Session V27: Focus Session: Attosecond Science and Strong Field Chemical Physics I

Attosecond dynamics of multi-electron re-arrangement during strong-field ionization

Nonlinear interaction of intense infrared laser light with atoms or molecules converts incident radiation into its high harmonics, increasing the incident frequency by orders of magnitude. High harmonics are generated during radiative recombination of an electron liberated by strong field ionization with the hole left in the molecule. High harmonic spectroscopy records and analyzes information about molecular structure\footnote{M. Lein, M. J. Phys. B, 40, R135 (2007), W. Boutu et al, Nature Physics 4, 545,(2008)} and dynamics\footnote{S. Baker, et al, Science 312, 424 (2006); S. Baker, et al, Phys. Rev. Lett., 101, 053901 (2008)}$^,$\footnote{O. Smirnova, et al, Nature, 460, 972 (2009)}, encoded in harmonic amplitudes, phases and polarizations. Potentially, tens of eV broad harmonic spectra encode dynamics between ionization and recombination with attosecond temporal resolution. For absorption of multiple photons, attosecond dynamics of multi electron rearrangement during strong field ionization is not understood. The shape and location of the hole after ionization are determined by the relative phases of the participating electronic states. We show that these phases, which are set up by ionization and reflect dynamics of core rearrangement, are naturally recorded in high harmonic emission. We use high harmonic spectroscopy to find the shape and location of the hole by reconstructing the relative phases between different ionization channels set up by strong field ionization.   

Strong Field Ionization Probing of the Transition from a Molecule to Atoms

We use strong-field ionization to probe electron orbital rearrangement in a dissociating molecule. By illuminating a bromine molecule with 400~nm light, the molecule is excited into a dissociative state. A strong 800 nm pulse is then used to directly probe the dissociative state by ionizing the molecule at different times during the breakup process. We observe time-dependent changes in the kinetic energy release spectra and the angular-dependent ionization yield as the molecular bond ruptures, which we attribute to electronic rearrangement within the molecule. Different orbitals are found to respond differently to the nuclear motion. These data are compared with theoretical predictions, which allow us to identify a well-defined transition from the molecular state into separated atoms for the first time, which occurs at the surprisingly long timescale of 140~fs and an internuclear separation of 5.6 {\AA}.   

IR-assisted ionization of Ar$^{+}$/Ar$^{++}$ by the attosecond XUV radiation

The use of attosecond XUV and femtosecond IR radiation to induce and control electron dynamics in small atoms and molecules is a central theme of attosecond science. Here we use a 42 eV photon to excite an Ar$^{+ }$Rydberg state just below the argon double-ionization threshold. A weak IR photon is then used to probe and control the excitation/ionization processes. The Ar+/Ar++ yields are monitored with sub-optical cycle attosecond relative delays between the XUV and IR pulses. When the IR probe arrives after the XUV pulse has excited well defined states of Ar$^{+\ast }$, a seven-times enhanced yield of Ar$^{++}$ yield is seen compared with ionization without the IR pulse present. An additional enhancement (x10) in the Ar$^{++}$ yield is seen when the XUV and IR pulses simultaneously excite Ar. Moreover, modulations in the Ar+/Ar++ yield are observed when the two beams are delayed on attosecond time scales. Photoelectrons are measured in coincidence with the ions using the COLTRIMS technique. The ionization mechanisms behind these observations will be discussed.   

Attosecond interferometry in strong field physics

The combination of an attosecond extreme ultraviolet (XUV) pulse with an infrared (IR) laser field has been the mainstay of attosecond metrology. It has been used to characterize both attosecond pulse trains and single attosecond pulses. Over the last few years it has been demonstrated that this same combination can also be used to study and control strong field processes such as high harmonic generation [1], strong field ionization [2] and electron rescattering [3]. We will illustrate these principles with examples from our most recent work on the absorption of XUV radiation in strong IR fields, and discuss recent experiments as well.\\[4pt] [1] K.J. Schafer, et al., ``Strong field quantum path control using attosecond pulse trains,'' Phys.\ Rev.\ Lett.\ 92 023003 (2004).\\[0pt] [2] P. Johnsson, et al., ``Attosecond control of ionization dynamics by wavepacket interference,'' Phys.\ Rev.\ Lett.\ 99, 233001 (2007).\\[0pt] [3] J. Mauritsson, et al., ``Coherent Electron Scattering Captured by an Attosecond Quantum Stroboscope,'' Phys.\ Rev.Lett.\ 100, 073003 (2008).   

