Session C05: Focus Session: Strong Field Physics in Atoms and Molecules

Ultrafast X-ray Spectroscopy of Conical Intersections

Conical intersections are general electronic degeneracies in molecules and are the origin of ultrafast electronic and nuclear dynamics in molecular excited states. We discuss dynamics at conical intersections, using small hydrocarbon molecules as examples, and show how such dynamics may be probed by Time-Resolved Photoelectron Spectroscopy (TRPES) and Time- Resolved X-Ray Absorption Spectroscopy (TRXAS). In particular, we propose that TRXAS will be a uniquely powerful probe of the conical intersection itself.   

Strong-field-induced dynamics in bound and dissociative states of the halomethanes

Halomethanes often serve as prototypical systems for the studies of the laser-controlled chemistry (e.g. bond breaking, bond formation, concerted elimination of the halogens). Here, we present the results of a pump-probe experiment aimed to characterize bound and continuum nuclear wave packets created in diiodomethane (CH$_{\mathrm{2}}$I$_{\mathrm{2}})$ and chloroiodomethane (CH$_{\mathrm{2}}$ICl) molecules irradiated with the intense near-infrared laser pulse. Employing channel-selective Fourier spectroscopy of the delay-dependent ion yields, we identify signatures of the vibrational motion in both, neutral and ionic states, and observe signatures of bending and stretching vibrations. Using coincident 3D ion momentum imaging to disentangle different pathways leading to the doubly and triply charged final states, we trace the spatio-temporal evolution of several dissociation channels triggered by the pump pulse. We focus on the pathways involving a new bond formation, in particular, I$_{\mathrm{2\thinspace }}$or ICl elimination, discuss possible mechanisms of these reactions and their correlation with the bound-state vibrational motion.   

Angle-dependence of strong-field ionization of singly-charged Chloromethane and Bromomethane

We have studied the ionization probability of CH$_3$Cl and CH$_3$Br molecules exposed to intense 780 nm laser pulses as a function of the angle between the molecular axis and the linear laser polarization. Experimentally, the molecules are exposed to two laser pulses. The first induces no ionization but, instead, creates a rotational wave packet within each molecule that exhibits preferential alignment in the laboratory frame at specific time delays. We measure the variation in the single ionization yield as a function of the delay between the two pulses. We obtain the angular dependent ionization probability by fitting the observed delay-dependent yield to moments of the angular distribution of the rotational wavepacket which can be accurately calculated. The experimentally determined angular distributions are compared to results of new Time-Dependent Density Functional Theory (TD-DFT) predictions. Both experiment and theory find that even though the molecules have Highest Occupied Molecular Orbitals (HOMO-s) that are very similar, the angle dependence of their ionization yields differ substantially.   

Strong Field Molecular Ionization Viewed with Coincidence Velocity Map Imaging

In this talk I will present measurements of strong field molecular ionization carried out with shaped octave spanning laser pulses and characterized with momentum resolved measurements of ions and electrons measured in coincidence. The measurements highlight non-adiabatic dynamics underlying strong field ionization of molecules.   

Exploring strong-field isomerization and dissociation of acetylene anion and cation targets

Over the past several years, acetylene has generated substantial interest as a prototype system for examining isomerization processes, specifically hydrogen migration. Using coincidence 3D momentum imaging, we investigate intense ultrafast laser-induced isomerization and two-body fragmentation of keV beams of various charge states of acetylene, including C$_2$H$_2^-$, C$_2$H$_2^+$, and C$_2$H$_2^{2+}$. Whereas the vast majority of previous work on strong field isomerization and fragmentation of acetylene has necessarily involved ionization step(s), by focusing on dissociation, we ensure that the dynamics ensue within a single molecular ion species, potentially simplifying interpretation. Also, we find the behavior of the branching ratios of the acetylene (CH$^{q_1}$+CH$^{q_2}$) and vinylidene (C$^{q_1}$+CH$_2^{q_2}$) dissociation channels to depend strongly upon the initial ionic species.   

Strong-field molecular alignment driven by nonadiabatic charge localization: rotational wavepacket modifications

Strong electric field of an intense, nonresonant ultrashort laser pulse can effect a substantial drop of electron potential energy across a molecule, which is comparable with the distances between field-free potential energy surfaces. This can result in a transient radical redistribution of electron density and thus can alter the mechanism of rotational wavepacket formation and subsequent alignment in the molecular ensemble. We consider model diatomic molecules in a tight-binding approximation, in a situation when the laser-induced potential energy shifts across the molecule are comparable with the original level splitting. Averaging over the carrier-frequency oscillations of the laser field produces an effective rotational Hamiltonian, which allows for studying systematically the dependence of the nonadiabatic alignment kick on the molecular parameters and the laser pulse characteristics. We trace the transition from the usual alignment mechanism, which relies on anisotropic molecular polarizability, to a new mode of the effective-torque interaction, which is related to the fledging nonadiabatic electron localization.   

Absolute strong-field ionization probabilities of ultracold alkali atoms

We report on precise measurements of absolute non-linear ionization probabilities obtained by exposing ultracold $^{\mathrm{87}}$Rb atoms to the field of an ultrashort laser pulse. We have investigated both, the non-resonant and resonant strong-field response in the demanding transitional regime where the Keldysh parameter is near unity and thus \textit{ab-initio} theory, based on solving the time-dependent Schr\"{o}dinger equation (TDSE), is required. Employing optically trapped ultracold atomic gases indeed allows retrieving absolute ionization yields since the target density is recorded simultaneously to the ionized atoms, seen as spatially resolved losses. The accurate data sets are in perfect agreement with ionization probabilities obtained by numerically solving the TDSE without any free parameters. The single outer-shell electron and the low ionization potential combined with a high polarizability compared to commonly used rare gas atoms make alkali atoms ideal model systems for studying strong-field physics.