Resonant collisional radiative transfer of the vibrational energy between molecules subjected to infrared laser multiphoton excitation in a two-component medium has been studied. Experiments have been carried out with BCl3 molecules of the natural isotopic composition in a mixture with CH3F molecules, which are optically active sensitizers and acceptors of radicals. Both types of molecules resonantly absorb laser radiation. Resonant collisional radiative energy transfer from CH3F molecules to 10BCl3 ones has been observed. It has been shown that this process significantly increases the dissociation yield of 10BCl3 and 11BCl3 molecules compared to the dissociation yield with neutral acceptors of radicals. Dependences of the dissociation yields of BCl3 molecules on the pump laser frequency have demonstrated a structure that closely correlates with the structure of the infrared absorption spectrum of CH3F molecules. The method has been described and the experimental results have been reported. Conditions for efficient resonant collisional radiative transfer of the vibrational energy between molecules subjected to infrared laser multiphoton excitation have been discussed.

The parameters of boron isotope-selective IR multiphoton (MP) dissociation of 2-chloroethenyl dichloroborane molecules (HClC=CHBCl2) under the action of pulsed CO2 laser radiation were measured. The laser radiation frequencies were tuned in resonance with the low-frequency wing of the IR absorption band of HClC=CH10BCl2 molecules. The dependences of the dissociation yields and selectivity of HClC=CH10BCl2 and HClC=CH11BCl2 molecules on the intrinsic gas pressure, laser radiation intensity and frequency, and N2 inert buffer gas and BCl3 resonance buffer gas pressures were obtained. At sufficiently high intrinsic pressures of HClC=CBCl2 (66.5−266 Pa) and moderate laser radiation intensity (2–3 J/cm2), high MP dissociation parameters are achieved: dissociation yields of HClC=CH10BCl2 molecules of up to 6–9

Significant interest in the laser separation of uranium isotopes at the turn of the century stimulated the search for and development of a number of methods and approaches that use both atomic and molecular mechanisms of isotope separation. Effective laser methods for the separation of uranium isotopes are currently under active development in many countries. New approaches to the molecular laser isotope separation (MLIS) of uranium are reviewed. They are based on resonant isotopeselective multiphoton excitation of high vibrational states (2v3 and 3v3) of 235UF6 and 238UF6 molecules in gas -dynamically cooled molecular flows by bichromatic IR laser radiation and the subsequent dissociation of the excited molecules by the same laser pulses. The foundations of these approaches are analyzed. The results of experiments on twoand three -photon excitation of SF6 molecules, whose spectroscopic properties are similar to those of UF6 molecules, into the 2v3 and 3v3 vibrational states by, respectively, twoand three -frequency radiation of pulsed CO2 lasers are presented and discussed. Specific setups and parameters of resonant twoand three -photon isotope -selective excitation of 235UF6 and 238UF6 molecules into the 2v3 and 3v3 vibrational states by bichromatic IR radiation from two pulsed CF4 lasers and two para-H2 lasers with emissions in the 16-mu m region are proposed and analyzed. A method is considered for the isotope -selective excitation and dissociation of UF6 molecules in a mixture with a sensitizer (SF6 molecules) under nonequilibrium thermodynamic conditions of a shock wave. Low -energy MLIS methods for uranium based on the proposed approaches are shown to be feasible.

A strong increase in the efficiency of the isotope-selective infrared laser multiphoton dissociation of 11 BCl 3 molecules in the natural mixture with 10 BCl 3 by radiation of a pulsed CO 2 laser in the case of admixture of SF 6 molecules, which serve as a sensitizer and simultaneously acceptors of radicals, Cl atoms formed in the dissociation of BCl 3 molecules, has been detected. The yield and selectivity of dissociation of 11 BCl 3 molecules increase by several times and the threshold energy density for the dissociation of molecules decreases significantly in the case of their irradiation in the mixture with SF 6 compared to irradiation without SF 6 . This property allows the single-frequency isotope-selective dissociation of 11 BCl 3 molecules by unfocused laser radiation at a moderate energy density (≈3–5 J/cm 2 ), which is important and relevant for the practical implementation of the laser separation of boron isotopes.

