Showing papers in "Journal of Physics B in 1976"
TL;DR: In this paper, reduced atomic matrix elements are derived for arbitrary field strengths and first-and second-order correlation functions in the scattered field are also obtained and discussed in relation to the scattered spectrum and intensity-fluctuation measurements.
Abstract: The description begins with an operator master equation for the atom plus incident field. Reduced atomic matrix elements are derived for arbitrary field strengths. First- and second-order correlation functions in the scattered field are also obtained and discussed in relation to the scattered spectrum and intensity-fluctuation measurements. This formalism has the appealing feature that all information is readily available from the one set of four coupled equations. The deficiencies in both the one-photon approximation and the semiclassical perspective are established in a natural and transparent fashion.
251Â citations
TL;DR: In this article, a cross-beam method was used to study vibrational excitation of HF and HCl by electron impact at very low collision energies, and the absolute cross sections reached values of 10-15 cm2 for the threshold peaks in the nu = 1 channel.
Abstract: Differential scattering experiments with the crossed-beam method have been performed to study vibrational excitation of HF and HCl by electron impact at very low collision energies. The excitation functions for nu =1 and nu =2 have been measured from threshold to some eV above. In each case, an unusually strong and sharp threshold peak is observed with a broad resonance region following thereafter. The absolute cross sections reach values of the order of 10-15 cm2 for the threshold peaks in the nu =1 channel. The angular distributions are isotropic in the whole resonance region. The threshold resonances are interpreted as quasi-bound 2 Sigma + states in the dipole field possibly correlated with H*+X-. The broad resonance region is attributed to a very short-lived resonance state which may be the HX- 2 Sigma + state correlated with ground-state fragments H+X-.
216Â citations
TL;DR: In this article, an electron spectrometer has been constructed for the study of elastic and inelastic electron scattering processes, which is used to measure differential cross sections of electrons elastically scattered by He, Ne, Ar and N2 at impact energies between 100 and 3000 eV and scattering angles between 5 degrees and 55 degrees.
Abstract: An electron spectrometer has been constructed for the study of elastic and inelastic electron scattering processes. Up to now the apparatus has been used to measure differential cross sections of electrons elastically scattered by He, Ne, Ar and N2. Direct absolute cross section measurements were performed on N2 at 500 eV impact energy and at scattering angles between 5 degrees and 9 degrees . Relative cross section measurements were done on He, Ne, Ar and N2 at impact energies between 100 and 3000 eV and scattering angles between 5 degrees and 55 degrees . The relative cross sections were put on an absolute scale by means of the apparatus calibration factor derived from the absolute measurements on N2. Analysis of the exponential behaviour of the differential cross section as a function of momentum transfer yielded apparent polarizabilities of the target.
197Â citations
TL;DR: In this paper, absolute differential elastic cross sections were measured for electrons scattered between 2 degrees and 150 degrees by static gas targets of argon, neon, nitrogen and carbon monoxide.
Abstract: Absolute differential elastic cross sections were measured for electrons scattered between 2 degrees and 150 degrees by static gas targets of argon, neon, nitrogen and carbon monoxide. The energy ranges were 20-800 eV for argon and nitrogen, 50-800 eV for neon and 200-800 eV for carbon monoxide. The present nitrogen cross sections agree with those of Bromberg (1974) and Jansen et al. (1976) at the higher energies and with Srivastava et al. (1975) at the lower energies. Within a relative uncertainty of 5% the isoelectronic nitrogen and carbon monoxide cross sections were equal at all angles for 500 and 800 eV. At 200 eV the nitrogen cross sections were generally 5-20% larger at angles up to 120 degrees .
162Â citations
TL;DR: The second-order correlation function for light scattered at resonance from a two-level atom is calculated in this article, and the measurement of intensity correlations employing either homodyne or heterodyne techniques provides a novel approach for the extraction of linewidth and splitting frequencies.
Abstract: The second-order correlation function for light scattered at resonance from a two-level atom is calculated The measurement of intensity correlations employing either homodyne or heterodyne techniques provides a novel approach for the extraction of linewidth and splitting frequencies
161Â citations
TL;DR: The results for the He++N2 reaction agree with the previously established branching ratio and considerable differences are apparent between the product distributions obtained and those previously reported and an explanation is attempted.
