Nuclear dynamics and ionization of diatomic molecules in intense laser fields

Master of ScienceDepartment of PhysicsUwe ThummIn this work we studied the dynamics of deuterium molecules in intense laser fields both experimentally and theoretically. For studying the dynamics of the molecule on a time scale that is less than the period of the laser field (2.7 fs for 800 nm), an advanced experimental technique: COLTRIMS (cold target recoil ion momentum spectroscopy) was used. COLTRIMS allows studying the nuclear dynamics without using attosecond laser pulses. This thesis consists of two main parts. In the first part we deduced the angular dependence of the ionization probability of the molecule without aligning the molecules, by measuring the relative angle between a deuteron resulting from field dissociation and an emitted electron using electron-ion coincidence measurements with circularly polarized light in COLTRIMS. We found out that for 50 fs pulses (1850 nm wavelength and 2 x10[superscript]14 W/cm[superscript]2 intensity), D[subscript]2 molecules are 1.15 times more likely to be ionized when the laser field is parallel to the molecular axis than when the laser field is perpendicular. This result agreed perfectly with the result from our ab initio theoretical model and also with predictions of the molecular Ammosov-Delone-Krainov (mo-ADK) theory. In the second part of this work we calculated the time evolution of an initial nuclear wave packet in D[subscript]2[superscript]+ generated by the rapid ionization of D[subscript]2 by an ultra short laser pulse. We Fourier transformed the nuclear probability density with respect to the delay between the pump and probe pulses and obtained two-dimensional internuclear-distance-dependent power spectra which serve as a tool for visualizing and analyzing the nuclear dynamics in D[subscript]2[superscript]+ in an external laser field. We attempt to model realistic laser pulses, therefore in addition to the main spike of the pulse we include the Gaussian pedestal. The optimal laser parameters for observing field-induced bond softening and bond hardening in D[subscript]2[superscript]+ can be achieved by varying the intensity, wavelength, and duration of the probe-pulse pedestal. Despite the implicit “continuum wave” (infinite pulse length) assumption the validity of the “Floquet picture” is tested for the interpretation of short-pulse laser-molecule interactions

Wavelength dependence of electron localization in the laser-driven dissociation of H2+_2^+

We theoretically investigate the laser wavelength dependence of asymmetric dissociation of H$_2^+$. It is found that the electron localization in molecular dissociation is significantly manipulated by varying the wavelength of the driving field. Through creating a strong nuclear vibration in the laser-molecular interaction, our simulations demonstrate that the few-cycle mid-infrared pulse can effectively localize the electron at one of the dissociating nuclei with weak ionization. Moreover, we show that the observed phase-shift of the dissociation asymmetry is attributed to the different population transfers by the remaining fields after the internuclear distances reach the one-photon coupling point.Comment: 11 pages, 7 figure

Large-Angle Electron Diffraction Structure in Laser-Induced Rescattering from Rare Gases

We have measured full momentum images of electrons rescattered from Xe, Kr, and Ar following the liberation of the electrons from these atoms by short, intense laser pulses. At high momenta the spectra show angular structure (diffraction) which is very target dependent and in good agreement with calculated differential cross sections for the scattering of free electrons from the corresponding ionic cores

Field-free orientation of CO molecules by femtosecond two-color laser fields

We report the first experimental observation of non-adiabatic field-free orientation of a heteronuclear diatomic molecule (CO) induced by an intense two-color (800 and 400 nm) femtosecond laser field. We monitor orientation by measuring fragment ion angular distributions after Coulomb explosion with an 800 nm pulse. The orientation of the molecules is controlled by the relative phase of the two-color field. The results are compared to quantum mechanical rigid rotor calculations. The demonstrated method can be applied to study molecular frame dynamics under field-free conditions in conjunction with a variety of spectroscopy methods, such as high-harmonic generation, electron diffraction and molecular frame photoemission

Attosecond correlated electron dynamics at C₆₀ giant plasmon resonance

Fullerenes have unique physical and chemical properties that are associated with their delocalized conjugated electronic structure. Among them, there is a giant ultra-broadband - and therefore ultrafast - plasmon resonance, which for C60 is in the extreme-ultraviolet energy range. While this peculiar resonance has attracted considerable interest for the potential downscaling of nanoplasmonic applications such as sensing, drug delivery and photocatalysis at the atomic level, its electronic character has remained elusive. The ultrafast decay time of this collective excitation demands attosecond techniques for real-time access to the photoinduced dynamics. Here, we uncover the role of electron correlations in the giant plasmon resonance of C60 by employing attosecond photoemission chronoscopy. We find a characteristic photoemission delay of up to 200 attoseconds pertaining to the plasmon that is purely induced by coherent large-scale correlations. This result provides novel insight into the quantum nature of plasmonic resonances, and sets a benchmark for advancing nanoplasmonic applications

Angular asymmetry and attosecond time delay from the giant plasmon resonance in C-60 photoionization

This combined experimental and theoretical study demonstrates that the surface plasmon resonance in C-60 alters the valence photoemission quantum phase, resulting in strong effects in the photoelectron angular distribution and emission time delay. Electron momentum imaging spectroscopy is used to measure the photoelectron angular distribution asymmetry parameter that agrees well with our calculations from the time-dependent local density approximation (TDLDA). Significant structure in the valence photoemission time delay is simultaneously calculated by TDLDA over the plasmon active energies. Results reveal a unified spatial and temporal asymmetry pattern driven by the plasmon resonance and offer a sensitive probe of electron correlation. A semiclassical approach facilitates further insights into this link that can be generalized and applied to other molecular systems and nanometer-sized metallic materials exhibiting plasmon resonances