V. N. Lokhman

Multiphoton dissociation of CF3I clusters by IR laser radiation

The results of investigation of dissociation of (CF3I)n clusters upon resonant excitation of vibrations of CF3I molecules by pulsed CO2 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 ΦIR 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 CF3I molecules formed during dissociation of clusters are measured, as well as the dependence of these parameters on ΦIR. It is concluded that, under these conditions of IR excitations, the dissociation of (CF3I)n clusters can be treated as consecutive evaporation of molecules.

Multiphoton ionization of CF3I clusters by ultraviolet laser radiation

The results of the investigation of the multiphoton ionization of (CF3I)n clusters by ultraviolet laser radiation are reported. The yields of the I2+ 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.

Reactions induced in (CF3I)n clusters by femtosecond UV laser pulses

The excitation and ionization of CF3I molecules and their clusters by femtosecond UV laser pulses is studied. It is concluded that the types of excitation of free CF3I molecules and their clusters by femtosecond UV laser pulses are different. The composition and kinetic energy of ion products observed upon the ionization of (CF3I)n clusters by femtosecond pulses are found to differ considerably from those obtained upon ionization by nanosecond pulses. It is shown that the molecular I2+ ion is produced in reactions induced in (CF3I)n clusters by UV radiation. Using the pump-probe method, we found the two channels of producing I2+ ions with characteristic times τ1 ≈ 1 ps and τ2 ≈ 7 ps. A model of the reactions under study proposed in the paper is consistent with our experimental results.

Ultrafast dissociation dynamics of [Fe(CO)5](n) clusters induced by femtosecond IR radiation.

Using the femtosecond time-resolved infrared pump-visible probe technique, we have measured for the first time the ultrafast dissociation dynamics of [Fe(CO)5]n clusters induced by IR resonant excitation of C≡O vibrational modes in the 5-μm region. Free Fe(CO)5 molecules formed as a result of the cluster dissociation have been ionized by the femtosecond laser radiation at λ = 400 nm and have been detected with a time-of-flight mass-spectrometer. The temporal dependence of the yield of free molecules has been measured under different conditions of the IR laser excitation. We have proposed a model that describes well experimental results and makes it possible to calculate the temporal profile of the cluster temperature in terms of the concept of the evaporative ensemble. The rates of the intramolecular and intracluster vibrational relaxation in [Fe(CO)5]n clusters have been estimated.

Laser ultraviolet fragmentation of homogeneous (CF3I)n clusters in a molecular beam and (CF3I)n clusters inside of large (Xe)m clusters or on their surface

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.

Direct observation of intracluster reactions induced in (CF3I)n clusters by femtosecond ultraviolet radiation

Intracluster reactions that are induced in (CF3I)n clusters by femtosecond ultraviolet radiation, including the reaction of the formation of the I2+ molecular ion, have been directly observed. It has been shown that there are two channels of the formation of I2+ ions with the characteristic times τ1 ≈ 1 ps and τ2 ≈ 7 ps. A model of these reactions has been proposed that is in good agreement with the experimental data.

Dynamics of photoprocesses induced by femtosecond infrared radiation in free molecules and clusters of iron pentacarbonyl

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.

Ultraviolet and infrared laser-induced fragmentation of free (CF3I)n clusters in a molecular beam and (CF3I)n clusters inside or on the surface of large (Xe)m clusters

The fragmentation of free homogeneous (CF3I)n clusters in a molecular beam (n ≤ 45 is the average number of molecules in the cluster) and (CF3I)n clusters inside or on the surface of large (Xe)m clusters (m ≥ 100 is the average number of atoms in the cluster) by ultraviolet and infrared laser radiations has been studied. These three types of (CF3I)n clusters are shown to have different stabilities with respect to fragmentation by both ultraviolet and infrared radiations and completely different dependences of the fragmentation probability on the energy of ultraviolet and infrared radiations. When exposed to ultraviolet radiation, the free (CF3I)n clusters fragment at comparatively low fluences (ΦUV ≤ 0.15 J cm−2) and the weakest energy dependence of the fragmentation probability is observed for them. A stronger energy dependence of the fragmentation probability is observed for the (CF3I)n clusters localized inside (Xe)m clusters, and the strongest dependence is observed for the (CF3I)n clusters located on the surface of (Xe)m clusters. When the clusters are exposed to infrared radiation, the homogeneous (CF3I)n clusters efficiently fragment at low fluences (ΦIR ≤ 25 mJ cm−2), higher fluences (ΦIR ≈ 75 mJ cm−2) are needed for the fragmentation of the (CF3I)n localized inside (Xe)m clusters, and even higher fluences (ΦIR ≈ 150 mJ cm−2) are needed for the fragmentation of the (CF3I)n clusters located on the surface of (Xe)m clusters. It has been established that small (CF3I)n clusters located on the surface of (Xe)m clusters do not fragment up to fluences ΦIR ≈ 250 mJ cm−2. The fragmentation efficiency of (CF3I)n clusters is shown to be the same (at the same fluence) when they are excited by both pulsed (τp ≈ 150 ns) and continuous-wave infrared laser radiations. Possible causes of such a pattern of ultraviolet and infrared laser-induced fragmentation of these clusters are discussed.

Control of the clustering process in molecular beams using IR lasers

The control of clustering in molecular beams with the help of IR lasers is experimentally studied. Studies were performed with a molecular CF3I gas diluted with argon or xenon using a cw CO2 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.

Determination of the composition and content of pulsed cluster beams from time-of-flight mass spectra of cluster fragments

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 (SF6)mArn (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.