Session D04: Time-Resolved Molecular Dynamics and Femtochemistry

Live

The practical tools to reconstruct real-space movies in the molecular frame from experimental time-resolved x-ray scattering (TRXS) measurement is an active field of research. We have developed frequency-resolved x-ray scattering (FRXS) as one such tool for producing molecular movies, which leverages the current strength of free-electron lasers: fine temporal resolution (50 fs) and fast data accumulation (4 mJ pulses at 120 Hz). We have used FRXS to characterize the simultaneous dissociations and vibrations on multiple electronic states in molecular iodine. This presentation will focus on the extension of FRXS to polyatomic molecules. For example, we will show how normal modes of motion may be recovered from a TRXS measurement using FRXS, and we will discuss the application of FRXS to chemically interesting dynamics like ring-opening.   

Live

Molecular bond stabilization is generally understood as a field-intensity-dependent effect, i.e., a decrease in the dissociation yield with respect to increasing laser field intensity [1,2]. In this study we revisit this effect in the strong-field dissociation of O$_2^+$ by numerically solving the time-dependent Schrödinger equation, within the Born-Oppenheimer approximation. Our results show that, for a fixed peak intensity, the total angle-integrated dissociation yields do not monotonically increase as the infrared-pulse duration is increased. This molecular stabilization effect is consistent with the transient population trapping in light-induced (bond-hardening) potential energy surfaces. In addition, we provide further evidence for the underlying bond-hardening mechanism by examining the power spectra of the evolving partial nuclear densities associated with two relevant cationic states of O$_2^+$, $a\,^4\Pi_u$ and $f\,^4\Pi_g$. [1] E. E. Aubanel, A. Conjusteau, and A. D. Bandrauk, Phys. Rev. A 48, R4011 (1993). [2] A. Giusti-Suzor, F. H. Mies, L. F. DiMauro, E. Charron, and B. Yang, J. Phys. B 28, 309 (1995). [3] P. M. Abanador, T. Pauly, and U. Thumm, Phys. Rev. A (submitted).   

Live

We describe the development of an apparatus to measure excited state molecular dynamics using UV pump and VUV probe pulses and velocity map imaging (VMI) of the photoelectrons and photoions generated by the pump and probe pulses. We apply a fast voltage switch on the VMI plates and a time-stamping camera (Timepix3) in order to measure the vector momenta of both the electrons and ions with a single detector. We highlight the capabilities of the instrument and present and interpret measurements of internal conversion in several molecules including 1,3-cyclohexadiene, \textit{cis,cis}-1,3-cyclooctadiene.   

Live

X-rays promote electrons from the core levels to vacant valence orbitals, thus endowing them with a unique element specificity. Moreover, the core level transitions can easily sense the shifts in the electron density in the proximity of the probed element. We produce soft X-rays around 280 eV driving high harmonics in a helium gas target with 1470 nm pulses. This table-top broad band X-ray source allows us to investigate the ultrafast dynamics in photoexcited pyrazine (C4H4N2). Pyrazine is excited to the second singlet excited state with 266 nm light, which is previously thought to convert in tens of femtoseconds to the first excited singlet state and then to the ground state in tens of picoseconds [1]. Recent experimental results with X-ray transient absorption indicate another major timescale of 1 ps, which can be assigned to a vibrationally excited electronic ground state or isomers of pyrazine (pyrimidine and pyridazine). An ongoing collaboration with theoreticians is aimed to disambiguate the nature of the observed transients. 1. V. Stert et al., J. Chem. Phys., 112, 4460 (2000).   

Live

Ultrafast electron diffraction measurements of isolated molecules have recently showed the capability to capture structural dynamics with great detail, including the motion of nuclear wavepackets in complex molecules. These advances have been facilitated by the use of relativistic electron pulses to reach sub-200 fs resolution, such as the MeV-UED setup at SLAC National Lab. We have developed a table-top setup for ultrafast electron diffraction which reaches femtosecond resolution using sub-relativistic electrons at an energy of 90 keV. Electrons are produced at a repetition rate of 5 kHz using a photocathode and a DC acceleration stage, and temporally compressed on the target using an RF cavity that is synchronized to the laser. We demonstrate an overall resolution of the instrument of 270 fs by capturing a rotational wavepacket in Nitrogen molecules. We have also characterized the time of arrival drift between laser and electron pulses to be on the order of 100 fs over several hours.   

