Ultrafast Strong-Field Vibrational Dynamics Studied by Femtosecond Extreme-Ultraviolet Transient Absorption Spectroscopy
Femtosecond time-resolved extreme-ultraviolet core-level absorption spectroscopy has developed into a powerful tool for investigating chemical dynamics due to its sensitivity for detecting changes in electronic structure. By probing the core-levels of atoms and molecules, dynamics may be monitored with elemental specificity, as well as localized sensitivity to the oxidation state around the atomic absorber. Previous experiments with this technique demonstrated the capability to quantitatively resolve strong-field ionization dynamics including the complete |j,m> quantum state distribution of Xe+ and the dissociative ionization of CH2Br2, both monitored in real-time with femtosecond resolution. In this thesis, the complete reconditioning of the experimental apparatus is described and its application to the study of vibrational wave packets created by strong-field ionization of Br2 is discussed. Initially, the experimental apparatus was meant as a proof of principle demonstration for table-top extreme ultraviolet transient absorption spectroscopy based on a high-order harmonic generation source. To achieve the experiments discussed here, substantial changes were required to its construction concerning the high harmonic source and overall stability of the apparatus, which are discussed in terms of increasing the operation of the experiment to several days as opposed to a few hours, yielding an unprecedented spectral resolution after Fourier Transform of the signal versus pump-probe time delay for a table-top core-level spectrometer, better than 1.5 cm-1. The improvements to the apparatus are exploited in terms of examining the vibrational wave packet dynamics in Br2 induced by optical strong-field ionization.
First, extreme-ultraviolet transient absorption is established as a bond-length specific probe capable of resolving vibrational motion in Br2 and due to a change in the transition amplitude and energy versus bond length of the 3d5/2,3/2 core-level transitions to unoccupied molecular orbitals. Furthermore, the degree of coherence and beat composition of the 1&Sigma+ g ground state vibrational wave packet is established. Second, the preparation of vibrational wave packets by strong-field ionization of Br2 is analyzed as a function of field intensity. At a field intensity near the threshold of ionization, selective depletion of the vibrational amplitude is observed near the inner turning point, corresponding to the minimum in the ionization energy for the unperturbed molecular potential energy curves. The observed superposition is determined to be primarily composed of the &nu0&nu1 vibrational beat. As the field intensity is increased, perturbation of the potential curves results in preparation of the wave packet at longer internuclear separations, and the distortion of the potentials is directly observed by a shift in the core-level transition energy within the intense laser pulse. Moreover, the composition of the wave packet is found to involve higher vibrational levels, indicating the involvement of a Raman pumping mechanism in the preparation of the superposition. Third, several ongoing experiments utilize the unique instrumental capabilities including the elemental specificity and polarization dependence of the absorption. The elemental specificity and nuclear motion dependence of the core-level probe is utilized to investigate the correlated detection of vibrational wave packet dynamics in IBr on the iodine 4d and bromine 3d core-level absorptions simultaneously. Also, polarization dependent absorption of Xe2+ looks to extend the quantitative measurements conducted for Xe+ to determine the complete |j,m> state populations after sequential ionization. Lastly, polarization dependent measurements of the strong-field ionization of Br2 suggest alignment between the molecular axis and the electric field of the ionizing pump at field intensities near the threshold for ionization, which is lost at higher field strengths. These studies expand the repertoire of capabilities for high-order harmonic generation beyond the proof of principle experiments and establish the technique as a practical tool for monitoring complex, ultrafast molecular dynamics by core-level probing.
The capabilities of femtosecond transient absorption have since been expanded here to include the investigation of strong-field generated vibrational wave packets in molecular bromine. To this end, a theoretical model for determining the sensitivity of XUV absorption to valence vibrational motion generated by strong-field ionization was developed utilizing Restricted Excitation Window Time-Dependent Density Functional Theory coupled with internuclear separation, vibrational state sensitive Ammosov-Delone-Krainov tunnel ionization rates. This theoretical model, including details of traditional Ammosov-Delone-Krainov tunnel ionization rates, is presented in Chapter 2. Chapter 3 describes the experimental changes incorporated into the apparatus for successful multi-day experiments for the determination of vibrational dynamics with ~1 cm-1 spectral resolution. Demonstration of the sensitivity of XUV transient absorption to non-stationary ground state vibrational dynamics is then presented in Chapter 4, investigating the vibrational superposition generated in Br2 after irradiation with a 1.6 x 1014 W/cm2 driving pump pulse. Chapter 5 expounds upon the dependence on field intensity to the observed vibrational dynamics in Br2 as well as Br2+. Chapter 6 discusses the currently ongoing experiments being conducted for ground state vibrational wave packet preparation in iodine monobromide, the determination of the complete |j,m> quantum state population of Xe2+, and the alignment of the molecular axis of Br2 with the electric field of the ionizing 800 nm field following strong-field ionization. Chapter 6 also presents the conclusions of the experimental work presented here as well as the future outlook of XUV transient absorption spectroscopy in the broader perspective of investigating chemical reaction and dynamics.