Ultrafast spectroscopy can initiate and probe electronic dynamics in molecules within femtoseconds to attoseconds timescale. Here, we report the use of strong field ionization pump-probe technique to detect multimode vibrational motions and electronic coherence in methyl iodide cation (CH3I+). For the first time, the coherence between the spin-orbit components of methyl iodide cation ground states and all symmetric vibrational modes were captured. The periodicities of the detected quantum beats vary between 6.0 fs and 117.0 fs. A few vibrational beatings from the first excited state (A state) were also detected. Furthermore, our approach reveals the time evolution of the quantum coherence in methyl iodide cation. The electronic coherence decays in the first picosecond while vibrational quantum beats persist. Our study further showed that rotational revival does not revive electronic coherence, suggesting both vibrational and rotational dephasings play a role in the decay of electronic quantum beatings. Theoretical analysis using a quantum model reveals intimate interaction between electronic and vibrational coherence in polyatomic systems.

Photochemical reactions of cyclic organic molecules are commonly used in many biological systems, solar energy resources, synthetic materials, and pharmaceuticals. Though energetically unfavored, highly strained ring structures produced from photolysis are of particular interest because their reaction mechanisms, once fully understood, could be leveraged for synthetic methods. In particular, we are interested in studying cyclopentadiene (CP), a small organic molecule whose photochemical pathway involves an electrocyclic process to form highly strained ring products upon UV excitation. We performed ultrafast time-resolved, gas-phase X-ray scattering experiments on cyclopentadiene at the CXI endstation of LCLS to explore the structural dynamics as the reaction evolves through a set of conical intersections.

The continuing progress of ultrafast sources such as X-ray free-electron lasers, high-repetition-rate near-infrared lasers, and Mega-electronvolt ultrafast electron diffraction facilities enable studies of electronic and structural dynamics in gas-phase molecules with unprecedented spatial and temporal resolution. I will present a series of experiments utilizing a variety of different spectroscopic techniques such as time-resolved photoelectron spectroscopy and Coulomb explosion imaging to study ring-opening and other isomerization reactions of molecules such a furan, toluene, thiophenone, and quadricyclane. The results are compared to experiments performed with other ultrafast techniques such as ultrafast electron and X-ray diffraction in order to highlight strengths and limitations of each technique.This work is supported by the Chemical Science, Geosciences, and Bioscience Division, Office of Basic Energy Science, Office of Science, U.S. Department of Energy, grants no. DE-FG02-86ER13491 and DE-SC0020276, and by the National Science Foundation grant no. PHYS-1753324.html:<hr /><h3>Footnotes: This work is supported by the Chemical Science, Geosciences, and Bioscience Division, Office of Basic Energy Science, Office of Science, U.S. Department of Energy, grants no. DE-FG02-86ER13491 and DE-SC0020276, and by the National Science Foundation grant no. PHYS-1753324.

Excited state intramolecular proton transfer (ESIPT) is key to many important biological mechanisms. However, direct observation of the structural dynamics of ESIPT has been limited given both the required spatial and temporal resolution. The combination of femtosecond temporal resolution and sub-Angstrom spatial resolution possible from mega-electronvolt ultrafast electron diffraction (MeV-UED) make it an ideal method for observing ESIPT. Furthermore, the neighboring -OH and -NO₂ groups on o-nitrophenol are known to undergo proton transfer upon excitation to the lowest singlet state (S₁). Using MeV-UED, the structural dynamics of proton transfer in o-nitrophenol have been resolved following excitation to the S₁ state. In contrast to the S₁ state, higher lying excited states are suspected to follow different relaxation pathways and their structural evolution could provide further insight into the dynamics of ESIPT in o-nitrophenol. This presentation will discuss previous findings of ESIPT following excitation of o-nitrophenol to the S₁ state and will present new findings related to the relaxation dynamics of the S₄ state.

Energy transfer across a heterogeneous interface is an important topic to understand detailed functioning mechanisms of solar cells. Here, we used mega-electronvolt ultrafast electron diffraction (MeV UED) as a sensitive time-resolved "thermometer" to simultaneously measure structural dynamics and energy transfer between a polymer (PTB7) and an atomic thin MoS₂ monolayer. Optical excitation of the polymer at 700 nm induces a short-lived temperature jump that relaxes quickly through the heterojunction interface to the monolayer MoS₂. The thermal energy transfers from the polymer to the atomic layer is described by the thermal transport model. The time-resolved structural dynamics of polymer suggests a bond dissociation located specifically at the C-O sidechain during the flattening motion of the two aromatic conjugated rings in the excited state, providing the fundamental mechanism of the photo-instability of a polymer in the applications of solar cell materials.

