Showing papers in "Journal of Physics B in 1982"

An empirical approach to K-shell ionisation cross section by electrons

Abstract: The working out of an empirical expression for the cross section for production of atomic K-shell vacancies by electrons is described. The basis is a critical analysis of the existing experimental data. The ranges for which the function is evaluated are 6

Population trapping and dispersion in a three-level system

Abstract: The population dynamics and spectra of a three-level system driven by two applied fields of arbitrary amplitudes and detunings are examined using dressed-state techniques. The authors report the existence of novel dispersive features in the variation with detuning of the intermediate-state population.

On the choice of translation factors for approximate molecular wavefunctions

Abstract: A critical analysis is presented for three different methods which introduce translation factors in the molecular approach to atomic collisions. Unlike other contributions in this field, approximate one- and many-electron wavefunctions are explicitly considered from the start. The authors' conclusions are illustrated with the calculation of the charge exchange cross section for He2++H(1s) to He+(2s,2p)+H+, which exhibits a strong origin dependence in the standard perturbed stationary states (PSS) approach. The authors' main (and new) conclusions are: (i) that the Bates-McCarroll method, and the method of projection on atomic components, are not necessarily convergent, and are exceedingly time consuming for approximate wavefunctions and (ii) that the common translation factor method is the most promising approach at present. However, before it can be used systematically in molecular calculations, more research is needed regarding its sensitivity to the analytical form of the common factor-otherwise some of the defects of the PSS approach reappear (in disguise) using this method.

Steady-state quantum interference in resonance fluorescence

Abstract: It is shown that when a monochromatic laser couples a single atomic ground level to two closely spaced excited levels the system can be driven into a state in which quantum interference prevents any fluorescence from the excited levels, regardless of the intensity of the exciting field. This steady-state interference occurs only at a particular excitation frequency which depends on the separation of the excited states and the relative size of the two transition dipole matrix elements. The results are derived from the density matrix equations of motion. It is shown that a correct description of the effect requires the inclusion of generalised Einstein A coefficients which are usually neglected in phenomenological damping theories. A dressed-state analysis is introduced to simplify the generalisation to atoms having more complex manifolds of excited states. Analogous interferences in multiphoton absorption and ionisation are also discussed briefly.

Rotational excitation of N2, CO and H2O by low-energy electron collisions

Abstract: Absolute total and differential rotational and rotational-vibrational cross sections have been measured for N2 (weak quadrupole), CO (weak dipole), and H2O (strong dipole). In the beam experiments, the range of collision energies of the electrons was chosen between 0.5 and 6 eV, and the range of scattering angles from 15 to 105 degrees was covered. The overall energy inhomogeneity of the electron spectrometer was between 10 and 18 meV FWHM, depending on the experiment. Direct and resonant rotational excitation has been measured. In the case of N2 the rotational branches with Delta J=0, +or-2, +or-4 have been fitted to energy-loss spectra and for each branch a cross section is given. The authors have proceeded similarly in the case of CO (for dipole excitation Delta J=0, +or-1, for excitation via the 2 Pi resonance Delta J=0, +or-1, ..., +or-4) and in the case of H2O ( Delta J=0, +or-1). For N2 and CO the results are compared with calculated angular dependences. Finally, the cross sections are listed according to the different types of interaction potentials and interaction mechanisms.

The effects of laser field fluctuations on coherent population trapping

Abstract: A three-level system driven by two coherent fields on two-photon resonance leads to the trapping of population in a superposition of initial and final states which is immune to further photoexcitation, manifesting itself in a narrow coherence minimum in the absorption spectrum. The authors analyse the effects of laser field bandwidths and cross-correlations on these phenomena. They find that the inclusion of laser bandwidths dephases atomic coherences which leads to the destruction of population trapping and of the narrow coherence minimum in the absorption spectrum. When the driving fields are not cross-correlated, the atomic coherences are similarly dephased in both lambda and ladder systems. If, however, the driving fields are critically cross-correlated, two-photon coherences in the lambda system are unaffected by laser fluctuations; trapping and the coherence minimum persist. For the ladder system, on the other hand, the cross-correlation does not restore the atomic coherences involved.

