Electronic Excited-state Dynamics Research Articles

Managing temperature in open quantum systems strongly coupled with structured environments.

In non-perturbative non-Markovian open quantum systems, reaching either low temperatures with the hierarchical equations of motion (HEOM) or high temperatures with the Thermalized Time Evolving Density Operator with Orthogonal Polynomials Algorithm (T-TEDOPA) formalism in Hilbert space remains challenging. We compare different ways of modeling the environment. Sampling the Fourier transform of the bath correlation function, also called temperature dependent spectral density, proves to be very effective. T-TEDOPA [Tamascelli et al., Phys. Rev. Lett. 123, 090402 (2019)] uses a linear chain of oscillators with positive and negative frequencies, while HEOM is based on the complex poles of an optimized rational decomposition of the temperature dependent spectral density [Xu et al., Phys. Rev. Lett. 129, 230601 (2022)]. Resorting to the poles of the temperature independent spectral density and of the Bose function separately is an alternative when the problem due to the huge number of Bose poles at low temperatures is circumvented. Two examples illustrate the effectiveness of the HEOM and T-TEDOPA approaches: a benchmark pure dephasing case and a two-bath model simulating the dynamics of excited electronic states coupled through a conical intersection. We show the efficiency of T-TEDOPA to simulate dynamics at a finite temperature by using either continuous spectral densities or only all the intramolecular oscillators of a linear vibronic model calibrated from abinitio data of a phenylene ethynylene dimer.

Triplet Excimer Formation in a DNA Duplex with Silver Ion-Mediated Base Pairs.

The dynamics of excited electronic states in self-assembled structures formed between silver(I) ions and cytosine-containing DNA strands or monomeric cytosine derivatives were investigated by time-resolved infrared (TRIR) spectroscopy and quantum mechanical calculations. The steady-state and time-resolved spectra depend sensitively on the underlying structures, which change with pH and the nucleobase and silver ion concentrations. At pH ∼ 4 and low dC20 strand concentration, an intramolecularly folded i-motif is observed, in which protons, and not silver ions, mediate C-C base pairing. However, at the higher strand concentrations used in the TRIR measurements, dC20 strands associate pairwise to yield duplex structures containing C-Ag+-C base pairs with a high degree of propeller twisting. UV excitation of the silver ion-mediated duplex produces a long-lived excited state, which we assign to a triplet excimer state localized on a pair of stacked cytosines. The computational results indicate that the propeller-twisted motifs induced by metal-ion binding are responsible for the enhanced intersystem crossing that populates the triplet state and not a generic heavy atom effect. Although triplet excimer states have been discussed frequently as intermediates in the formation of cyclobutane pyrimidine dimers, we find neither computational nor experimental evidence for cytosine-cytosine photoproduct formation in the systems studied. These findings provide a rare demonstration of a long-lived triplet excited state that is formed in a significant yield in a DNA duplex, demonstrating that supramolecular structural changes induced by metal ion binding profoundly affect DNA photophysics.

Rydberg-type electronic excited state dynamics in small sodium–water clusters

Equilibrium geometries and thermodynamic potentials of the neutral and ionised species of the Na ⋯ ( H 2 O ) n ( n = 1 , . . , 8 ) mixed clusters were computed at MN15/def2-TZVPD level of density functional theory (DFT). The vertical and adiabatic ionisation energies and enthalpies were computed and their cluster size dependence was discussed. Laser-induced ionisation involves electronic excitation through Rydberg-type excited states, which have been characterised using the TDDFT method, including the ωB2PLYP double-hybrid exchange-correlation functional. Ab initio molecular dynamics calculations were performed on a time scale of 20 picoseconds. Fluctuations of the charge and the sodium–oxygen atomic distances predict that, the 3 s 1 electron of the sodium atom are transferred from the delocalised Rydberg orbitals to the Rydberg orbitals around the water molecules and the sodium atom becomes positively charged with around 0.6e after the first 10 ps . On the other hand, some of the water molecules can move away up to 5 Å from the sodium with a significant negative charge on them. It has been shown that non-radiative relaxation cannot be excluded, they can mostly occur for cases n ⩾ 4 . The results confirm that the adiabatic photo-ionisation can occur on the basis of cluster disintegration.