Live

Ultrafast Electron Diffraction (UED) has emerged as a powerful tool to investigate photochemistry on its natural femtosecond timescale. We present two recent time-resolved studies in which UED, coupled with $\textit{ab initio}$ molecular dynamics simulations, provides two unique probing capabilities. In spectroscopically similar isomeric cyclohexadiene (CHD) derivatives, we observe steric effects which affect the mechanisms, efficiencies, and timescales of ultrafast ring opening. In ammonia (NH$_3$), the independent atom model for molecular diffraction breaks down as the valence electronic structure plays a substantial role. We use this to follow the ultrafast dissociation including two conical intersections following UV photoexcitation, with simultaneous sensitivity to the evolving nuclear and electronic degrees of freedom.   

On Demand

In the last few years we reported a series of numerical simulations proving that it is possible to create an electronic wave packet and subsequent electronic motion in a neutral molecule photoexcited by a UV pump pulse within a few femtoseconds. The ozone molecule has been served as sample system. By using extreme ultraviolet probe pulse the splitting of the excited B state nuclear wavepacket were followed into a path leading to molecular fragmentation and an oscillating path, revolving around the Franck-Condon point with 22-fs wave packet revival time. Recent experiment was strongly corroborated with our full quantum-mechanical ab initio simulations.   

On Demand

UV pump -- XUV probe femtosecond transient absorption spectroscopy is used to study the dynamics of 2-iodothiophene (C$_{\mathrm{4}}$H$_{\mathrm{3}}$IS). Iodine N-edge core-to-valence transitions probe changes in valence electronic structure from the perspective of the iodine atom, with ultrafast time resolution. A key question we wish to address is, to what extent ring-opening occurs prior to C-I bond fission. In 2-iodothiophene, the initial $\pi \pi $* electronic excitation is localized on the thiophene ring, rather than the halogen atom, resulting in singly occupied molecular orbitals (SOMO) that are ``dark'' to our probe. Significant electronic and nuclear rearrangement (and perhaps ring-opening) must occur before the localized I(4d) core-to-valence transitions are observed. While 2-photon ionization of 2-iodothiophene leads to the prompt appearance of I$^{\mathrm{+}}$ ions, within the instrument response function, the onset of neutral atomic I fragments is delayed by over 100~fs relative to the I$^{\mathrm{+}}$ and parent bleach signals. A global fit approach is used to separate the spectral contributions of each fragmentation channel. The observation of delayed absorption features is similar to that reported in iodobenzene, where the parent depletion occurs 50~fs prior to the emergence of atomic iodine [L.~Drescher et al., J.~Chem.~Phys. 145, 011101 (2016)].   

Not Participating

Implemented in a pump-probe scheme, the scattering patterns provide a direct view of the structural evolution of the molecules on a femtosecond timescale. Our early study of 1,3-cyclohexadiene used excitation at 267 nm to record time-resolved scattering patterns that show the structural evolution during the electrocyclic ring opening reaction on a sub-100 fs time scale. We now explore the photo-induced dynamics of medium-sized organic molecules upon excitation at 200 nm by ultrafast x-ray scattering using the Linac Coherent Light Source. We show that the scattering patterns inherently reflect the nature of the initially excited state. The nature of the excited electronic state is identified and in good agreement with theoretical predictions. The optical excitation to excited electronic states changes the charge density distribution of the molecule. This leads to a measurable change in the total scattering signal, which needs to be considered in the analysis.   

Unraveling the Photochemical Ring-Opening Dynamics of 1,3-cyclohexadiene with Ultrafast Electron Diffraction and \textit{Ab initio} Multiple Spawning

While the photoinduced ring-opening of 1,3-cyclohexadiene (CHD) has been the topic of numerous experimental and theoretical studies, none have achieved the temporal and spatial resolution needed to unambiguously observe its ring-opening dynamics. Here, we directly observe the photoinduced ring-opening of CHD on the femtosecond and sub-{\AA}ngstr\"{o}m scales using MeV Ultrafast Electron Diffraction (UED) and Ab Initio Multiple Spawning (AIMS). We show that CHD experiences a substantial acceleration of the ring-opening motion upon relaxation to the ground state via a conical intersection, which is transformed into rotation of the terminal ethylene groups in 1,3,5-hexatriene (HT). Additionally, we observe a coherent oscillation between the cZc, tZc/cZt, and tZt rotamers of HT upon relaxation to the ground state.