Ultrafast Relaxation Dynamics of the Ethylene Cation C(2)H(4)+

Dissociation and Isomerization Following Ionization of Ethylene: Insights from Nonadiabatic Dynamics Simulations

Photoionized and electronically excited ethylene C2H4+ can undergo H-loss, H2-loss, and ethylene-ethylidene isomerization, where the latter entails a hydrogen migration. Recent pioneering experiments with few-femtosecond extreme ultraviolet pulses and complementary theoretical studies have shed light on the photodynamics of this prototypical organic cation. However, no theoretical investigation based on dynamics simulations reported to date has described the mechanisms and time scales of dissociation and isomerization. Herein, we simulate the coupled electron-nuclear dynamics of ethylene following vertical ionization and electronic excitation to its four lowest-lying cationic states. The electronic structure is treated at the CASSCF level, with an active space large enough to describe bond breaking and formation. The simulations indicate that dissociation and isomerization take place mainly on the cationic ground state and allow the probing of previous hypotheses concerning the correlation between the photochemical outcome and the traversed conical intersections. The results, moreover, support the long-standing view that H2-loss may occur from the ethylidene form. However, the ethylene-ethylidene isomerization time predicted by the simulations is considerably longer than those previously inferred from indirect experimental measurements.

Sub-50 fs temporal resolution in an FEL-optical laser pump-probe experiment at FLASH2

High temporal resolution is essential for ultra-fast pump-probe experiments. Arrival time jitter and drift measurements, as well as their control, become critical especially when combining XUV or X-ray free-electron lasers (FELs) with optical lasers due to the large scale of such facilities and their distinct pulse generation processes. This paper presents the application of a laser pulse arrival time monitor that actively corrects the arrival time of an optical laser relative to the FEL's main optical clock. Combined with post-analysis single pulse jitter correction this new approach improves the temporal resolution for pump-probe experiments significantly. Benchmark measurements on photo-ionization of xenon atoms performed at FLASH beamline FL26, demonstrate a sub-50 fs FWHM overall temporal resolution.

Auger-Meitner and X-ray Absorption Spectra of Ethylene Cation: Insight into Conical Intersection Dynamics

We present a theoretical investigation of the near-edge X-ray absorption fine structure and the Auger-Meitner decay spectra of ethylene and its cation. Herein, we demonstrate that our method, coupled with the nuclear ensemble approach, successfully reproduces the natural bandwidth structure of the experimental resonant Auger-Meitner decay spectra of ethylene, which is not very well reproduced within the Franck-Condon approximation. Furthermore, we analyze the Auger-Meitner decay spectra of the ethylene cation in light of minimum energy conical intersection structures involving the two lowest cationic states (D1 and D0), providing valuable insights into the ultrafast D1/D0 relaxation dynamics. Our results suggest that Auger-Meitner electron spectroscopy can help elucidate the mechanism behind the initial 20 fs of the relaxation dynamics.

Few-Femtosecond C2H4+ Internal Relaxation Dynamics Accessed by Selective Excitation

Dissociation of the ethylene cation is a prototypical multistep pathway in which the exact mechanisms leading to internal energy conversions are not fully known. For example, it is still unclear how the energy is exactly redistributed among the internal modes and which step is rate-determining. Here we use few-femtosecond extreme-ultraviolet pulses of tunable energy to excite a different superposition of the four lowest states of C₂H₄⁺ and probe the subsequent fast relaxation with a short infrared pulse. Our results demonstrate that the infrared pulse photoexcites the cationic ground state (GS) to higher excited states, producing a hot GS upon relaxation, which enhances the fragmentation yield. As the photoexcitation probability of the GS strongly depends on the molecular geometry, the probing by the IR pulse provides information about the ultrafast excited-state dynamics and the type of conical intersection (planar or twisted) involved in the first 20 fs of the nonradiative relaxation.

Few-Femtosecond Isotope Effect in Polyatomic Molecules Ionized by Extreme Ultraviolet Attosecond Pulse Trains

Following ionization by an extreme ultraviolet (XUV) attosecond pulse train, a polyatomic molecule can be promoted to more-than-one excited states of the residual ion. The ensuing relaxation dynamics is often facilitated by several reaction coordinates, making them difficult to disentangle by the usual spectroscopic means. Here, we show that in atto-chemistry isotope labeling can be an efficient tool for unraveling the relaxation pathways in highly excited photoionized molecules. Employing an XUV pump pulse and a near-infrared probe pulse, we found the nuclear as well as coupled electron-nuclear dynamics in ethylene to be almost 40% faster compared to that of its deuterated counterpart. The findings, which are supported by advanced nonadiabatic dynamics calculations, led to the identification of the relevant nuclear coordinates controlling the relaxation. Our experiment highlights the relevance of ultrashort XUV pulses to capture the isotopic effect in few-femtosecond molecular photodynamics.

