M Crance

Measurement of the three-photon detachment cross sections of the negative ions of iodine, bromine and fluorine at the wavelength 1.0642 mu m

The three-photon detachment cross sections of the ions F-, Br- and I- are measured, using the fundamental 1064.2 nm wavelength of a single-mode Nd:YAG laser. The detachment signal is measured by detecting the neutral atoms produced from an ionic beam of kinetic energy 1.2 keV. The data analysis takes into account the laser pulse time profile, the actual spatial profile of the laser beam in the interaction region, and the fast motion of the ions through the laser beam during the laser pulse. Saturation of the three-photon process is observed and used to normalise the detachment signal. The authors obtain sigma (F-)=6.1

Theory of two- and three-photon ionisation of metastable helium atoms

+2.6-1.8*10-95 s2 m6, sigma (Br-)=1.6+0.8-0.6*10-94 m6 and sigma (I-)=3.3+16-1.1*10-94 s2 m6. These measured cross sections are compared with theoretical predictions. The result obtained on F- agrees with a very recent measurement performed with a different technique.

Two-photon ionisation of atomic hydrogen in the presence of one-photon ionisation

Calculations of discrete oscillator strengths, of AC Stark shifts and of single and multiphoton ionisation cross sections are presented for helium atoms. The wavefunctions of excited (bound and continuum) states are determined in the framework of the central field approximation; two optimal parametric potentials are determined for singlet and triplet systems. The reliability of the wavefunctions is shown by comparing oscillator strengths for various transitions and photoionisation cross sections for the 21S, 23S, 21P and 23P states with previous accurate theoretical results. Cross sections for two- and three-photon ionisation of 21S and 23S metastable states are calculated for linearly polarised light for a large range of photon frequencies. The AC Stark shifts of resonant transitions are evaluated. Cross sections and shifts are calculated by means of perturbation theory; the infinite summations over intermediate states are evaluated by means of perturbed-function solutions of single-particle equations. The results are compared with experiment as well as with previous calculations.

Four-photon ionisation in Cs I near the 6f resonance

The authors propose a numerical method to calculate N-photon ionisation cross sections, when ionisation by N-1 photons is energetically possible. They present some results obtained for N=2. Cross sections obtained for two-photon ionisation of ns states of hydrogen by circularly polarised light are in excellent agreement with previous theoretical ones. As an example the method is also applied to lithium atoms.

Multiphoton ionisation of hydrogen. A non-perturbative calculation of strong-field effects

Presents the first application to a physical case of the effective-operators method introduced by Armstrong, Beers and Feneuille (1975) to interpret resonant multiphoton ionisation. In particular, the four-photon ionisation of Cs I near the 6f resonance is considered. Atomic quantities needed for the evaluation of effective operators are obtained by using a non-relativistic central-potential model. Theoretical results are compared with experimental data recently obtained by Morellec, Normand and Petite (1976). Taking into account the uncertainty of the measurements, the main experimental features and, in particular, the shapes of the resonance curves are reproduced well by the calculation described here.

Multiphoton detachment from negative ions of halogens

The multiphoton ionisation probability and branching ratios towards different continua are calculated for the hydrogen ground state by applying the complex dilatation method with a finite basis of square integrable wavefunctions. The evolution of the process is studied and interpreted at lambda =0.53 mu m up to an intensity of 1.3*1014 W cm-2.

A new method for measuring relative oscillator strengths using a CW dye laser

Multiphoton detachment cross sections are calculated by applying the complex dilatation method with a finite basis of square integrable wavefunctions. The ion ground state is represented by Hartree-Fock wavefunctions. Excited states are taken as a Hartree-Fock core plus a free electron. Scaling laws are obtained for photodetachment cross sections. Two-photon detachment cross sections are in agreement with previous results.

Excess-photon absorption in a negative ion

A new method for measuring relative oscillator strengths is described. The basic idea is the same as in the classical hook method, i.e. the study of anomalous dispersion of a vapour in the neighbourhood of an absorption line by using a two-beam interferometer. The improvement consists in measuring directly the optical path difference as a phase. Therefore in principle very precise values of relative oscillator strengths can be obtained, provided that the analysis of the transmission is performed with a sufficiently high resolving power. In order to solve this problem, the two transitions 1s5-2p6 and 1s5-2p4 of Ne I have been studied. The ratio of oscillator strengths (f1s5-2p6)/(f1s5-2p4)=2.60+or-0.15 was obtained.

Measurements of line strengths in the 2-0 band of CO

A time-of-flight and angular selection of the electrons ejected by multiphoton detachment of F- at the wavelength 1064 nm yields the first experimental evidence for excess-photon absorption in a short-range potential. This makes the coherent character of excess-photon ionization or detachment conspicuous, for no photons could be absorbed by an electron which has untied its bonds to the atomic core. The experimental angular distributions are compared to theoretical angular distributions which were obtained within the free-electron approximation. The time-of-flight spectra of the ejected electrons show that slow detached electrons can receive a super-acceleration of more than 1 eV from ponderomotive effects.

The influence of space charge on the angular distribution of electrons ejected after multiphoton ionisation

A high resolution Fourier transform spectrometer has been used to measure the band strength of the first overtone of CO by studying anomalous dispersion along the band. The value obtained is S20=2.11+or-0.03 cm-2 atm-1 at 273K.