Increase in the probability of passing molecules through a cooled multichannel plate as induced by a high-power infrared laser

It has been found that molecules (e.g., SF₆, CF₃I) excited in a molecualr beam by intense infrared laser radiation into high vibrational states (with energy E _v ≥ 0.5–2.0 eV) pass through a multichannel metal plate, which is cooled to T _s ? 80–85 K and inclined to the beam axis, much more efficiently than unexcited (vibrationally cold) molecules. This property provides the possibility of separating excited and unexcited molecules in the beam. The method is described and the first experimental results are reported.     

Spectral and energetic characteristics of passing vibrationally excited SF6 molecules through a cooled multichannel plate

We study the spectral and energetic characteristics of SF₆ molecules excited in a pulsed molecular beam by intense IR laser radiation into high-lying vibrational states (with the energy E _v ≥ 0.3–2.0 eV) that pass through a multichannel metal plate that is cooled to T _s ? 80–85 K and is inclined to the beam axis. From comparison of the measured characteristics with the spectral and energetic characteristics of the IR multiphoton absorption of these molecules, we find that they correlate rather well with each other. The results obtained allow us to conclude that, due to different probabilities of passing vibrationally excited and unexcited molecules through the multichannel plate, it is possible to separate molecules in the beam with respect to their isotopic (component) composition.     

Disintegration of argon clusters in collisions with highly vibrationally excited SF6 molecules in crossed molecular and cluster beams

It has been found that collisions of highly vibrationally excited SF₆ molecules (with the vibrational energy E _vib ≥ 0.5–2.0 eV) with Ar _N clusters (where N ≤ 30–40 is the number of atoms in a cluster) in crossed molecular and cluster beams result in capture of molecules followed by complete disintegration of the clusters. Possible applications of the effect for selective doping of clusters with molecules, laser separation of isotopes, and selective transport of molecules to the surface are discussed.     

Laser control of the capture of chromophore molecules by nanoclusters of noble gases in crossed molecular and cluster beams

The infrared-laser-radiation-controlled capture of chromophore molecules (on an example of SF₆) by cold nanoclusters of noble gases (Xe _N , N ≥ 100–1000 is the number of atoms in a cluster) in the crossed molecular and cluster beams has been investigated by a new method based on the selective vibrational excitation of molecules by an intense infrared laser pulse before their capture by clusters, which leads to a significant increase in the probability of their desorption from the surface of clusters as compared to the unexcited molecules. The possibility of using the proposed method for the selective doping of clusters with molecules, laser separation of isotopes, and selective transport of molecules to the surface has been discussed.     

On the mechanism of formation of helium molecules in a low-temperature plasma

The glow of He₂ molecules in a low-pressure (P ≤ 2 Torr) helium plasma excited by a monochromatic electron beam is detected and studied. The experimental data point to the existence of two basic mechanisms of formation of an excited He₂ molecule, namely, the collision of an excited helium atom with two unexcited atoms and the recombination of a molecular ion.     

Disintegration of Ar N,Kr N,and (N2) N clusters during collisions with highly vibrationally excited SF6 molecules

It is shown that, when highly vibrationally excited SF₆ molecules (with vibrational energy E _vib ≥ 0.5–2 eV) collide with weakly bound van der Waals Ar _N , Kr _N , or (N₂) _N (N ≤ 30–40 atoms in a cluster) clusters in intersecting molecular and cluster beams, the molecules are trapped by the clusters, the clusters then undergo full disintegration, and the trapped molecules become free. The method of studying this process and the results obtained are described. The possibilities of application of this method for selective doping of clusters by molecules, laser separation of isotopes, and selective transportation of molecules to a surface are discussed.     

Selecting molecules embedded in nanodroplets (clusters) of superfluid helium

A method of selecting molecules embedded in nanodroplets (clusters) of superfluid helium is proposed, which is based on the selective vibrational excitation of embedded molecules by intense IR laser radiation. This action leads to a significant decrease in size of the excited clusters, after which these clusters are separated with respect to size via scattering of the cluster beam on a crossing atomic beam. The method is described in detail and the possibility of selecting SF₆ molecules in liquid helium nanodroplets using the excitation by CO₂ laser radiation and the angular separation via scattering on a xenon atomic beam is demonstrated. The results show that, by using this technique, it is possible to separate molecules with respect to isotope (element) composition. Advantages and drawbacks of the method are analyzed.     

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.     