Using the spectroscopic data on the 235UF6 and 238UF6 molecules and on the lasing frequencies of CF4 and para-H2 lasers and recent results, a method has been proposed to increase the efficiency of the isotope-selective infrared laser dissociation of 235UF6 molecules under nonequilibrium thermodynamic shock conditions. The method involves two processes: (i) the resonant multiphoton excitation of 235UF6 molecules to the 3ν3 or 2ν3 vibrational states by the bichromatic infrared radiation of two CF4 or para-H2 lasers and (ii) the irradiation of 235UF6 molecules with SF6 molecules serving as sensitizers resonantly absorbing the radiation of these lasers. The essence of the method has been described. Schemes and parameters for isotope-selective dissociation of 235UF6 molecules using this method has been presented.

Based on spectroscopic data for the overtone states of the ν 3 vibration in UF 6 molecules and lasing frequency of CF 4 - and para-H 2 -lasers, which emit near 16 μm, the possibility of resonance two-photon isotope-selective excitation of 2ν 3 vibrational states in UF 6 molecules by bichromatic IR radiation from these lasers has been analyzed. Schemes and parameters for the excitation of 238 UF 6 and 235 UF 6 molecules in the 2ν 3 state by two lasers with lasing frequencies detuned by 3.5–13.0 cm –1 from the Q branches in the linear absorption spectra of UF 6 molecules in a gasdynamically cooled molecular flow are suggested. At the same time, the sum of these frequencies (ν L 1 + ν L 2 ) is equal to the frequency of the 0ν 3 → 2ν 3 vibrational transition in UF 6 molecules. If both lasers act on molecules simultaneously, there appears the possibility of their selective excitation from the ground vibrational state 0ν 3 to excited states 2ν 3 . The isotope-selective excitation of overtone vibrational states 2ν 3 in 238 UF 6 and 235 UF 6 molecules using the method suggested here may form a basis for low-energy laser separation of uranium isotopes.

Effective radiation–collision involvement of molecules that do not absorb laser radiation in resonance with a laser field in a two-component molecular medium that includes molecules absorbing laser radiation and is subjected to intense infrared laser radiation has been detected. Experiments have been performed with the CF2HCl/CF3Br mixture (at the 1/1 pressure ratio), where molecules are under nonequilibrium thermodynamic conditions in a shock wave, which has been formed in front of the solid surface on which a gas-dynamically cooled intense pulsed molecular beam is incident. Molecules have been excited by the pulsed radiation of a СО2 laser. A method has been described and the first results have been reported. Effective dissociation of CF2HCl molecules (with the yield β ≥ 10–15%) in the CF2HCl/CF3Br mixture irradiated by the CO2 laser detuned by more than 15–25 cm–1 from the center of the infrared absorption band of CF2HCl molecules vibrationally cooled in the shock wave has been detected at quite low excitation energy densities (Φ ≤ 0.5–1.0 J/cm2) at which dissociation of CF2HCl molecules in a pure gas hardly occurs. The results can be applied to separate isotopes using the method of isotope-selective infrared laser dissociation of molecules.

About half a century ago, active research began on the use of lasers for the separation of isotopes, including uranium isotopes. In a number of highly developed countries, projects on laser separation of uranium isotopes were initiated. However, in the late 1990s, these projects were closed. At the same time, research on laser separation of uranium isotopes is being carried out in a number of countries at the present time. Research today is mainly focused on the development of low energy methods. A review of the results of studies on molecular laser isotope separation (MLIS) of uranium is presented. MLIS methods, including low-energy methods, as well as the physicochemical processes underlying them, are considered. The infrared and ultraviolet absorption spectra of UF6 molecules, which form the basis of uranium MLIS methods, are presented and dis-cussed. Lasers that have been developed and used to excite and dissociate UF6 molecules are briefly described. The results of studies on laser dissociation of UF6 molecules using different excitation schemes and laser separation of uranium isotopes are presented and analyzed. Applications of low-energy MLIS methods, including SILEX technology, for the separation of uranium isotopes are considered. In the context of molecular laser separation of uranium isotopes using low-energy methods, the recently obtained results on the separation of isotopes in SF(6 )and CF3Br molecules, which are close analogs of UF6 molecules in a number of spectroscopic properties, are presented and analyzed. Approaches that can be a promising alternative to low-energy MLIS methods are discussed. Data on the efficiency of some laser methods and their comparison with each other are given. A brief historical excursion into research on laser separation of uranium isotopes is given.