Attosecond spectroscopy for the investigation of ultrafast dynamics in atomic, molecular and solid-state physics

Since the first demonstration of the generation of attosecond pulses (1 as = 10^-18s) in the extreme-ultraviolet spectral region, several measurement techniques have been introduced, at the beginning for the temporal characterization of the pulses, and immediately after for the investigation of electronic and nuclear ultrafast dynamics in atoms, molecules and solids with unprecedented temporal resolution. The attosecond spectroscopic tools established in the last two decades, together with the development of sophisticated theoretical methods for the interpretation of the experimental outcomes, allowed to unravel and investigate physical processes never observed before, such as the delay in photoemission from atoms and solids, the motion of electrons in molecules after prompt ionization which precede any notable nuclear motion, the temporal evolution of the tunneling process in dielectrics, and many others. This review focused on applications of attosecond techniques to the investigation of ultrafast processes in atoms, molecules and solids. Thanks to the introduction and ongoing developments of new spectroscopic techniques, the attosecond science is rapidly moving towards the investigation, understanding and control of coupled electron-nuclear dynamics in increasingly complex systems, with ever more accurate and complete investigation techniques. Here we will review the most common techniques presenting the latest results in atoms, molecules and solids.

Multi-channel photodissociation and XUV-induced charge transfer dynamics in strong-field-ionized methyl iodide studied with time-resolved recoil-frame covariance imaging

The photodissociation dynamics of strong-field ionized methyl iodide (CH3I) were probed using intense extreme ultraviolet (XUV) radiation produced by the SPring-8 Angstrom Compact free electron LAser (SACLA). Strong-field ionization and subsequent fragmentation of CH3I was initiated by an intense femtosecond infrared (IR) pulse. The ensuing fragmentation and charge transfer processes following multiple ionization by the XUV pulse at a range of pump-probe delays were followed in a multi-mass ion velocity-map imaging (VMI) experiment. Simultaneous imaging of a wide range of resultant ions allowed for additional insight into the complex dynamics by elucidating correlations between the momenta of different fragment ions using time-resolved recoil-frame covariance imaging analysis. The comprehensive picture of the photodynamics that can be extracted provides promising evidence that the techniques described here could be applied to study ultrafast photochemistry in a range of molecular systems at high count rates using state-of-the-art advanced light sources.

The quantum-Ehrenfest method with the inclusion of an IR pulse: Application to electron dynamics of the allene radical cation

We describe the implementation of a laser control pulse in the quantum-Ehrenfest method, a molecular quantum dynamics method that solves the time-dependent Schrödinger equation for both electrons and nuclei. The oscillating electric field-dipole interaction is incorporated directly in the one-electron Hamiltonian of the electronic structure part of the algorithm. We then use the coupled electron-nuclear dynamics of the π-system in the allene radical cation (•CH₂=C=CH₂)⁺ as a simple model of a pump-control experiment. We start (pump) with a two-state superposition of two cationic states. The resulting electron dynamics corresponds to the rapid oscillation of the unpaired electron between the two terminal methylenes. This electron dynamics is, in turn, coupled to the torsional motion of the terminal methylenes. There is a conical intersection at 90° twist, where the electron dynamics collapses because the adiabatic states become degenerate. After passing the conical intersection, the electron dynamics revives. The IR pulse (control) in our simulations is timed to have its maximum at the conical intersection. Our simulations show that the effect of the (control) pulse is to change the electron dynamics at the conical intersection and, as a consequence, the concomitant nuclear dynamics, which is dominated by the change in the torsional angle.

Robustness of the ePIE algorithm for the complete characterization of femtosecond, extreme ultra-violet pulses

Frequency-resolved optical gating for the complete reconstruction of attosecond bursts (FROG-CRAB) is a well-known technique for the complete temporal characterization of ultrashort extreme ultraviolet (XUV) pulses, with durations down to a few tens of attoseconds. Recently, this technique was extended to few-femtosecond XUV pulses, produced by high-order harmonic generation (HHG) in gases, thanks to the implementation of a robust iterative algorithm: the extended ptychographic iterative engine (ePIE). We demonstrate, by using numerical simulations, that the ptychographic reconstruction technique is characterized by an excellent degree of convergence and robustness. We analyse the effects on pulse reconstruction of various experimental imperfections, namely, the jitter of the relative temporal delay between the XUV pulse and a suitably delayed infrared (IR) pulse and the noise of the measured FROG-CRAB spectrograms. We also show that the ePIE approach is particularly suitable for the reconstruction of incomplete FROG-CRAB spectrograms (i.e., spectrograms with a reduced number of measured time delays) and of spectrograms acquired with a reduced spectral resolution, particularly when relatively high-intensity IR pulses are employed.

Reduction of laser-intensity-correlated noise in high-harmonic generation

We present a scheme for correcting the spectral fluctuations of high-harmonic radiation. We show that the fluctuations of the extreme-ultraviolet (XUV) spectral power density can be predicted solely by monitoring the generating laser pulses; this method is in contrast with traditional balanced detection used in optical spectroscopy, where a replica of the signal is monitored. Such possibility emerges from a detailed investigation of high-harmonic generation (HHG) noise. We find that in a wide parameter range of the HHG process, the XUV fluctuations are dominated by a spectral blueshift, which is correlated to the near-infrared (NIR) driving laser intensity variation. Numerical simulations support our findings and suggest that non-adiabatic blueshift is the main source of XUV fluctuations. A straightforward post-processing of the XUV spectra allows for noise reduction and improved precision of attosecond transient absorption experiments. The technique is readily transferable to attosecond transient reflectivity and potentially to attosecond photoelectron spectroscopy.

