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.

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.

We present the results of our investigation into the isotope-selective control of the clusterization of SF6 molecules during their gasdynamic expansion in a mixture with an argon carrier gas using IR laser radiation. The method is based on the selective vibrational excitation of molecules of a chosen isotopic composition by an IR laser near the nozzle edge, which leads to the suppression of their clusterization with each other and with argon atoms. The experimental setup and the method of investigation are described. Results are presented on determining the efficiency and the selectivity of the clusterization suppression of molecules at different gas compositions above the nozzle, as well as at different distances of the irradiation region of particles from the nozzle edge. Dependences of the clusterization suppression efficiency on the frequency and power of the exciting laser radiation are obtained. Experiments on the selective suppression of the clusterization of SF6 molecules in mixtures with other carrier gases have also been performed. It is found that controlling the clusterization process of SF6 molecules in a mixture with the argon carrier gas makes it possible to achieve relatively high selectivity values. Thus, in the case of using an SF6/Ar mixture at a 1/200 pressure ratio, the obtained selectivity of the clusterization suppression of (32)SF6 molecules with respect to (34)SF6 molecules under jet irradiation at the 10P(16) CO2 laser line (at a frequency of 947.74cm(-1)) was alpha((SF6)-S-32/(SF6)-S-34) >= 10-20. Conditions are found under which optimal values of the efficiency and selectivity of the clusterization suppression of molecules are realized.

We report the results of research on the experimental control of CF3Br molecule clustering under gas-dynamic expansion of the CF3Br – Ar mixture at a nozzle exit by using IR laser radiation. A cw CO2 laser is used for exciting molecules and clusters in the beam and a time-of-flight mass-spectrometer with laser UV ionisation of particles for their detection. The parameters of the gas above the nozzle are determined (compositions and pressure) at which intensive molecule clustering occurs. It is found that in the case of the CF3Br gas without carrier when the pressure P0 above the nozzle does not exceed 4 atm, molecular clusters actually are not generated in the beam. If the gas mixture of CF3Br with argon is used at a pressure ratio 1 : N, where N ⩾ 3, and the total pressure above the nozzle is P0 ⩾ 2 atm, then there occurs molecule clustering. We study the dependences of the efficiency of suppressing the molecule clustering on parameters of the exciting pulse, gas parameters above the nozzle, and on a distance of the molecule irradiation zone from the nozzle exit section. It is shown that in the case of resonant vibrational excitation of gas-dynamically cooled CF3Br molecules at the nozzle exit one can realise isotope-selective suppression of molecule clustering with respect to bromine isotopes. With the CF3Br – Ar mixtures having the pressure ratio 1 : 3 and 1 : 15, the enrichment factors obtained with respect to bromine isotopes are kenr ≈ 1.05 ± 0.005 and kenr ≈ 1.06 ± 0.007, respectively, under jet irradiation by laser emission in the 9R(30) line (1084.635 cm−1). The results obtained let us assume that this method can be used to control clustering of molecules comprising heavy element isotopes, which have a small isotopic shift in 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.

The dynamics of dissociation of (IF2CCOF) n , (CF3I) n , and (CH3I) n clusters induced by femtosecond UV radiation (λ = 266 nm) is investigated. The method of UV excitation and photoionization probing (λ = 400 nm) combined with the time-of-flight mass spectrometry is applied. It is found that clusters excited to Rydberg states lying below the ionization threshold are decomposed, as a result of subsequent relaxation processes, giving rise to free neutral molecules. A mechanism of electronic–vibrational relaxation process is proposed, and data on the characteristic times of different stages of this process are obtained.

The composition and content of pulsed neutral cluster beams are determined by a specially proposed method from ion signals of cluster fragments in time-of-flight spectra of clusters in these beams. For beams of different-size mixed clusters (SF 6 ) m Ar n (where 1 ≤ m ≤ 4 and 0 ≤ n ≤ 9 are the number of molecules and atoms in clusters, respectively), it is shown that the proposed method makes it possible to determine the composition and content of neutral van der Waals molecular and atomic–molecular clusters in beams.

