Time-Resolved Photoelectron Imaging with a VUV Low-Order-Harmonic Source: From Femtochemistry to Femtochirality

Abstract

The topic of this doctoral thesis is the study of time-resolved phenomena in molecular systems using the novel possibilities opened by harmonic generation in gases. These systems have different \textit{sizes}, ranging from small molecules to large clusters with hundreds of molecules, and \textit{properties}, covering simple triatomic molecules through chiral molecules to hydrogen-bonded clusters representing a complex environment in which interesting processes can be studied. This broad spectrum of dynamics can be initiated and also detected using femtosecond (1~fs = $1\cdot 10^{-15}$~s) laser pulses in vacuum- and extreme-ultraviolet (VUV/XUV) spectral range. In other words, the central part of this thesis is devoted to a time-resolved perspective on molecular dynamics arising on sub-100~fs timescales. The dynamics are elucidated using a pump-probe photoelectron spectroscopy approach. The investigated systems are photo-excited and then photo-ionized. After photo-ionization, a photoelectron is detected which carries information about the parent molecule. The information is encoded in the kinetic-energy and angular distribution of the emitted photoelectron. One of the widely used experimental setups to collect this information is a velocity-map-imaging (VMI) spectrometer, which measures energy and angular distributions of these electrons simultaneously. The most complete picture about the molecular dynamics is recorded when the VMI detection is combined with temporal information obtained by precisely timed laser pulses. To achieve this goal, all experiments have been realized in a $(1 + 1^\prime)$-photon pump-probe scheme using a two-color interferometer combined with a VMI spectrometer. The novelty of the experimental setup is a low-order-harmonic generation (LOHG) source generating femtosecond pulses of radiation in the VUV/XUV region of the electromagnetic spectrum. Such a light source is a much desired extension to explore molecular dynamics of highly excited molecular or Rydberg states which were previously inaccessible by single-photon processes. Moreover, this type of VUV/XUV source is compatible with the generation of circularly polarized harmonics, opening up a new branch of study with which molecular chirality can be studied on the ultrafast timescale. The main part of the thesis describes three case studies selected such that they demonstrate the capabilities of the presented experimental setup. They are: i) femtosecond dynamics of SO2, ii) dynamics of the solvated electron in water clusters, and iii) time-resolved photoelectron circular dichroism. The first two cases belong to the realm of femtochemistry as introduced by A. Zewail in the last two decades of the last century. The last topic opens a new avenue in ultrafast science by applying femtosecond circularly polarized pulses to study the real-time evolution of chirality during photochemical reactions. Therefore the last case is opening up the new field of femtochirality. In experiments with the SO2 molecule, the excited-state dynamics in the $\tilde{\text{A}}/\tilde{\text{B}}$- and $\tilde{\text{F}}$-bands were investigated using time-resolved photoelectron imaging and photoion yield measurements. SO2 represents a small molecular system with complex non-adiabatic coupling between multiple close-lying electronic states. Moreover, it serves as a benchmark in many theoretical studies because its full-dimensional potential-energy surfaces can be calculated. The solvated electron, which can be prepared in large water clusters, is an important product during radiolysis of liquid water and as such it has attracted a lot of attention in both experimental and theoretical studies for more than two centuries. In this thesis, the solvated electron is studied by means of time-resolved photoelectron imaging. An increase of the electron binding energy together with the narrowing of the photoelectron band of the solvated electron have been observed. This is the first direct observation of the solvation dynamics from electron creation and solvation to its decay in sub-ps timescales. Moreover, anisotropy parameters as a function of binding energy and time have been measured showing a small, but persistent anisotropy of the solvated electron. The novel experimental approach is the implementation of circularly-polarized harmonics. The combination of circularly-polarized VUV radiation with an angle-resolved photoelectron detection opens up a new way to study molecular chirality in the time domain by utilizing the so-called photoelectron circular dichroism (PECD). In this thesis, the first time-resolved PECD experiments are demonstrated on time-dependent chirality associated with a photo-induced C-I bond breaking, thereby introducing a general experimental approach for chiral femtochemistry. The experimental results are supported by high-level \textit{ab initio} calculations. The broad applicability of the time-resolved PECD measurement scheme is demonstrated by studying two chiral molecules: CHFBrI and 2-iodobutane. Whereas CHFBrI displays a non-vanishing PECD at long pump-probe delays, the PECD decays to zero after the photodissociation of 2-iodobutane, reflecting the effective chirality of the product radicals on long timescales. In addition to demonstrating the broad applicability of LOHG to ultrafast molecular dynamis, these results also pave the way to attosecond time-resolved studies in the VUV/XUV domain, such as the role of electronic coherences in excited-state dynamics, the very earliest steps in photoionization of water, or the attosecond electron-scattering dynamics in chiral molecular potentials.