First-excited Electronic States Research Articles

Analytical Nonadiabatic Couplings and Gradients within the State-Averaged Orbital-Optimized Variational Quantum Eigensolver.

We introduce several technical and analytical extensions to our recent state-averaged orbital-optimized variational quantum eigensolver (SA-OO-VQE) algorithm (see Yalouz et al. Quantum Sci. Technol. 2021, 6, 024004). Motivated by the limitations of current quantum computers, the first extension consists of an efficient state-resolution procedure to find the SA-OO-VQE eigenstates, and not just the subspace spanned by them, while remaining in the equi-ensemble framework. This approach avoids expensive intermediate resolutions of the eigenstates by postponing this problem to the very end of the full algorithm. The second extension allows for the estimation of analytical gradients and nonadiabatic couplings, which are crucial in many practical situations ranging from the search of conical intersections to the simulation of quantum dynamics, in, for example, photoisomerization reactions. The accuracy of our new implementations is demonstrated on the formaldimine molecule CH2NH (a minimal Schiff base model relevant for the study of photoisomerization in larger biomolecules), for which we also perform a geometry optimization to locate a conical intersection between the ground and first-excited electronic states of the molecule.

Surface chemical trapping of optical cycling centers.

Quantum information processors based on trapped atoms utilize laser-induced optical cycling transitions for state preparation and measurement. These transitions consist of an electronic excitation from the ground to an excited state and a decay back to the initial ground state, associated with a photon emission. While this technique has been used primarily with atoms, it has also recently been shown to work for some divalent metal hydroxides (e.g. SrOH) and alkoxides (e.g. SrOCH3). This extension to molecules is possible because these molecules feature nearly isolated, atomic-like ground and first-excited electronic states centered on the radical metal atom. We theoretically investigate the extension of this idea to a larger scale by growing the alkyl group, R, beyond the initial methyl group, CH3, while preserving the isolated and highly vertical character of the electronic excitation on the radical metal atom, M. Theory suggests that in the limit as the size of the ligand carbon chain increases, it can be considered a functionalized diamond (or cubic boron nitride) surface. Several requirements must be observed for the cycling centers to function when bound to the surface. First, the surface must have a significant band gap that fully encapsulates both the ground and excited states of the cycling center. Second, while the surface lattice imposes strict limits on the achievable spacing between the SrO- groups, at high coverage, SrO- centers can interact, and show geometric changes and/or electronic state mixing. We show that the coverage of the diamond surface with SrO- cycling centers needs to be significantly sub-monolayer for the functionality of the cycling center to be preserved. Having the lattice-imposed spatial control of SrO- placements will allow nanometer-scale proximity between qubits and will eliminate the need for atom traps for localized cycling emitters. Our results also imply that a functionalization could be done on a scanning microscope tip for local quantum sensing or on photonic structures for optically-mediated quantum information processing.

Ab initio electronic structure and prospects for the formation of ultracold calcium–alkali-metal-atom molecular ions

Experiments with cold ion–atom mixtures have recently opened the way for the production and application of ultracold molecular ions. Here, in a comparative study, we theoretically investigate ground and several excited electronic states and prospects for the formation of molecular ions composed of a calcium ion and an alkali-metal atom: CaAlk+ (Alk = Li, Na, K, Rb, Cs). We use a quantum chemistry approach based on non-empirical pseudopotentials, operatorial core-valence correlation, large Gaussian basis sets, and full configuration interaction method for valence electrons. Adiabatic potential energy curves, spectroscopic constants, and transition and permanent electric dipole moments are determined and analyzed for the ground and excited electronic states. We examine the prospects for ion-neutral reactive processes and the production of molecular ions via spontaneous radiative association and laser-induced photoassociation. After that, spontaneous and stimulated blackbody radiation transition rates are calculated and used to obtain radiative lifetimes of vibrational states of the ground and first-excited electronic states. The present results pave the way for the formation and spectroscopy of calcium–alkali-metal-atom molecular ions in modern experiments with cold ion–atom mixtures.

