Electronic States Of Molecules Research Articles

Direct observation of geometric-phase interference in dynamics around a conical intersection.

Conical intersections are ubiquitous in chemistry and physics, often governing processes such as light harvesting, vision, photocatalysis and chemical reactivity. They act as funnels between electronic states of molecules, allowing rapid and efficient relaxation during chemical dynamics. In addition, when a reaction path encircles a conical intersection, the molecular wavefunction experiences a geometric phase, which can affect the outcome of the reaction through quantum-mechanical interference. Past experiments have measured indirect signatures of geometric phases in scattering patterns and spectroscopic observables, but there has been no direct observation of the underlying wavepacket interference. Here we experimentally observe geometric-phase interference in the dynamics of a wavepacket travelling around an engineered conical intersection in a programmable trapped-ion quantum simulator. To achieve this, we develop a technique to reconstruct the two-dimensional wavepacket densities of a trapped ion. Experiments agree with the theoretical model, demonstrating the ability of analogue quantum simulators-such as those realized using trapped ions-to accurately describe nuclear quantum effects.

Ultrafast X-Ray Probes of Elementary Molecular Events

Elementary events that determine photochemical outcomes and molecular functionalities happen on the femtosecond and subfemtosecond timescales. Among the most ubiquitous events are the nonadiabatic dynamics taking place at conical intersections. These facilitate ultrafast, nonradiative transitions between electronic states in molecules that can outcompete slower relaxation mechanisms such as fluorescence. The rise of ultrafast X-ray sources, which provide intense light pulses with ever-shorter durations and larger observation bandwidths, has fundamentally revolutionized our spectroscopic capabilities to detect conical intersections. Recent theoretical studies have demonstrated an entirely new signature emerging once a molecule traverses a conical intersection, giving detailed insights into the coupled nuclear and electronic motions that underlie, facilitate, and ultimately determine the ultrafast molecular dynamics. Following a summary of current sources and experiments, we survey these techniques and provide a unified overview of their capabilities. We discuss their potential to dramatically increase our understanding of ultrafast photochemistry.

Adsorchromism: Molecular Nanoarchitectonics at 2D Nanosheets-Old Chemistry for Advanced Chromism.

Chromism induced by changes in the electronic states of dye molecules due to surface adsorption is termed “adsorchromism” in this article. These changes of molecular electronic states are induced by protonation, aggregation, intramolecular structural changes, and other processes, depending on the surface environment. Intramolecular structural changes, such as co‐planarization and decreased molecular motion are the most characteristic and interesting behavior of dye molecules at the surfaces, resulting in spectral shift and/or emission enhancement. In this review, adsorchromism at the surfaces of layered materials are summarized since their flexibility of interlayer distance, surface flatness, and transparency is suitable for a detailed observation. By understanding the relationship between adsorchromism and the electronic states of molecules on the surfaces, it will be possible to induce some desired functions which can be realized simply by adsorption, instead of complicated organic syntheses. Thus, adsorchromism has potential applications such as effective solar energy harvesting systems, or biological/chemical sensors to visualize environmental changes.

Dynamic Nuclear Polarization Enhanced Nuclear Spin Optical Rotation.

Nuclear spin optical rotation (NSOR) has been investigated as a magneto-optical effect, which holds the potential for applications, including hybrid optical-nuclear magnetic resonance (NMR) spectroscopy and gradientless imaging. The intrinsic nature of NSOR renders its detection relatively insensitive, which has prevented it moving from a proof of concept to a method supporting chemical characterizations. In this work, the dissolution dynamic nuclear polarization technique is introduced to provide nuclear spin polarization, increasing the signal-to-noise ratio by several thousand times. NSOR signals of 1 H and 19 F nuclei are observed in a single scan for diluted compounds, which has made this effect suitable for the determination of electronic transitions from a specific nucleus in a large molecule.

Attosecond XUV probing of vibronic quantum superpositions in Br2+

Ultrafast laser excitation can create coherent superpositions of electronic states in molecules and trigger ultrafast flow of electron density on few- to sub-femtosecond time scales. While recent attosecond experiments have addressed real-time observation of these primary photochemical processes, the underlying roles of simultaneous nuclear motions and how they modify and disturb the valence electronic motion remain uncertain. Here, we investigate coherent electronic-vibrational dynamics induced among multiple vibronic levels of ionic bromine (Br$_2^+$), including both spin-orbit and valence electronic superpositions, using attosecond transient absorption spectroscopy. Decay, revival, and apparent frequency shifts of electronic coherences are measured via characteristic quantum beats on the Br-$3d$ core-level absorption signals. Quantum-mechanical simulations attribute the observed electronic decoherence to broadened phase distributions of nuclear wave packets on anharmonic potentials. Molecular vibronic structure is further revealed to be imprinted as discrete progressions in electronic beat frequencies. These results provide a future basis to interpret complex charge-migration dynamics in polyatomic systems.