Abstract: The reactions of He+ ions with N2, O2, CO2, and CH4, and of C+ and N+ ions with O2 are presented. The measurements were carried out in the SIFT apparatus (selected ion flow tube), which involves the injection of a mass-selected positive-ion beam into a flowing neutral gas into which a reactant gas is introduced at a position downstream in the flow. Data acquisition and analysis are made by the flowing afterglow technique. The results for the He++N2 reaction agree with the previously established branching ratio. For the other reactions, considerable differences are apparent between the product distributions obtained and those previously reported and an explanation is attempted. A brief discussion of the reaction mechanism is presented for each reaction in the light of the determined product distributions, and the anticipated development of the SIFT technique for ion-molecule reaction mechanisms is outlined.
151Â citations
TL;DR: In this paper, an analysis of the data, leads to partial oscillator strengths for a variety of dissociative processes involving excitations of two-electron states and deep valence-shell states.
Abstract: Dipole oscillator strengths for absorption and for formation of parent and fragment ions of N2 and CO in the energy range of 10-60 eV have been obtained, using techniques of energy-loss electron spectroscopy and electron-ion coincidence. A quantitative collection has been made of the fragment ions regardless of their initial kinetic energy. An analysis of the data, leads to partial oscillator strengths for a variety of dissociative processes involving excitations of two-electron states and deep valence-shell states. In addition, the branching of some of these dissociative states of CO+ over the various fragmentation channels is reported. Evidence is presented for the validity of the 'shake-up' model which has been used in calculating relative intensities of the C2 Sigma + states with respect to those of the B2 Sigma + states of the two molecules.
132Â citations
TL;DR: In this article, a Wannier-like stationary configuration of slow electrons in the field of a residual ion is shown and solutions of the Schrodinger equation in this limited region of space are obtained and applied to the investigation of the cross section for multiple ionization near threshold.
Abstract: Many slow electrons in the field of a residual ion are shown to have Wannier-like stationary configurations. Solutions of the Schrodinger equation in this limited region of space are obtained and applied to the investigation of the cross section for multiple ionization near threshold. Numerical results are presented for double escape in several spin and angular momentum states and for triple escape in the 2S and 4S states.
122Â citations
TL;DR: In this paper, differential and integral cross sections for the excitation of the (100, 001) and (010) modes are given in absolute units, and two distinct resonance regions are observed: strong and sharp threshold resonances and a broad resonance region centred around 6-8 eV.
Abstract: For pt.I see ibid., vol.9, no.14, p.2521 (1976). Differential scattering experiments with the crossed-beam method have been performed to study vibrational excitation of H2O by electron impact for collision energies from threshold to 10 eV. Differential and integral cross sections for the excitation of the (100, 001) and 010 modes are given in absolute units. Two distinct resonance regions are observed which are superimposed on a background of direct excitation: strong and sharp threshold resonances and a broad resonance region centred around 6-8 eV. The broad enhancement of the cross sections around 6 eV is attributed to a very short-lived 2A1 state of H2O-. The threshold resonances are interpreted as quasi-bound states in the dipole field of H2O. They are expected to be of general importance in electron-polar-molecule interactions.
118Â citations
TL;DR: In this paper, cross sections for s, p and d have been computed in the framework of a single-electron model (non-relativistic) by using a parametric central potential.
Abstract: Cross sections for s, p and d have been computed in the framework of a single-electron model (non-relativistic) by using a parametric central potential. The evolution of the non-hydrogenic behaviour of photoionization cross sections near threshold is studied along Rydberg series; non-hydrogenic characters are particularly large for all the ns series and for the np series of K. The validity of the method is discussed and it appears that sigma sigma np and sigma nd values are more reliable than sigma ns. The results are compared to experimental data and other theoretical results, mainly provided by the quantum-defect theory. The agreement between the various results is generally very good for sigma np and sigma nd; the agreement is satisfactory even for the most unfavourable cases provided by sigma ns of Na and K.
105Â citations
TL;DR: In this article, the authors presented a new method of calculating the dispersion force between two atoms, which is expressed in terms of physical properties of the separated atoms and which approaches a well known inverse-power series as the nuclear distance goes to infinity.
Abstract: Presents a new method of calculating the dispersion force between two atoms. This force has usually been calculated by expanding the electric interaction into the interaction between the multipole moments; this method, however, has the defect of giving a divergent series for the dispersion force. The new method gives a convergent series which is expressed in terms of physical properties of the separated atoms and which approaches a well known inverse-power series as the nuclear distance goes to infinity. The method is applied to hydrogen atoms.
