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.     

Direct observation of intracluster reactions induced in (CF<Subscript>3</Subscript>I)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> 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.     

Detection of SF<Subscript>6</Subscript> molecules sublimating from the surface of (CO<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> nanoparticles in a cluster beam by the infrared multiphoton excitation method

It has been found that SF₆ molecules captured by large van der Waals clusters (CO₂) _N (where N ≥ 10² is the number of monomers in a cluster) in intersecting molecular and cluster beams sublimate from the surface of clusters after a certain time and carry information on the velocity and temperature (internal energy) of clusters. Experiments have been carried out for detecting these molecules by means of a pyroelectric detector and the infrared multiphoton excitation method. The multiphoton absorption spectra of molecules sublimating from the surface of clusters have been obtained. The temperature of the (CO₂) _N nanoparticles in the cluster beam has been estimated using these spectra and comparing them with the infrared multiphoton absorption spectra of SF₆ in the initial molecular beam.     

甲醛的态选择性多光子电离研究

我们在XeCl准分子激光多光子电离甲醛分子的研究中,获得了甲醛经过2+1和2+2光子共振激发过程而产生的母体离子和高能离解通道的离子CO⁺。在电子轰击和单光子电离中占优势的低能阈离解通道离子HCO⁺,由于态选择性激发的结果而未出现。 关键词：     

(CH3I)n(n=1,2,3,4)的同步辐射光电离研究

用VUV同步辐射辐照在连续的超声射流冷却束中产生的（CH₃I)_n（n=1,2，3，4）团簇分子，通过测量其光电离及解离电离产生的各种离子的光电离效率（PIE）曲线，获得了（CH₃I)_n⁺（n=1，2，3，4）的绝热电离势及各种碎片离子的出现势，估算了有关分子的键能．在CH₃I⁺的PIＥ曲线上观察到CH₃I分子的自电离结构，并对其进行了标识，归属为收敛于CH₃I⁺（²E_1/2）态的4组Rydberg系，即ns，npσ，npπ和nd． 关键词：     

On mechanism of population transfer to ungerade ion-pair states of I<Subscript>2</Subscript> molecule

We discuss the population transfer to the D 0 _u ⁺ and F 0 _u ⁺ ungerade ion-pair states of the I₂ molecule upon optical excitation of the E0 _g ⁺ and f 0 _g ⁺ gerade states by the optical-optical double resonance method. This effect is explained by multiphoton ionization of iodine vapor and subsequent collisions with photoelectrons.     

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.     

Double ionization of Sc<Superscript>+</Superscript>ions by electron impact

Electron impact double ionization cross-sections of Sc⁺ions have been calculated in the binary encounter approximation (BEA). Accurate expression of σ_ΔE(cross-section for energy transfer ΔE) and Hartree-Fock velocity distributions for the target electrons have been used throughout the calculations. Direct double ionization from ejection of 3d and 4s electrons has been investigated in the modified double binary encounter model incorporating the focusing action of the target ion on the incident electron. The identification of the 3p shell whose ionization provides a major contribution to double ionization through ionization-autoionization is an interesting aspect of the present investigation. The theoretical results show satisfactory agreement with the experimental observations.     

Ellipticity-dependent of multiple ionisation methyl iodide cluster using 532 nm nanosecond laser

The dependence of multiply charged ions on laser ellipticity in methyl iodide clusters with 532 nm nanosecond laser was measured using a time-of-flight mass spectrometer. The intensities of multiply charged ions I^q+(q = 2–4) with circularly polarised laser pulse were clearly higher than those with linearly polarised laser pulse but the intensity of single charged ions I⁺ was inverse. And the dependences of ions on the optical polarisation state were investigated and a flower petal and square distribution for single charged ions (I⁺, C⁺) and multiply charged ions (I²⁺, I³⁺, I⁴⁺, C²⁺) were observed, respectively. A theoretical calculation was also proposed to simulate the distributions of ions and theoretical results fitted well with the experimental ones. It indicated that the high multiphoton ionisation probability in the initial stage would result in the disintegration of big clusters into small ones and suppress the production of multiply charged ions.     