The dynamics of photoprocesses induced by femtosecond infrared radiation in free Fe(CO)5 molecules and their clusters owing to the resonant excitation of vibrations of CO bonds in the 5-μm range has been studied. The technique of infrared excitation and photoionization probing (λ = 400 nm) by femtosecond pulses has been used in combination with time-of-flight mass spectrometry. It has been found that an infrared pulse selectively excites vibrations of CO bonds in free molecules, which results in a decrease in the yield of the Fe(CO)5 + molecular ion. Subsequent relaxation processes have been analyzed and the results have been interpreted. The time of the energy transfer from excited vibrations to other vibrations of the molecule owing to intramolecular relaxation has been measured. The dynamics of dissociation of [Fe(CO)5] n clusters irradiated by femtosecond infrared radiation has been studied. The time dependence of the yield of free molecules has been measured under different infrared laser excitation conditions. We have proposed a model that well describes the results of the experiment and makes it possible, in particular, to calculate the profile of variation of the temperature of clusters within the “evaporation ensemble” concept. The intramolecular and intracluster vibrational relaxation rates in [Fe(CO)5] n clusters have been estimated.

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 .

We have studied the fragmentation of homogeneous (CF3I)(n) clusters (n <= 45 is the average number of molecules per cluster) and of (CF3I)(n) clusters that are located either inside of or on the surface of large (Xe)(m) clusters (m >= 100 is the average number of atoms per cluster) as a result of resonant excitation of clusterized molecules by IR laser radiation. We have detected that these three types of (CF3I)(n) clusters are differently stable with respect to the fragmentation and exhibit quite different dependences of the fragmentation probability on the IR radiation energy fluence. Whereas homogeneous (CF3I)(n) clusters efficiently fragment at comparatively low laser energy fluences (Phi(IR) <= 25 mJ cm(-2)), to fragment (CF3I)(n) clusters that are localized inside (Xe)(m) clusters, higher energy fluences (Phi(IR) approximate to 75 mJ cm(-2)) are necessary, and even higher energy fluences are required (Phi(IR) approximate to 150 mJ cm(-2)) to fragment (CF3I) n clusters localized on the surface of (Xe)(m) clusters. We have shown that small (CF3I)(n) clusters localized on the surface of (Xe)(m) clusters do not fragment up to energy fluences of Phi(IR) approximate to 250 mJ cm(-2). Possible reasons why the considered clusters have this fragmentation character are discussed.

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 results of the investigation of the multiphoton ionization of (CF3I) (n) clusters by ultraviolet laser radiation are reported. The yields of the I (2) (+) and I+ ions, which are the products of the multiphoton ionization, have been measured as functions of the intensity of the ultraviolet radiation at the wavelengths of 308 and 232.5 nm. The degree of multiphoton ionization has been determined and appears to depend on the wavelength of radiation. The velocity distributions of the products have been measured in detail for various wavelengths and various polarizations of radiation. The anisotropy parameters of the velocity distributions of the produced ions and their kinetic energy have been determined. After analysis of the data, a mechanism of the multiphoton ultraviolet ionization of the clusters under investigation has been proposed. This mechanism depends on the used wavelengths.

The results of investigation of dissociation of (CF 3 I) n clusters upon resonant excitation of vibrations of CF 3 I molecules by pulsed CO 2 laser radiation are reported. The kinetics of dissociation of such clusters is studied and the dissociation yield is measured. It is shown that its value is completely controlled by fluence Φ IR of transmitted IR radiation energy and its time dependence is exponential. The results of measurements show that the dissociation lifetime of clusters is shorter than 10 −8 s. The velocity and translational temperature of free CF 3 I molecules formed during dissociation of clusters are measured, as well as the dependence of these parameters on Φ IR . It is concluded that, under these conditions of IR excitations, the dissociation of (CF 3 I) n clusters can be treated as consecutive evaporation of molecules.