Behaviour of Raman resonance scattering across the K X-ray absorption edge

Abstract: A generalisation of the Kramers-Heisenberg formula is used to demonstrate the evolution of inelastic X-ray scattering into fluorescence across the K absorption edge. As an illustration the results of a recent synchrotron radiation study of the inelastic scattering in the vicinity of the Mn K absorption edge in KMnO4 by Briand et al. (1981) are analysed and found to be in qualitative agreement with the theory.

Angular distribution of photoelectrons from polarised atoms exposed to polarised radiation

Abstract: The structure of the angular distribution of photoelectrons from polarised targets exposed to polarised radiation has been analysed in the dipole approximation. It is shown in particular that a measurement of this angular distribution without a spin polarisation measurement of the photoelectron constitutes a complete experiment for polarisable one-electron atoms and many other atoms. The completeness of the type of experiment proposed permits one to extract both magnitude and phase of all participating dipole transition amplitudes from an experimentally observed angular distribution, and represents therefore the basis for a sensitive comparison between experimental and theoretical work.

Multiphoton above-threshold ionisation of xenon at 0.53 and 1.06?m

Abstract: Standard techniques of photoelectron spectrometry have been used to study multiphoton ionisation of xenon. At 0.53 mu m, up to ten-photon ionisation of the 5p shell is detected above the six-photon ionisation threshold for both the Xe+ 2P1/2 and 2P3/2 states. The intensity dependence is compared with the predictions of perturbation theory. The branching ratio 2P3/2:2P1/2 has been measured as a function of the ionisation order. At 1.06 mu m up to 21-photon ionisation has been detected above the eleven-photonionisation threshold.

The quadratic zeeman effect in hydrogen rydberg series: application of sturmian functions

Abstract: The authors describe a method of calculating highly excited states of hydrogen in uniform magnetic fields of moderate strength ( approximately 5T). Results for Balmer emission lines and an approximate photoabsorption spectrum for Cs are presented. The authors comment on the use of adiabatic approximations and the role of the spin-orbit interaction.

The observation of electric-dipole-forbidden inner-shell transitions in N2 and Ar by the electron energy-loss technique

Abstract: By using low-energy electron impact excitation the authors have observed the first example of an inner-shell single-to-triplet transition, namely the transition from the ground state to the (1s)-1( pi 2p) 3 Pi state of N2. The parameters omega e and Re of the triplet state have been deduced from the observed vibrational structure of the transition. The parity-forbidden transition to the (2p32/)-14p state of Ar has also been studied, and the intensity of this transition relative to that of the electric-dipole-allowed transition to the (2p32/)-14s state has been measured as a function of the incident electron energy.

Refined orthogonal variational treatment of continuum distorted waves

Abstract: Two-state coupled equations within the continuum distorted-wave representation (CDW) are derived using the time-dependent second-order Euler-Lagrange variational principle of Sil (1980). Matrix elements are simplified using generalised non-orthogonal coordinates. It is shown that the original CDW theory of Cheshire (1964) is a variation-perturbation approximation, analogous to the Oppenheimer-Brinkman-Kramers approximation (OBK) in the travelling atomic eigenstate representation. It is also shown that unlike OBK the CDW wavefunctions are in general not normalised and the CDW wavefunction normalisation integrals are therefore calculated numerically using a complex Laplace transform. The implications of a variational CDW theory of ionisation are briefly considered.

Chemical sensitivity of the Kβ/Kα X-ray intensity ratio for 3d elements

Abstract: Chemical influences on the relative Kbeta /Kalpha X-ray intensity ratio up to five percent were found for different chemical constitutions of Cr, Mn, Fe and Cu by very precise PIXE measurements. In the discussion of these effects, screening of 3p electrons by a varying 3d valence charge, as well as polarisational effects are of importance.

