The authors show that the effects of saturation on the ionisation probability at small impact parameters can be exhibited in ion-atom collisions where simultaneous electron capture and ionisation occur. Such effects show up for this process because capture probabilities are only appreciable at small impact parameters, whereas saturation effects hardly appear in a simple ionisation process where significant contributions come from large impact parameters. When such effects are accounted for accurately, good agreement with experiments can be obtained within an independent-electron model.
The shape of the Na D emission line wings is measured in a high-pressure sodium lamp with xenon buffer gas operated at two low values of the arc power. Under these conditions the collisional broadening of the quasi-static far wings by xenon is dominant. The author has calculated the shape of the far wings with several available sets of Na-Xe X 2 Sigma 1/2, A 2 Pi 1/2,3/2 and B 2 Sigma 1/2 interaction potentials using the classical quasi-static line-broadening theory. None of these sets gives completely satisfactory agreement with the measured Na D wings. The author proposes modifications to the available potentials and obtains good agreement with the measured blue and red Na D line wings (550-750 nm) and with the measured positions of the satellites at 560.2 and 590.9 nm. The proposed Na-Xe interaction potentials are valid for Na-Xe internuclear distances between about 0.3 and 0.9 nm.
From measurements of total, ionisation and positronium formation cross sections the authors have determined the 'total cross section without ionising channels' and compared this with calculations of the angle-integrated elastic and excitation cross sections. The comparison shows that the theoretical results for elastic scattering and excitation are in satisfactory agreement with experimental data. The measurements and calculations of positron ionisation indicate that the positron cross section is smaller than the electron cross section between threshold and about 35 eV, a behaviour which is in accordance with threshold laws derived from the Wannier theory. The partitioning of the total cross section also elucidates the 'convergence problem' of electron and positron scattering on helium, that is, the fact that measured electron and positron total cross sections merge at 200 eV, whereas for the elastic cross sections theory predicts convergence at a much higher energy.
Dipole transition moments have been calculated for the transitions A2+ to X2+, B2 to X2+, C2+ to X2+, D2+ to X2+, E2 to X2+ and B2 to A2+ in HeH. The spectroscopic constants obtained in this work support an experimental assignment for the C 2+ to A 2+ transition.
Laser excitation in the far red wing of the second principal series doublet of Cs mixed with Xe revealed a diffuse structure associated with the 2P(3/2) component. The structure is thought to originate from a reflection type of spectrum between the weakly bound E 2Sigma(1/2) excited state and the X 2Sigma(1/2) repulsive ground state of CsXe.
The authors studied in an earlier paper (1985) the production of a fast beam of H(3s) atoms. They investigated the particular problem of a coherent laser excitation of the 3s1/2 and 3p1/2 Stark states from the 2s1/2 metastable Stark state. They measured the 3s1/2 fluorescence outside the electric field and quantal interference effects have been observed as a function of the electric field strength. A simple theoretical interpretation is proposed which takes into account the quasi-adiabatic evolution of the atom in the electric field under the authors' experimental conditions.
The ionisation of highly excited Rydberg atoms in collisions with ground-state atoms is considered. The stationary diffusion-like mechanism of the ionisation is presented. The investigation is based on the Fokker-Planck equation. Analytical expressions for the probabilities of stationary diffusive ionisation are obtained and applied for the explanation of experimental results on the ionisation of highly excited Rydberg states of caesium in collisions with ground-state Cs atoms. It is concluded that the diffusive mechanism is the main mechanism for collisional ionisation of Rydberg atoms with n>or approximately=25-30.
Presents the first detailed theoretical calculation of the absorption spectrum of Sr out of the 5s2 1S0 ground state. The autoionisation region below the 5p3/2 threshold is investigated, photoionisation cross sections being calculated by combining the multichannel quantum defect theory (MQDT) with the eigenchannel R-matrix method. Small-scale variational R-matrix calculations enable one to evaluate LS-coupled short-range reaction matrices. A jj-coupled reaction matrix is then deduced using a geometrical frame transformation. Diagonalisation of this matrix gives the energy-dependent MQDT parameters required for the description of the 13 relevant interacting channels. The 13-channel MQDT model built with theoretical R-matrix parameters provides an accurate description of the positions and the shapes of the 4d3/2,5/2 np, nf and 5p1/2,3/2 ns, nd J=1 autoionising lines. Particular attention is paid to high lying resonances observed near the spin-orbit split 4d3/2,5/2 and 5p1/2,3/2 thresholds.
Measurements of superelastic scattering of 2 eV electrons from oriented Na(3P) atoms have been carried out over the angular range 10 to 120'. Results are presented in terms of Lperpendicular to , the angular momentum transferred perpendicular to the scattering plane. Comparison is made with previous experiments at small angles, and with close-coupling calculations. Good agreement is seen with the earlier experimental work, but significant disagreement is seen with theory at angles beyond 40'.
It is pointed out that, when considering electron capture in collisions of highly charged ions with atoms or molecules, a qualitative discussion in terms of a common 'reaction window' is valid only when fully stripped ions are involved. In the case of partly stripped ions with well separated subshell levels it becomes essential to define individual reaction windows for the final states of the captured electron. These reaction windows generally appear at smaller internuclear separations than in the corresponding bare-ion case. The shift is particularly pronounced for final states of high angular momentum. This observation is supported by available experimental data.
