Molecules In Excited Electronic States Research Articles

Determination of excited state molecular structures from time-resolved gas-phase X-ray scattering.

We present a comprehensive investigation of a recently introduced method to determine transient structures of molecules in excited electronic states with sub-ångstrom resolution from time-resolved gas-phase scattering signals. The method, which is examined using time-resolved X-ray scattering data measured on the molecule N-methylmorpholine (NMM) at the Linac Coherent Light Source (LCLS), compares the experimentally measured scattering patterns against the simulated patterns corresponding to a large pool of molecular structures to determine the full set of structural parameters. In addition, we examine the influence of vibrational state distributions and find the effect negligible within the current experimental detection limits, despite that the molecules have a comparatively high internal vibrational energy. The excited state structures determined using three structure pools generated using three different computational methods are in good agreement, demonstrating that the procedure is largely independent of the computational chemistry method employed as long as the pool is sufficiently expansive in the vicinity of the sought structure and dense enough to yield good matches to the experimental patterns.

Computation of Molecular Ionization Energies Using an Ensemble Density Functional Theory Method.

Computation of the ionization energies and of the respective Dyson orbitals based on the use of the extended Koopmans theorem (EKT) is implemented in connection with an ensemble density functional theory (eDFT) method, the state-interaction state-averaged spin-restricted ensemble-referenced Kohn-Sham (SI-SA-REKS or SSR) method. The new methodology enables fast computation of the ionization energies and evaluation of the respective Dyson orbitals, the square norms of which are related with the ionization probabilities, in the ground and excited electronic states of molecules. As the application of EKT recycles the intermediate quantities from the SSR analytical energy gradient, evaluation of the ionization energies and probabilities can be carried out on-the-fly during the nonadiabatic molecular dynamics simulations. This opens up a perspective for fast theoretical simulation of the time-resolved photoelectron spectroscopy observations. In the present work, the new methodology is tested in the computation of the ionization energies and Dyson orbitals of several molecules in the ground and excited electronic states, including strongly correlated species, such as the ozone molecule, dissociating chemical bonds, and conical intersections.

Saturation of photoassociation in NaCs dark magneto-optical trap

We report the production of ultracold heteronuclear NaCs molecules in excited electronic states by photoassociation (PA) of ultracold Na and Cs atoms. The saturation of the photoassociation scattering probability is observed from the dependence of the trap loss probability on the photoassociation laser intensity. Based on the scattering theory, we estimate the saturation intensity of photoassociation, which is deduced by fitting the experimental data to a saturation model.

Ab Initio Nonadiabatic Quantum Molecular Dynamics.

The Born-Oppenheimer approximation underlies much of chemical simulation and provides the framework defining the potential energy surfaces that are used for much of our pictorial understanding of chemical phenomena. However, this approximation breaks down when the dynamics of molecules in excited electronic states are considered. Describing dynamics when the Born-Oppenheimer approximation breaks down requires a quantum mechanical description of the nuclei. Chemical reaction dynamics on excited electronic states is critical for many applications in renewable energy, chemical synthesis, and bioimaging. Furthermore, it is necessary in order to connect with many ultrafast pump-probe spectroscopic experiments. In this review, we provide an overview of methods that can describe nonadiabatic dynamics, with emphasis on those that are able to simultaneously address the quantum mechanics of both electrons and nuclei. Such ab initio quantum molecular dynamics methods solve the electronic Schrödinger equation alongside the nuclear dynamics and thereby avoid the need for precalculation of potential energy surfaces and nonadiabatic coupling matrix elements. Two main families of methods are commonly employed to simulate nonadiabatic dynamics in molecules: full quantum dynamics, such as the multiconfigurational time-dependent Hartree method, and classical trajectory-based approaches, such as trajectory surface hopping. In this review, we describe a third class of methods that is intermediate between the two: Gaussian basis set expansions built around trajectories.