Spectroscopic diagnostic for ablative compression experiments

Abstract: A spectroscopic diagnostic which will be useful for laser-driven thick-shelled ablative compression targets is demonstrated. The diagnostic which uses spectral line emission from lithium-like bromine provides information from several allowed lines in a small spectral range and can be used to estimate ground state number density as well as electron density in the compressed core.

Differential cross sections for elastic scattering of electrons by CH4 in the energy range of 3 to 20 eV

Abstract: Differential, integral and momentum transfer cross sections have been determined for the elastic scattering of electrons by CH4. With the help of a crossed electron beam, molecular beam apparatus using the relative flow technique, the ratios of the elastic differential cross sections (DCS) of CH4 to those of He were measured at electron impact energies of 3, 5, 6, 7.5, 9, 10, 15, and 20 eV for scattering angles from 30 to 140 degrees . From the ratios, the absolute DCS were determined by utilising the known DCS of He. Because pure rotational excitation could not be resolved, the elastic DCS are the sum of elastic and pure rotational excitations at room temperature. Integral and momentum transfer cross sections were calculated from the DCS. The CH4 DCS are similar in size and in angular behaviour to those of Ar at impact energies below 7.5 eV, and also agree approximately with a close-coupling calculation for electron scattering from polyatomic molecules. A broad maximum in the integral and momentum transfer cross sections is located at 7.5 eV. The angular distributions at energies up to about 7.5 eV are characteristic of a d-wave type of resonance.

Relativistic effects in electron scattering by atoms. III. Elastic scattering by krypton, xenon and radon

Abstract: For pt.II see ibid., vol.14, p.559 (1981). The elastic scattering of electrons by krypton, xenon and radon atoms in the low-energy region up to 30 eV has been investigated. Direct and indirect relativistic effects are taken into account. The increase in the contribution of the direct effect with the nuclear charge of the target has been studied. Polarisation is included using a procedure based on the Pople-Schofield approximation. Partial-wave phaseshifts with l=0-8 are presented. The calculation predicts results which are in quite good agreement with measurements on total, momentum-transfer and elastic differential cross sections for krypton and xenon. There is poor agreement between theory and experiment on spin polarisation for xenon at energies of 25 and 30 eV. The present work indicates that radon exhibits the Ramsauer-Townsend effect also.

Close-coupling calculations for inelastic processes in intermediate energy ion-atom collisions

Abstract: A modified two-centre atomic orbital expansion is proposed for close-coupling calculations of inelastic electronic processes in ion-atom collisions at intermediate energy, and electron transfer cross sections are calculated for H++H and He+H+ collisions. For close collisions, the inclusion of united-atom orbitals in the expansion is shown to be essential for reproducing experimental data as well as earlier calculations based on multi-state molecular orbital or other pseudostate expansions. The convergence of two-state atomic orbital expansion calculations for KK charge transfer is assessed for various asymmetries of the collision system. It is argued that the present expansion is convenient and fast converging over a broad range of collision energies, particularly for close collisions. In another application, K-shell excitation in Ne++O collisions is modelled. At low collision velocities, no deviations from the predictions of 2p pi -2p sigma molecular orbital studies are found even with extended basis sets.

Elastic scattering of slow electrons by CH4 and H2O using a local exchange potential and new polarisation potential

Abstract: A parameter-free polarisation potential is proposed for the elastic scattering of electrons by polyatomic molecules, based on the method of Pople and Schofield and the work of Temkin. Cross sections are obtained for CH4 and H2O using this polarisation potential and a Hara local exchange potential. Agreement with experiment is good.