Collisional radiative involvement of molecules not absorbing IR laser radiation into resonance with a laser field has been investigated for the case when nonabsorbing and absorbing molecules are laser-irradiated in a two-component medium. Experiments have been conducted with a CF2HCl/CF3Br mixture at a 1/1 pressure ratio. Molecules have been excited by a pulsed CO2 laser. Two types of experiments have been considered: (i) molecules were irradiated under nonequilibrium thermodynamic conditions due to a compression shock wave arising before the solid surface when it was subjected to a supersonic pulsed gasdynamically cooled molecular flux and (ii) molecules were irradiated under static conditions with a gas in the cell kept at room temperature. It has been found that when molecules vibrationally cooled in the compression shock wave (in this case, their IR absorption bands are narrow; the FWHM is 7–8 cm–1) are irradiated, initially nonabsorbing CF2HCl molecules get effectively involved into resonance with the laser field. Their effective dissociation (with dissociation yield β ≥ 10–15%) has been discovered when the CF2HCl/CF3Br mixture was irradiated under the condition of CO2 laser frequency detuning by more than 15–25 cm–1 from the center of the IR absorption band of molecules vibrationally cooled in the compression shock wave. If molecules are irradiated at room temperature (the gas temperature in the cell), in which case the IR absorption band of molecules is rather wide (25–30 cm–1), the collisional radiative involvement of CF2HCl molecules into interaction with the laser field, albeit less pronounced, persists.

We consider the possibility of resonant isotope-selective population of states with υ = 3 of the vibrational mode ν 3 of UF6 molecules as a result of three-photon bichromatic excitation of the molecules by IR radiation from two pulsed CF4 lasers and two pulsed para-H2 lasers. We propose specific schemes and analyse the possibility of exciting the 238UF6 and 235UF6 molecules into states with υ = 3 of the vibrational mode ν 3 using two CF4 lasers (or para-H2 lasers) generating in the region of 16 μm at frequencies that are significantly (by ∼3.5 – 17 cm−1) detuned from the linear absorption bands at the frequencies of ν 3 vibration of these molecules in a gas-dynamically cooled molecular flow. However, the sum of the doubled lasing frequency 2ν L1 of the first laser and the frequency ν L2 of the second laser is equal to a frequency of the transition υ = 0 → υ = 3 of the vibrational mode ν 3 of the UF6 molecules. When both laser pulses coincide in time, the possibility of selective excitation of UF6 molecules from the ground vibrational state with υ = 0 to the state with υ = 3 of the vibrational mode ν 3 is realised. The proposed schemes for the excitation of UF6 molecules to states with υ = 3 are compared with the previously implemented schemes for the effective excitation of states with υ = 3 of the vibrational mode ν 3 (F 1) and states with υ = 2 of the vibrational mode ν 3 (A 1) of SF6 molecules by three- and two-frequency radiation of pulsed CO2 lasers, respectively.