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 I ₂ ⁺ 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 I ₂ ⁺ ions with characteristic times ??₁ ?? 1 ps and ??₂ ?? 7 ps. A model of the reactions under study proposed in the paper is consistent with our experimental results.     

I. Electron-impact ionization and dissociation of C<Subscript>2</Subscript>H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and C<Subscript>2</Subscript>D<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>

Absolute cross-sections for electron-impact ionization and dissociation of C₂H₂⁺ and C₂D₂⁺ have been measured for electron energies ranging from the corresponding thresholds up to 2.5 keV. The animated crossed beams experiment has been used. Light as well as heavy fragment ions that are produced from the ionization and the dissociation of the target have been detected for the first time. The maximum of the cross-section for single ionization is found to be (5.56 ± 0.03)× 10^-17 cm² around 140 eV. Cross-sections for dissociation of C₂ H₂⁺ (C₂D₂⁺) to ionic products are seen to decrease for two orders of magnitude, from C₂D⁺ (12.6 ± 0.3) × 10^-17 cm² over CH⁺(9.55 ± 0.06) × 10^-17 cm², C⁺ (6.66 ± 0.05) × 10^-17 cm², C₂⁺ (5.36 ± 0.27) × 10^-17 cm², H⁺ (4.73 ± 0.29) × 10^-17 cm² and CH₂⁺ (4.56 ± 0.27) × 10^-18 cm² to H₂⁺ (5.68 ± 0.49) × 10^-19 cm². Absolute cross-sections and threshold energies have been compared with the scarce data available in the literature.     

II. Electron-impact dissociative excitation of C<Subscript>2</Subscript>H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and C<Subscript>2</Subscript>D<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>

Absolute cross-sections for electron-impact dissociative ionization of C₂ H₂⁺ and C₂ D₂⁺ to CH⁺, C⁺, C₂⁺ , H⁺, CH₂⁺ and C₂D⁺ fragments are determined for electron energies ranging from the corresponding threshold to 2.5 keV. Results obtained in a crossed beams experiment are analyzed to estimate the contribution of dissociative ionization to each fragment formation. The dissociative ionization cross sections are seen to decrease for more than an order of magnitude, from CH⁺ (5.37±0.10) × 10^-17 cm² over C⁺ (4.19± 0.16) × 10^-17 cm², C₂D⁺ (3.94±0.38) × 10^-17 cm², C₂⁺ (3.82±0.15) × 10^-17 cm² and H⁺ (3.37±0.21) × 10^-17 cm² to CH₂⁺ (2.66±0.14) × 10^-18 cm². Kinetic energy release distributions of fragment ions are also determined from the analysis of the product velocity distribution. Cross section values, threshold energies and kinetic energies are compared with the data available from the literature. Conforming to the scheme used in the study of the dissociative excitation of C₂H₂⁺ ( C₂ D₂⁺ )\left( {\rm C}_2 {\rm D}_2^+ \right), the cross-sections are presented in a format suitable for their implementation in plasma simulation codes.     

Spin dynamics in LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> and NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript> low-dimensional helical magnets

Comprehensive NMR investigation of low-frequency spin dynamics of LiCu₂O₂ (LCO) and NaCu₂O₂ (NCO) low-dimensional helical magnets in the paramagnetic state has been carried out for the first time. Temperature dependences of the spin–lattice relaxation rate and anisotropy on various LCO/NCO nuclei have been determined at various orientations of single crystals in an external magnetic field. The spatial asymmetry of spin fluctuations in LCO multiferroic has been discovered. The quantitative analysis of the anisotropy of spin–lattice relaxation in LCO/NCO has allowed estimating the contributions of individual neighboring Cu²⁺ ions to the transferred hyperfine field on Li⁺(Na⁺) ions.     

