Session G7: Invited Session: Ultrafast Electron Dynamics

Attosecond pre-determination of reaction dynamics in polyatomic molecules

An important aim of Ultrafast Laser Science and Attosecond Physics is the measurement of valence electron dynamics in molecules during complex restructuring and fragmentation reactions. Such reactions may be triggered by the removal of electrons, e.g., by ionization with intense, ultrashort laser pulses. Depending on the valence-shell from which the electrons are removed, the molecular ion might be put into a binding state or a certain dissociative state. With control over the ionization process it might thus be possible to gain control over the subsequent restructuring and fragmentation processes on a purely electronic level. Detailed insight into the process of electron removal can be obtained from interferometric measurements using two bound state wavepackets released at different times within a sub-cycle of a laser field. I will present results of experiments that exploit this kind of electron wavepacket interferometry with sub-10 attosecond resolution for tracing the evolution of the phase of the bound state of an atom or molecule during the removal of an electron. I will furthermore present two conceptually similar schemes that allow pre-determining the outcome of molecular restructuring and fragmentation processes in polyatomic molecules on sub-femtosecond time-scales by gaining control over the process of electron removal. The first method involves recollision double ionization in intense few-cycle laser fields with a known carrier-envelope phase (CEP). Tuning of the CEP allows controlling the removal of inner-valence electrons and the controlled population of dissociative excited states. Using this method I will show experimental CEP-control over various fragmentation reactions of a series of polyatomic molecules. The second control scheme uses the strong preponderance of ionization from specific molecular orbitals to the alignment of the molecular axis with respect to the laser polarization direction for determining which valence level the electrons are removed from. I will demonstrate experimental control over different fragmentation pathways of the acetylene molecule using the field-free alignment angle as a control knob.   

New attosecond spectroscopies for correlation-induced electron hole dynamics

In this talk I will present two of our recent ideas for new attosecond time-resolved measurements of electron hole dynamics [1,2]: \begin{itemize} \item \textit{Single-photon }laser enabled Auger decay (\textit{sp}LEAD) spectroscopy \end{itemize} and \begin{itemize} \item High-harmonic generation (HHG) spectroscopy of Auger-type transitions. \end{itemize} Unlike the well-known attosecond streaking, the proposed spectroscopies do not rely on photo- or secondary electron emission and are applicable to ultrafast electronic processes involving bound-bound transitions, such as electron correlation-driven charge migration. We simulate the new attosecond spectroscopies using both model and \textit{ab initio }methods. Specific applications include hole migration in glycine, atomic Auger and Coster-Kronig decays as well as quasi-exponential dynamics of molecular orbital breakdown in \textit{trans-}butadiene and propanal. \\[4pt] [1] B. Cooper and V. Averbukh, \textit{Phys. Rev. Lett. }\textbf{111}, 083004 (2013). \\[0pt] [2] J. Leeuwenburgh, B. Cooper, V. Averbukh, J. P. Marangos and M. Ivanov, \textit{Phys. Rev. Lett. }\textbf{111}, 123002 (2013).   

Coherent motion of an electron hole near a conical intersection: Using time-resolved XUV spectroscopy to study charge dynamics in polyatomics

The quantum mechanical motion of charge across a molecule is at the heart of complex biological and chemical processes occurring in nature. Ultrafast pump-probe studies of valence electron phenomena in neutral molecules suggest that such charge dynamics are often mediated through coupled electronic and nuclear motion or through electronic correlations. However, the real-time evolution of an electron hole in a photoionized molecule remains a relatively unexplored facet of the charge transfer phenomena. Investigation of such processes in time-domain requires preparation and monitoring of a superposition of quantum states in the ionized molecule. We report our recent investigations of the coherent motion of an electron hole wavepacket created near a conical intersection in CO2 molecule. Using extreme ultraviolet (XUV) attosecond pulse train as a pump and femtosecond near-infrared (NIR) pulse as a probe, we resolved the oscillation of the electron hole density between sigma and pi orbitals. We found that these charge dynamics are driven by the coupled bending and asymmetric stretch vibrations of the molecule. We also quantified the mixing between electron hole states and found that the degree of electronic coherence decreases with time due to thermal dephasing. The experimental and theoretical results we obtained for the linear triatomic molecule represent the first steps in elucidating the inner workings of coherent charge migration processes and pave the way for the application of attosecond and femtosecond XUV spectroscopies in the measurement and control of charge dynamics in complex biochemical systems.   

Dynamics of electrons and holes measured by high-harmonic spectroscopy

The dynamics of molecular valence-shell electrons offer a rich variety of phenomena ranging from simple two-level quantum beats to complex relaxation phenomena [1]. These dynamics are both of fundamental interest and relevant for understanding the properties and chemical reactivity of molecules. Molecular valence-shell dynamics take place on femto- to attosecond time scales and therefore require novel techniques that offer both an extreme temporal resolution and a high sensitivity. This lecture will present two novel schemes based on high-harmonic spectroscopy (HHS) that we have developed for measuring valence-shell electron dynamics of molecules. The first scheme [2] gives access to electronic dynamics in neutral molecules with unprecedented sensitivity: a 0.1\% excitation fraction results in a 20\% modulation of the high-harmonic signal, directly tracing the temporal evolution of the electronic density. This scheme exploits coherent cross-channels in high-harmonic generation that result from ionization and recombination in distinct quantum states. The intensity of the emission from these channels directly probes the electronic coherence and its evolution under coupling to nuclear degrees of freedom. In the second scheme we apply HHS as a unified pump-probe experiment to follow attosecond charge migration in a molecule following strong-field ionization. We impulsively orient the molecules using a recently developed all-optical technique [3] and measure both the amplitude and the phase of high-harmonic emission using multiple wavelengths and intensities of the driving laser pulse. We further introduce a new theoretical framework that accounts for correlation-driven electron-hole dynamics in high-harmonic spectroscopy. The comparison between theory and experiment reveals the signatures of the attosecond relaxation of the electronic valence shell following ionization. \\[4pt] [1] J. Breidbach and L. S. Cederbaum. J. Chem. Phys. 118, 3983 (2003).\newline [2] P. M. Kraus, S. B. Zhang, A. Gijsbertsen, R. R. Lucchese, N. Rohringer, and H. J. W\"orner, PRL 111, 243005 (2013).\newline [3] P. M. Kraus, A. Rupenyan and H. J. W\"orner, PRL 109, 233903 (2012)