Interaction of intense beams of highly vibrationally excited molecules with molecules (clusters) condensed on a cold surface

The interaction of intense beams of SF₆ and CF₃I molecules, excited by powerful IR laser radiation to high vibrational states (0.3 eV ≤ E _vib ≤ 2.0 eV), with molecules (clusters) condensed on a cold surface (T _s ≈ 80–85 K) has been studied. The probability that the excited and unexcited molecules are reflected from the cold metal surface covered by condensed molecules (clusters), as well as the probability that such excited and unexcited molecules are transmitted through a cooled multichannel metal plate and a converging cone oriented at an angle relative to the molecular beam axis, has been determined. Expressions for these probabilities of reflection and transmission as functions of the angle of incidence and the parameters of the exciting laser radiation and the molecular beam are obtained. It is shown that highly vibrationally excited molecules are reflected from the surface and transmitted through the plates and cones with a much higher probability than unexcited molecules. The results suggest that this phenomenon can be used for the separation of molecules in a beam with respect to isotope (or atomic) composition.     

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.     

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.     

High-efficiency Zn isotope separation in a photochemical reaction induced by two-photon excitation

High-efficiency laser isotope separation was accomplished for zinc atoms in the situation when isotope shifts are much smaller than the Doppler width. The atoms were excited via two-photon absorption of two counter-propagating waves slightly detuned from the intermediate state, so that all “0requisite” atoms were excited. The isotope separation results from a photochemical reaction between the selectively excited zinc isotope and the CO₂ molecules. The reaction rate is 3–5 orders of magnitude higher than for the unexcited atoms. The mechanisms of two-photon resonance line broadening are considered both theoretically and experimentally, and the possibilities of the method for obtaining isotopes in quantitative yields are estimated.     

Change in the states of optically excited Rb atoms near sapphire surface

The changes in the states of excited Rb atoms approaching a single-crystal sapphire surface have been investigated by methods of laser-excitation spectroscopy and luminescence of Rb vapor in a cell with sapphire windows, the gap between which was varied from 250 to 500 nm. Upon resonant excitation of Rb atoms by two semiconductor lasers with powers of 20 and 40 mW, luminescence from optically excited 5D _3/2 and 5D _5/2 states and optically unexcited 6P _1/2 and 6P _3/2 states is observed. It is established that the luminescence intensity from unexcited states is only a few times lower than that from excited states, with allowance for the fact that excited atoms are rapidly and almost completely quenched on the sapphire surface. The found anomalously strong luminescence from optically unexcited 6P _J states is explained by their nonradiative occupation near the sapphire surface from optically excited 5P _J states, in which atoms fail to reach the sapphire surface because of the repulsion from it. This repulsion is due to the polarization interaction between sapphire and the atoms in the 5P _J states near the surface. Nonradiative transition from the 5P _J state to the 6P _J ^?1 state is accompanied by excitation of two optical phonons in sapphire.     

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)     

Photoinduced electron transfer in solutions of A Pd-porphyrin-quinone complex

Photoinduced electron transfer (PET) in a Pd-porphyrin-quinone complex (Pd-P-Q) was investigated using the flash photolysis method in microsecond and picosecond range and by luminescence. The investigations were performed for toluene solutions. Intramolecular PETs in the lower excited singlet state (k_CT=1.7·10¹⁰ sec⁻¹) and in the triplet state (k_CT=2.5·10⁸sec⁻¹) were observed. For a quantum yield of the triplet state that is close to 1 in Pd-porphyrin (Pd-P) this yield is equal to 0.4 in Pd-P-Q. This decrease is the result of PET in the excited state and, possibly, in an unrelaxed triplet state. Triplet Pd-P-Q molecules were efficiently quenched by unexcited molecules, and this process is related to intermolecular charge transfer. This electron transfer is likely to occur from the Pd-P protion of an excited molecule to the quinone protion of an unexcited molecule in the collisional complex (Pd-P-Q)₂. Charged radicals, formed once the collisional complex is transferred and separated, have a wide absorption band with a maximum of about 960 nm. The average lifetime of the radicals was about 2 msec. Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No 1 pp. 61–70, January–February, 1997.     