TL;DR: In this paper, the angular momentum reduction to a sum of radial integrals is carried out completely in an approximation valid for all problems except those involving very tightly bound electrons, and the coefficients are presented in a form suitable for computation.
Abstract: The Breit interaction which gives the leading correction to the Coulomb potential in quantum electrodynamics, is required for precision calculations of binding energies of inner electrons in heavy atoms for calculations of multiplet structure, and for other applications. The correct form for use in multi-configuration calculations is discussed, and the angular momentum reduction to a sum of radial integrals is carried out completely in an approximation valid for all problems except those involving very tightly bound electrons. The coefficients are presented in a form suitable for computation; a computer program for matrix elements of the Breit interaction for general atomic states is at present under test.
TL;DR: In this paper, some forms of asymptotic expansions are suggested for wavefunctions of charged particles scattering on systems with charge, multipole moments and inner degrees of freedom, and the recurrence relations necessary for the calculations are obtained.
Abstract: Some forms of asymptotic expansions are suggested for wavefunctions of charged particles scattering on systems with charge, multipole moments and inner degrees of freedom. The recurrence relations necessary for the calculations are obtained. Formulae are written for collisions of electrons with ions or atoms. The methods suggested may also be used in the investigation of collisions between other particles.
TL;DR: In this paper, the hook method was used to measure the gf values for Ca I, Sr I, Ba I and Ba I in the principal series of Mg I, Ca I and Sr I to obtain a value of photoionization cross section at the threshold.
Abstract: The hook method (distortion of interferometric channel-spectra in the neighbourhood of absorption lines by anomalous dispersion) has been used to measure the gf values for Ca I, Sr I and Ba I The gf values of lines in the principal series of Mg I, Ca I and Sr I have been plotted by the quantum-defect method to obtain a value of the photoionization cross section at the threshold Extrapolations of the bound state gf values for Mg I, Ca I and Sr I yield cross sections at threshold of 236, 275 and 112 Mb respectively, all about two times larger than previous values based on vapour-pressure data
TL;DR: In this article, the gross spectral behaviour of dipole oscillator strengths for absorption, ionization and dissociation of H2, HD and D2 was measured over the energy range of 0 to 70 eV, using the technique of detection of forward-scattered 8 keV electrons in delayed coincidence with the ions.
Abstract: The gross spectral behaviour of the dipole oscillator strengths for absorption, ionization and dissociation of H2, HD and D2 has been measured over the energy range of 0 to 70 eV, using the technique of detection of forward-scattered 8 keV electrons in delayed coincidence with the ions. A quantitative collection of ionic fragments is ensured up to kinetic energies of 20 eV. The absorption results are in excellent agreement with those of Samson and Cairns (1965). By subtracting the total ionization spectrum from absorption, a region about 1 eV wide is found above the ionization potential where part of the oscillator strength goes into neutral formation. No appreciable isotope effect is observed in this region.
TL;DR: In this article, cross sections for the ionization of metastable helium atoms, predominantly in the 23S state, at electron energies from the threshold to 1000 eV have been measured using the crossed electron and fast atom beam technique.
Abstract: Cross sections for the ionization of metastable helium atoms, predominantly in the 23S state, at electron energies from the threshold to 1000 eV have been measured using the crossed electron and fast atom beam technique. Corrections are made for the presence of ground-state atoms and a very small concentration of highly excited atoms. The semiquantal binary encounter data agree satisfactorily up to 20 eV, but less well at higher energies. From 200 to 1000 eV, the measured data agree well with Bethe-Born cross sections of Briggs and Kim (1971) for He(23S) provided a correction is made to take account of a spurious contribution to the ionization signal which is only significant at the higher energies. All the various Born cross sections, at high energies, are substantially lower than the present measurements and also the Bethe-Born data, but they are either comparable with or smaller than the measured cross section at its peak.
TL;DR: In this paper, measurements of electron scattering at low energy from Mg, Zn, Cd, and Hg have been carried out using the electron transmission method and a large shape resonance is observed for each element which is identified with the (ns2np)2P ground state of the negative ion.
Abstract: Measurements of electron scattering at low energy from Mg, Zn, Cd and Hg have been carried out using the electron transmission method. A large shape resonance is observed for each element which is identified with the (ns2np)2P ground state of the negative ion. The electron affinities in eV are found to be Mg(-0.15), Zn(-0.49), Cd(-0.33) and Hg(-0.63) with an error of +or-0.04 eV. The results are compared with the available theoretical predictions.