Absolute cross sections and kinetic energy release distributions for electron-impact ionization and dissociation of CD<Superscript>+</Superscript><Subscript>4</Subscript>

Absolute cross sections for electron-impact dissociative excitation and ionization of CD⁺ ₄ leading to formation of ionic products (CD²⁺ ₄, CD⁺ ₃, CD⁺ ₂, CD⁺, C⁺, D⁺ ₃, D⁺ ₂, and D⁺) have been measured. The animated crossed-beams method is applied in the energy range from the reaction threshold up to 2.5 keV. Around 100 eV, the maximum cross sections are found to be (3.8±0.2) ×10^-19 cm²,  cm², (7.1±0.8) ×10^-17 cm², (9.0±0.8) × 10^-17 cm² and (3.7±0.4) ×10^-17 cm² for the heavy carbonaceous ions CD²⁺ ₄, CD⁺ ₃, CD⁺ ₂, CD⁺ and C⁺ respectively. For the light fragments, D⁺ ₃, D⁺ ₂, and D⁺, the cross sections around the maximum are found to be (5.0±0.6) ×10^-19 cm², (1.7± 0.2) ×10^-17 cm² and (10.6±1.0) ×10^-17 cm², respectively. The cross sections are presented in closed analytic forms convenient for implementation in plasma simulation codes. The analysis of ionic product velocity distributions allows determination of the kinetic energy release distributions which are seen to extend from 0 to 9 eV for heavy fragments, and up to 14 eV for light ones. The comparison of present energy thresholds and kinetic energy release with available published data gives information about states contributing to the observed processes. Individual contributions for dissociative excitation and dissociative ionization are determined for each detected product. A complete database including cross sections and energies is compiled for use in fusion application.     

Electron-impact ionization and dissociation of C<Subscript>2</Subscript>D<Superscript>+</Superscript>

Absolute cross sections for electron-impact single ionization, dissociative excitation and dissociative ionization of the ethynyl radical ion (C₂D⁺)^+) have been measured for electron energies ranging from the corresponding reaction thresholds to 2.5 keV. The animated crossed electron-ion beam experiment is used and results have been obtained for the production of C₂D²⁺, C²⁺, C₂⁺_2^+ , CD⁺, C⁺ and D⁺. The maximum of the cross section for single ionization is found to be (2.01 ± 0.02) × 10^-17 cm², at the incident electron energy of 105 eV. Absolute total cross sections for the various singly charged fragments production are observed to decrease by a factor of almost three, from the largest cross-section measured for C⁺, over C₂⁺_2^+ and CD⁺ down to that of D⁺. The maxima of the cross sections are obtained to be (14.5 ± 0.5) × 10^-17 cm² for C₂⁺_2^+, (12.1 ± 0.1) × 10^-17 cm² for CD⁺, (27.7 ± 0.2) × 10^-17 cm² for C⁺ and (11.1 ± 0.8) × 10^-17 cm² for D⁺. The smallest cross section is measured to be (1.50 ± 0.04) × 10^-18 cm² for the production of the doubly charged ion C²⁺. Individual contributions for dissociative excitation and dissociative ionization are determined for each singly-charged product. The cross sections are presented in closed analytic forms convenient for implementation in plasma simulation codes. Kinetic energy release distributions of dissociation fragments are seen to extend from 0 to 6 eV for the heaviest fragment C₂⁺_2^+, up to 11.0 eV for CD⁺, 14.2 eV for C⁺ and 11.2 eV for D⁺ products.     

