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We introduce a perturbative approximation to the combined density functional theory and multireference configuration interaction (DFT/MRCI) approach. The method, termed DFT/MRCI(2), results from the application of quasi-degenerate perturbation theory (QDPT) and the Epstein–Nesbet partitioning to the DFT/MRCI Hamiltonian matrix. The application of QDPT obviates the need to diagonalize the large DFT/MRCI Hamiltonian; electronic energies are instead obtained as the eigenvalues of a small effective Hamiltonian, affording an orders of magnitude savings in the computational cost. Most importantly, the DFT/MRCI(2) approximation is found to be of excellent accuracy, furnishing excitation energies with a root mean squared deviation from the canonical DFT/MRCI values of less than 0.03 eV for an extensive test set of organic molecules.

International audience; Time-resolved photo-electron spectroscopy (TRPES) has become one of the most widespread techniques for probing nonadiabatic dynamics in the excited electronic states of molecules. Furthermore, the complementary development of ab initio approaches for the simulation of TRPES signals has enabled an interpretation of these transient spectra in terms of the underlying coupled electronic-nuclear dynamics. In this perspective we discuss the current state of the art, including efforts to push femtosecond pulses into the vacuum ultraviolet and soft X-ray regimes as well as the utilization of novel polarizations to use time-resolved optical activity as a probe of nonadiabatic dynamics. We close this perspective with a forward-looking prospectus on the new areas of application for this technique.

We demonstrate attosecond transient-absorption spectroscopy (ATAS) at the carbon K-edge as a powerful technique to detect few-femtosecond electronic dynamics driven by conical intersections in gas-phase organic molecules, revealing the fastest electronic relaxation measured to date., International Conference on Ultrafast Phenomena, July 18–22, 2022, Montreal, QC, Canada

We present the first vacuum ultraviolet time-resolved photoelectron spectroscopy (VUV-TRPES) study of photoisomerization dynamics in the paradigmatic molecule cis-stilbene. A key reaction intermediate in its dynamics, known as the phantom state, has often been invoked but never directly detected in the gas phase. We report direct spectral signatures of the phantom state in isolated cis-stilbene, observed and characterized through a combination of VUV-TRPES and ab initio multiple spawning (AIMS) nonadiabatic dynamics simulations of the channel-resolved observable. The high VUV probe photon energy tracks the complete excited-state dynamics via multiple photoionization channels, from initial excitation to its return to the "hot" ground state. The TRPES was compared with AIMS simulations of the dynamics from initial excitation, to the phantom-state intermediate (an S1 minimum), through to the ultimate electronic decay to the ground state. This combination revealed the unique spectral signatures and time-dependent dynamics of the phantom-state intermediate, permitting us to report here its direct observation.

Using time-resolved Coulomb explosion imaging (CEI) in an ultraviolet (UV) pump near infrared (NIR) probe experiment, we directly image the different dissociation dynamics in the formaldehyde molecule. Different pathways are distinguished from each other, despite their statistical nature. To extract such dynamics, hidden in a statistical background, calls for an elimination of noise. In our approach, we take advantage of CEI being a quasi-background-free technique., International Conference of Computational Methods in Sciences and Engineering ICCMSE 2020, April 29 – May 3, 2020, Crete, Greece (Virtual), Series: AIP Conference Proceedings

Functional group substituents are a ubiquitous tool in ground-state organic chemistry often employed to fine-tune chemical properties and obtain desired chemical reaction outcomes. Their effect on photoexcited electronic states, however, remains poorly understood. To help build an intuition for these effects, we have studied ethylene, substituted with electron acceptor (cyano) and/or electron donor (methoxy) substituents, both theoretically and experimentally: using ab initio quantum molecular dynamics and time-resolved photoelectron spectroscopy. Our results show the consistent trend that photo-induced ethylenic dynamics is primarily localized to the carbon with the greater electron density. For doubly substituted ethylenes, the trend is additive when both substituents are located on opposite carbons, whereas the methoxy group (in concert with steric effects) dominates when both substituents are located on a single carbon atom. These results point to the development of rules for structure–dynamics correlations; in this case, a novel mechanistic ultrafast photochemistry for conjugated carbon chains employing long-established chemical concepts.

