Measurement of L x-ray production cross sections by 60 keV photons and average L shell fluorescence yields of lanthanides

L x-ray production cross sections have been measured for lanthanides with 60 keV. The measured L x-ray production cross section values for the lanthanides are in good agreement with the theoretical ones evaluated using L _i subshell fluorescence yields ω_i, Coster-Kronig transition probabilities ? _ij based on the Relativistic-Hartree-Slater theory, K to L _i subshell vacancy transfer probabilities n _KLi, fractions of the ratiative width of the subshell F _ny and L _i subshell photoionisation cross section σ_Pi. The average L shell fluorescence yields ω_L have also been derived using the presently measured total L x-ray production cross section values and the theoretical K to L shell vacancy transfer probabilities. These results are compared with theoretically predicted values.     

Resonantly enhanced two-photon excitation of sodium atoms in a heat pipe

We report the results of a study of two-color laser excitation, ω₁ + ω₂, in dense sodium vapor where ω₁ was tuned in the vicinity of the 3P fine structure doublet and ω₂ was tuned to highernl states. Resonant excitation by ω₂ of higherns andnd states from one member of the 3P doublet when ω₁ is tuned near the other is attributed to collisional excitation transfer. Anomalous dipole forbiddenp-p transitions are observed when ω₂ is tuned to highnP states (≥20) and are believed to be DC electric field induced. Sample spectra for the processes considered are calculated and compared with experiment.     

Polarization in elastic scattering: close-coupling studies on ArN2

We present the results of close-coupling calculations of m_j-dependent differential and integral cross sections forj₁ = 2 → j₂ = 2 rotationally elastic ArN₂ collisions. Two potential surfaces were used with differing long-and short-range anisotropies. If the anisotropy is long-ranged the scattering of an isotropic beam results in a significant angle dependent polarization of the elastically scattered products. To a certain extent this reflects a selective loss of m_j-state population due to rotationally inelastic transitions. For quantization along the initial relative velocity vector or perpendicular to the scattering plane, the depolarization of an initially m_j-state selected beam vanishes in the forward direction and is significantly less than the statistical limit at all angles, which indicates a dynamical conservation of the direction of the molecular rotational angular momentum. By contrast, in the helicity frame depolarization is much more pronounced. The oscillatory structure present in the rotationally inelastic differential cross section does not appear to be quenched by the interference between various m → m′ transitions.     

Quasi-resonant energy transfer in collisions: Na2(A1Σ u + )+K(4S)

Cross sections for electron energy transfer from the initial rotational stateJ′of the two lowest vibrational levelsv′=0 andv′=1 of excited dimers Na₂(A) to potassium atoms as described by Na₂(A¹Σ _u ⁺ ,v′J′)+K(4S)→Na₂ (X¹Σ _g ⁺ ,v″J″)+K(4P)+ΔE have been examined by laser-induced fluorescence. A strong increase of the cross section by as much as an order of magnitude has been observed for those dimerv′J′-levels for which the dipole transitions are close to resonance of the 4S-4P transitions in the atom (ΔE<4 cm^?1). The absolute cross sections for energy transfer have been calculated by the Rabitz approximation of first-order perturbation theory. In the case of closest energy resonance (ΔE=0.9 cm^?1) the cross section is Q=7.8×10^?13 cm².     

Complex formation in proton—D2 collisions

Classical trajectory calculations are used to compute the formation cross section (suitably defined) for strongly interacting collision complexes formed in H⁺ + D₂ collisions in the kinetic energy range from 0.1 to 4 eV. This cross section corresponds to the usual Langevin cross section only if the kinetic energy is less than 0.2 eV, and provided that little initial excitation is present, while for higher kinetic energies it drops exponentially. It is in much better agreement with absolute integral cross sections observed experimentally than the latter. Further study shows that it is the contribution from large orbital anglular momenta, which the Langevin cross section overestimates. Orbiting complexes (of H⁺ around D₂) play a negligible role, and are very short-lived. The lifetime of strongly coupled complexes is estimated to be 450 E^?1.3 fs, where E is the total energy in eV. The use of trajectory data to improve Light's phase space theory is discussed.     