We report on the results of investigation of laser isotope-selective IR dissociation of molecules (CF3Br and CF2HCl as examples) characterized by a small (less than 0.25 cm–1) isotopic shift in the IR absorption spectra in nonequilibrium thermodynamic conditions of a compression shock (shock wave) formed in front of the solid surface under the action of an intense incident pulsed gasdynamically cooled molecular beam. Experiments were made using pure CF3Br and CF2HCl gases as well as the CF3Br/CF2HCl mixture with a pressure ratio of 1/1 for the formation of the molecular flow and a shock wave. It is found that the efficiency of dissociation of molecules in the shock wave and in the beam incident on the surface is much higher than in an unperturbed flow. It is shown (for CF3Br as an example) that the dissociation yield in the case of excitation of molecules in the shock wave and in the beam incident on the surface increases strongly (by 5–10 times) as compared to the dissociation yield in the unperturbed flow, while the dissociation threshold decreases substantially (by 3–5 times). It is also established that in the case of irradiation of molecules in the mixture, a mutual strong increase in the efficiency of their dissociation is observed as compared to the case when molecules are exposed to radiation separately. This makes it possible to induce isotope-selective laser IR dissociation of molecules for low excitation energy densities (Φ ≤ 1.0–1.5 J/cm2), thus improving the selectivity of the process. This is demonstrated for dissociation of these molecules in the shock wave, which is selective in chlorine and bromine isotopes. We obtained enrichment coefficients Kenr(35Cl/37Cl) = 0.90 ± 0.05 in the residual CF2HCl gas and Kenr(79Br/81Br) = 1.20 ± 0.09 in the formed product (Br2) under laser irradiation of the CF2HCl/CF3Br = 1/1 mixture and CF3Br molecules at the 9R(30) CO2-laser line (at a frequency 1084.635 cm–1) for energy density Φ ≈ 1.3 J/cm2. The application of the results for laser isotope separation is considered.

This chapter concerns one of the new directions of investigation which have been developed during recent years, namely laser infrared (ir) spectroscopy of highly vibrationally excited molecules. It considers transitions between vibrational molecular levels lying near or above the dissociation energy D in the ground electronic state, that is, in the energy range E≳D. ir spectroscopy of transitions between such strongly excited vibrational states has become possible owing to the development of methods of producing highly excited molecules, in particular the method of multiphoton excitation of polyatomic molecules in a strong ir laser field. ir spectroscopy is one of the sources of most complete information about strongly vibrationally excited molecules, since a linear absorption spectrum allows the characterization of any quantum mechanical system. In spite of some pitfalls which are quite natural in initial work, the experiments provided the foundation-stone for the ir spectroscopy of polyatomic molecules having vibrational energies of the order of the dissociation limit.

A method for isotope-selective infrared laser photodissociation of molecules characterized by a small (less than 0.25 cm−1) isotopic shift in the infrared absorption spectra is demonstrated by the example of the CF3Br molecule. The method is based on infrared laser excitation of molecules in a pulsed gas-dynamically cooled molecular flow interacting with a solid surface. The dissociation of molecules has been studied under three different conditions of their optical excitation: (i) in an undisturbed molecular flow, (ii) in a flow incident on the surface, and (iii) in the shock wave region formed in front of the surface upon its interaction of the incident molecular flow. It is shown that the dissociation yield upon the excitation of molecules in the shock wave and in the flow incident on the surface is 5–10 times higher than the dissociation yield upon excitation in the undisturbed flow, whereas the dissociation threshold in the former case is 3–5 times lower than that in the latter case. This allows the observation of the isotope selective dissociation at low energy densities of the exciting laser pulse (Φ ≤ 1.5–2.0 J/cm2). The enrichment factor Kenr(79Br/81Br) for the dissociation product Br2 is 0.85 ± 0.07 and 1.20 ± 0.09 for excitation of molecules in the incident flow and in the shock wave, respectively.