Light-induced conversion of nuclear spin modifications of molecules

We report a new effect in which one nuclear spin modification of molecules is converted into another in a resonance laser radiation field. The effect is based on selective (with respect to modifications) optical excitation and the difference of the conversion rates of spin modifications for excited and unexcited molecules. The effect is expected to be very substantial and should occur for all molecules capable of absorbing the radiation of existing lasers. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 498–503 (25 October 1999)     

Multiphoton fragmentation and ionization of CF2HCl molecules and clusters by UV radiation

The results of experimental studies of multiphoton ionization of CF₂HCl molecules and clusters by UV laser radiation in the wavelength range 217–236 nm are reported. In the case of molecules, the main reaction products are CF₂H⁺ and CF⁺ ions as well as atomic chlorine. It is found that the spectra of the products of ionization of free molecules and molecules condensed into clusters differ qualitatively: multiphoton ionization of clusters does not yield CF₂H⁺ ions. The dependences of the ion yield on the intensity of laser radiation and its wavelength are measured. The effect of a constant electric field and the radiation spectral width on the multiphoton ionization process is demonstrated. The shape of the velocity distributions is determined for a number of products. A strong anisotropy is detected in the reaction of formation of CF₂H⁺ ions. Possible mechanisms for these processes are discussed.     

The formation of an intense secondary pulsed molecular beam and obtaining accelerated molecules and radicals in it

A method for obtaining an intense secondary pulsed molecular beam is described. The kinetic energy of molecules in the beam can be controlled by vibrational excitation of the molecules in the source under high-power IR laser radiation. A compression shock (shock wave) is used as a source of secondary beams. The shock wave is formed in interaction between an intense pulsed supersonic molecular beam (or flow) and a solid surface. The characteristics of the secondary beam were studied. Its intensity and the degree of gas cooling in it were comparable with the corresponding characteristics of the unperturbed primary beam. Vibrational excitation of molecules in the shock wave and subsequent vibrational-translational relaxation, which occurs when a gas is expanded in a vacuum, allow the kinetic energy of molecules in the secondary beam to be substantially increased. Intense [≥10²⁰ molecules/(sr s)] beams of SF₆ and CF₃I molecules with kinetic energies approximately equal to 1.5 and 1.2 eV, respectively, were generated in the absence of carrier gases, and SF₆ molecular beams with kinetic energies approximately equal to 2.5 and 2.7 eV with He (SF₆/He=1/10) and H₂ (SF₆/H₂=1/10) as carrier gases, respectively, were obtained. The spectral and energy characteristics of acceleration of SF₆ molecules in the secondary beams were studied. The optimal conditions were found for obtaining high-energy molecules. The possibility of accelerating radicals in secondary molecular beams was demonstrated.     

Multiphoton excitation of molecules by single mode CW lasers

An intracavity laser interaction zone with molecules expanded in a molecular beam set-up permits multiphoton excitation under collision-free conditions by narrow band radiation. Two- and three-laser experiments are performed to study rapid adiabatic passage processes, stimulated de-excitation and hole burning effects. The detection of excitation occurs by means of a sensitive bolometric molecular beam detector. The essential information obtained for SF₆ concerns the vibrational bottleneck, the conservation of excited eigenstate characteristics for at least 1 ms, the ease with which about 15 photons can be deposited in the molecules with the help of two cw CO₂ lasers and the influence of simultaneous two-frequency radiation, yielding a significant extra-excitation. Other molecules where multiphoton excitation is observed utilizing the same set-up are CF₃I and CF₃Br.     

Vibrational predissociation of SF6 dimers and trimers

IR photo-dissociation spectra of SF₆ clusters have been studied. A He-seeded molecular beam has been attenuated by crossing it with a line tunable cw CO₂ laser of moderate power. — In the electron bombardment beam ionizer (E _el=100eV) small neutral clusters are found to fragment predominantly to the main monomer mass (SF ₅ ⁺ ). — Predissociation spectra have been calculated for clusters containing up to six SF₆-molecules invoking the dipole-dipole resonance force to lift the degeneracy of the molecule — excited molecule interaction. On the basis of these spectra, dimer and trimer concentrations have been determined quantitatively, for different molecular beam conditions.     

Direct measurement of lifetimes of infrared multiple-photon dissociated molecules

A beam of mass selected SF ₅ ⁺ ions is crossed with a pulsed CO₂ laser beam. The distribution of lifetimes of infrared multiple-photon pumped SF ₅ ⁺ ion-molecules prior to dissociation into SF ₄ ⁺ +F has been measured directly using a time-of-flight ion-beam technique. The data provide new information on the distribution of the total internal energy of molecules excited by absorption of infrared radiation.     

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.     

