B. Srigengan

Fast-beam study of H2+ ions in an intense femtosecond laser field

A fast beam of H2+ ions, produced from a low energy ion accelerator, has been used for the first time in intense laser field experiments. The technique has enabled neutral dissociation products to be analysed and detected for the first time in such studies. Energy spectra of neutral and ionized fragments, product yields as a function of focused laser intensity and angular distributions of neutral dissociation products have been measured. Significant differences are observed between the present results and those obtained from experiments involving neutral H2 molecules. These differences are indicative of the precursor H2 molecule playing an important and hitherto neglected formative role in the laser-induced fragmentation processes.

Geometry- and diffraction-independent ionization probabilities in intense laser fields : Probing atomic ionization mechanisms with effective intensity matching

We report an experimental technique for the comparison of ionization processes in ultrafast laser pulses irrespective of pulse ellipticity. Multiple ionization of xenon by 50 fs 790 nm, linearly and circularly polarized laser pulses is observed over the intensity range 10 TW/cm{sup 2} to 10 PW/cm{sup 2} using effective intensity matching (EIM), which is coupled with intensity selective scanning (ISS) to recover the geometry-independent probability of ionization. Such measurements, made possible by quantifying diffraction effects in the laser focus, are compared directly to theoretical predictions of multiphoton, tunnel and field ionization, and a remarkable agreement demonstrated. EIM-ISS allows the straightforward quantification of the probability of recollision ionization in a linearly polarized laser pulse. Furthermore, the probability of ionization is discussed in terms of the Keldysh adiabaticity parameter {gamma}, and the influence of the precursor ionic states present in recollision ionization is observed.

Experimental measurements of low-energy elastic scattering of electrons from ions

First experimental results are presented from a new electrostatic spectrometer designed to study electron scattering from positive ions. Measurements of elastic scattering from have been carried out over the centre-of-mass energy interval 12 - 24 eV for fixed scattering angles of , and . Results are compared with the Rutherford formula and a phaseshift calculation based on a semiempirical potential.

A technique for absolute measurements in near threshold excitation of positive ions: application to excitation of

An electron-ion crossed beams experiment has been developed in which the energy loss of scattered electrons is used to determine cross sections for excitation in positive ions. Scattered electrons are confined by an axial magnetic field and are channelled into a detector using reflecting potentials and an (trochoidal) deflection. Complete collection of these electrons along with monitoring of the overlap and current of the beams enables absolute total cross sections to be determined. This technique has been used to study near threshold excitation of the D resonance transition in . , CII, trochoidal spectrometer

Superelastic scattering of electrons from Ar3

A crossed-beams energy-loss spectrometer has been used to study, for the first time, angular distributions for the superelastic scattering of an electron from a positive ion. It is often a distinct disadvantage for collision studies that ECR sources tend to produce ions in a mixture of ground and metastable levels. We have however used this to our advantage to study superelastic scattering from metastables, observing energy gain spectra using an electrostatic hemispherical analyser fitted with a position sensitive detector. First results are presented for Ar3+ ions, with observation of the 3s23p3 2D° metastable state at 2.61 eV above the ground 3s23p3 4S° level.

Dynamic effects in low-energy electron - ion scattering

Angular differential measurements from a new electron - ion crossed-beams instrument suggest dynamic effects play only a minor role in forward scattering from almost fully dressed ions. This is contrary to the predictions of a recent many-body theory, and contrary to the case of low-energy backscattering where large dynamic effects have recently been postulated.

Low-energy elastic backscattering of electrons from Ar+

Measurements of absolute partial cross sections, integrated over the angular range 90 degrees -180 degrees , are reported for the elastic scattering of electrons from Ar+ at impact energies 3.5-6.5 eV. These constitute the first measurements of backward scattering and for energies below the inelastic threshold in the elastic scattering of free electrons from positive ions. Comparison with the predictions of the Rutherford formula, and partial wave analysis using semi-empirical static potentials, suggests that low-energy, large-angle scattering from singly charged positive ions provides an excellent regime for the testing of dynamic effects in electron-ion collisions.

On the recollision-free excitation of krypton during ultrafast multi-electron tunnel ionization

The probability of multiple ionization of krypton by 50 femtosecond circularly polarized laser pulses, independent of the optical focal geometry, has been obtained for the first time. The excellent agreement over the intensity range 100 TW cm−2 to 100 PW cm−2 with the recent predictions of Kornev et al (2003 Phys. Rev. A 68 043414) provides the first experimental confirmation that non-recollisional electronic excitation can occur in strong-field ionization. This is particularly true for higher stages of ionization, when the laser intensity exceeds 10 PW cm−2 as the energetic departure of the ionized electron(s) diabatically distorts the wavefunctions of the bound electrons. By scaling the probability of ionization by the focal volume, we discuss why this mechanism was not apparent in previous studies.

Intense laser field studies of positive ions

There is considerable current interest in the study of atoms and molecules in very strong electric fields. The advent of short pulse high power lasers has enabled new experimental studies in this area leading to the discovery of new effects. We describe an experiment designed to extend these studies to targets of positive ions produced from conventional low energy accelerators. First results with H2+ molecular ions are presented, and noted differences with previous measurements using neutral H2 molecules are discussed.

Elastic scattering of electrons from multiply charged ions

A crossed-beams energy-loss spectrometer has been used to investigate angular distributions for electron scattering from Nq+ ions, q = 1-3, at a collision energy of 10 eV. Results are compared with the predictions of a partial waves calculation based on a semi-empirical potential, and with the classical Rutherford formula.