Electronic coherences in molecules are ultrafast charge oscillations on the Molecular Frame MF and their direct observation and separation from electronic population dynamics is challenging. Here we present a valence shell Lab Frame LF scattering method suited to probing electronic coherences in isolated systems. MF electronic coherences lead to LF electronic anisotropies observable by ultrafast angle resolved scattering. Moment analysis of the LF anisotropy completely separates electronic coherences from population dynamics, demonstrated in excited state NH3 using ultrafast time energy angle resolved photoelectron spectroscopy. This general approach applies equally to attosecond femtosecond electronic coherences in isolated systems

We present a framework for the calculation of diabatic states using the combined density functional theory and multireference configuration interaction (DFT/MRCI) method. Due to restrictions present in the current formulation of the DFT/MRCI method (a lack of analytical derivative couplings and the inability to use non-canonical Kohn–Sham orbitals), most common diabatization strategies are not applicable. We demonstrate, however, that diabatic wavefunctions and potentials can be reliably calculated at the DFT/MRCI level of theory using a propagative variant of the block diagonalization diabatization method (P-BDD). The proposed procedure is validated via the calculation of diabatic potentials for LiH and the simulation of the vibronic spectrum of pyrazine. In both cases, the combination of the DFT/MRCI and P-BDD methods is found to correctly recover the non-adiabatic coupling effects of the problem.We present a framework for the calculation of diabatic states using the combined density functional theory and multireference configuration interaction (DFT/MRCI) method. Due to restrictions present in the current formulation of the DFT/MRCI method (a lack of analytical derivative couplings and the inability to use non-canonical Kohn–Sham orbitals), most common diabatization strategies are not applicable. We demonstrate, however, that diabatic wavefunctions and potentials can be reliably calculated at the DFT/MRCI level of theory using a propagative variant of the block diagonalization diabatization method (P-BDD). The proposed procedure is validated via the calculation of diabatic potentials for LiH and the simulation of the vibronic spectrum of pyrazine. In both cases, the combination of the DFT/MRCI and P-BDD methods is found to correctly recover the non-adiabatic coupling effects of the problem.

The photochemical dynamics of double-bond-containing hydrocarbons is exemplified by the smallest alkenes, ethylene and butadiene. Chemical substituents can alter both decay timescales and photoproducts through a combination of inertial effects due to substituent mass, steric effects due to substituent size, and electronic (or potential) effects due to perturbative changes to the electronic potential energy surface. Here, we demonstrate the interplay of different substituent effects on 1,3-butadiene and its methylated derivatives using a combination of ab initio simulation of nonadiabatic dynamics and time-resolved photoelectron spectroscopy. The purely inertial effects of methyl substitution are simulated through the use of mass 15 "heavy-hydrogen" atoms. As expected from both inertial and electronic influences, the excited-state dynamics is dominated by pyramidalization at the unsubstituted carbon sites. Although the electronic effects of methyl group substitution are weak, they alter both decay timescales and branching ratios by influencing the initial path taken by the excited wavepacket following photoexcitation.