Multiphoton ionization of iodine atoms and CF<Subscript> 3</Subscript>I molecules by XeCl laser radiation

We report about effective ionization of iodine atoms and CF₃I molecules under the action of intense XeCl laser radiation (308 nm). The only ion fragment resulting from the irradiation of the CF₃I molecules is the I⁺ ion. We have studied the influence of the intensity, spectral composition, and polarization of the laser radiation used on the intensity of the ion signal and the shape of its time-of-flight peak. Based on the analysis of the results obtained, we have suggested the mechanism of this effect. The conclusion drawn is that the ionization of the iodine atoms by the ordinary XeCl laser with a nonselective cavity results from a three- (2 + 1)-photon REMPI process. This process is in turn due to the presence of accidental two-photon resonances between various spectral components of the laser radiation and the corresponding intermediate excited states of the iodine atom. The probability of ionization of the atoms from their ground state I(²P_3/2) by the radiation of the ordinary XeCl laser is more than two orders of magnitude higher than the probability of their ionization from the metastable state I^*(²P_1/2). The ionization of the CF₃I molecules by the XeCl laser radiation occurs as a result of a four-photon process involving the preliminary one-photon dissociation of these molecules and the subsequent (2 + 1)-photon REMPI of the resultant neutral iodine atoms.     

Measurement of nanoparticle temperature in a (CO2) N cluster beam using SF6 molecules as tiny probe thermometers

A temperature measurement technique using SF₆ molecules as tiny probe thermometers is described, and results are presented, for large (CO₂) _N van der Waals clusters (with N ≥ 10²) in a cluster beam. The SF₆ molecules captured by (CO₂) _N clusters in crossed cluster and molecular beams sublimate (evaporate) after a certain time, carrying information about the cluster velocity and internal temperature. Experiments are performed using detection of these molecules with an uncooled pyroelectric detector and infrared multiphoton excitation. The multiphoton absorption spectra of molecules sublimating from clusters are compared with the IR multiphoton absorption spectra of SF₆ in the incoming beam. As a result, the nanoparticle temperature in the (CO₂) _N cluster beam is estimated as T _cl < 150 K. Time-of-flight measurements using a pyroelectric detector and a pulsed CO₂ laser are performed to determine the velocity (kinetic energy) of SF₆ molecules sublimating from clusters, and the cluster temperature is found to be T _cl = 105 ± 15 K. The effects of various factors on the results of nanoparticle temperature measurements are analyzed. The potential use of the proposed technique for vibrational cooling of molecules to low temperatures is discussed.     

Van der Waals molecules and collision pairs of noble-gas atoms ahead of a shock wave front

The effect of creation of an excess concentration of free electrons in an anomalously thick layer (≈5 cm) ahead of an explosively driven shock wave in noble gases is discussed and interpreted. This effect is the ionization of excited 1_u-state molecules produced due to the absorption of a small intensity flux (as compared to the ionization one) of photons (with energies substantially lower than the atom ionization threshold) by unexcited colliding complexes and van der Waals molecules. A model is proposed which explains the excitation of xenon molecules ahead of the radiationless shock wave of an open discharge. The absorption spectra of colliding complexes and van der Waals molecules adjacent to each other near the atomic absorption lines can be resolved into two spectra, and these spectra can be changed by an increase in gas temperature. As a result, radiation capable of exciting van der Waals molecules penetrates through the shock wave of an open discharge and excites xenon molecules there. The present work develops further the knowledge concerning the radiation energy transport in the shock wave front. It also proves that in front of an explosively driven shock wave a great number of excited molecules of noble gases are actually formed, and this means considerable progress toward a VUV laser with optical pumping. Translated from Preprint No. 56 of the P. N. Lebedev Physical Institute, Moscow, 1993.     

On the mechanism of light-induced orientation of molecules in absorbing nematic liquid crystals

A new mechanism is proposed for the collective light-induced rotation of orientationally ordered molecules of absorbing nematic liquid crystals. The mechanism derives from the noncentral character of the light-induced change in the interaction potential of the excited and unexcited molecules and also from the anisotropy of their correlation function. Rotation is produced by the difference of the torques exerted by the intermolecular forces orienting the ensembles of excited and unexcited molecules. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 410–414 (10 September 1998)     

The quenching of Pb(6p 2 1 D 2) atoms in collisions with molecules

The collisions of metastable Pb(6p ^{2 1} D ₂) atoms with various molecules were studied by the diagnostics of radiation from a hollow cathode lamp and a laser on lead vapor. Experiments were performed for a gas flow of lead atoms with argon. The Pb(6p ^{2 1} D ₂) states were excited in a gas discharge in the presence of reagent gas molecules. The absolute rate constants for the quenching and chemical reactions of lead atoms in the ground and excited states were determined. The quantum efficiency of chemical reactions was close to one for the N₂O, CH₂Cl₂, SF₆, and CuBr molecules. Long-lived chemical compounds were formed in these reactions.