Photoassociation of ultracoldLiRb*molecules: Observation of high efficiency and unitarity-limited rate saturation

We report the production of ultracold heteronuclear $^{7}\mathrm{Li}^{85}\mathrm{Rb}$ molecules in excited electronic states by photoassociation (PA) of ultracold $^{7}\mathrm{Li}$ and $^{85}\mathrm{Rb}$ atoms. PA is performed in a dual-species $^{7}\mathrm{Li}$-$^{85}\mathrm{Rb}$ magneto-optical trap (MOT) and the PA resonances are detected using trap loss spectroscopy. We identify several strong PA resonances below the Li (2$s$ ${}^{2}$${S}_{1/2}$) + Rb (5$p$ ${}^{2}{P}_{3/2}$) asymptote and experimentally determine the long range ${C}_{6}$ dispersion coefficients. We find a molecule formation rate (${P}_{\mathrm{LiRb}}$) of 3.5 \ifmmode\times\else\texttimes\fi{} 10${}^{7}$ s${}^{\ensuremath{-}1}$ and a PA rate coefficient (${K}_{\mathrm{PA}}$) of 1.3 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}10}$ cm${}^{3}$/s, the highest among heteronuclear bi-alkali-metal molecules. At large PA laser intensity we observe the saturation of the PA rate coefficient (${K}_{\mathrm{PA}}$) close to the theoretical value at the unitarity limit.

Resonance Raman and Resonance Hyper-Raman Intensities: Structure and Dynamics of Molecular Excited States in Solution

Resonance Raman scattering is discussed as a vibronic spectroscopy that can provide detailed information about the structure and dynamics of excited electronic states of molecules. The emphasis is on molecules in liquid solution. The theory of resonance Raman intensities and experimental and interpretive methods are discussed both in a historical context and in their present and future implementations. The related but much less developed technique of two-photon-resonant hyper-Raman scattering is also discussed in a similar context.

Double-hybrid density functional theory for excited electronic states of molecules

Double-hybrid density functionals are based on a mixing of standard generalized gradient approximations (GGAs) for exchange and correlation with Hartree-Fock (HF) exchange and a perturbative second-order correlation part (PT2) that is obtained from the Kohn-Sham (GGA) orbitals and eigenvalues. This virtual orbital-dependent functional (dubbed B2PLYP) contains only two empirical parameters that describe the mixture of HF and GGA exchange (ax) and of the PT2 and GGA correlation (ac), respectively. Extensive testing has recently demonstrated the outstanding accuracy of this approach for various ground state problems in general chemistry applications. The method is extended here without any further empirical adjustments to electronically excited states in the framework of time-dependent density functional theory (TD-DFT) or the closely related Tamm-Dancoff approximation (TDA-DFT). In complete analogy to the ground state treatment, a scaled second-order perturbation correction to configuration interaction with singles (CIS(D)) wave functions developed some years ago by Head-Gordon et al. [Chem. Phys. Lett. 219, 21 (1994)] is computed on the basis of density functional data and added to the TD(A)-DFTGGA excitation energy. The method is implemented by applying the resolution of the identity approximation and the efficiency of the code is discussed. Extensive tests for a wide variety of molecules and excited states (of singlet, triplet, and doublet multiplicities) including electronic spectra are presented. In general, rather accurate excitation energies (deviations from reference data typically <0.2 eV) are obtained that are mostly better than those from standard functionals. Still, systematic errors are obtained for Rydberg (too low on average by about 0.3 eV) and charge-transfer transitions but due to the relatively large ax parameter (0.53), B2PLYP outperforms most other functionals in this respect. Compared to conventional HF-based CIS(D), the method is more robust in electronically complex situations due to the implicit account of static correlation effects by the GGA parts. The (D) correction often works in the right direction and compensates for the overestimation of the transition energy at the TD level due to the elevated fraction of HF exchange in the hybrid GGA part. Finally, the limitations of the method are discussed for challenging systems such as transition metal complexes, cyanine dyes, and multireference cases.

Step-ladder Rabi oscillations in molecules exposed to intense ultrashort vuv pulses

We theoretically investigate the production of Rabi-type oscillations in two-photon resonant ionization of ${\mathrm{H}}_{2}$ by intense ultrashort vuv pulses. We show that, with such pulses, oscillations proceed through a step-ladder mechanism that successively involves higher and higher vibrational levels of the lower and upper electronic states. The consequences of this mechanism are (i) the relatively large proportion of ${\mathrm{H}}_{2}$ molecules in excited electronic states not accessible by direct photon absorption, (ii) the splitting in the energy distribution of ionized electrons, and (iii) the unusually broad (non-Franck-Condon) vibrational distribution of both ${\mathrm{H}}_{2}$ and $\mathrm{H}_{2}{}^{+}$.