Photostimulated electron transfer and its action on paramagnetism of Sp<Subscript>3</Subscript>Cr(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The effect of irradiation by ultraviolet light on the effective magnetic moment of a paramagnetic single crystal based on photochrome spiropyran (Sp) and chromium oxalates Sp₃Cr(C₂O₄)₃ molecules is detected. It is shown that the deviation of the temperature dependence of the magnetic moment from the Curie law is caused not by the exchange interaction, but by electron redistribution between Cr³⁺ and Cr⁴⁺ ions and spiropyran molecules Sp⁰ and Sp⁺. Analysis of the angular dependence of EPR spectra makes it possible to determine the contribution of Cr³⁺ ions to the magnetic properties of the crystals and to determine the crystal field parameters D = 0.619 cm⁻¹ and E = 0.024 cm⁻¹. Irradiation of hydrated samples by ultraviolet light leads to intensity redistribution of EPR lines attributed to Cr³⁺ and Sp⁰. Thermally stimulated paramagnetism of triplet states of spiropyran ions Sp⁺ and the SpI salt is observed.     

Independent electron theory of multiple ionization of xenon by intense laser pulses

The intensity dependence of the multiphoton ionization spectra of Xe atoms has been investigated with an improved accuracy and well-controlled laser parameters. In particular, we have examined the ionization rates for X³⁺, X²⁻, X⁺ as functions of the laser intensity and the pressure in the target chamber. The apparatus used for these measurements is characterized by a high-energy resolution (better than 200 meV) and a completely digital acquisition system. The time-of-flight spectra clearly show the contributions of the different isotopes present in Xe gas. The laser pulses have been characterized with great accuracy by monitoring the energy, pulse width and divergence shot by shot. The ionization rates of the different ions have been used for testing the basic assumption of the Geltman theory of multiple ionization based on the single electron ionization model. We have found that for the small intensity range investigated the quantity (dXe ⁺/dI)·(dXe ³⁺/dI)/(dXe ²⁺/dI)² appears to be quite close to the value 0.5 predicted by this model.     

Cascade mechanism of formation of doubly charged ions upon multiphoton ionization of barium atoms by linearly and circularly polarized radiation (ω = 16800–18700 cm<Superscript>−1</Superscript>)

The process of formation of doubly charged ions upon multiphoton ionization of barium atoms by linearly and circularly polarized radiation of dye lasers is studied in the frequency range from 16800 to 18700 cm^?1. A large number of resonance maxima in the yield of Ba⁺ and Ba²⁺ ions were observed upon tuning the radiation frequency. It is unambiguously proved that most resonance maxima in the yield of Ba²⁺ ions are associated with the resonance transitions in the spectrum of Ba⁺ ions. This fact confirms the cascade mechanism of formation of Ba²⁺ ions in the frequency range of localization of these maxima.     

H<Superscript>+</Superscript><Emphasis Type="Bold">and He</Emphasis><Superscript>2+</Superscript> impact single and double ionization of lead

H⁺ and He²⁺ impact single and double ionization cross sections of ground state lead atoms have been calculated in the binary encounter approximation. Calculations of direct double ionization cross sections have been performed in the modified double binary encounter model. The accurate expressions of σ_ΔE (cross-section for energy transfer ΔE) and Hartree-Fock velocity distributions for the target electrons have been used throughout the calculations. Contributions to double ionization from Auger effect following ionization of inner shells have been considered in the present work. Our H⁺ impact single and double ionization cross sections are in good agreement with the experimental observations. In calculations of He²⁺ impact cross sections, the present theoretical approach shows limited success in the experimentally investigated region (50–350 keV amu^-1).     

VUV luminescence of Er<Superscript>3+</Superscript> ions in LiYF<Subscript>4</Subscript> and BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystals

Vacuum ultraviolet luminescence of Er³⁺ ions in LiYF4 and BaY₂F₈ crystals has been investigated. It is revealed that under excitation by 193 nm radiation from an ArF excimer laser the interconfigurational 5d–4f radiative transitions in Er³⁺ ions are observed. It is shown that from the LiYF₄:Er crystal only the spin-forbidden luminescence (λ = 165 nm) is detected, whereas both the spin-forbidden (λ = 169 nm) and spin-allowed (λ = 160.5 nm) components are observed from the BaY₂F₈:Er crystal.