Session M4: Focus Session: Recollision Physics

Attosecond electron emission probes of ultrafast nanolocalized fields

Ongoing experimental and theoretical work on the temporal and spatial characterization of nanolocalized plasmonic fields will be presented. Because of their broad spectral bandwidth, plasmons in metal nanoparticles undergo ultrafast dynamics with timescales as short as a few hundred attoseconds. So far, the spatiotemporal dynamics of optical fields localized on the nanoscale has been hidden from direct access in the real space and time domain. Our ultimate goal is to characterize the nanoplasmonic fields not only on a nanometer spatial scale but also on $\sim$100 attosecond temporal scale. Information about the nanoplasmonic fields, which are excited by few-cycle laser pulses with stable electric field waveform, can be obtained by the measurement of photoemitted electrons. We will present recent results on the large acceleration of recollision electrons in nanolocalized fields near dielectric nanoparticles following the excitation by 5-fs near-infrared laser pulses with controlled electric field waveforms.   

Prevalence of different double ionization pathways in driven atomic and molecular systems

We present in a three-dimensional quasiclassical framework a unified way of exloring double ionization in strongly driven atomic and molecular systems as a function of total electron escape energy and as a function of intensity [1]. Exploring the double ionization pathways the electrons follow to escape we identify the regimes of total electron energy for which the Direct pathway (simultaneous ejection of both electrons after recollision) prevails over the Delayed one (one electron ionizes with a delay with respect to the recollision time) and vise versa [2]. We also identify new double ionization pathways and explore the differences between double ionization in atoms and molecules. We also show that in the case of N$_{2}$ for intensities in the over the barrier regime that are still in the non-sequential ionization regime the correlated momenta with a characteristic square structure and the probability distribution as a function of total energy with a two peak structure probe the tunneling phase of the re-colliding electron [3]. Finally, we present in a consistent way the double ionization pathways (enhanced ionization and re-scattered double ionization) as well as single ionization pathways in the full fragmentation of the H$_{2}$ molecule in very good agreement with experimental results. [1] A. Emmanouilidou et al. arXiv:1005.3126 submitted 2010. [2] A. Emmanouilidou, Phys. Rev. A 83, 023403 (2011). [3] A. Emmanouilidou et al. arXiv:1101.4960 submitted 2011.   

Phase-tagged non-sequential double ionization of N$_2$, O$_2$, and CO in 4-fs laser fields

Being widely regarded as a prototype process for correlated dynamics, non-sequential double ionization (NSDI) has been the subject of numerous experimental and theoretical studies. It is generally understood in the framework of a recollision model. Using reaction microscope detection combined with a single shot phase meter, we phase-tag each double ionization event and thus study the sub-cycle dynamics of the NSDI by exposing the target particles to known near-single cycle waveforms. In a recent study on NSDI in argon using this technique, we obtained the CEP dependence of the total double ionization yield and the asymmetric longitudinal recoil momentum, from which our understanding of the NSDI process in atoms can be rigorously tested. Here we extend our studies to NSDI in N$_{2}$, O$_{2}$, and CO in 4-fs laser fields and gain further insight into the recollision process for molecules.   

Recollisions and correlated double ionization with circularly polarized light

One of the most striking surprises of recent years in intense laser-matter interactions has come from multiple ionization by intense short laser pulses: nonsequential double ionization rates were found to be several orders of magnitude higher than the sequential mechanism allows. This discrepancy has made the characteristic ``knee'' shape in the double ionization yields versus intensity plot one of the most dramatic manifestations of electron-electron correlation in nature. The mechanism that regulates such correlated ionizations is now settled, for linear polarization, and follows the so-called recollision or three step model. This model which works so well for linear polarization is much harder to justify in elliptic or circularly polarized fields, where the ionized electron is expecting to spiral out from the core. As a result, a common wisdom in the strong field community is that recollision is suppressed in circularly polarized fields. The matter would rest there if there were not for contradictory experiments. I will show how the recollision model has to be adapted for circular polarization and explain the two apparently contradictory experiments, the absence of recollision for helium and its presence for magnesium [1]. \\[4pt] [1] Phys. Rev. Lett. - {\bf 105}, 083002 (2010).   

Probing the giant resonance in Xe via HHG with sub-two cycle 1.8 $\mu$m laser pulses

We present high harmonic spectra of xenon obtained with a 1.8 $\mu$m sub 2 cycle laser source [1]. These spectra contain features due to collective multi-electron effects involving inner shell electrons, in particular the giant resonance at 100eV. This demonstrates a new class of collective electronic dynamics, induced and probed by the recombining electron. The large enhancement seen at 100 eV is recognized from photoionization studies as the xenon giant resonance [2]. In HHG, this enhancement is the result of a Coulomb interaction between the returning continuum electron and a bound 4d electron which is subsequently promoted to fill the 5p hole. The hole is later filled by the continuum electron and an XUV photon is emitted. This represents the first time that e-e correlations and excitation of the ion have been observed in gas phase HHG [3].\\[4pt] [1] B.E. Schmidt et al. App. Phys. Lett. 96, 071111 (2010).\\[0pt] [2] M. Ya Amusia and J-P. Connerade, Rep. Prog. Phys. 63, 41 (2000).\\[0pt] [3] A. D. Shiner et al. accepted to Nature Phys. (2011).   

Dependence of carbon fragments from methane in strong and ultrastrong elliptically polarized laser fields

We present the ellipticity dependence of the ultrafast photoionization for C$^{n+}$ fragments from methane. The study extends from the strong field (C$^{+}$, C$^{2+})$ at 10$^{14}$ W/cm$^{2}$ to the ultrastrong field (C$^{5+})$ at 10$^{18}$ W/cm$^{2}$. The measurements show that C$^{+}$ and C$^{2+}$ ionization have limited sensitivity to the field polarization. As the laser intensity and corresponding degree of ionization increase (C$^{4+}$, C$^{5+})$, the dependence on the field polarization increases. Comparison to a semi-classical field ionization model shows that the ellipticity dependence of the relative ion yield for higher charge states comes from the field dependence of tunnelling ionization rather than nonsequential ionization due to rescattering. A movement from a molecule-like response to an atom-like response with the increase in intensity is observed. This work is supported by the Army Research Office under award no W911NF-09-1-0390 and the National Science Foundation under award no 0757953.   

Electron recollisions or precollisions in elliptically polarized laser fields

We have recently predicted [1] that the degree of elliptical polarization of intense short laser pulses is related to the timing of strong-field non-sequential double ionization (NSDI) of atoms, and that some of the correlated electron effects of NSDI unexpectedly show up in sequential double ionization (SDI). The agreement with recent ETH experimental SDI data [2] using elliptically polarized pulses calls into question the uncorrelated-electron assumption that drives the usual tunneling characterization of high-field ionization. We will report explanations [3] for an SDI ``knee" and other intensity-dependent indications of pre-ionization correlation of the two electrons. Among these with observable consequences is a striking oscillation discovered [2] in the ratio of parallel to antiparallel peak heights of two SDI electrons emitted in and out of phase [4].\\[4pt] [1] X. Wang and J.H. Eberly, Phys. Rev. Lett. 105, 083001 (2010).\\[0pt] [2] A.N. Pfeiffer, et al. (submitted).\\[0pt] [3] X. Wang and J.H. Eberly, arXiv submit/0189750.\\[0pt] [4] X. Wang and J.H. Eberly, Phys. Rev. Lett. 103, 103007 (2009).