Transcript Overview
Ultrafast Laser Interactions with Atoms, Ions and Molecules I.D.Williams B. Srigengan, M. Suresh, J. McKenna W.R. Newell W.A. Bryan, S.L. Watson, E.M.L. English Research Interest…. • Strong field ionization of atoms (noble gases), ions and small molecules (H2, H2+, CO2, N2) • Tests of non-sequential ionization by comparing linear to circular laser polarization effects • Pulse lengthening experiments on ionization of small molecules, namely H2+ • Ionization of metastably produced ionic targets (C+, Ar+, Xe+) Strong Field Ionization…. Laser Intensity ~ 1016 Wcm-2 Electric field ~ 2x1011 Vm-1 Field Free i.e. similar to atomic Coulomb field strength Alternating electric field gradient lowers the Coulomb barrier Tunnelling Barrier width decreases hence there is an increased probability of the electron tunnelling through it Over-the-Barrier Ionization If barrier height is lowered below electron potential level, the electron escapes over the barrier Sequential Ionization…. Walker et al PRL, 73, 1227 (1994) An example for He…… The normal Sequential ionization of an atom is indicated by the blue curves Initially the He atom is ionized through tunnel or multiphoton ionization to form He+ He He+ + e- S The newly formed He+ ion is then further field ionized to form He2+ He+ He2+ + eThis is a sequential stepwise process Nonsequential Ionization…. Walker et al PRL, 73, 1227 (1994) The observed double ionization yield is enhanced (red curve) by orders over magnitude above the sequential yield. This is Nonsequential ionization. Both electrons are believed to be ejected in a single step process He He2+ + 2eSeveral models have been proposed to explain this enhanced ionization; S NS • Collective Tunneling • Shake-up model • Rescattering model Rescattering model is generally most widely accepted. Rescattering model…. Linear Polarized Field The first ejected electron, before it escapes the vicinity of the atom, is driven back by the reversal of the laser field direction. Over a number of optical cycles it may recollide with a secondary electron. This electron may be ejected directly or excited to a high lying state from which it tunnels free giving double ionization. This model predicts a strong laser polarization dependence. Rescattering model…. Circular Polarized Field Unlike with linear polarization, with circular polarization the electron will be driven around the core region as opposed to back through it. The probability of a recollision is therefore much reduced for circular. Experiments have shown that the Nonsequential component is strongly suppressed for circular polarization giving credence to the Rescattering model.