We will present a quantum dynamics theory for modelling the structural damage of molecules following the autoionization decay of molecular core-hole states generated by X-rays. Our theory will model the fundamental processes underlying the distortion of metalloprotein geometries characterized by X-ray crystallography, as well as the decay of radionucleotides used in highly targeted cancer-therapies. Both the absorption of X-rays and radionucleotide decay creates unstable core-hole states which can decay via autoionization involving the ejection of an electron and the collapse of another to the core-hole. The structural damage occurs via the Coulomb explosion of highly charged molecular cations created by the autoionization. Hence, simulations coupling the autoionization to structural dynamics will provide fundamental insight into this complex phenomenon. However, the required theoretical treatment is challenging, due to the complexity of autoionization and exponential scaling of possible decay channels with respect to the system size. Here we propose a quantum molecular dynamics method, which uses a time-dependent set of trajectory functions for modelling autoionization decay across multiple potential energy surfaces. The initial implementation of our method uses autoionization rates from atomic simulations, and our results are benchmarked against experimental x-ray/ion coincidence data of IBr molecule. This work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division.

l0pt Figure Newly developed X-ray sources offer valuable insight on molecular dynamics with unprecedented spatiotemporal resolutions and element sensitivity. Significant advances made in the control of the polarization of X-ray beams enables real-time monitoring of molecular chirality, which is an indispensable subject for understanding and controlling biological process. We theoretically apply time- and frequency-resolved X-ray Circular dichroism (TRXCD) spectroscopy to discern the time evolution of molecular chirality at different element windows during the photodissociation of 2-iodobutane. Following an optical excitation, the iodine atom dissociates from the chiral center, which we capture by quantum nuclear dynamics simulations. A resonant X-ray pulse then probes the iodine or carbon atom through an element-specific core-to-valence transition. The TRXCD signal at the iodine L₁-edge captures the timing of chirality loss, c.a 70 fs. The strong electric dipole–electric quadrupole (ED−EQ) response at this high X-ray regime makes this signal sensitive to vibronic coherences. At the carbon K-edges, the signals re-capture the chirality of 2-butyl radical and the spin state of the iodine atom. The stronger electric-magnetic dipole response make the signals more intuitive for the electronic population than coherence. Overall, the element-specific TRXCD signal offers a unique window into the time-dependent chirality of molecules.

X-ray free-electron lasers (XFELs) generate high-intensity x-ray pulses which enable x-ray nonlinear spectro-scopies. The extension of nonlinear spectroscopies to the x-ray domain promises the observation of electronic dynamics on their natural timescales with atomic spatial resolution. Stimulated x-ray Raman spectroscopy is an especially powerful tool, which in a propagation geometry combines large signal enhancement through stimulated emission with ultrahigh energy resolution that overcomes core-hole lifetime broadening. We present high-resolution stimulated Raman spectroscopy realized using stochastic XFEL pulses and correlation techniques. A covariance map between the transmitted SASE pulse and the stimulated Raman scattering produces a high-resolution x-ray Raman spectrum. This promising tool could be applied to study ultrafast electronic and molecular dynamics such as charge transfer in complex systems.

Molecular systems can be used for a wide range of applications but for a complete description knowledge about their transient electronic properties is often required. Optical-pump soft X-ray-probe NEXAFS spectroscopy is an ideal technique to investigate these systems due to its elemental and orbital selectivity which allows probing the time evolution of the electronic structure. Our laboratory transient NEXAFS spectrometer A. Jonas et al., Opt. Express 27, 2019, 36524-36537 s based on a laser-produced plasma source covering an energy range between 200 – 1500 eV with an energy resolving power of ≥ 1000 and 500 ps time resolution. Due to the high efficiency of the setup, the investigation of absorption changes as small as 10⁻⁴ is possible A. Jonas et at., Anal. Chem. 92, 2020, 15611-15615 These parameters allow obtaining high-quality time-resolved NEXAFS spectra formerly only attainable at synchrotron radiation sources. Static and transient NEXAFS measurements in transmission of solid samples and measurements in the liquid phase with a flatjet system at the Carbon and Nitrogen K-edge as well as 3d metal L-edges will be presented. Possible synergies of these laboratory-based measurements in combination with synchrotron instrumentation will be discussed.

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Footnotes:

A. Jonas et al., Opt. Express 27, 2019, 36524-36537 i A. Jonas et at., Anal. Chem. 92, 2020, 15611-15615.