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.

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.

The infrared (IR) laser radiation control of the clustering of CF3Br molecules during the gas-dynamic expansion of a CF 3 Br/Ar mixture at the exit from a nozzle is investigated. Prominence is given to studying the possibility of bromine-isotope-selective suppression of the clustering of CF 3 Br molecules due to their resonance vibrational excitation in the gas-dynamic expansion zone near the nozzle. A continuous CO 2 laser is used in experiments to excite molecules and clusters in a beam, and a quadrupole mass spectrometer is used to detect them. The experimental setup and the experimental technique are described. The dependences of the efficiency of molecule clustering suppression on the exciting laser radiation parameters, the gas parameters (composition, pressure) above the nozzle, and the distance from the nozzle exit section to a molecule irradiation zone are obtained. Bromine-isotope-selective suppression of molecule clustering is shown to occur at the exit from the nozzle due to the resonance vibrational excitation of gas-dynamically cooled CF 3 Br molecules. When CF 3 Br/Ar mixtures are used at pressure ratios p (CF 3 Br): p (Ar) = 1: 10 and 1: 30, the enrichment of a cluster beam by bromine isotopes are K enr ( 81 Br) ≈ 1.18 ± 0.09 and 1.12 ± 0.07 during the 9 R (30) laser line (1084.635 cm –1 ) irradiation of a jet. The clustering suppression selectivity is α ≈ 1.18 when the mixture at the pressure ratio p (CF 3 Br): p (Ar) = 1: 10 is used. These results suggest that the proposed method can selectively control the clustering of the molecules containing the heavy element isotopes that have a small isotope shift in IR absorption spectra (OsO 4 , WF 6 , UF 6 ).

We present the study of suppressing the clustering of CF3Br molecules with argon atoms under gas-dynamic expansion of the CF3Br -Ar mixture at a nozzle exit by a cw CO2 laser. The main attention is paid to the analysis of possibility of implementing the selective control of bromine isotope clustering. The method relies on the vibrational excitation of molecules with the laser radiation, which leads to the suppression of their clustering with argon atoms. The experimental setup and the method of study are briefly described. The results of evaluation of the efficiency and selectivity of suppression of molecular - atomic clustering for different compositions of the gas above the nozzle and separations between the irradiation zone and the nozzle exit section are presented. It is shown that under the resonance vibrational excitation of gas-dynamically cooled CF3Br molecules at the nozzle exit one can implement bromine isotope selective suppression of clustering of molecules with argon atoms. It is found that by controlling the clustering of CF3Br molecules with argon atoms one can implement considerably higher enrichment and selectivity factors than in the case of controlling the clustering of CF3Br molecules with each other. Thus, using the CF3Br -Ar mixture (the ratio of pressures 1 : 100 and 1 : 200) the enrichment factors for the bromine isotopes K-enr(Br-81) = 1.50 +/- 0.13 and 1.30 +/- 0.09, respectively, were obtained under the conditions of irradiating the jet at the 9R(30) line of the laser (v = 1084.635 cm(-1)). The selectivity a of the suppression of clustering of CF3Br molecules with argon atoms, achieved in this case, amounted to 4.02 +/- 0.19 and 2.31 +/- 0.11, respectively. The results show that the method allows one to selectively control the clustering of molecules, comprising isotopes of heavy elements that possess a small isotopic shift in the IR absorption spectra.

The control of clustering in molecular beams with the help of IR lasers is experimentally studied. Studies were performed with a molecular CF 3 I gas diluted with argon or xenon using a cw CO 2 laser. The control of clustering is based on the resonance vibrational excitation of molecules or clusters by the IR laser radiation near the nozzle exit. Depending on the distance from the excitation region to the nozzle cut, the irradiation of the molecular beam either suppresses the clustering of resonantly excited molecules (when the beam is irradiated near the nozzle exit where clustering occurs) or causes the dissociation of small clusters (when the beam is irradiated away from the nozzle where clustering is at the growth stage). The suppression of molecular clustering and the dissociation of clusters in beams were studied by measuring and analyzing the integrated intensities of ion peaks of cluster fragments with a time-of-flight spectrometer. The efficiencies of clustering suppression and cluster dissociation were studied as functions of exciting laser radiation parameters, the beam irradiation geometry, gas parameters over the nozzle, and the nozzle construction. It is shown that the efficiencies of clustering suppression and cluster dissociation strongly depend on the exciting laser radiation intensity, the nozzle construction, and the distance from the irradiation region to the nozzle cut. Parameters providing the most efficient control of clustering suppression and cluster dissociation are found.