Parametric fits in the atomic d shell

Abstract: A study is made of the effective operator Xi that reproduces (to second order in perturbation theory) the displacements of the energy levels of dNs2 due to the interaction of this configuration with dN+1s. The matrix elements of Xi are broken up into separate n-electron parts of all the states of dN, and it is shown that Xi corresponds to quasi-spin ranks of 0 and 1 only. Contributions from the n=1 and n=2 parts of Xi to the classic electrostatic parameters Fk, alpha and beta are worked out, and it is shown that an orthogonalisation of the n=3 part to the electrostatic operators leads to corrections to the Fk but none to alpha and beta . This last result generalises to the f shell and also to all types of configuration interaction in which a single electron is excited. An orthogonalisation of the operators associated with alpha and beta is also carried out and a fit to the experimental levels of Cr IV 3d3 demonstrates that the parameters associated with the orthogonalised operators have smaller mean errors than the conventional ones.

The cross section for symmetric charge exchange of Ne+ in Ne and Ar+ in Ar at low energies

Abstract: The mobilities of Ne+ in Ne and Ar+ in Ar have been measured at 293 K at E/N values up to 1000 Td using a variable length drift tube employing Tyndall-Powell grids. Measurements were also made of the mobility of Ar+in Ar at 216K over the range 100-500 Td. The room temperature data were used to obtain the charge exchange cross section over the energy ranges 0.12-1.25 eV (lab) for Ne+in Ne and 0.11 to 0.63 (lab) for Ar+ in Ar with an uncertainty of less than +or-5%. Before use in the analysis the data were corrected for the effect of thermal motion by using a relation based on the work of Forsth and Skullerud (1979). The accuracy of this procedure was checked by comparing the predicted and experimental values at 216K and also by a Monte-Carlo calculation. Sources of error in the analysis are discussed and the derived cross sections compared with previous results.

Photoionisation mass spectrometric study of acetylene in the VUV region

Abstract: Photoionisation mass measurements on acetylene have been made between 1100 and 400 AA by using synchrotron radiation as a light source. Photoionisation efficiency curves are obtained for the molecular ion C2H2+ and also for fragment ions C2H+, C2+, CH2+ and CH+. To assign the structures of C2H2+, ab initio calculations have been performed for the excitation from the second (3 sigma g) and third (2 sigma u) occupied molecular orbitals. Two broad maxima at 930 and 810 AA between the first and second ionisation thresholds are assigned to the autoionisation from valence-type excited states. New Rydberg series (np pi u) converging to the second ionisation threshold were found. The appearance potentials of fragment ions by photoexcitation are given and discussed in terms of the heats of formation of molecular ions. For each of C2+ and CH+, two values of the appearance potential are obtained from a double-step structure. The maximum heat of formation of C2+ is estimated from the appearance potential to be 1980 kJ mole-1.

Milne's differential equation and numerical solutions of the Schrodinger equation. II. Complex energy resonance states

Abstract: For pt.I see ibid., vol.14, p.4213 (1980). The combination of Milne's theory for calculating bound-state energies and wavefunctions with the complex rotation method yields an appealingly simple and powerful tool for the computation of complex-valued resonance (Siegert) energies and wavefunctions. The method provides an unambiguous assignment of a quantum number n=0,1,... to a resonance state and permits a unified picture of bound-state and resonance properties. The numerical technique is sufficiently fast and stable to enable reliable calculations of higher resonances. This is illustrated for some model potentials. Furthermore the complex energy WKB quantisation is discussed and tested numerically.

Population trapping in photoionisation to dressed continuum states

Abstract: Continuum states in photoionisation may be dressed by laser coupling to bound states. The authors show that the photoexcitation of such dressed continuum states can lead to the trapping of part of the population in a coherent superposition state and the diminution of total ion yields.

Rydberg to plasma evolution in a dense gas of very excited atoms

Abstract: A Rydberg atom gas quickly evolves into a plasma above a threshold density. The process is attributed to Rydberg-Rydberg and Rydberg-electron collisions occurring faster than the medium expansion. Observations are found in good agreement with a simple rate equation model. This effect is different from a Mott transition. Conditions for the occurrence of this latter effect in a Rydberg atom gas are briefly discussed.