Following on from previous experimental and theoretical work [Neville et al., Nat. Commun. 7, 11357 (2016)], we report the results of a combined electronic structure theory and quantum dynamics study of the excited state dynamics of the pyrrole dimer following excitation to its first two excited states. Employing an exciton-based analysis of the A(π3s/σ*) and B(π3s/3p/σ*) states, we identify an excited-state electron transfer pathway involving the coupling of the A(π3s/σ*) and B(π3s/3p/σ*) states and driven by N–H dissociation in the B(π3s/3p/σ*) state. This electron transfer mechanism is found to be mediated by vibronic coupling of the B state, which has a mixed π3s/3p Rydberg character at the Franck-Condon point, to a high-lying charge transfer state of the πσ* character by the N–H stretch coordinate. Motivated by these results, quantum dynamics simulations of the excited-state dynamics of the pyrrole dimer are performed using the multiconfigurational time-dependent Hartree method and a newly developed model Hamiltonian. It is predicted that the newly identified electron transfer pathway will be open following excitation to both the A(π3s/σ*) and B(π3s/3p/σ*) states and may be the dominant relaxation pathway in the latter case.

The efficient, yet accurate, simulation of X-ray absorption spectra represents a significant challenge for ab initio electronic structure methods. Conventional approaches involve the explicit calculation of all core-excited states spanning the energy range of interest, even though only a small number of these states will contribute appreciably to the spectrum. We here report a different approach, based on a time-independent Chebyshev filter diagonalization scheme, which allows for the X-ray absorption spectrum to be computed without the explicit calculation of the core-excited eigenstates. Furthermore, in a subsequent postprocessing calculation, selected peaks may be analyzed via the calculation of natural transition orbitals, if desired. The scheme presented here is based on a refinement of the time-independent Chebyshev filter diagonalization approach. Previous formulations of this method have been characterized by a requirement for significant "user input" via the (sometimes unintuitive) tuning of various numerical parameters. To circumvent this, we introduce a new class of filters based on discrete prolate spheroidal sequences. We demonstrate that the resulting method, which we term Chebyshev-Slepian filter diagonalization, makes filter diagonalization essentially a black-box procedure. The Chebyshev-Slepian filter diagonalization method is implemented at the second-order algebraic diagrammatic construction level of theory and validated through the calculation of the X-ray absorption spectra of trifluoroacetonitrile and 1,4-benzoquinone.

We report how imaginary time wavepacket propagation may be used to efficiently calculate the lowest-lying eigenstates of the electronic Hamiltonian. This approach, known as the relaxation method in the quantum dynamics community, represents a fundamentally different approach to the solution of the electronic eigenvalue problem in comparison to traditional iterative subspace diagonalization schemes such as the Davidson and Lanczos methods. In order to render the relaxation method computationally competitive with existing iterative subspace methods, an extended short iterative Lanczos wavepacket propagation scheme is proposed and implemented. In the examples presented here, we show that by using an efficient wavepacket propagation algorithm the relaxation method is, at worst, as computationally expensive as the commonly used block Davidson-Liu algorithm, and in certain cases, significantly less so.

The excited state non-adiabatic dynamics of the smallest polyene, trans 1,3-butadiene (BD), has long been the subject of controversy due to its strong coupling, ultrafast time scales and the difficulties that theory faces in describing the relevant electronic states in a balanced fashion. Here we apply Ab Initio Multiple Spawning (AIMS) using state-averaged complete active space multistate second order perturbation theory [SA-3-CAS(4/4)-MSPT2] which describes both static and dynamic electron correlation effects, providing a balanced description of both the initially prepared bright 11Bu (ππ*) state and non-adiabatically coupled dark 21Ag state of BD. Importantly, AIMS allows for on-the-fly calculations of experimental observables. We validate our approach by directly simulating the time resolved photoelectron-photoion coincidence spectroscopy results presented in Paper I [A. E. Boguslavskiy et al., J. Chem. Phys. 148, 164302 (2018)], demonstrating excellent agreement with experiment. Our simulations reveal that the initial excitation to the 11Bu state rapidly evolves via wavepacket dynamics that follow both bright- and dark-state pathways as well as mixtures of these. In order to test the sensitivity of the AIMS results to the relative ordering of states, we considered two hypothetical scenarios biased toward either the bright 1Bu or the dark 21Ag state. In contrast with AIMS/SA-3-CAS(4/4)-MSPT2 simulations, neither of these scenarios yields favorable agreement with experiment. Thus, we conclude that the excited state non-adiabatic dynamics in BD involves both of these ultrafast pathways.