Steering the outcome of a photochemical reaction-An in silico experiment on the H2CSO sulfine using few-femtosecond dump pulses

We propose a pump-dump control scheme using sub-10 fs pulses to enhance the photochemical formation of the three-membered C-S-O ring oxathiirane from the parent H₂CSO sulfine molecule. The ultrashort nature of the pulses is essential to promptly alter the photoinduced dynamics, e.g., while a bond is elongating, which is key to selectively form the oxathiirane by radiative dumping. We carried out an in silico pump-dump experiment with excited-state dynamics simulations that include the interaction with electric field of the pump and dump pulses. By applying the dump pulse when the CS bond is elongating, the population transferred to the ground state will form the oxathiirane with a branching ratio of 4, much higher than the one solely due to nonradiative relaxation (0.66). The overall oxathiirane yield can be increased by up to 17% when the 6 fs IR dump pulse is applied at a delay time of 47 fs.

Ultrafast nuclear dynamics of the acetylene cation C2H2+ and its impact on the infrared probe pulse induced C–H bond breaking efficiency

We track the few-femtosecond excited-state dynamics of the acetylene cation through modulations of the C₂H⁺ photofragment yield.

Ultrafast mapping of relaxation dynamics of ethylene cation

The complex ultrafast molecular relaxation dynamics of ethylene, initiated by tunable vacuum-ultraviolet ~10-fs pulses, was measured. Exploiting state selectivity, an unprecedented time-energy mapping of the process was demonstrated on a few-femtosecond temporal scale.

Few-femtosecond extreme-ultraviolet pulses fully reconstructed by a ptychographic technique

Sub-10-fs pulses tunable in the extreme-ultraviolet (XUV) spectral region are particularly important in many research fields: from atomic and molecular spectroscopy to the study of relaxation processes in solids and transition phase processes, from holography to free-electron laser injection. A crucial prerequisite for all applications is the accurate measurement of the temporal characteristics of these pulses. To fulfill this purpose, many phase retrieval algorithms have been successfully applied to reconstruct XUV attosecond pulses. Nevertheless, their extension to XUV femtosecond pulses is not trivial and has never been investigated/reported so far. We demonstrate that ultrashort XUV pulses, produced by high-order harmonic generation, spectrally filtered by a time-delay compensated monochromator, can be fully characterized, in terms of temporal intensity and phase, by employing the ptychographic reconstruction technique while other common reconstruction algorithms fail. This allows us to report on the generation and complete temporal characterization of XUV pulses with duration down to 5 fs, which constitute the shortest XUV pulse ever achieved via a time-delay compensated monochromator.

Charge migration and charge transfer in molecular systems

The transfer of charge at the molecular level plays a fundamental role in many areas of chemistry, physics, biology and materials science. Today, more than 60 years after the seminal work of R. A. Marcus, charge transfer is still a very active field of research. An important recent impetus comes from the ability to resolve ever faster temporal events, down to the attosecond time scale. Such a high temporal resolution now offers the possibility to unravel the most elementary quantum dynamics of both electrons and nuclei that participate in the complex process of charge transfer. This review covers recent research that addresses the following questions. Can we reconstruct the migration of charge across a molecule on the atomic length and electronic time scales? Can we use strong laser fields to control charge migration? Can we temporally resolve and understand intramolecular charge transfer in dissociative ionization of small molecules, in transition-metal complexes and in conjugated polymers? Can we tailor molecular systems towards specific charge-transfer processes? What are the time scales of the elementary steps of charge transfer in liquids and nanoparticles? Important new insights into each of these topics, obtained from state-of-the-art ultrafast spectroscopy and/or theoretical methods, are summarized in this review.

Few-femtosecond passage of conical intersections in the benzene cation

Observing the crucial first few femtoseconds of photochemical reactions requires tools typically not available in the femtochemistry toolkit. Such dynamics are now within reach with the instruments provided by attosecond science. Here, we apply experimental and theoretical methods to assess the ultrafast nonadiabatic vibronic processes in a prototypical complex system-the excited benzene cation. We use few-femtosecond duration extreme ultraviolet and visible/near-infrared laser pulses to prepare and probe excited cationic states and observe two relaxation timescales of 11 ± 3 fs and 110 ± 20 fs. These are interpreted in terms of population transfer via two sequential conical intersections. The experimental results are quantitatively compared with state-of-the-art multi-configuration time-dependent Hartree calculations showing convincing agreement in the timescales. By characterising one of the fastest internal conversion processes studied to date, we enter an extreme regime of ultrafast molecular dynamics, paving the way to tracking and controlling purely electronic dynamics in complex molecules.