Attosecond photoelectron spectroscopy at surfaces

We apply the attosecond streaking technique for time-resolved studies of electron emission from solids. Employing isolated attosecond XUV pulses in combination with sub-4 femtosecond NIR pulses, the relative timing of photoemission from different electronic states can be determined with a temporal resolution approaching 10 attoseconds. In the quest of measuring the absolute time between photoexcitation an emission of an electron from a solid, we investigate clean metal surfaces in comparison to xenon covered surfaces. In addition, we study the relative emission time of 5d valence electrons and 4d core electrons of atomic xenon in comparison to condensed xenon.   

Ultrafast molecular and materials dynamics probed by attosecond coherent x-rays

The x-ray bursts generated during high harmonic generation represent the fastest strobe light in existence, fast enough to capture electron dynamics in atoms, molecules, and solids. Bright, attosecond, beams of coherent x-rays now span from the VUV to $>$ 0.5 keV [1,2], with the prospect of reaching the hard x-ray region in the near future. Exciting applications of attosecond science and technology will be discussed, including capturing the coupled motions of electrons and atoms in molecules, high-resolution imaging, nanoscale heat transport as well as ultrafast, element-specific, dynamics in magnetic materials [3-6]. \\[4pt] [1] Popmintchev et al., PNAS 106, 10516 (2009); Nature Photonics, to be published.\\[0pt] [2] Thomann et al., Optics Express 17, 4611 (2009).\\[0pt] [3] Siemens et al., Nature Materials, , to be published.\\[0pt] [4] La-O-Vorakiat et al., Physical Review Letters, , to be published.\\[0pt] [5] Li et al. Science 322, 1207 (2008).\\[0pt] [6] Murnane et al., Nature 460, 1088 (2009).   

Inducing and Probing Attosecond-Time-Scale Electronic Wavefunction Beating

Much of the current interest in the field of ultrafast science focuses on the observation of attosecond dynamics of electronic wavepackets. These experiments typically require attosecond pulses either for pumping or probing such dynamics and/or are limited to observing electronic states embedded in the ionization continuum of atoms. Here, we present numerical evidence---based on solutions of the time-dependent Schr\"odinger equation for a 1-dimensional model atom---that a pump--probe scheme with two few-cycle femtosecond laser pulses provides interferometric access to sub-femtosecond electron wavepacket dynamics. Both continuum- and bound-state electronic wavepacket interference can be simultaneously observed by recording and analyzing time-delay dependent interferences in the ATI spectrum of an atom. Both dipole-allowed and forbidden electronic transition information can be extracted from the data, making this approach a versatile and comprehensive spectroscopic method for probing the bound electronic level structure of an atom.   

Multichannel coherence in strong-field ionization

Atomic and molecular ions generated by a strong optical laser pulse are not in general in the electronic ground state. The density matrix for such ions is characterized by the electronic quantum-state populations and by the coherences among the electronic quantum states. Nonvanishing coherences signal the presence of coherent electronic wave-packet dynamics in the laser-generated ions. For noble-gas atoms heavier than helium, the most important channels populated via strong-field ionization are the outer-valence single-hole states with a total angular momentum of $j=3/2$ or $j=1/2$. For this case, we develop a time-dependent multichannel theory of strong-field ionization. We derive the ion density matrix and express the hole density in terms of the elements of the ion density matrix. Our wave-packet calculations demonstrate that neon ions generated in a strong optical field (800 nm) are almost perfectly coherent. In strong-field-generated xenon ions, however, the coherence is substantially suppressed.