We have demonstrated a generally applicable tabletop approach utilizing a 266nm femtosecond laser pulse pump, 800nm pulse probe, coupled with Coulomb explosion imaging (CEI). We have investigated two simple chemical reactions in C2H2 + simultaneously: proton transfer and C=C bond-breaking, triggered by multiphoton ionization to excited states. Too and fro proton migration results are in excellent agreement with new ab initio trajectory simulations which predict isomerization timescales and pathways., 29th International Conference on Photonic, Electronic and Atomic Collisions, ICPEAC 2015, 22 July 2015 through 28 July 2015

We investigate the competition between intersystem crossing (ISC) and internal conversion (IC) as nonradiative relaxation pathways in cyclic alpha,beta-unsaturated enones following excitation to their lowest lying (1)pi pi* state, by means of time-resolved photoelectron spectroscopy and ab initio computation. Upon excitation, the (1)pi pi* state of 2-cyclopentenone decays to the lowest lying (1)pi pi* state within 120 +/- 20 fs. Within 1.2 +/- 0.2 ps, the molecule subsequently decays to the triplet manifold and the singlet ground state, with quantum yields of 0.35 and 0.65, respectively. The corresponding dynamics in modified derivatives, obtained by selective methylation, show a decrease in both IC and ISC rates, with the quantum yields of ISC varying between 0.35 and 0.08. The rapid rates of ISC are explained by a large spin orbit coupling of 45-60 cm(-1) over an extended region of near degeneracy between the singlet and triplet state. Furthermore, the rate of IC is depressed by the existence of a well-defined minimum on the (1)n pi* potential energy surface. The nonadiabatic pathways evinced by the present results highlight the fact that these molecular systems conceptually represent "intermediate cases" between ultrafast dynamics mediated by vibrational motions at conical intersections versus those by statistical decay mechanisms.

Photoinduced dynamics tend to localize at a confined region of a molecule, called dynamophore. Here, we show examples from time-resolved photoelectron spectroscopy, 18th International Conference on Ultrafast Phenomena, UP 2012, July 8-13, 2012, Lausanne, Switzerland, Series: EPJ Web of Conferences

Wavepacket Dynamics (Born–Oppenheimer wavepacket dynamics; Nonadiabatic wave-packet dynamics; Selection of experimental observables) Principles of Time-Resolved Photoelectron Spectroscopy (Photoelectron spectroscopy as a probe (Motivation; Description of photoionization); Description of electronic and nuclear degrees of freedom (Complementary Koopmans' correlations (Type I); Corresponding Koopmans' correlations (Type II); Franck–Condon Spectra)) Applications of Time-Resolved Photoelectron Spectroscopy (Experimental demonstration of TRPES; Interrogation of molecular dynamics employing TRPES (Excited state intramolecular proton transfer; Photostability of DNA bases)), Series: Advanced Series in Physical Chemistry

Conical intersections allow electronically excited molecules to return to their electronic ground state. Here, we observe the fastest electronic relaxation dynamics measured to date by extending attosecond transient-absorption spectroscopy (ATAS) to the carbon K-edge. We selectively launch wave packets in the two lowest electronic states (D0 and D1) of C2H4+. The electronic D1 → D0 relaxation takes place with a short time constant of 6.8 ± 0.2 femtoseconds. The electronic-state switching is directly visualized in ATAS owing to a spectral separation of the D1 and D0 bands caused by electron correlation. Multidimensional structural dynamics of the molecule are simultaneously observed. Our results demonstrate the capability to resolve the fastest electronic and structural dynamics in the broad class of organic molecules. They show that electronic relaxation in the prototypical organic chromophore can take place within less than a single vibrational period., Science, 371 (6528), ISSN:0036-8075, ISSN:1095-9203