TL;DR: In this paper, general expressions for the angular distribution and polarization of photoelectrons in the region of autoionizing states have been obtained, and it is shown that when an autoencoder has two or more channels of decay, a resonance structure can occur in both the angular distributions and the polarization.
Abstract: General expressions for the angular distribution and polarization of photoelectrons in the region of autoionizing states have been obtained. It is shown that when an autoionizing state has two or more channels of decay a resonance structure can occur in both the angular distribution and the polarization. For the autoionizing states nsnp6mp of Ne and Ar, possible shapes for the energy dependence of the angular distribution have been discussed. This dependence is sensitive to the relation between the matrix elements which characterize excitation and decay of the autoionizing state through the different channels.
TL;DR: In this article, a scaling law for excitation via 2p sigma -2p pi coupling in ion-atom collisions was derived. But the scaling law was not tested with ab initio calculations using both Hartree-Fock and hydrogenic molecular orbitals.
Abstract: For pt.II see ibid., vol.8, no.11, p.1909 (1975). A law is derived which allows universal curves to be given for excitation via 2p sigma -2p pi coupling in ion-atom collisions. The scaling law is tested by comparison with ab initio calculations using both Hartree-Fock and hydrogenic molecular orbitals, and with experiment.
TL;DR: In this paper, the Stark energies for different states of a hydrogen atom in a uniform electric field calculated by the Rayleigh-Schrodinger perturbation theory and diagonal Pade approximants are compared with the results of exact numerical calculations.
Abstract: For pt.III see ibid., vol.12, p.2637 (1979). Stark energies for different states of a hydrogen atom in a uniform electric field calculated by the Rayleigh-Schrodinger perturbation theory and diagonal Pade approximants are compared with the results of exact numerical calculations. Certain general conclusions are formulated.
TL;DR: In this paper, the low-lying energy spectrum of Hf III has been studied in terms of wavefunctions including the 5d2, 5d6s, 6s2 even-parity manifolds and the 6s6p oddparity manifold, and the results show that the valence orbitals are substantially modified by direct and indirect relativistic effects.
Abstract: Methods of implementing ab initio relativistic self-consistent field techniques for atomic configurations containing open shells are discussed with a view to economical calculation of a whole group of atomic states. The low-lying energy spectrum of Hf III has been studied in terms of wavefunctions including the 5d2, 5d6s, 6s2 even-parity manifolds and the 5d6p, 6s6p odd-parity manifolds. The results show that the valence orbitals are substantially modified by direct and indirect relativistic effects, so that non-relativistic models must require substantially longer and more complex calculations to give results of comparable precision.
TL;DR: In this paper, the rotational excitation of a rigid rotor is formulated in a body-fixed frame of reference, and a simple form of the close-coupling equations to reduce the amount of computing time is taken.
Abstract: The problem of rotational excitation of a rigid rotor is formulated in a body-fixed frame of reference. Full account is taken of the simple form of the close-coupling equations to reduce the amount of computing time. In particular, the excitation of carbon monoxide (1 Sigma ground electronic state) by helium (1S ground electronic state) is considered, in the temperature conditions of the interstellar medium. Also, the validity of the centrifugal decoupling approximation is examined. It is found that good agreement is obtained in the partial cross sections for low J, the total angular momentum, and low rotor levels; for high J, the centrifugal decoupling cross sections converge too quickly.
TL;DR: In this paper, the Wannier theory for double escape near threshold (1965) makes detailed predictions for the energy and angular distributions of the outgoing electrons and the numerical coefficient in front that governs these distributions can be derived as a function of the charge Z on the residual core.
Abstract: The Wannier theory for double escape near threshold (1965) makes detailed predictions for the energy and angular distributions of the outgoing electrons. Not only the dependence on the excess energy above threshold but even the numerical coefficient in front that governs these distributions can be derived as a function of the charge Z on the residual core. These are of interest with the advent of recent accurate experiments and computer calculations. Such coefficients are presented and it is pointed out that there is a dramatic disappearance of some correlations for Z>or=3. Recent computer calculations for Z=2 can, perhaps, be extended to verify this result. Similar non-monotonic dependences on Z are to be expected for triple and other multiple escape.
TL;DR: In this paper, the collision damping width and shift of the D1 line due to noble gases were measured and an analysis of the shape of the red wings was performed in terms of the currently available interatomic potentials.
Abstract: Measurements of the collision damping width and shift of the sodium D lines due to noble gases are reported together with an analysis of the shape of the red wings of the D1 line when perturbed with neon, argon, nitrogen, krypton and xenon. The results are discussed in terms of the currently available interatomic potentials and are compared with recent broadening and shift calculations.