Dichroism in the ultrafast ARPES response and valence band orbital nature of $$\textrm{BaNiS}_2$$

Time-resolved ARPES gives access to the band structure and ultrafast dynamics of excited electronic states in solids. The orbital character of the bands close to the Fermi level is essential to understand the origin of several exotic phenomena in quantum materials. By performing polarization dependent time- and angle-resolved photoemission spectroscopy and by analyzing the chirality of the photoelectron yield for two different crystal orientations, we identify the orbital character of bands below and above the chemical potential for the Dirac semimetal BaNiS2. Our results illustrate how the control and understanding of matrix elements effects in time-resolved photoemission spectroscopy can be a powerful tool for the study of quantum materials.

Observation of site-selective chemical bond changes via ultrafast chemical shifts

The concomitant motion of electrons and nuclei on the femtosecond time scale marks the fate of chemical and biological processes. Here we demonstrate the ability to initiate and track the ultrafast electron rearrangement and chemical bond breaking site-specifically in real time for the carbon monoxide diatomic molecule. We employ a local resonant x-ray pump at the oxygen atom and probe the chemical shifts of the carbon core-electron binding energy. We observe charge redistribution accompanying core-excitation followed by Auger decay, eventually leading to dissociation and hole trapping at one site of the molecule. The presented technique is general in nature with sensitivity to chemical environment changes including transient electronic excited state dynamics. This work provides a route to investigate energy and charge transport processes in more complex systems by tracking selective chemical bond changes on their natural timescale.

MRCI+Q study on SnF+ including spin-orbit coupling

The low-lying electronic states correlated to the lowest dissociation limit of SnF+ are investigated using the multireference configuration interaction method including the core-valence correlations and the spin-orbit coupling effects. Based on the computed potential energy curves of the Λ–S and Ω states, the spectroscopic constants of the bound states of SnF+ are determined. The Λ–S components of the five Ω states (X1Σ+0+, a3Π0+, a3Π0-, a3Π1 and a3Π2) are shown as a function of bond length considering the avoided crossing rules. The a3Π-X1Σ+ transition properties have been reported in detail. Our study is expected to be helpful for understanding the spectroscopy and dynamics of electronic excited states of the SnF+ ion.

Methods to Calculate Electronic Excited-State Dynamics for Molecules on Large Metal Clusters with Many States: Ensuring Fast Overlap Calculations and a Robust Choice of Phase.

We present an efficient set of methods for propagating excited-state dynamics involving a large number of configuration interaction singles (CIS) or Tamm-Dancoff approximation (TDA) single-reference excited states. Specifically, (i) following Head-Gordon et al., we implement an exact evaluation of the overlap of singly-excited CIS/TDA electronic states at different nuclear geometries using a biorthogonal basis and (ii) we employ a unified protocol for choosing the correct phase for each adiabat at each geometry. For many-electron systems, the combination of these techniques significantly reduces the computational cost of integrating the electronic Schrodinger equation and imposes minimal overhead on top of the underlying electronic structure calculation. As a demonstration, we calculate the electronic excited-state dynamics for a hydrogen molecule scattering off a silver metal cluster, focusing on high-lying excited states, where many electrons can be excited collectively and crossings are plentiful. Interestingly, we find that the high-lying, plasmon-like collective excitation spectrum changes with nuclear dynamics, highlighting the need to simulate non-adiabatic nuclear dynamics and plasmonic excitations simultaneously. In the future, the combination of methods presented here should help theorists build a mechanistic understanding of plasmon-assisted charge transfer and excitation energy relaxation processes near a nanoparticle or metal surface.

Time-Resolved Optical Pump-Resonant X-ray Probe Spectroscopy of 4-Thiouracil: A Simulation Study.