Oscillator strength measurements of the 5s5p 3P1 → 5snd 3D2 Rydberg transitions of cadmium

We report oscillator strengths distribution in the bound region of cadmium corresponding to the 5s5p ³P₁ → 5snd ³D₂ (21 ≤ n ≤ 52) Rydberg transitions. The experiment was performed using two frequency doubled dye lasers simultaneously pumped by a common Nd:YAG laser in conjunction with an atomic beam set-up. The absolute photoionization cross section of the 5s5p ³P₁ intermediate state at the 5s ²S_1/2 ionization threshold has been measured as 20 (4) Mb using the saturation technique. The measured value of the photoionization cross section at threshold is used to extract the f-values of the above mentioned Rydberg transitions.     

A two-photon absorption cross section measurement in nitric oxide

We report here the measurement of a two-photon absorption cross section for the R₂₂ + S₁₂ (J″ = 9.5) [A ²Σ⁺ (ν′ = 0) - X ²Π (ν′ = 0)] transition in the gamma band system of nitric oxide by measuring the third-order susceptibility using a four-wave mixing technique. A value of σ⁽²⁾ = (1.0 ± 0.6) X 10^?38 πg(2ω₁ ? ω_f) cm⁴ s was obtained.     

Infrared-microwave double resonance study of the ν4 fundamental band of POF3

Three-level and four-level infrared-microwave double resonance effects have been observed in a POF₃ sample contained in a microwave waveguide Stark cell that was modified to permit transmission of radiation from a CO₂ infrared gas laser. In three-level double resonance experiments laser pumping of rotational states in the ν₄ = 1 vibrational state greatly increased the signal/noise for observation of Stark-shifted rotational transitions in the excited vibrational state. The frequencies of the observed excited state transitions were used to confirm the assignment of laser Stark spectra and to obtain independent measures of the rotational constant B and the dipole moment for ν₄ = 1. The observation of four level double resonances could be explained qualitatively by the assumption of dipole selection rules for collision-induced transitions. However, the intensities of the double-resonance effects could not be explained by this simple model.     

Study of rydberg atoms using spectral profiles in sum frequency generation

Spectral profilesI ₃(ω₃) in sum frequency generation (SFG) ω₃=2ω₁+ω₂ in gases using two dyelasers ω₁, ω₂ are analyzed to study atomic Rydberg states. They are measured by locking laser 1 to a two-photon transition 2ω₁=Ω _ge and shifting laser 2 across Rydberg transitions Ω _en (g=ground-,e=excited-,n=Rydberg state). From optically thin line wings (|ω₃?Ω _gn |»Γ _gn =homogeneous line width) dipole moment products μ _en μ _ng may be derived, from optically thick line cores (|ω₃?Ω _gn |≦Γ _gn ) oscillator strengths ≈|μ _ng |². Both quantities yield the individual dipole moments μ _en , μ _ng and phase information of the wavefunctions involved. Explicit evaluation procedures based on the small signal theory of the SFG process are presented and discussed. The potentialities of the method are demonstrated at the SFG spectrum of Cadmium I in the near Vacuum-UV involving Rydberg states5snp in the rangen=12?28.     

Measurements of the oscillator strengths for the 6p7s (1/2,1/2)1 → 6pnp (1/2,3/2)2 Rydberg transitions of lead

We report experimentally determined oscillator strength distribution in the bound region of lead corresponding to the 6p7s (1/2, 1/2)₁ → 6pnp (1/2, 3/2)₂ (20 ≤ n ≤ 52) Rydberg transitions. The absolute value of the photoionization cross section from the 6p7s (1/2, 1/2)₁ excited state at the first ionization threshold 6p (²P_1/2) has been determined as 27 ± 4 Mb using the saturation absorption technique. The threshold value of the photoionization cross section is used to calibrate the oscillator strengths of the above mentioned Rydberg transitions. Moreover, oscillator strengths in the bound region smoothly connect with the differential oscillator strength density at the first ionization threshold, and verify the fundamental condition of quantum defect theory.     