TL;DR: In this paper, the doubly differential cross section for electron loss from the projectile in fast atomic collisions is obtained from a transformation to the laboratory frame of the corresponding Born cross-section for ionization of electrons in the projectile frame.
Abstract: The doubly differential cross section for electron loss from the projectile in fast atomic collisions is obtained from a transformation to the laboratory frame of the corresponding Born cross section for ionization of electrons in the projectile frame. The ejected-electron velocity distribution is always centred close to the projectile speed but the character of the ejected electrons is different in forward and backward directions. In forward directions a cusp-shaped velocity distribution arises. In backward directions the description of the distribution as due to elastic scattering of the ejected electrons from the target nucleus is justified.
TL;DR: The absolute differential cross sections of the n=2 and 33S states of helium were obtained from threshold up to 3.6 eV above at 30 degrees, 60 degrees, 90 degrees and 120 degrees scattering angles as discussed by the authors.
Abstract: The absolute differential cross sections of the n=2 and 33S states of helium are obtained from threshold up to 3.6 eV above at 30 degrees , 60 degrees , 90 degrees and 120 degrees scattering angles. The energetically-dependent transmission of the post-collision analyser of the electron impact spectrometer is deduced from the known characteristics of the ionization process near threshold. The results are made absolute by normalization to accurate elastic cross sections of helium. Integral cross sections for the excitation of the 23S and 21S states, and of the 23P and 21P states are given at 1.6 and 2 eV above threshold, respectively.
TL;DR: In this paper, a cross-beam interaction region was used and the metastable atoms were detected by a channel electron multiplier placed in their direction of drift, to avoid detection of scattered electrons and UV photons by the metastables detector.
Abstract: Cross sections for the excitation of metastable states by electron impact have been studied as functions of incident energy up to 5 eV above the lowest metastable thresholds. The FWHM of the energy spread of the incident beam was typically 20 meV. A crossed-beam interaction region was used and the metastable atoms were detected by a channel electron multiplier placed in their direction of drift. Precautions were taken to avoid detection of scattered electrons and UV photons by the metastables detector. The incident energy scale could be calibrated from the onset positions of the metastable states, whose energies are accurately known from spectroscopic data. The energies of resonance structures in the cross sections could therefore be obtained, and are compared with the results of previous metastable-excitation and electron-transmission experiments.
TL;DR: In this article, the core polarization potential of a two-electron model Hamiltonian was derived using a frozen-cores method and the results were obtained for all bound states.
Abstract: A two-electron model Hamiltonian, used for Be I, includes a core polarization potential Vpol(1,2) which has asymptotic form Vpol approximately - alpha ((r1/r12)+(r2/r22))2 where alpha is the polarizability of Be III 1s1. Solutions are obtained using a frozen-cores method. Two approximations are considered: (a) uses an expansion with core states Be II 2s and 2p; (b) includes in addition a 3d core orbital with a parameter determined variationally. For the series 1S, 3S, 1P0, 3P0, 1D, 3D, 1F0 and 3F0, results are obtained for all bound states ( epsilon 0. For the series 1P, 3P, 1D0, 3D0 converging to Be II 2p, results are obtained for n
TL;DR: In this article, the ionization cross section per unit energy range and the total ionisation cross sections of CO2, CO, H2O, CH4 and NH3 due to electron impact ionization for energies varying from the ionisation threshold up to 10 keV are calculated.
Abstract: Following the semi-empirical approach of Khare (1969 and 1970) and Khare and Padalia (1970), the ionization cross section per unit energy range and the total ionization cross sections of CO2, CO, H2O, CH4 and NH3 due to electron impact ionization for energies varying from the ionization threshold up to 10 keV are calculated. The results are in fair agreement with the experimental data and other theoretical investigations.
TL;DR: In this paper, a comparative numerical study has been made in which phase shifts for the elastic scattering of electrons by H, He+ and He using these potentials are tested against those calculated from the exchange integro-differential equations.
Abstract: A number of local potentials have been suggested to represent the non-local exchange kernel in electron-atom scattering. A comparative numerical study has been made in which phase shifts for the elastic scattering of electrons by H, He+ and He using these potentials are tested against those calculated from the exchange integro-differential equations. The influence of additional long range polarization potentials is also investigated. It is shown that, except at the lowest energies, exchange effects can be represented accurately by the Furness-McCarthy and related potentials.