Watson Approximation Research Articles

Dynamics of Above-Threshold Ionization and Laser-Assisted Electron Scattering inside Helium Nanodroplets.

Laser-assisted electron scattering (LAES) is a fundamental three body interaction process that enables energy transfer between electrons and photons in the presence of matter. Here, we focus on the multiscattering regime of electrons generated by above-threshold ionization (ATI) of In atoms inside a high-density nanostructure, helium nanodroplets (HeN) of ∼40 Å radius. The stochastic nature of the multiscattering regime results in photoelectron spectra independent of laser polarization. Numerical simulations via tunnel-type ionization followed by applying the Kroll-Watson approximation for LAES are in agreement with experimental spectra and yield a mechanistic description of electron generation and the LAES energy modulation processes. We find a negligible influence of the electron start position inside the helium droplet on the simulated electron energy spectrum. Further, our simulations shine light on the interplay of electron time of birth, number of LAES gain/loss events, and final kinetic energy; early ionization leads to the largest number of scattering events and thereby the highest electron kinetic energy.

Effects of polarization direction on laser-assisted free–free scattering

This work will detail the effects of laser polarization direction (relative to the momentum transfer direction) on laser-assisted free–free scattering. Such processes play a role in the gas breakdown that occurs in electric discharges as well as providing a method for the laser heating of a plasma (Musa et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 175201, Mason 1993 Rep. Prog. Phys. 56 1275). Experimental results will be presented for electron-helium scattering in the presence of an Nd:YAG laser field ( eV) where the polarization direction was varied in a plane that is perpendicular to the scattering plane. To date, all of our experimental results are well described by the Kroll–Watson approximation (KWA) (Kroll and Watson 1973 Phys. Rev. A 8 804). The good agreement between our experiments and calculations using the KWA includes the case where the polarization is perpendicular to the momentum transfer direction, for which the KWA predicts vanishing cross section; other workers have found that the KWA tends to be inaccurate for cases where it predicts small cross sections (e.g. Musa et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 175201). We also present simulations of the effects that multiple scattering might have on experimental measurements. In particular, we examine conditions that are expected to be similar to those of the experiments reported by Wallbank and Holmes (Wallbank and Holmes 1993 Phys. Rev. A 48 R2515).

Electron-impact elastic scattering of helium in the presence of a laser field: non perturbative approach

We report a detailed analysis of electron-helium scattering in the presence of a laser field; focusing on the elastic process of helium atoms from the ground state 11S. The process under investigation is dealt with a nonperturbative approach using the Volkov wave function to describe the incident and scattered electrons, while the laser–target interaction is treated by using the Floquet method. The interaction of the incident electron with the atomic target is treated within the first Born approximation. Our results are perfectly consistent with the experimental data of DeHarak et al and with the Kroll–Watson approximation results for both one and two photon emission. We have investigated the effect of nonresonant and near resonant laser field on the electron–helium elastic collision process. It was found that the differential cross section is sensitive to the intensity and the frequency of the laser field. In the case of a non resonant laser field, dressing effects are important at small scattering angles. For a near-resonant laser photon energy, those effects are strongly reduced in the forward direction.

Eletron-helium laser-assisted free-free scattering for incident energies from 30 - 200 eV: effects of polarization direction

SynopsisWe report on experiments that examine electron-helium scattering in the presence of an Nd:YAG laser field of 1.17 eV photons. At each incidentelectron energy (30, 60, and 200 eV), the laser polarization direction is varied within a plane perpendicular to the Watson approximation calculations.

Laser-induced free–free transitions in elastic electron scattering from CO2

This paper presents measurements of laser-induced free–free transitions of electrons scattered from CO2 molecules in the ground electronic state at incident electron energies of 3.8 and 5.8 eV under a pulsed CO2 laser field. The differential cross section of free–free transitions involving absorption and emission of up to two photons was measured at various scattering angles with the polarization of the laser either parallel with or perpendicular to the momentum change vector of the scattered electrons. The results of the parallel geometry are found to be in qualitative agreement with the predictions of the Kroll–Watson approximation within the experimental uncertainty whereas those of the perpendicular geometry show marked discrepancy with the Kroll–Watson predictions.

Two-body off-shell effects in the second-order Faddeev–Watson approximation for atomic ionization processes

We investigate the behaviour of the off-shell Coulomb T-matrices in the second-order Faddeev–Watson approximation to atomic ionization processes. In addition to the well-known on-shell singularities we show that, under certain kinematic conditions, there are additional singularities as the two-body ionization threshold is approached. A method for dealing with this problem is suggested and the results are applied to three different processes.

Low-energy electron - helium scattering in a laser field

We report measurements for the scattering of electrons by helium atoms through in the presence of a laser. The incident energy of the electrons was in the range 0.2 - 2.5 eV and the polarization of the laser was arranged to be parallel to the direction of the electron momentum transfer. The data were compared with the results of simple calculations assuming the Kroll - Watson approximation.

`Small angle scattering of slow electrons by helium atoms in a laser field: a collective model' by S Varró and F Ehlotzky

Varró and Ehlotzky (1995) have employed an effective laser-induced polarizability to explain the breakdown of the Kroll - Watson approximation observed for small-angle electron - helium scattering in a laser field. Their derivation omits a cancelling electronic contribution so that the coherent polarization potential they introduce is much too weak to explain the breakdown.

Low-energy electron - argon scattering in a low-frequency laser field

We have used several non-perturbative Floquet methods to calculate the differential and total cross sections for electron scattering from argon in a laser field. One method utilizes the mixed-gauge R-matrix technique to calculate the scattering cross section. A diabatic approximation adopted from molecular applications is also applied to this low-frequency limit. Results are compared with the well known Kroll - Watson approximation. We find that all three theoretical treatments agree well with each other but do not agree with some experimental results. Possible explanations for the discrepancy between the calculations and experiments are examined.

A kinematically complete experiment on the reaction n+d → n+n+p

The energy of the three nucleons produced in the break-up reaction 2H(n, nnp) at 14.1 MeV is measured in coincidence for an experimental configuration which enhances the final-state interaction of one n-p pair and gives a two-fold redundancy, essential for the rejection of false events. The analysis of the shape of the distribution in the Watson approximation, with due consideration of the influence of the other pairs in triplet or singlet states, gives the value −23.0±1.7 fm for the singlet n-p scattering length. The absolute value ofd3σ/dE1dΩ1dΩ2 at the peak of the n-p pole is 5.1±0.7 mb sd −2 · MeV−1.

The neutron-neutron scattering length from reactions with tritium targets

The 3H(n, d)2n reaction was studied at 15.1 MeV neutron energy. The deuteron energy spectrum at 0° is well reproduced by using the Watson approximation with a n−n = −17 fm. A satisfactory agreement has been found among the a n−n values derived, by the same procedure, from all but one published work with tritium targets and two neutrons in the final state.