Molecular electronic decoherence following attosecond photoionisation

Attosecond pulses can be used to generate coherent superpositions of cationic electronic states in molecules through photoionisation. These can drive coherent electronic dynamics, which may decay within a few femtoseconds due to nuclear motion. In this work, we study the impact of the photoelectron on decoherence in the valence electron system of molecules following attosecond photoionisation. To this end, we include the photoelectron as a classical point charge in a quantum–classical simulation of light-induced ultrafast molecular dynamics and consider ionisation by sub-femtosecond pulses with distinct qualities. By disentangling the contributions of photoelectron and nuclei to the overall electronic decoherence, we find that the photoelectron causes partial decoherence within the first 50 attoseconds. This timescale is noticed to be independent of the ionising pulse. Full electronic decoherence is only seen when the spatial extension of the nuclear wave packet is considered.

Bi-stable electronic states of cobalt phthalocyanine molecules on two-dimensional vanadium diselenide

Hybrid organic/2D interfaces combine the wide spectrum of 2D material properties with the major advantages of organic materials, such as low cost, mechanical flexibility, and chemical tunability. Here, we report the electronic properties of cobalt phthalocyanine (CoPc) molecules adsorbed on molecular beam epitaxy-grown monolayer vanadium diselenide (VSe2). Using scanning tunneling microscopy/spectroscopy, we provide evidence of highly ordered molecular assembly on monolayer VSe2, with two distinctive bright and regular molecular contrasts, which are not observed on graphite. These contrasts also lead to a distinct difference in the electronic state of the molecule’s central Co atom, for which density functional theory calculations indicate the regular state as the ground state. A correlation between these two molecular states and the charged states of individual molecules is postulated, as demonstrated by the possibility of switching the bright state to the regular state using a negative tip voltage pulse.

Coherent interference of molecular electronic states in NO by two-color femtosecond laser pulses

Quantum coherence interference between electronic states in molecules is an important factor for controlling electronic and nuclear dynamics in photoinduced physics and chemistry. We measure and model molecular angle-dependent photoionization yields to explore coherent interference dynamics between quasiequal energy electronic-vibrational states of the nitric oxide molecule, NO, by ultrafast phase-controlled two-color femtosecond laser pulses. We demonstrate by experiment and theory that the photoelectron angular distribution of NO, where two excited electronic states are coherently combined by ultrafast pulses, is a function of the relative phase of the pulses, and the photoelectron kinetic energy. The modification of photoelectron emission angular patterns encodes the information of the coherence via electronic state interference. The result allows access to phase-dependent state correlations and ultrafast vibrationic dynamics in molecules.

Imaging charge migration in the asymmetric molecule with the holographic interference in strong-field tunneling ionization

Coherent superposition of several electronic states in molecules induces attosecond charge migration. We theoretically demonstrate a way to observe this attosecond charge migration in the asymmetric molecule HeH2+ using strong-field photoelectron holography, complementary to our previous work (He M et al 2018 Phys. Rev. Lett. 120, 133204) where the charge migration in the symmetric molecule was monitored. We disentangle the holographic interference from other types of interference in the photoelectron momentum distribution with the Fourier frequency filter method. By employing the holographic fringes originating from the ground state of HeH2+ as a reference, we develop a feasible approach to visualize the charge migration in the asymmetric molecule with attosecond temporal resolution.

High resolution spectroscopy on Te2: New lines for reference

Ro-vibrational spectra of different electronic states of molecules are often used as absolute wavelength or frequency standards. These standards are also used to mitigate any slow drift of laser frequency during an experiment. In the precision experiment, the two most commonly used molecular standards are iodine and tellurium, both are homo-nuclear diatomic molecules. The former is mostly used as standard for the long wavelength (600-900 nm) region, while the tellurium spectrum is widely used in short wavelength (400-550 nm) including near ultraviolet. A comprehensive data on tellurium spectra can be obtained from the tellurium atlas [1]. However near the 455 nm range where a number of important atomic resonance line, the atlas provides no significant data. We have performed high resolution modulation transfer spectroscopy (MTS) on tellurium molecule in a hot cell in the region close to 455 nm wavelength thereby obtained more than 100 new spectral lines which were not observed before. The linewidth of each of these peaks is about a few tens of MHz, making them suitable for laser frequency locking.

