Session Y05: Ultrafast Molecular Dynamics

Investigating Ultrafast Dynamics in Polyatomic Molecules with Inner-shell Ionization

The invention of free-electron lasers capable of producing intense pulses of short wavelength light has enabled new time-resolved studies into photo dynamics in a site-selective manner. By choosing a suitable photon energy, photoabsorption can be highly localized at an individual atomic orbital within a molecule. My talk will focus on a series of investigations into ultrafast processes using site-selective soft x-ray ionization and how these novel techniques offer unique information of phenomena like charge transfer and non-adiabatic coupling in polyatomic molecules. The talk will conclude with new opportunities to extend these techniques to understand the evolving molecular structures via correlation analysis.   

Nonadiabatic dynamics around a conical intersection between electronic states of the same symmetry studied with the thawed Gaussian Ehrenfest dynamics

We present a combination of the Ehrenfest dynamics with the thawed Gaussian wavepacket dynamics. The resulting "thawed Gaussian Ehrenfest dynamics" preserves the molecular energy exactly and partially includes both nuclear quantum and electronically nonadiabatic effects. First, we show that the failure of single-trajectory mean-field methods applied to realistic models of molecules can be explained by the coupled electronic states usually having different symmetry. From this observation, we conclude that our method can be useful to study nonadiabatic dynamics close to conical intersection of electronic states of the same symmetry, which have been understudied due to the difficulty to locate them. Using a diabatic model to simulate such situation, we compare the dynamics of our method with the exact result. We find that the approximate solution provides a good qualitative picture for the first 100 femtoseconds, which include two passages across the conical intersection.   

Time-resolved X-ray Absorption Spectroscopy of aromatic Carbonyls using Spectral Domain Ghost Imaging

We studied UV-induced ultrafast photochemistry of aromatic carbonyls using time-resolved soft X-ray absorption spectroscopy. By utilizing the specific sensitivity of near edge X-ray absorption fine structure spectroscopy (NEXAFS) to the electronic character of the excited states (e.g., ππ* and nπ* states) we were able to elucidate the initial steps in the photodissociation mechanisms of Norrish Type-I reactions involving internal conversion and a subsequent interplay between the singlet and triplet states in one of the smallest aromatic carbonyls, Acetophenone (C6H5COCH3). We excited the gas phase sample with a 266 nm pump pulse and probed the reaction by scanning the X-ray photo energy across around the O K-edge (~520 to 540 eV). We monitored the X-ray absorbance through the measure of Auger-Meitner electron yield together with the photon spectrum. The experiment  was carried out at the TMO instrument of the LCLS. The X-ray pulses generated by the free-electron laser (FEL) driven by self-amplified spontaneous emission (SASE) exhibit a strong shot to shot spectral jitter with a bandwidth of several eV. To overcome this spectral bandwidth limitation to resolve dynamics in the fine structure spectrum at an order of less than 1 eV we exploit the correlation present in the shot-to-shot fluctuations in the incoming X-ray pulses and the detected electron signal by employing spectral domain ghost imaging. Utilizing this technique we are able to resolve a pre-edge absorption feature, around 527 eV,  which is attributed to the population of the nπ* state. The initial formation of this feature is observed with a rise time in the order of 100 fs. Subsequently a slow decay combined with spectral shift can be seen persisting for over 20ps, indicating dynamics to the triplet manifolds before dissociation. Our experimental results are corroborated by theoretical simulations.   

The Surprising Dynamics of the McLafferty Rearrangement

The McLafferty rearrangement in radical cations is a well-known reaction that occurs in mass spectrometry. In this rearrangement, a γ-hydrogen transfers to a double-bonded atom through a six-membered transition state. Subsequently, the β-carbon bond breaks, resulting in π-electron rearrangement and the formation of a neutral olefin as well as a positive charged enol. However, the timescale of this reaction was not known, and literature supported a stepwise or concerted mechanism. This work used disruptive probing following strong-field tunnel ionization to find the timescale of the McLafferty rearrangement for 2-pentanone, 4-methyl-2-pentanone, and 4,4 dimethyl-2-pentanone. The transient ion yield of the McLafferty product, m/z 58, fits to a biexponential function, with a fast (~100 fs) and slow (~10 ps) component, thus this study supports the stepwise nature of the rearrangement. Furthermore, the fast timescale is attributed to the internal rotation of the molecule and proton transfer. The slow timescale, the π-electron rearrangement, is surprising. If this step only required electron motion, it would be orders of magnitude faster. We find that this step requires extensive exploration of internal degrees of freedom to attain the molecular geometry necessary for electron transfer. These findings were corroborated with ab initio molecular dynamic simulations.   

Ultrafast Dynamics on Many Electronic States

Many important problems in chemistry and materials science involve nonadiabatic dynamics on large numbers of electronic states.  Phenomena important for strong-field physics, energy conversion, hot carrier cooling, and relaxation of plasmonic excitations fall into this category.  Some of these phenomena involve long lived coherences, which are challenging to accurately model with many mixed quantum-classical methods.  We will present recent theoretical developments towards an accurate and broadly applicable simulation method for modeling dynamics in this regime.  Specifically, we will present the development of the Ehrenfest with collapse to a block (TAB) method and a derivative designed for dense manifolds of states (DMS).  The primary achievement of TAB-DMS is that it is able to accurately describe decoherence effects without requiring explicit computation of individual electronic eigenstates.  Coupling to graphics processing unit accelerated time-dependent configuration interaction software to TAB and TAB-DMS enables ab initio nonadiabatic molecular dynamics simulations on many electronic states, in full nuclear dimensionality, and without prior knowledge of reaction mechanism.  The utility of this approach will be demonstrated by application to long-lived electronic coherences observed in recent ultrafast experiments.   

Light-induced photodissociation on the lowest three electronic states of NaH molecule

It has been known that electronic conical intersections in a molecular system can also be created by laser light even in diatomics. The direct consequence of these light-induced degeneracies is the appearance of a strong mixing between the electronic and vibrational motions, which has a strong fingerprint on the ultrafast nuclear dynamics. In the present work pump and probe numerical simulations have been performed with the NaH molecule involving the first three singlet electronic states (X¹ Σ⁺(X), A¹ Σ⁺(A) and B¹ Π(B)) and several light-induced degeneracies in the numerical description. To demonstrate the impact of the multiple light-induced non-adiabatic effects together with the molecular rotation on the dynamical properties of the molecule, the dissociation probabilities, kinetic energy release spectra (KER) and the angular distributions of the photofragments were calculated by discussing the role of the permanent dipole moment as well.   

Electronic excited-state dynamics of cytosine and its derivatives studied by time-resolved photoelectron spectroscopy.

Cytosine derivatives, which exist as multiple tautomers in the gas phase, provide an ideal platform to investigate the effects of small structural changes on the photostability of cytosine. To date, most time-resolved spectroscopic studies have been performed in the solution phase where the tautomer ratio is shifted toward the keto form and intersystem crossing dynamics have been associated with an active role of the carbonyl group. Using gas-phase time-resolved photoelectron spectroscopy, this work focuses on the photodynamics of enol and keto cytosine derivatives as well as thionation (thiocytosine), switching of exocyclic groups (isocytosine), and a ring-substitution (azacytosine).