We theoretically monitor the photoinduced ππ* → nπ* internal conversion process in 4-thiouracil (4TU), triggered by an optical pump. The element-sensitive spectroscopic signatures are recorded by a resonant X-ray probe tuned to the sulfur, oxygen, or nitrogen K-edge. We employ high-level electronic structure methods optimized for core-excited electronic structure calculation combined with quantum nuclear wavepacket dynamics computed on two relevant nuclear modes, fully accounting for their quantum nature of nuclear motions. We critically discuss the capabilities and limitations of the resonant technique. For sulfur and nitrogen, we document a pre-edge spectral window free from ground-state background and rich with ππ* and nπ* absorption features. The lowest sulfur K-edge shows strong absorption for both ππ* and nπ*. In the lowest nitrogen K-edge window, we resolve a state-specific fingerprint of the ππ* and an approximate timing of the conical intersection via its depletion. A spectral signature of the nπ* transition, not accessible by UV-vis spectroscopy, is identified. The oxygen K-edge is not sensitive to molecular deformations and gives steady transient absorption features without spectral dynamics. The ππ*/nπ* coherence information is masked by more intense contributions from populations. Altogether, element-specific time-resolved resonant X-ray spectroscopy provides a detailed picture of the electronic excited-state dynamics and therefore a sensitive window into the photophysics of thiobases.

Structure and dynamics of electronically excited molecular systems

AbstractStructure and dynamics of electronic excited states of molecular systems is of contemporary interest to understand a rich variety of processes ranging from molecular spectroscopy to reaction dynamics. Developments over the past decades have established that coupling of electronic and nuclear motion is a generic feature in polyatomic molecular systems which causes a breakdown of the Born–Oppenheimer approximation. Such a coupling leads to conical intersections in non‐linear and glancing degeneracy in linear polyatomic systems. While the former case is quite general and has wide variances depending on the nature of the coupling on the symmetry ground, the latter one is specific to the linear systems having axial symmetry. The Jahn–Teller (JT) systems form a subclass in the former category and the latter category constitutes the Renner–Teller (RT) systems. These systems are characterized by high point group symmetry and orbital degeneracy. The latter splits upon distortion along suitable symmetry reducing nuclear displacement. The JT or the RT split component states may undergo symmetry allowed coupling among another JT or RT split component states or with some non‐degenerate states. These are termed as the pseudo‐JT and pseudo‐RT coupling, respectively. We have studied a variety of photo‐induced molecular processes with the aid of ab initio quantum chemistry and first principles quantum dynamics methods. We will document here some of the important developments in the field focusing on the work done on the JT and RT systems primarily in our own laboratory.This article is categorized under: Theoretical and Physical Chemistry > Spectroscopy Software > Quantum Chemistry Structure and Mechanism > Molecular Structures

First‐principles dynamics of photoexcited molecules and materials towards a quantum description

AbstractThe past decades have witnessed the success of ground‐state density functional theory capturing static electronic properties of various materials. However, for time dependent processes especially those involving excited states, real‐time time‐dependent density functional theory (rt‐TDDFT) and advanced nonadiabatic algorithms are essential, especially for practical simulations of molecules and materials under the occurrence of ultrafast laser field. Here we summarize the recent progresses in developing rt‐TDDFT approaches within numerical atomic orbitals and planewave formalisms, as well as the efforts combining rt‐TDDFT and ring polymer molecular dynamics to take into account nuclear quantum effects in quantum electronic‐nuclear dynamic simulations. Typical applications of first‐principles dynamics of excited electronic states including high harmonic generation, charge density wave, photocatalytic water splitting, as well as quantum nuclear motions in ozone and graphene, are presented to demonstrate the features and advantages of these methods. The progresses in method developments and practical applications provide unprecedented insights into nonadiabatic dynamics of excited states in the Ehrenfest scheme and beyond, towards a comprehensive understanding of excited electronic structure, electron–phonon interactions, photoinduced charge transfer and chemical reactions, as well as quantum nuclear motions in excited states.This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Electronic Structure Theory > Density Functional Theory Molecular and Statistical Mechanics > Molecular Dynamics and Monte‐Carlo Methods

Simultaneous observation of nuclear and electronic dynamics by ultrafast electron diffraction.