Effective production of Xe<Subscript>2</Subscript>I excimer molecules by high-energy charged particles in Xe containing a small amount of C<Subscript>3</Subscript>F<Subscript>7</Subscript>I

An anomalously high efficiency of generating Xe₂I* excimer molecules in dense Xe–C₃F₇I gaseous mixtures with a small amount of C₃F₇I that are excited by a pulsed beam of fast electrons is discovered. The electron energy is 150 keV, and the beam current amplitude and duration are, respectively, 5 A and 5 ns. The temporal–spectral characteristics of spontaneous radiation from XeI* and Xe2I* excimer molecules are measured. Also, the luminescence times of the upper level for the B–X transition in the XeI* molecule (λ_max = 253 nm) and the upper level for the 4²Γ–1²Γ transition in the Xe₂I* molecule (λ = 352 nm), as well as the rate constants of quenching these levels by the gaseous mixture components, are determined. Based on the characteristics of the track structure of a high-energy plasma produced by charged particles in the dense gaseous medium, a model of plasma-chemical processes leading to the formation of XeI* (λ_max = 253 nm) and Xe₂I* (λ = 352 nmnm) excimer molecules in a Xe–C₃F₇I mixture with a small amount of an iodine atom donor is suggested.     

Effects of pressure ratio on population inversion in a DF chemical laser with concurrent lasing

A numerical simulation is presented for investigating the effects of pressure ratio of D₂ injector to supersonic nozzle on the population inversion in the DF chemical laser cavity, while a lasing concurrently takes place. The chemical laser is generally used for the industrial process and manufacturing as well as the military weapon system, which requires high power characteristic of laser system rather than the others. The population inversion is absolutely needed to generate the laser beam and is non-equilibrium process. The laser beam is generated between the mirrors in the cavity and it is important to obtain stronger population inversion and more uniform distribution of the excited molecules in the laser cavity in order to produce high-power laser beam with good quality. In this study, these phenomena are investigated by means of analyzing the distributions of the DF excited molecules and the F atom used as an oxidant, while simultaneously estimating the maximum small signal and saturated gains and power in the DF chemical laser cavity. For the numerical solution, a fully conservative implicit method and a second order total variation diminishing (TVD) scheme are used with the finite-volume method (FVM). An 11-species (including DF molecules in various excited states of energies), 32-step chemistry model is adopted for the chemical reaction of the DF chemical laser system. The results are discussed by comparison with two D₂ injector pressure cases; 192 and 388.64 torr. Major results reveal that in the resonator, stronger population inversions occur in the all transitions except DF(1)-DF(0), when the D₂ injection pressure is lower. But, the higher D₂ injection pressure provides a favorable condition for DF(1)-DF(0) transition to generate the higher power laser beam. In other words, as the pressure of D₂ injector increases, the maximum small signal gain in the v_1-0 transition, which is in charge of generating most of laser power, becomes higher. Therefore, the total laser beam power becomes higher.     

Long-Range Energy Transfer Combined with Isotropic or Anisotropic Diffusion

Abstract

The effect of diffusion on long-range energy transfer by dipole-dipole interaction^1,2 from electronically excited donor molecules D? to (unexcited) acceptor molecules A may be important in solutions^3–7 and in molecular crystals^7–11. In solutions usually diffusion of matter occurs (the molecules perform a random walk) whereas in molecular crystals an excitation (exciton) migrates. Exciton migration can often be described in terms of an effective exciton diffusion tensor which may be highly anisotropic^12–17. Because of the restricted space only the time dependence of the donor fluorescence after δ-excitation will be given. The extension to general excitation forms and to the acceptor flouorescence is a straightforward matter^18,19.     

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.     

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.     