We have revealed a strong (by a factor of 2 to 5) mutual increase in the yield of IR molecular dissociation (by the example of CF 2 HCl and CF 3 Br) and a significant (by a factor of 1.5 to 3) lowering of dissociation thresholds in the nonequilibrium thermodynamic conditions of compression shock in the irradiation of the molecules by resonance IR laser radiation in the bimolecular mixture in comparison with their individual irradiation. This opens up the possibility to perform efficient isotope-selective IR dissociation of molecules at lower excitation energy densities ( Φ ⩽ 1.5 – 2.0 J cm −2 ) and thereby to improve the dissociation selectivity. This was demonstrated by the example of chlorine- and bromine-isotope selective dissociation of the specified molecules, which are characterised by quite small (less than 0.25 cm −1 ) isotope shifts in the IR vibrational absorption spectra excited by laser radiation. The enrichment coefficients K enr ( 35 Cl / 37 Cl) = 0.90 ± 0.05 in the residual CF 2 HCl gas and K enr ( 79 Br / 81 Br) in the resultant Br 2 product are obtained when the CF 2 HCl : CF 3 Br = 1 : 1 molecular mixture and CF 3 Br molecules, respectively, are irradiated by the 9R(30) CO 2 laser line (frequency, 1084.635 cm −1 ) at an energy density Φ ≈ 1.3 J cm −2 .

A strong increase in the efficiency of isotope-selective infrared laser dissociation of molecules (by an example of CF2HCl) under nonequilibrium thermodynamic conditions in a shock wave has been reached by using a mixture with a resonantly absorbed gas (CF3Br). It has been shown that the dissociation yield of CF2HCl molecules irradiated in a mixture with CF3Br molecules is more than a factor of 5–10 higher than the dissociation yield in the case of their irradiation in the form of pure gas. The dissociation threshold of CF2HCl molecules in the mixture with CF3Br molecules also decreases significantly from ≈1.5−2.0 to 0.2−0.3 J/cm2). This allows the efficient isotope-selective infrared dissociation of molecules at low exciting energy densities (Φ≤ 1.0−1.5 J/cm2), as well and increasing the selectivity of the process. The method has been described and the first results have been presented. The dissociation of CF2HCl molecules selective in the 35Cl and 37Cl isotopes with the enrichment factor Kenr(35Cl/37Cl) = 0.90 ± 0.05 is implemented when a CF2HCl/CF3Br = 1/1 mixture is irradiated by the 9R(30) (1084.635 cm−1) line of a CO2-laser at an energy density of Φ ≈ 1.3 J/cm2.

AbstractWe present the results of an investigation into the interaction of SF_6 molecules and clusters in a molecular beam with resonant IR laser radiation at different stages of the beam evolution along the axis of its propagation. The beam has been formed as a result of gas-dynamic expansion of a mixture of SF_6 with argon carrier gas during expansion from a pulsed nozzle. The experimental setup and the investigation method are described. It has been shown that selective vibrational excitation of SF_6 molecules with a specific sulfur isotope by a CO_2 laser near the nozzle edge causes suppression of the clustering process of these isotopic molecules. Selective IR excitation of clusters under the conditions of the formed cluster beam leads to isotopically selective dissociation of clusters. Depending on the experimental conditions including different distances of the irradiation zone of particles from the nozzle edge, the results of measuring the efficiency and selectivity of molecular clustering suppression and cluster dissociation processes are presented. It has been shown that both of these processes make it possible to achieve high selectivity values for the ^32S and ^34S sulfur isotopes. In the case in which the clustering of SF_6 molecules was selectively suppressed, selectivity values α ≥ 25–30 have been obtained. Upon selective dissociation of (SF_6)_2 dimers under similar expansion conditions of the gas mixture, selectivity values α ≥ 20–25 for ^32SF_6^32SF_6 dimers with respect to ^34SF_6^32SF_6 dimers have been obtained. Particular attention has been paid to measurements at a high dilution of SF_6 in argon under conditions of predominant formation of (SF_6)_ m Ar_ n mixed clusters. The potential of using studied processes as a basis for the technology of the laser isotope separation are discussed.