Simultaneous observation of nuclear and electronic motion is crucial for a complete understanding of molecular dynamics in excited electronic states. It is challenging for a single experiment to independently follow both electronic and nuclear dynamics at the same time. Here we show that ultrafast electron diffraction can be used to simultaneously record both electronic and nuclear dynamics in isolated pyridine molecules, naturally disentangling the two components. Electronic state changes (S1→S0 internal conversion) were reflected by a strong transient signal in small-angle inelastic scattering, and nuclear structural changes (ring puckering) were monitored by large-angle elastic diffraction. Supported by ab initio nonadiabatic molecular dynamics and diffraction simulations, our experiment provides a clear view of the interplay between electronic and nuclear dynamics of the photoexcited pyridine molecule.

Electronic and nuclear dynamics in one

Chemical Physics Because of the complex, ultrafast interplay between nuclear and electronic degrees of freedom, probing both nuclear and electronic dynamics in excited electronic states within a single time-resolved experiment is a great challenge. Yang et al. used ultrafast electron diffraction in combination with ab initio nonadiabatic molecular dynamics and diffraction simulations to study the relaxation dynamics of isolated pyridine molecules after photoexcitation to the S1 state (see the Perspective by Domcke and Sobolewski). They showed that electronic state evolution and molecular structural changes can be recorded simultaneously and independently by tracing a transient signal in small-angle inelastic scattering and large-angle elastic diffraction, respectively. Science , this issue p. [885][1]; see also p. [820][2] [1]: /lookup/doi/10.1126/science.abb2235 [2]: /lookup/doi/10.1126/science.abb9937

Time-resolved ultrafast transient polarization spectroscopy to investigate nonlinear processes and dynamics in electronically excited molecules on the femtosecond time scale

We report a novel experimental technique to investigate ultrafast dynamics in photoexcited molecules by probing the 3rd-order nonlinear optical susceptibility. A non-collinear 3-pulse scheme is developed to probe the ultrafast dynamics of excited electronic states using the optical Kerr effect. Optical homodyne and optical heterodyne detections are demonstrated to measure the 3rd-order nonlinear optical response for the S1 excited state of liquid nitrobenzene, which is populated by 2-photon absorption of a 780 nm 40 fs excitation pulse.

Ultrafast Dynamics of Excited Electronic States in Nitrobenzene Measured by Ultrafast Transient Polarization Spectroscopy.

We investigate ultrafast dynamics of the lowest singlet excited electronic state in liquid nitrobenzene using ultrafast transient polarization spectroscopy, extending the well-known technique of optical Kerr effect spectroscopy to excited electronic states. The third-order nonlinear response of the excited molecular ensemble is measured using a pair of femtosecond pulses following a third femtosecond pulse that populates the S1 excited state. By measuring this response, which is highly sensitive to details of the excited state character and structure, as a function of time delays between the three pulses involved, we extract the dephasing time of the wave packet on the excited state. The dephasing time, measured as a function of time delay after pump excitation, shows oscillations indicating oscillatory wave packet dynamics on the excited state. From the experimental measurements and supporting theoretical calculations, we deduce that the wave packet completely leaves the S1 state potential energy surface after three traversals of the intersystem crossing between the singlet S1 and triplet T2 states.

Ab Initio Linear and Pump-Probe Spectroscopy of Excitons in Molecular Crystals.

Linear and nonlinear spectroscopies are powerful tools used to investigate the energetics and dynamics of electronic excited states of both molecules and crystals. While highly accurate ab initio calculations of molecular spectra can be performed relatively routinely, extending these calculations to periodic systems is challenging. Here, we present calculations of the linear absorption spectrum and pump-probe two-photon photoemission spectra of the naphthalene crystal using equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD). Molecular acene crystals are of interest due to the low-energy multiexciton singlet states they exhibit, which have been studied extensively as intermediates involved in singlet fission. Our linear absorption spectrum is in good agreement with experiment, predicting a first exciton absorption peak at 4.4 eV, and our two-photon photoemission spectra capture the qualitative behavior of multiexciton states, whose double-excitation character cannot be captured by current methods. The simulated pump-probe spectra provide support for existing interpretations of two-photon photoemission experiments in closely related acene crystals such as tetracene and pentacene.