Accurate theoretical study on the ground and first-excited states of Na2: potential energy curves, spectroscopic parameters, and vibrational energy levels

Potential energy curves (PECs) for the ground and first-excited electronic states of Na2 are obtained by fitting the ab initio energies calculated at the MRCI(Q)/aug-cc-pVXZ (X = T, Q, 5) level of theory, which are subsequently extrapolated to the complete basis set limit. The relativistic effect and core–valence correlation are also considered. The PECs are accurate at both short and long internuclear distances with the root-mean-squared deviations being 0.72 cm−1 for Na2 [Formula: see text] and 0.36 cm−1 for Na2 [Formula: see text]. Utilizing the obtained PECs, we calculate the spectroscopic parameters, vibrational energy levels, classical turning points, inertial rotation, and centrifugal distortion constants, which are in good agreement with other theoretical and experimental work.

Materials for third-order nonlinear optics

Abstract

Full-electron calculation of effective electronic couplings and excitation energies of charge transfer states: Application to hole transfer in DNA pi-stacks.

In this work I develop and apply a theoretical method for calculating effective electronic couplings (or transfer integrals) between redox sites involved in hole or electron transfer reactions. The resulting methodology is a refinement and a generalization of a recently developed approach for transfer integral evaluation. In fact, it holds for any overlap between the charge-localized states used to represent charge transfer (CT) processes in the two-state model. The presented theoretical and computational analyses show that the prototype approach is recovered for sufficiently small overlaps. The method does not involve any empirical parameter. It allows a complete multielectron description, therefore including electronic relaxation effects. Furthermore, its theoretical formulation holds at any value of the given reaction coordinate and yields a formula for the evaluation of the vertical excitation energy (i.e., the energy difference between the adiabatic ground and first-excited electronic states) that rests on the same physical quantities used in transfer integral calculation. In this paper the theoretical approach is applied to CT in B-DNA base dimers within the framework of Density Functional Theory (DFT), although it can be implemented in other computational schemes. The results of this work, as compared with previous Hartree-Fock (HF) and post-HF evaluations, support the applicability of the current implementation of the method to larger pi-stacked arrays, where post-HF approaches are computationally unfeasible.

Accurate, two-state ab initio study of the ground and first-excited states of He2+, including exact treatment of all Born–Oppenheimer correction terms

Born-Oppenheimer (BO) potentials for the ground and first-excited electronic states of He2+ are determined using high level ab initio techniques for internuclear separations R of 1.2-100 bohrs and accurately fit to analytical functions. In the present formulation, the BO potentials are nuclear mass independent, and the corresponding BO approximation is obtained by ignoring four terms of the full rovibronic Hamiltonian. These four Born-Oppenheimer correction (BOC) terms are as follows: (1) mass polarization, (2) electronic orbital angular momentum, (3) first derivative with respect to R, and (4) second derivative with respect to R. In order to enable an exact rovibronic calculation, each of the four BOC terms are computed as a function of R, for the two electronic states and for their coupling, without any approximation or use of empirical parameters. Each of the BOC terms is found to make a contribution to the total energy over at least some portion of the range of R investigated. Interestingly, the most significant coupling contribution arises from the electronic orbital angular momentum term, which is evidently computed for the first time in this work. Although several BOC curves exhibit a nontrivial dependence on R, all are accurately fit to analytical functions. The resulting functions, together with the BO potentials, are used to compute exact rovibronic energy levels for 3He 3He+,3He 4He+) and 4He 4He+. Comparison to available high quality experimental data indicates that the present BOC potentials provide the most accurate representation currently available of both the low- and high-lying levels of the ground electronic state and the bound levels of the excited state.

Effect of CO2 on Nonthermal-Plasma Reactions of Nitrogen Oxides in N2. 1. PPM-Level Concentrations

PPM-level concentrations of CO2 are added to NO/N2 mixtures to determine the effect of CO2 on plasma generation and to investigate the mechanism of CO2 reactions. Addition of 599.9 ppm CO2 does not affect the electric discharge in NOx in N2 nonthermal plasma. However, CO2 slightly dissociates, with CO2 conversion reaching a maximum of about 5.5% at the power input at which NO conversion ceases. NO(A), the first-excited electronic state of NO, is detected by corona-induced optical emission and is found to contribute to CO2 dissociation. A kinetic model including 38 reactions is required to adequately model decomposition of NOx and CO2 in the presence of CO2. About 18% of the NO(A) that is formed reacts with CO2 to form NO, CO, and O.

Optimal control theory for a target state distributed in time: optimizing the probe-pulse signal of a pump-probe-scheme.