Collision dynamics of excited NaK. I. Reactive KNaK and elastic HeNaK collisions

The collision dynamics of excited NaK have been studied using the technique of circularly polarised laser fluorescence. A very unusual m-dependent rate is observed for KNaK* reactive or quenching collisions which is different depending on whether the initial excitation is via Q or P, R transitions. A model is presented attributing this to the spatial distribution of the ¹Π Λ-doublet components and the influence of the nuclear function. Elastic HeNaK collisions are observed and a reorientation cross section for f = 30 was evaluated. This was found to have an upper limit of 0.3 Ų. Comparison of this with HeLi₂ reorientation cross sections indicate that the presence of a permanent dipole moment has little effect on the reorientation rate in rare gas-alkali dimer collisions.     

Simulated ion trajectory and induced signal in ion cyclotron resonance ion traps. Effect of ion initial axial position on ion coherence,induced signal,and radial or z ejection in a cubic trap

The effects of ion initial axial position on coherence of ion motion, induced ion cyclotron resonance (ICR) signal. and radial and z ejection have been evaluated by numerical simulation for a cubic Fourier transform-ion cyclotron resonance ion trap. For a given initial ion cyclotron phase and radius, ions of different initial z position are shown to be excited to significantly different ion cyclotron radii (and ultimately radially ejected at significantly different excitation amplitude-duration products). Ion initial z displacement from the trap midplane affects observed ICR signal magnitude in two ways: (1) for the same postexcitation cyclotron radius, an ion with larger initial z displacement induces a smaller ICR signal and (2) an ion with larger initial z displacement is excited to a smaller cyclotron radius. We also evaluate the induced ICR signal as a function of excitation amplitude-duration product for spatially uniform or Gaussian ion initial z distributions. In general, if the excitation waveform contains components at frequency, 2 ω_z or (ω₊ + 2 ω_z, in which ω_z is the axial C“trapping”) oscillation frequency, then ejection occurs axially. However, the resulting excitation amplitude-duration product for such axial ejection is significantly higher (factor of, ～ 4) than that required for radial ejection (at ω₊) for ions of small initial radius. The present results offer the first explanation of how, even if the ion is initially at rest on the z axis (i.e., zero excitation electric field amplitude on the z axis), z ejection (axial ejection) may nevertheless occur if the excitation waveform contains frequency components at ω₊ + 2ω_z and/or 2_{w z} Namely, our simulations reveal that off-resonant excitation pushes ions away from the z axis, after which the ions are exposed to z excitation and eventual z ejection.     

The born approximation as a simple diagnostic method for direct molecular collisions with applications to Cl + HI and F + H2 reactions

Born approximation computations are presented and discussed for the Cl + HI → I + HCl and F + H₂ → H + HF reactions and their isotopic analogues. Most aspects of the role of reagent energy or the energy disposal in the products previously deduced from experiment or trajectory computations can be accounted for the Born approximation. The procedure used here neglects the interaction between non-bonded atoms. It does thereby provide a very simple computational scheme which requires as input only the spectroscopic constants of the reactants and products. In addition it offers simple qualitative interpretations of the trends in the results. The overall satisfactory agreement between the present results and past studies lends credibility to the basic propensity rule provided by the Born approximation: The most probable transitions are those that minimize the momentum transfer to the nuclei. The principle is discussed with special reference to exothermic (E′_T ? E_T) and endothermic transitions.The computations for Cl + HI indicate a decline of the reaction cross section with increasing kinetic energy and a strong enhancement by HI rotational energy. The surprisal analysis confirms the absence of vibrational population inversion for endothermic transitions. For the F + H₂ (and isotopic variants) reactions, the product-rotational state distribution extends nearly to the energy cut-off. The vibrational state distribution is somewhat different for para- and normal H₂ and, in general, the collision outcome is very sensitive to the initial rotational state of H₂ particularly at low translational energies. The HF/DF branching ratio is F + HD collisions is increasing with increase of the HD rotational state. The vibrational surprisal is essentially isotopically invariant.     

