Journal of Physics B: Atomic and Molecular Physics

A dressed-atom approach to resonance fluorescence in intense laser fields is presented. Simple and general results are derived which include the now well known predictions concerning two-level atoms but are not restricted to such simple cases. The positions of the various components of the fluorescence and absorption spectra are given by the allowed Bohr frequencies of the total system: atom+laser mode (dressed atom). The master equation, describing spontaneous emission from the dressed atom is solved in the limit of high intensities. Simple expressions, taking into account the effect of cascades, are derived for the widths of the components.

A pulsed crossed-beam technique incorporating time of flight spectroscopy has been successfully developed and applied to measurements of the electron impact ionisation cross sections of atomic hydrogen in the range 14.6-4000 eV. The method has been derived from a crossed-beam coincidence technique. By substituting a pulsed proton beam for the pulsed electron beam in the present work, measured electron impact ionisation cross sections have been normalised by reference to known equivelocity proton impact cross sections for both ionisation and charge transfer. The results resolve the discrepancy between earlier less accurate cross sections measured using the modulated cross-beam technique. They provide a reliable check on the range of validity of theoretical predictions over a wide energy range.

Values of the van der Waals interaction between the 2 ¹S and 2 ³S metastable states of helium and the systems Ne, Ar, Kr, Xe, H₂, N₂, O₂, CH₄, Li, Na, K, Rb, Cs and O are computed. The values are used to determine Penning ionization probabilities from the experimental data with results differing significantly from an earlier analysis. Estimates are made of Penning ionization cross sections for He (2 ³S) in the alkali metals, in atomic oxygen and in methane, for which no experimental data exist.

It is shown that the on-energy-shell single-particle scattering amplitudes obtained from the minimal-coupling and the multipolar forms of the Hamiltonian in non-relativistic quantum electrodynamics are equal to order e²; both forms of the Hamiltonian thus lead to the same differential cross section for Kramers-Heisenberg scattering. The proof given is based directly on the relations which hold between the two transition matrices and is simpler than a previous proof which relied on sum rules derived from the canonical commutation relations. The proof is also more general in that the multipolar Hamiltonian is defined in terms of line integrals along paths which are not assumed to be straight lines but which can be chosen from a large class of possible curves.

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.

The multiphoton ionisation of xenon has been studied using a picosecond Nd:YAG laser and a time-of-flight electron spectrometer with a collection angle of 2 pi sr. In the absence of space charge effects, suppression of the first electron peak is observed at an intensity of 5*10¹² W cm^-2 in linear polarisation, while saturation occurs at 1.3*10¹³ W cm^-2. In circular polarisation, the same effect is observed at a slightly lower intensity. The intensity dependences of the first three peaks have been measured.

Non-relativistic rates for the decay of 2s hydrogen atoms to the ground state by single-photon and two-photon emission in the presence of a homogeneous magnetic field of arbitrary strength (0<or=B<or=4.7*10⁶ T) are calculated by variational procedures. Over the whole range of B, two-photon emission is the dominant process. As the magnetic field grows, the two-photon decay rate increases. It is found that the Markov approximation can be applied to the two-photon decay for magnetic fields of strength B>or=4.7*10³ T.

Differential cross sections for the one-photon emission free-free process are measured as a function of laser intensity when 10.55 eV electrons are scattered from argon atoms in the presence of a pulsed CO₂ laser. The cross sections are reported for both single longitudinal-mode and multimode laser pulses up to an intensity of 2*10⁷ W cm^-2. For intensities less than 2*10⁶ W cm^-2, the cross sections for both laser pulses are found to be linear with intensity, in agreement with perturbation theory. At higher intensities comparisons are made with predictions based on the low-frequency approximation and two laser models. The linear parts of the cross sections are used to obtain estimates of the spatial inhomogeneities in the electron-laser interaction region. Reasonable agreement is found between the experimental cross sections and those predicted by the two laser models.

Stark widths and shifts of the Ar I 696.5 nm and of the Ar II 480.6, 484.7 and 434.8 nm lines have been measured in the range of electron density and temperature 0.6-1.5*10¹⁸ cm^-3, 16200-18700 K. These high-density plasmas are created in linear flash-tubes. The plasma parameters are principally determined by measurements of the continuum radiation, from the intensity of optically thick lines in their centre and by the condition of local thermodynamic equilibrium of the plasma. The electron density and temperature radial profiles so deduced are found to be practically flat over more than half of the tube radius. In this quasi-stationary stage of the plasma, the experimental line profiles are recorded by an optical multichannel analyser coupled with a high-dispersion spectrometer. The profiles are analysed and fitted to a Lorentzian function. The Stark parameters, width and shift, show a non-linear dependence against the electron density.

The following applications of spectral line broadening theory to astrophysics are briefly reviewed: (i) understanding qualitative effects visible on spectrograms; (ii) quantitative understanding of hydrogen-line profiles for the determination of stellar atmospheric parameters; (iii) effects of line broadening on the determination of stellar chemical composition.