The authors have investigated the magneto-optical rotation in caesium vapour near the 6S1/2 to 6P3/2 resonance (Macaluso-Corbino effect) at 40 and 50 G, using a single-mode laser. The measurements were performed at low light intensities in order to avoid non-linear effects due to saturation or optical pumping. They find excellent agreement between their results and a complete theoretical treatment with energies and wavefunctions obtained by diagonalising a Hamiltonian that includes the hyperfine interaction. Allowance for wavefunction perturbation, neglected in most treatments, is found to be essential.
Expansion of the recently suggested relativistic two-body equation (Pilkuhn) to the order 1/c2 does not reproduce the Breit potential. Nevertheless, the equation gives the known result for the quadrupole moment in the ground state of the hydrogen atom contrary to Pilkuhn's assertion (Ibid., vol.17, p.4061, 1984).
An ejected-electron spectroscopic study has been carried out on the potassium autoionising state created by charge transfer excitation in collisions of the potassium ion with the neon atom. The ion impact energy was varied between 2.1 and 7.2 keV. A detailed analysis has been performed for the spectral profiles of autoionisation of the potassium 3p54s2 2P1/2 2 and 2P3/2 states. Their complete alignment with respect to the internuclear axis has been observed.
Photoions of H2O+, OH+ and H+ from H2O and those of D2O+, OD+ and D+ from D2O have been observed in coincidence with threshold photoelectrons near the ionic B2B2 states. It is confirmed that above the dissociation limits to OH+ and OD+, the 2B2 states of H2O+ and D2O+ predissociate to OH++H and H++OH and to OD++D and D++OD, respectively.
This is the latest in a sequence of R-matrix calculations on electron-helium scattering in which target states with successively higher principal quantum number are explicitly included in the close-coupling expansion. The 5-state (n
The spectra of atoms exposed to black-body radiation suffer temperature-dependent frequency shifts predicted earlier and now becoming measurable, whose thermodynamic status is clarified. To a good approximation, they are calculable as differences between the coupling-induced shifts in the Helmholtz free energies of the coupled system atom plus quantised electromagnetic field, in thermal equilibrium subject to the constraint that the atom is in a given state li). Effectively, the state label i plays the same role as do macroscopically controlled variables in ordinary thermodynamics. With kT well below the electron rest-mass energy, such calculations require only the forward scattering amplitude of light from the atom; some previously overlooked but theoretically interesting relativistic features of the exact expressions are pointed out. For a harmonic oscillator, uniquely, the constrained free-energy shift is the same for all levels (and therefore also the same as the shift in unconstrained equilibrium); consequently there is no thermal frequency shift at all. Recent comments on black-body-induced Stark shifts by Ford et al. (1985-6) are based on their calculation of the free energy of a harmonic oscillator in unconstrained equilibrium with the field. It is shown that as regards frequency shifts their comments are incompatible with, and misrepresent, previous correct work on such shifts.
Two synchronised pulsed single-mode tunable laser systems are used to excite atomic hydrogen in an atomic beam. A vacuum ultraviolet (VUV) laser populates first the 2p 2P3/2 sublevel from the ground 1s 2S1/2 state. A second-step UV laser selects substates of the n=33 manifold in the presence of a weak electric field (23.6 V cm-1). Stark patterns of this manifold are recorded for different polarisation configurations of the exciting light beams. The n=44 substates are detected by the standard field ionisation technique. The measured relative intensities of the Stark components agree with the theoretical excitation probabilities in the two-step process, calculated to the first order of perturbation theory.
The authors have determined the rate coefficients beta for the dissociative attachment reactions of electrons with HBr and HI molecules in their VT-FALP apparatus, and in a parallel study they have determined the rate coefficients k for the exact reverse of these reactions, namely the associative detachment reactions of Br- and I- (and additionally F- and Cl-) with H atoms using their VT-SIFT apparatus. The ratios of beta /k for these reactions yield values for the free energy changes in the reactions which, combined with their calculated entropy changes, provide values for the enthalpy changes in the reactions. The results obtained indicate, in accordance with expectations, that the HI reaction is closely thermoneutral and that the dissociative attachment reaction of electrons with HBr is endothermic. The derived bond energies in HI and HBr are within error bounds equal to those previously determined using thermochemical data. Some aspects of the kinetics of the reactions are discussed.
The generation of a squeezed state via non-degenerate four-wave mixing in a system of three-level atoms is discussed. The condition for receiving a nearly perfect squeezing is shown.
A simple relationship between the electron spin polarisation function and the electron-photon coherence and correlation parameters associated with the excitation of n3P target states by an unpolarised incident electron beam is derived for LS-coupled targets initially in a 1S0 state. This relationship is useful because it facilitates the experimental determination of the electron spin polarisation functions associated with this particular type of excitation process through the measurement of the lambda and chi coherence and correlation parameters, thus avoiding the necessity of measuring the spin of the scattered electrons directly. This relationship is illustrated by using first-order many-body theory (FOMBT) to predict the electron spin polarisation functions Sp(k, k') associated with the excitation of the n3P/sub /J (n=2, . . ., 8; J=0, 1, 2) states of helium by electron impact.