Vibrational and rotational energy transfer upon molecular collisions

The cross section of the rotational and vibrational energy transfer is derived by using the first Born approximation which quantizes the translational motion of the colliding particles. The theory developed here integrates the intermolecular potential V(R) over all regions of the internuclear distance R by obtaining a Fourier transform of V(R). This differs from previous semiclassical (impact-parameter) treatments which either considered the short-range repulsive interaction or expanded V(R) into a long-range multipole expansion. The cross section obtained here is expressed in a very simple algebraic expression which can be readily calculated. This will be illustrated by examples of CO₂(001)+N₂(υ=0)=CO₂(000)+N₂(υ=1)+18.6 cm^?1 and CO(υ=1)+CO(υ=1)=CO(υ=0)+CO(υ=2)+27 cm^?1. Calculations have been made both for the exothermic and for the endothermic reactions. The comparison of the present results with experimental results as well as with previously calculated results will be discussed.     

Energy dependence of the cross section for alkali—methyl iodide reactions

The dependence of the reaction cross section for all the alkali—methyl iodide reactions on the translational energy in the range E_T = 0–40 kcal/mole has been discussed in an idealized collision model of hard-sphere interaction between colliding particles. In all reaction of the family, the cross section increases sharply with E_T showing an Arrhenius-like positive energy dependence for E_T just past threshold and then takes a maximum value. The maximum value is largest for the Cs reaction and decreases with the alkali mass except that it slightly increases from Rb to K, and the peak becomes broader as the mass decreases. In the post-maximum region the cross section decreases slowly with E_T.     

Water proton spin-lattice relaxation behaviour in heterogeneous biological systems

The water proton spin-lattice relaxation rate has been measured for a condensed 50% deuterated erythrocyte water system. Nuclear relaxation data, obtained in non-selective and selective modes, indicate that cross relaxation between erythrocyte and water protons occurs. The observed selective relaxation enhancement is interpreted in terms of intermolecular nuclear interactions modulated by motions which satisfy the ω_oT_c > 1 condition. Selective relaxation rates are here proposed to be more sensitive to interface characteristics than the non-selective ones.     

Multiple collisions in molecular beam scattering experiments

Due to the forward peaked differential cross section for elastic atom—atom scattering the effect of multiple collisions has to be considered in the analysis of crossed beam measurements of the total cross section and especially of the small angle differential cross section at large values of the beam attenuation. At angles θ ≈ θ₀, with θ₀ the quantum mechanical scaling angle of the elastic differential cross section, the correction for the latter case amounts to 20% at beam attenuations I/I₀ = exp(?1). Firstly, a careful analysis of the probabilities for single and multiple scattering is given, resulting in an expression for the measured beam signals which is correct for all values of the beam attenuation. The probability for multiple scattering is then calculated for an inverse power potential V(r) = ?C_sr^?s, with s = 4 through s = 7, which include both the case of ion—atom scattering (s = 4) and atom—atom scattering (s = 6). The results are given as effective differential cross sections σ_n(θ) for n-fold scattering. They are described by a single, simple analytical function with four free parameters that have been determined for n = 2, 3 and 4 by a least squares method. The σ_n(θ) are normalised to the total cross section Q.     

K-shell ionization induced by 30 MeV/u argon and neon beams

We have studiedK-shell ionization induced by 30 MeV/u Ne and Ar projectiles on target atoms with atomic numbers ranging from 27 to 90. X-ray production cross sections and energy shifts were measured with Si(Li) detectors. In most cases satisfactory agreement between measurements and theoretical direct ionization cross sections is obtained when the contribution of electron capture is included. The influence of multiple ionization on the fluorescence yield ω _K is discussed.     

Multiquantum rotational transitions in the mechanism of vibrational energy exchange: Application to CO*-CO

The present study introduces a simplified method for treating method for treating vibrational-rotational coupling during the collisional exchange of energy. This rotational coupling modifies the vibrational energy mismatch, ω₀ and is a dominant effect, working to make the actual energy mismatch smaller than the apparent energy discrepancy. The calculation is applied to the analysis of the CO^*-CO system and it demonstrates the effect of collisions involving transitions in which ΔJ>1.