The results of studies on the isotope-selective dissociation of homogeneous and mixed van der Waals (SF6)mArn clusters induced by infrared laser radiation are presented. The method is based on the selective vibrational excitation of clusters by an infrared laser, which leads to their heating and dissociation. An experimental setup and procedure are described. The results of measurements of the efficiency and selectivity of cluster dissociation at different pressures and gas compositions above the nozzle, as well as various parameters of laser radiation, are presented. The dependences of the efficiency and selectivity of cluster dissociation on the frequency and power of the exciting laser radiation on the composition and pressure of gas above the nozzle, as well as on the distance of the irradiation zone of the particles on the nozzle exit are obtained. It is shown that the frequency of laser radiation and the composition and pressure of gas above the nozzle significantly affect the parameters of selective infrared dissociation of clusters. Experiments on isotope-selective dissociation of both homogeneous (SF6)m clusters and mixed (SF6)mArn clusters are performed. It was found that, with the dissociation of (SF6)mArn clusters, a rather high selectivity can be reached. Thus, in the case of an SF6/Ar mixture at a pressure ratio of 1/200, for the selectivity of dissociation of (32SF6)2 clusters with respect to $$^{{34}}{\text{SF}}_{6}^{{32}}$$ SF6 clusters, the values ≥10–20 are obtained under conditions of irradiation by a beam on the 10P(32) laser line (at a frequency of 932.96 cm–1). Conditions are found when the values of efficiency and selectivity of dissociation of homogeneous (SF6)m clusters and mixed (SF6)mArn clusters are optimal.

The paper presents the results of studies of IR laser-induced bromine-isotope-selective dissociation of small mixed (CF3Br)(m)Ar-n van der Waals clusters (m = 1, 2 and 1 <= n <= 5 is the number of molecules and atoms in the clusters, respectively). The experiments used a pulsed CO2 laser to excite clusters and a quad-rupole mass spectrometer to detect a molecular cluster beam. The research method is based on the selective vibrational excitation of clusters by IR laser radiation, leading to their predissociation. The possibilities of implementing bromine-isotope-selective dissociation of clusters are considered. The experimental setup and research method are briefly described. The results of determining the cluster dissociation efficiency and selectivity as functions of the energy and frequency of laser radiation are presented. It is shown that resonant vibrational excitation of clusters makes it possible to induce their isotope-selective dissociation. Thus, using a CF3Br/Ar mixture with a pressure ratio of 1/200, the enrichment factors of the Br-79 isotope are found to be K-enr(Br-79) = 1.15 +/- 0.04 and 0.95 +/- 0.03 under cluster irradiation conditions on the 9R(30) (v = 1084.635 cm(-1)) and 9R(24) (v = 1081.087 cm(-1)) lines, respectively The achieved dissociation selectivities for the clusters are, respectively, 1.16 +/- 0.05 and 0.95 +/- 0.04. The results obtained suggest that this method can be used to separate isotopes in molecules containing isotopes of heavy elements, which have a slight isotopic shift in the IR absorption spectra.

We present the results of an investigation into the interaction of SF6 molecules and clusters in a molecular beam with resonant IR laser radiation at different stages of the beam evolution along the axis of its propagation. The beam has been formed as a result of gas-dynamic expansion of a mixture of SF6 with argon carrier gas during expansion from a pulsed nozzle. The experimental setup and the investigation method are described. It has been shown that selective vibrational excitation of SF6 molecules with a specific sulfur isotope by a CO2 laser near the nozzle edge causes suppression of the clustering process of these isotopic molecules. Selective IR excitation of clusters under the conditions of the formed cluster beam leads to isotopically selective dissociation of clusters. Depending on the experimental conditions including different distances of the irradiation zone of particles from the nozzle edge, the results of measuring the efficiency and selectivity of molecular clustering suppression and cluster dissociation processes are presented. It has been shown that both of these processes make it possible to achieve high selectivity values for the 32S and 34S sulfur isotopes. In the case in which the clustering of SF6 molecules was selectively suppressed, selectivity values α ≥ 25–30 have been obtained. Upon selective dissociation of (SF6)2 dimers under similar expansion conditions of the gas mixture, selectivity values α ≥ 20–25 for 32SF632SF6 dimers with respect to 34SF632SF6 dimers have been obtained. Particular attention has been paid to measurements at a high dilution of SF6 in argon under conditions of predominant formation of (SF6)mArn mixed clusters. The potential of using studied processes as a basis for the technology of the laser isotope separation are discussed.