Study of the energy release region of a heavy-ion flux in nanomaterials by X-ray spectroscopy of multicharged ions

X-ray spectroscopic data are reported on the state of a silicon aerogel medium (SiO₂) irradiated by a Ni ion flux with an energy of 4.7 MeV/nucleon. Radiation from the electron transitions to the K shell in multicharged Si ions is detected with spatial resolution along the direction of the beam propagation in the medium. Calculations performed for the ionization state and population of the levels excited in the silicon atoms of the target provide the estimate T_e = 70–120 eV for the medium electron temperature in the ion tack region. Two-dimensional hydrodynamic calculations provide the estimate T_e = 60–116 eV for the same conditions and densities of 0.5–2.5 g/cm³ of the excited target medium.     

Evolution of molecular states studied in collisions of ions with laser excited atoms by a crossed beam experiment

Differential scattering cross sections in the energy rangeE _CM=25–50 eV have been measured in a crossed beam machine for the scattering of Na⁺ by laser excited Na(3² P _3/2). The superelastically scattered ions resulting from the collisional deexcitation process 3² P _3/2-² S _1/2 are sharply peaked at a constant reduced scattering angle of 180eV degrees. Through the choice of either linearly or circularly polarized light, one can selectively excite the magnetic sublevels of the Na atoms. This ability to align or orient the Na^* allows us to discuss in considerable detail both the transition from the space-fixed (atomic) to the body-fixed (molecular) system and the dynamics in the merging region of the two frames. We find that the symmetry axis of the excited electron charge cloud is locked to the internuclear axis and hence to the body-fixed reference frame at an internuclear separation of about 35 a.u. A pronounced asymmetry of the scattering from atoms excited with LHC or RHC polarized light is discussed in terms of simple models.     

Molecule formation and energy transfer processes in a vapor with high density of 3P-excited sodium atoms

A rich fluorescence spectrum extending between 4000 and 8200 Å has been observed whenever sodium vapor is excited by dye laser light tuned to the 3²S → 3²P transition. Molecule formation due to collisions between excited and unexcited atoms is manifested by the presence of an emission band of sodium in the spectral range 4160–4570 Å.     

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.     

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.     

Laser spectroscopy of NO2 under very high resolution: Fluorescence spectra from selectively excited hfs levels

Absorption spectra of NO₂ with reduced Doppler width and resolved hyperfine structure have been obtained by crossing the beam from a single mode argon laser, tunable around 4880 and 5145 Å, with a well-collimated NO₂ beam at low pressures. With the laser frequency stabilized onto selected NO₂ absorption lines the fluorescence spectra of NO₂ molecules in selectively populated hfs levels of an excited electronic state were examined through a grating monochromator which resolved the different rotational lines. From the fluorescence spectra the quantum numbers N′ and K′ of the emitting levels could be determined. The analysis proved that some emitting levels have K′-values differing from that of the primarily excited level. This implies radiationless transitions between different electronic states in the free molecule, which can change the value of K′ but preserve that of N′.     

Ionic composition of a humid air plasma under ionizing radiation

A kinetic model is proposed for ion–molecular processes involving charged particles of a humid air plasma produced by a fast electron beam. The model includes more than 600 processes involving electrons and 41 positive and 14 negative ions, including hydrated ions H₃O⁺ (H₂O) _n and O ₂ ^? (H₂O) _n with n = 1, 2, …, 12. The energy costs of production of electron–ion pairs and electronic and vibrational (for water molecules, also rotational) excitation of molecules are calculated in nitrogen, oxygen, water vapor, air, and humid air. A method is proposed for calculating the energy costs in mixtures by the calculation data in pure gases. The evolution of the plasma composition is studied by the numerical solution of a system of 56 time-dependent balance equations for the number of charged particles of plasma by the fourth-order Runge–Kutta method. The steady-state composition of plasma is determined by solving nonlinear steady-state balance equations for the ionization rates of humid air from 10 to 10¹⁶ cm^–3/s and the fraction of water molecules from 10^–3% to 1.5%. It is established that, for water vapor content (the ratio of the number density of water molecules to the total number density of air molecules) of 0.015–1.5% in air at atmospheric pressure and room temperature, the main ion species are two types of positive ions H₃O⁺ (H₂O) _n with the number of water molecules n = 5, 6 and three species of negative ions O ₂ ^? (H₂O) _n with n = 5, 8, 9.     

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)