We have investigated the processes of excitation and ionisation of monomers and clusters of and molecules under the action of femtosecond laser radiation at the wavelengths of 266, 400 and . It is concluded that the nature of the excitation of free molecules and clustered molecules by femtosecond pulses is different. The simulation of the ionisation yield of the objects under study has shown that the multiphoton ionisation is the key mechanism in the case of free molecules, while the field ionisation may play a significant role for clusters, in particular, in the case of ionisation at the wavelength of .

The fragmentation of homogeneous (CF3I) n clusters (where n ≤ 45 is the average number of molecules in a cluster) in a molecular beam, as well as (CF3I) n clusters inside of large (Xe) m clusters (where m ≥ 100 is the average number of atoms in a cluster) or on their surface, by laser ultraviolet radiation has been studied. It has been found that the indicated three types of (CF3I) n clusters have different stabilities with respect to fragmentation and strongly different dependences of the fragmentation probability on the energy of ultraviolet radiation. Fragmentation at low energies and the weakest energy dependence of the probability of fragmentation are observed for homogeneous clusters, a stronger dependence is characteristic of (CF3I) n clusters localized inside (Xe) m clusters, and the strongest dependence is observed for (CF3I) n clusters on the surface of (Xe) m clusters. Possible reasons for such a character of the observed dependences have been discussed.

The excitation and ionization of CF 3 I molecules and their clusters by femtosecond UV laser pulses is studied. It is concluded that the types of excitation of free CF 3 I molecules and their clusters by femtosecond UV laser pulses are different. The composition and kinetic energy of ion products observed upon the ionization of (CF 3 I) n clusters by femtosecond pulses are found to differ considerably from those obtained upon ionization by nanosecond pulses. It is shown that the molecular I 2 + ion is produced in reactions induced in (CF 3 I) n clusters by UV radiation. Using the pump-probe method, we found the two channels of producing I 2 + ions with characteristic times τ 1 ≈ 1 ps and τ 2 ≈ 7 ps. A model of the reactions under study proposed in the paper is consistent with our experimental results.

Intracluster reactions that are induced in (CF 3 I) n clusters by femtosecond ultraviolet radiation, including the reaction of the formation of the I 2 + molecular ion, have been directly observed. It has been shown that there are two channels of the formation of I 2 + ions with the characteristic times τ 1 ≈ 1 ps and τ 2 ≈ 7 ps. A model of these reactions has been proposed that is in good agreement with the experimental data.

The separation of nitrogen isotopes is studied upon successive single-photon IR excitation and UV dissociation of ammonia molecules. The excitation selectivity was provided by tuning a CO2 laser to resonance with 14NH3 molecules [the 9R(30) laser line] or with 15NH3 molecules [the 9R(10) laser line]. Isotopic mixtures containing 4.8% and 0.37% (natural content) of the 15NH isotope were investigated. The dependences of the selectivity and the dissociation yield for each isotopic component on the buffer gas pressure (N2, O2, Ar) and the ammonia pressure were obtained. In the limit of low NH3 pressures (0.5—2 Torr), the dissociation selectivity α(15/14) for 15N was 17. The selectivity mechanism of the IR+UV dissociation is discussed and the outlook is considered for the development of the nitrogen isotope separation process based on this approach.

The one-photon IR excitation and subsequent UV dissociation of ammonia molecules selective with respect to nitrogen isotopes were studied. The selectivity of vibrational excitation is achieved by tuning CO 2 laser radiation to resonance with 14 NH 3 or 15 NH 3 molecules. The dependences of the yield of dissociation for each isotopic component and the selectivity on the buffer gas (N 2 , O 2 , Ar) pressure, the partial pressure of ammonia, and the time of delay between IR and UV laser pulses were established. At low pressures (67–270 Pa) of the isotopic mixture with a 15 N concentration of 4.8%, the dissociation selectivity for 15 N was 17. The mechanisms responsible for the selectivity of IR + UV-initiated dissociation are discussed. The phenomenological model has been developed that takes into consideration the effect of the interisotopic V-V exchange and V-T relaxation on the formation of the yield and selectivity of the two-stage IR+UV dissociation of ammonia.

We have studied the dynamics of intramolecular vibrational energy redistribution (IVR) from the initially excited mode ν1 (acetylene-type H–C bond) in H–CC–CH2Cl molecules in the gaseous phase by means of anti-Stokes spontaneous Raman scattering. The energy relaxation from ν1 due to IVR was estimated to occur on the time-scale τ≈750ps, which is one of the slowest IVR time-scales reported so far. The deactivation due to molecular collisions occurred with a rate constant of ≈8.5μs−1Torr−1, and was slower than the IVR process. A theoretical model which is based on the idea of statistical nature of couplings and incompleteness of IVR (expressed in the form of the so-called dilution factor) is proposed. The model provides rationalization for the observed kinetics and suggests that the IVR process is mediated by not only anharmonic but also vibrational–rotational interactions.