Optimal control theory (OCT) is formulated for the case of a two-color pump-probe experiment. The approach allows to calculate the pump-pulse shape in such a way that the probe-pulse absorption signal is maximized. Since the latter quantity is given by the time-averaged expectation value of a time dependent operator (the probe-pulse field-strength times the dipole operator) a version of OCT has to be used where the target state is distributed in time. The method is applied to a molecular three-level system with the pump-pulse driving the transition from the electronic ground state into the first-excited electronic state and the probe-pulse connecting the first-excited state with a higher lying electronic state. Depending on the probe-pulse duration, the vibrational wave packet becomes localized or at least highly concentrated in the Franck-Condon window for the transition into the higher-excited state. The dependence on the probe-pulse duration and on the delay time between the optimized pump-pulse and the probe-pulse is discussed in detail. The whole study demonstrates the feasibility of laser pulse induced temporal wave packet localization and the use of spectroscopic quantities as target states in experiments on femtosecond laser pulse control.

Cone states of tri-hydrogen isotopomers and criterion for the geometric phase effect

We report calculations of the vibrational spectrum of μH 2/HD/DT as well as D 3 in their first-excited electronic state, both with the inclusion of the geometric phase effect and without including it. The results show that, especially for μHD and μDT, it plays a minor role for the first 30 vibrational levels of such systems. This can be rationalized from the tilting of the C 3-axis due to mass effects. A simple phenomenological criterion for the relative role of the GP effect in tri-hydrogen isotopomers has been suggested.

Vibrational Calculations for the HD2 First-Excited Electronic State Using a Coordinate-Transformation Technique

We report calculations of the vibrational spectrum of HD2 in its first-excited electronic state by using a coordinate-transformation technique. The implications of the geometric phase (GP) effect in the spectrum have been investigated. The results show that the lowest 45 vibrational levels calculated using the traditional Longuet−Higgins phase (φ/2, where φ is the pseudorotation angle) are in good agreement with those obtained by using a single-surface generalized Born−Oppenheimer equation previously reported by the authors.

Intramolecular proton transfer in the first-excited electronic states of 4-methyl-2,6-diformyl phenol in some hydrocarbon solvents

Proton translocation in the singlet excited state of 4-methyl-2,6-diformyl phenol (MFOH) results in an interesting fluorescence band at 535 nm. The fluorescence emission and excitation spectra of MFOH are recorded and a time-correlated single-photon counting technique is used to determine the fluorescence lifetimes in some hydrocarbon solvents at room temperature. The stable molecular structure in the ground state of MFOH is an intramolecularly hydrogen bonded closed conformer from which proton transfer takes place in the first excited 1(ππ *) state. The Stokes-shifted fluorescence band observed in the yellow—green region is likely to originate from the enol tautomer which is found to exist even at 77 K. From nanosecond measurements and the quantum yields of fluorescence we have determined proton transfer rates. It is suggested that the proton transfer in MFOH is relatively slow in these solvents and MFOH can have only one ground state conformation, except in benzene and toluene.

Raman spectra of IVB and VIB transition metal disulfides using laser energies near the absorption edges

In this paper, we present Raman spectra of ZrS 2, HfS 2, MoS 2 and WS 2 using laser energies near the energies of the absorption edges. The Raman spectra probe the properties of the first-excited electronic state and the nature of the electron-phonon coupling. The spectra of the IVB disulfides are independent of the laser excitation energy, suggesting weak electron-phonon interaction. In contrast, additional Raman bands appear in the spectra of the VIB disulfides as the laser energy approaches the band gap energy. The new modes in the spectra of MoS 2 and WS 2 cannot be assigned as first-order processes nor as combination bands of the phonons with zero momentum. The resonance Raman scattering of MoS 2 is analyzed in terms of second-order scattering due to the coupling of phonon modes of nonzero momentum with an electronic transition associated with excitonic states.

Vibrational Assignment of Anthracene-d10 from the Fluorescence and Absorption Spectra

The fluorescence and absorption spectra of anthracene-d10 in a fluorene matrix at 4.2°K have been measured and analyzed. In the ground electronic state seven ag and two b3g fundamentals have been assigned, while in the first-excited electronic state (1B1u) six ag and four b3g fundamentals were located. Other possible fundamentals are indicated. Earlier data concerning ag and b3g fundamentals of anthracene-h10 are critically reviewed and compared with the present results.