Electronic structure and spectroscopy of the low-lying electronic states of thallium fluoride: MRCI + Q study including spin-orbit coupling

The low-lying electronic states of thallium fluoride (TlF) are investigated using the multi-reference configuration interaction method including the core-valence and the spin-orbit coupling effects. Based on the calculated potential energy curves, the spectroscopic constants of the bound states of TlF are determined, which are in accordance with available theoretical and experimental results in the literature. The transition dipole moments, Franck-Condon factors, and the radiative lifetimes of the a3Π0+- X1Σ+0+and a3Π1- X1Σ+0+ transitions have been reported. Our study is expected to be helpful for understanding the spectroscopy and dynamics of electronic excited states of the TlF molecule.

On the origin of spatially dependent electronic excited-state dynamics in mixed hybrid perovskite thin films

The fundamental photophysics underlying the remarkable performance of organic-inorganic hybrid perovskites in optoelectronic device applications has been increasingly studied using complementary spectroscopic techniques. However, the spatially heterogeneous polycrystalline morphology of the solution-processed thin films is often overlooked in conventional ensemble measurements and therefore the reported results are averaged over hundreds or even thousands of nano- and micro-crystalline grains. Here, we apply femtosecond transient absorption microscopy to spatially and temporally probe ultrafast electronic excited-state dynamics in chloride containing mixed lead halide perovskite (CH3NH3PbI3–xClx) thin films. We found that the electronic excited-state relaxation kinetics are extremely sensitive to the spatial location probed, which was manifested by position-dependent transient absorption signal amplitude and decay behaviour, along with an obvious rise component at some positions. The analysis of transient absorption kinetics acquired at several distinct spatial positions enabled us to identify Auger recombination as the dominant mechanism underlying the initial portions of the spatially dependent dynamics with variable rate constants. The different rates observed suggest occurrence of distinct local electronic structures and variable contributions from impurities/defects and phonons in this nonlinear dynamical process.

Revisiting the spectroscopy of xanthine derivatives: theobromine and theophylline.

We explore the influence of the relative position of the methyl substituent on the photophysics of theophylline and theobromine, two molecules that are structurally related to the DNA bases. Using a combination of spectroscopic techniques and quantum mechanical calculations, we show that moving the methyl group from N1 in theophylline to N7 in theobromine causes significant differences in their excited state properties, i.e., it produces pyramidalization of N7 in the excited state of the latter. Paradoxically, this modification seems to have little effect on the structural properties of the cation and the ionization process. It is suggested that similar effects may exist in the excited state properties of DNA bases.

Imaging the Dynamics of CH2BrI Photodissociation in the Near Ultraviolet Region.

The photodissociation dynamics of jet-cooled CH2BrI were investigated in the near-ultraviolet (UV) region from 280-310 nm using velocity map imaging. We report the translational and internal energy distributions of the CH2Br radical and ground state I (2 P3/2) or spin-orbit excited I (2 P1/2) fragments determined by velocity map imaging of the ionized iodine fragments following 2 + 1 resonance-enhanced multiphoton ionization of the nascent neutral iodine products. The velocity distributions indicate that most of the available energy is partitioned into the internal energy of the CH2Br radical with only modest translational excitation imparted to the cofragments, which is consistent with a simple impulsive model. Furthermore, from extrapolation of the velocity distribution results, the first determination of the C-I bond dissociation energy of CH2BrI is presented in this work to be D0 = 16 790 ± 590 cm-1. The ion images appear anisotropic, indicative of a prompt dissociation, and the derived anisotropy parameters are consistently positive. Additionally, the angular distributions report on the electronic excited state dynamics, which validate recent works characterizing the electronic states responsible for the first absorption band of CH2BrI. In the current work, photolysis of CH2BrI on the red edge of the absorption spectrum reveals an additional channel producing I (2 P3/2) fragments.

Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2'-bipyridine)2(CN)2

We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,2′-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2′-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)2(CN)2] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)2(CN)2] complement prior measurement performed on [Fe(bpy)3]2+ and [Fe(bpy)(CN)4]2− in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)N(CN)6–2N]2N-4, where N = 1–3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.