Influence of molecular oxygen addition on gain and generation characteristics of a cryogenic slab RF-discharge-pumped overtone CO laser

The influence of molecular oxygen addition to an active medium of a cryogenic slab RF-discharge-pumped overtone CO laser operating without active medium circulation on the threshold generation conditions and spectral and energy characteristics of laser radiation is calculated. Results of the modelling well agree with experimental data obtained at the initial period of laser operation and substantially differ from data obtained after reaching a stationary operation regime. The model sufficiently well describes characteristics of a single-pulse electron-beam controlled CO laser operated on gas mixtures comprising oxygen. In this model, the generation pulse profiles of a cryogenic slab CO laser operated on the fundamental band transitions of the CO molecule under pulsed-periodic RF pumping well agree with experimental data. The agreement is also observed for the parts of the RF-discharge power spent on exciting vibrational and electronic states of molecules. Reasons for distinctions between some other calculated and experimental data are analysed.

Application of Connection in Molecular Dynamics

The evolution of electronic states in molecule has two origins: dynamical one produced by Schrödinger equation and kinematical one caused by base transformation due to nuclear motion. In current theories, the former gets analytic expression; the latter depends on heavy numerical calculation, which contains uncertainty. By using connection of fiber bundles, this paper establishes an analytic formula for the latter, and the numerical work is simplified. It shows the mathematical structure of molecule is fiber bundle.

Taking six-dimensional spectra in finite time.

Clever data acquisition can probe how vibrations couple to electronic states in molecules

Probing the excited state relaxation dynamics of pyrimidine nucleosides in chloroform solution

Ultrafast transient electronic and vibrational absorption spectroscopy (TEAS and TVAS) of 2'-deoxy-cytidine (dC) and 2'-deoxy-thymidine (dT) dissolved in chloroform examines their excited-state dynamics and the recovery of ground electronic state molecules following absorption of ultraviolet light. The chloroform serves as a weakly interacting solvent, allowing comparisons to be drawn with prior experimental studies of the photodynamics of these nucleosides in the gas phase and in polar solvents such as water. The pyrimidine base nucleosides have some propensity to dimerize in aprotic solvents, but the monomer photochemistry can be resolved clearly and is the focus of this study. UV absorption at a wavelength of 260 nm excites a 1ππ* ← S0 transition, but prompt crossing of a significant fraction (50% in dC, 17% in dT) of the 1ππ* population into a nearby 1nπ* state is too fast for the experiments to resolve. The remaining flux on the 1ππ* state leaves the vertical Franck-Condon region and encounters a conical intersection with the ground electronic state of ethylenic twist character. In dC, the 1ππ* state decays to the ground state with a time constant of 1.1 ± 0.1 ps. The lifetime of the 1nπ* state is much longer in the canonical forms of both molecules: recovery of the ground state population from these states occurs with time constants of 18.6 ± 1.1 ps in amino-oxo dC and ∼114 ps in dT, indicating potential energy barriers to the 1nπ*/S0 conical intersections. The small fraction of the imino-oxo tautomer of dC present in solution has a longer-lived 1nπ* state with a lifetime for ground state recovery of 193 ± 55 ps. No evidence is found for photo-induced tautomerization of amino-oxo dC to the imino-oxo form, or for population of low lying triplet states of this nucleoside. In contrast, ∼8% of the UV-excited dT molecules access the long-lived T1 (3ππ*) state through the 1nπ* state. The primary influence of the solvent appears to be the degree to which it destabilizes the states of 1nπ* character, with consequences for the lifetimes of these states as well as the triplet state yields.

Theoretical Calculation of the Low-Lying Electronic States of the Molecule ScSe

Theoretical investigation of the lowest electronic states of ScSe molecule, in the representation 2s+1Λ(+/-), has been performed via CASSCF and MRCI + Q (single and double excitations with Davidson correction) calculations. The calculated potential energy curves (PECs), permanent dipole moment curves (PDMCs), and spectroscopic constants are reported for the 14 lowest electronic states. The comparison of the present results with the rare available theoretical data in literature shows an overall good agreement. To the best of our knowledge, 13 electronic states of the ScSe molecule are not yet investigated either experimentally or theoretically; they are investigated in the present work for the first time.

