Relativistic rescattering and multi-electron ionization of atoms and molecules in ultra-strong laser fields

We have defined and analytically derived a new parameter called “rescattering deflection parameter” ΓR to gauge the transition from strong field to ultra-strong field rescattering dynamics. We also have developed a 3D semiclassical relativistic rescattering model, including tunneling ionization and trajectory ensemble to quantify the laser-atom interaction at intense laser fields. Our analytical results for the rescattering deflection parameter and the numerical results obtained using the rescattering model are in very good agreement with each other. For 800 nm laser field, the ultra-strong field onset happens at 3 1016 W/cm2 and for longer wavelength radiation Lorentz force becomes significant even at lower intensities.;Ion yields from neon (Ne+ to Ne+8) and xenon (Xe+7 to Xe+12) at intensities from 10 14 W/cm2 to 1018 W/cm2 are reported along with the calculated ion yields from the rescattering model. For all the charge states measured here, the ADK model agrees very well with the measurement near saturation. Below saturation, the measurement clearly shows the characteristic “knee structure” (non-sequential ionization) in the measured ion yields from neon and xenon for charge states that saturate below an intensity of 3 1016 W/cm2 and also the presence of cross-shell ionization in xenon from L-shell to k-shell. The gradual decrease in the observed NSI yields below 3 1016 W/cm 2 is attributed to the decrease in the ionization cross section as one moves to higher charge states. Finally, for intensities beyond 3 10 16 W/cm2, the Lorentz deflection due to laser magnetic field becomes significant and start to suppresses the observed NSI yield by moving the returning electron wavepacket along the laser propagation direction. Below saturation, the model predicts the general trend in the non-sequential ionization, but the actual agreement is anywhere between 1% to 15%.;We have measured carbon fragmental (C+2 to C+5 ) ion yields from methane from both the linear and circular polarized laser fields from the onset of Coulomb regime (1014 W/cm 2) up to relativistic regime (1018 W/cm2). The linear polarization data clearly shows the presence of “knee structure” with all the charge states measured here and the knee structure is partially and completely suppressed for C+4 and C+5 respectively in circular polarized field. This result indicates C+2 and C+3 are produced through Coulomb explosion mechanism and the higher charge states C+4 and C+5 are increasingly produced through atomic-like tunneling and rescattering mechanism as one moves to ultra-strong laser fields. We have also measured photoelectron spectra from methane at intensities 8 1015 W/cm2 and 7 1018 W/cm2 in energies up to 0.6 MeV. The measured photoelectron spectra are in very good agreement with an atomic carbon ionization model at ultra-strong laser field. This result corroborates the results from carbon ion measurements that methane responses to ultra-strong laser field in atomic-like manner. The non-sequential ionization yield calculated for atomic carbon ionization is in good agreement with measured C+5 ion yield from methane, further corroborating our earlier results. This result can be generalized to larger molecules as the measured carbon ion yields from ethane, butane, and octane are nearly similar to the methane results.;The rescattering dynamics are expected to change significantly in a 400 nm laser field when compared to the same from 800 nm laser field. The returning photoelectron flux at the parent ion is enhanced in a 400 nm laser field. In that case, it is the kinetic energy of the returning photoelectron that determines whether the rescattering ionization is feasible or not. We have calculated the ion yields for neon (Ne+2 to Ne+8) and xenon (Xe+4 to Xe+12) from 400 nm laser field at intensities from 1014 W/cm2 to 10 18 W/cm2. Compared to the similar results from an 800 nm laser field, we observe suppression of non sequential ion yields up to ∼10 16 W/cm2 and enhancement beyond that. This suppression and enhancement in the ion yields are attributed to the return electron energy being smaller or higher than the ionization potential of the ion in question. (Abstract shortened by UMI.).