model using a 0.5 cm.sup.-1 spectral window; the dashed curve is the spectrum of a SF.sub.6 --AR mix (X(SF.sub.6) = 0.005) obtained on a Beckman IR-20AX spectrometer with a 0.8 cm.sup.-1 spectral window; and the solid circles show spectral results obtained with a CO.sub.2 laser. The rationale for choosing the various CO.sub.2 lines was based upon recent examination of the temperature dependence of absorption of CO.sub.2 laser lines by SF.sub.6. This work indicated that the P(20) and P(32) lines are absorbed primarily in hot bands thus they should greatly enhance the vibrational energy of the absorbing molecules. On the other hand, very high resolution spectroscopy of cooled SF.sub.6 reveals that the P(16) line is coincident with the ground state .nu..sub.3 Q-branch. Thus it falls in a region where the density of states is very high. It was felt that the variations of .beta. among the selected lines might be indicative of the enrichment mechanism. However, from the results in Table I the variation with wavelength is not great and may simply be the result of line-to-line variations in laser output.

FIG. 6 displays .beta. as a function of laser pulses parametric in gas pressure. There is a near linear increase in .beta. with the number of laser pulses. In addition the fractional increase in .beta. per radiative event is approximately equal to the ratio of focal volume to total cell volume.

The greatest improvement in enrichment is achieved by reducing the total gas pressure as shown in FIG. 7. This effect is partially accounted for by the greater extent of reaction at low pressure (Table I). It is also indicative of a collisional process (most probably V-V transfer in SF.sub.6) inducing scrambling of the isotopically selective excitation. Phenomenologically one observes that .beta. is proportional to exp(1/p). Scrambling may also be induced by chemical reactions of highly excited SF.sub.6 produced just outside the region where the power exceeds the multiple photon absorption threshold.

In this example, the method of the invention produces better than a factor of 33 increase in the ratio of .sup.34 S/.sup.32 S in SF.sub.6 that has been irradiated in the presence of H.sub.2 by an intensely focused CO.sub.2 laser. The mechanism of enrichment appears to be a multiple photon dissociation of .sup.32 SF.sub.6 with the liberated F being scavenged by H.sub.2 to form HF.

From the data in Table I the threshold power required to produce enrichment appears to be greater than 5 MW/cm.sup.2. A peak power of 200 MW/cm.sup.2 does produce some enrichment but is not nearly as efficient as the 6 GW/cm.sup.2 pulses.

Mixtures of natural BCl.sub.3 and hydrogen were irradiated with focused CO.sub.2 laser radiation from a CO.sub.2 laser operating on the P(20) line using the same setup as described in Example I for SF.sub.6 irradiation. For some irradiations the admixture consisted of 20% BCl.sub.3 and for others 40% BCl.sub.3 with the ramainder hydrogen. Natural BCl.sub.3 contains 80% .sup.11 B and 20% .sup.10 B. It also contains 75% .sup.35 Cl and 25% .sup.37 Cl. Thus mass spectrometric analysis of natural BCl.sub.3 reveals the following mass peaks corresponding to the indicated isotopic ratio.

Total pressures of the hydrogen -- BCl.sub.3 mixture were varied from 1.5 to 5 torr for each mixture. Where the pressure was greater than 3 Torr, little or no chemical reaction occurred. At pressures of 3 torr or less, however, mass peaks 115, 117 119, 120, 121, and 122 are removed after 1000 pulses and the residual remaining BCl.sub.3 contains 70% .sup.11 B.sup.35 Cl.sub.3 and 30% .sup.11 B.sup.35 Cl.sub.2.sup.37 Cl. Further, the isotopic combinations which are removed were shown to be a function of gas pressure and gas temperature. That is to say, by a proper choice of these parameters, the CO.sub.2 radiation will provide selective synthesis of isotopic compounds. Thus selective enrichment and separation of desired boron and/or chlorine isotopes may be achieved in accordance with the method of this example.

The various features and advantages of the invention are thought to be clear from the foregoing description. However, various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise will many variations and modifications of the preferred embodiment, illustrated, all of which may be achieved without departing from the spirit and scope of the invention as defined in the following claims.