On entropy/information continuity in molecular electronic states

ABSTRACTIn the information-theoretic (IT) description of electronic states in molecules, the current and source concepts of the continuity relations for densities of quantum descriptors of the state resultant entropic content combining the probability and current/phase contributions are examined and interpreted. The resultant Shannon- and Fisher-type functionals, isomorphic in the position and momentum representations of quantum mechanics, are explored and the corresponding information sources are established. All these thermodynamic-like expressions in terms of the relevant IT intensities, affinities, fluxes and probability sources predict the vanishing entropy source for the zero intensities associated with the state modulus (probability) and phase (current) degrees of freedom.

Theoretical Study of the Triplet Electronic States of the BP Molecule

The Complete Active Space Self Consistent Field (CASSCF) with Multi Reference Configuration Interaction (single and double excitation with Davidson correction) MRCI + Q method has been used to investigate the potential energy curves of the 17 low-lying triplet electronic states of the molecule BP. The harmonic vibrational frequency ωe, the inter-nuclear distance at equilibrium Re, the rotational constant Be, the electronic energy with respect to the minimum ground state energy Te, and the permanent dipole moment have been also calculated. A literature review shows a strong correlation between our investigated data and those previously published either theoretically or experimentally. This work introduces, for the first time, a study of 14 new electronic states. Our spectroscopic data can be a conducive to further work on BP molecule in both experimental and theoretical research.

On the angular momentum transfer upon excitation of electronic states of molecules by electrons of various energies

The intensities of lines IQ(1) and IQ(3) of the vibrational transition v′ = 0 → v″ = 0 of the Fulcher band system upon excitation of hydrogen molecules by an electron beam are measured. The beamelectron energy is ɛe = 15…30 eV (Δɛe ∼ 1 eV). The effective cross sections of H2 (d3Πu, v = 0) state excitation by electron impact are estimated both without changing themolecule angular momentum and with an appreciable angular momentum transfer. Processes with an appreciable angular momentum transfer are most efficient in the subthreshold excitation energy range (Ethr = 14 eV < Ee ≤ 18 eV) and are related to the formation of an intermediate composite system, i.e., a negative ion in the quasi-discrete state.

On phase/current components of entropy/information descriptors of molecular states

Quantum-generalised descriptors of the information content of electronic states in molecules are proposed, in which non-classical (current) terms complement classical (probability) functionals of the ordinary information theory. The relation between densities of the familiar classical Fisher and Shannon information/entropy measures is applied to determine their non-classical complements. The quantum supplement of the classical Shannon entropy describes the average magnitude of the phase distribution, while the current term in the Fisher measure accounts for the gradient content of the state phase function. Illustrative applications of these quantum information concepts are presented and thermodynamical analogies are commented upon. The particle-density-constrained (vertical) and -unconstrained (horizontal) equilibria in molecules and their fragments are explored and the corresponding equilibrium ‘thermodynamic’ phases are determined. A separation of the density (modulus) and current (phase) factors of general many-electron states is effected using the Harriman–Zumbach–Maschke construction of antisymmetric states yielding the specified electron density. The phenomenological framework in spirit of the non-equilibrium thermodynamical description is proposed. It accounts for both the density and current degrees of freedom of molecular states. The associated entropy source in the information continuity equation is derived.

Theoretical study of LiK and LiK+ in adiabatic representation

The potential energy curves have been calculated for the electronic states of the molecule LiK within the range 3 to 300 a.u., of the internuclear distance R. Using an ab initio method, through a semiempirical spin-orbit pseudo-potential for the Li (1s2) and K (1s22s22p63s23p6) cores and core valence correlation correction added to the electrostatic Hamiltonian with Gaussian basis sets for both atoms. The core valence effects including core-polarization and core-valence correlation are taken into account by using an l-dependent core-polarization potential. The molecular orbitals have been derived from self-consistent field (SCF) calculation. The spectroscopic constants, dipole moments and vibrational levels of the lowest electronic states of the LiK molecule dissociating into K (4s, 4p, 5s, 3d, and 5p) + Li (2s, 2p, 3s, and 3p) in 1, 3Σ, 1, 3Π, and 1, 3Δ symmetries. Adiabatic results are also reported for 2Σ, 2Π, and 2Δ electronic states of the molecular ion LiK+ dissociating into Li (2s, 2p, 3s, and 3p) + K+ and Li+ + K (4s, 4p, 5s, 3d, and 5p). The comparison of the present results with those available in the literature shows a very good agreement in spectroscopic constants of some lowest states of the LiK and LiK+ molecules, especially with the available theoretical works. The existence of numerous avoided crossing between electronic states of 2Σ and 2Π symmetries is related to the charge transfer process between the two ionic systems Li+K and LiK+.