Ionisation of H2 by fast protons and multiply charged ions of He, Li, C, N and O

Abstract: A crossed-beam coincidence technique, previously developed in this laboratory and used in measurements of the ionisation of H by protons and multiply charged ions, has now been applied to studies of non-dissociative and dissociative ionisation of H2. Cross sections sigma ni for nondissociative ionisation and cross sections sigma di for the sum of the dissociative ionisation channels have been separately determined for 38-1500 keV amu-1 H+, 31-550 keV amu-1 He2+, 30-388 keV amu-1 Liq+ with 1

High-resolution spectra from inner-shell transitions in highly ionised chromium (Cr XIX-XXIII)

Abstract: High-resolution spectra of chromium in the wavelength interval from 2.18 to 2.24 AA have been obtained from TFR plasmas using a bent-crystal spectrometer. The authors observe the characteristic resonance, forbidden and intercombination lines of the helium-like Cr XXIII ion together with 1s2sq2pm-1s22sq2pm-1 transitions from Cr XXII down to Cr XIX. All observed features are identified as the most intense lines obtained from ab initio calculations. Electron temperatures and ion abundances are derived from relative line intensities.

Total electron detachment cross sections for collisions of H- and He and F- with atomic and molecular targets

Abstract: Absolute total electron detachment cross sections for collisions of H- and D- with He, and F- with He, Ne, Ar, CO, N2 O2 and CO2 have been measured in the energy range from about 1 up to 300 eV. The results for H-(D-)+He are found to be in very good agreement with the recent theoretical calculations of Gauyacq (1980) and Taylor and Delos (1982). The thresholds for electron detachment in F- collisions are found to be approximately 7 eV for all targets except O2, which has a threshold of about 4 eV. For this latter case, evidence of some charge transfer producing O2- (or O-+O) is also observed.

The boron K photoabsorption spectra of BF3, BCl3, and BBr3

Abstract: The photoabsorption cross sections of BF3, BCl3, and BBr3 were obtained in the region between 190 and 280 eV using synchrotron radiation. Above the B K ionisation threshold, a broad band due to shape resonance was observed for BF3, while no counterpart was observed for BCl3 and BBr3. Instead, a new strong band was found below the B K ionisation threshold in the spectra of BCl3 and BBr3. The origin of the shape resonances is discussed by comparing the spectra of these three molecules. Assignments of the B K spectra of BF3, BCl3 and BBr3 and of the Cl L spectrum of BCl3 are given with the aid of ab initio calculations.

Spectroscopic and kinetic analysis of the VUV emissions of argon and argon-xenon mixtures. I. Study of pure argon

Abstract: The VUV luminescence spectra of argon excited by alpha particles at a few hundred Torr present two continua, at 180 and 127 nm. The 127 nm emission is very intense, it corresponds to transitions from the lowest vibrational levels of the A 1y(3 Sigma u+) and B Ou+(1 Sigma u+) states of Ar2 towards the dissociative X Og+(1 Sigma g+) ground state. The shape of the pulse corresponding to the emission of the second continuum was reconstituted by time amplitude conversion. Three exponential terms can be seen. The variation of the time constants with pressure allows a kinetic scheme for the formation and decay of the radiative molecule species to be established. The excited states of the argon molecule are created by three-body collisions, from the (1)()J=1 and (1)()J=2 states of the 3p54s configuration, the collision constants being 1.6 and 13.8 Torr-2 s-1 respectively. The radiative lifetime of the A 1u(3 Sigma u+) state is 2.86 mu s.

Threshold effects in strong-field photodetachment

Abstract: Introduces a simple model of laser-induced negative ion photodetachment, incorporating the Wigner power law, in order to discuss non-perturbatively the effects of strong exciting laser fields near to threshold. The authors find that the threshold can be smeared out and shifted slightly, as might be expected. More interesting is the possibility of observing power law (t-3) and other deviations from the exponential decay of bound electron probability near to threshold.