Catalytic enantioselective reactions driven by photoinduced electron transfer

Photoinduced electron transfer is an essential step in the conversion of solar energy into chemical energy in photosystems I and II (ref. 1), and is also frequently used by chemists to build complex molecules from simple precursors. During this process, light absorption generates molecules in excited electronic states that are susceptible to accepting or donating electrons. But although the excited states are straightforward to generate, their short lifetimes makes it challenging to control electron transfer and subsequent product formation-particularly if enantiopure products are desired. Control strategies developed so far use hydrogen bonding, to embed photochemical substrates in chiral environments and to render photochemical reactions enantioselective through the use of rigid chiral complexing agents. To go beyond such stoichiometric chiral information transmission, catalytic turnover is required. Here we present a catalytic photoinduced electron transfer reaction that proceeds with considerable turnover and high enantioselectivity. By using an electron accepting chiral organocatalyst that enforces a chiral environment on the substrate through hydrogen bonding, we obtain the product in significant enantiomeric excess (up to 70%) and in yields reaching 64%. This performance suggests that photochemical routes to chiral compounds may find use in general asymmetric synthesis.

Temperature dependent non-radiative effects in the disorder-to-order transition in cyclopentanone and cyclohexanone films on Al 2O 3 (0001)

The excited electronic states of molecules can serve as sensitive probes of the microenvironment. Cyclopentanone and cyclohexanone thin films were prepared by vacuum deposition of these molecules onto a cryostatically cooled Al 2O 3 (001) surface. As the surface was heated, the films underwent several glass and crystalline transitions that were clearly identifiable by the large increase in the radiative intensity. The surface temperature was ramped to these transition temperatures, and the emission was found to exhibit decays that were kinetically different. Very importantly, the non-radiative relaxation in these films is thermally sensitive, and competes effectively with the radiative process.

Theory of femtosecond stimulated Raman spectroscopy.

Femtosecond broadband stimulated Raman spectroscopy (FSRS) is a new technique that produces high-resolution (time-resolved) vibrational spectra from either the ground or excited electronic states of molecules, free from background fluorescence. FSRS uses simultaneously a narrow bandwidth approximately 1-3 ps Raman pump pulse with a continuum approximately 30-50 fs Stokes probe pulse to produce sharp Raman gains, at positions corresponding to vibrational transitions in the sample, riding on top of the continuum Stokes probe spectrum. When FSRS is preceded by a femtosecond actinic pump pulse that initiates the photochemistry of interest, time-resolved Raman spectroscopy can be carried out. We present two theoretical approaches to FSRS: one is based on a coupling of Raman pump and probe light waves with the vibrations in the medium, and another is a quantum-mechanical description. The latter approach is used to discuss the conditions of applicability and limitations of the coupled-wave description. Extension of the quantum-mechanical description to the case where the Raman pump beam is on resonance with an excited electronic state, as well as when FSRS is used to probe a nonstationary vibrational wave packet prepared by an actinic pump pulse, is also discussed.

Vibronic intensities in diatomic molecules

AbstractThe object of this work is to use vibronic intensity distributions to determine the bond lengths of diatomic molecules in excited electronic states without having to evaluate Franck–Condon (FC) integrals. More specifically, the intent is to derive simple linear or quadratic expressions that provide good correspondence of bond lengths obtained from experimental vibronic intensity ratios with the experimental bond lengths. To that end, we will construct power‐series expressions for the FC integral in the harmonic approximation in terms of a parameter that approaches zero as the vibrational frequencies of the two combining electronic states approach equality, show how these expressions may be truncated, and evaluate the range of bond length changes over which the various approximate equations are valid. The study covered 60 different electronic transitions in 40 different diatomics. Of these, 40 electronic transition data sets were considered good enough to merit analysis. It will be shown that both approximations perform well within the limit of their applicability. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

On the energies of low-lying excited electronic states of molecules

Two methods for calculating the energies of low-lying excited electronic states of molecules of the same symmetry as the ground state are compared. We term these the constrained variational method and the optimized trace method . Approximate solutions of the Schrodinger equation can be obtained by introducing finite basis set expansions. For each method two possibilities arise: (i) a common basis set can be used for the ground and the excited state; (ii) different basis sets can be employed for different states. Prototype calculations are reported for the hydrogen molecular ion using distributed basis sets of s-type Gaussian functions.

Determination of the rate constant of oxygen quenching of the excited states of molecules from the singlet oxygen luminescence

We have pioneered a method of determining the rate constant of quenching of the excited electronic states of molecules by molecular oxygen from measurements of the kinetics of photosensitized luminescence of singlet molecular oxygen (lδg). The method can be used in the case where the lifetime of the excited electronic state in an air-saturated solution is comparable with or larger than the luminescence time of the singlet molecular oxygen in the given solvent. It is shown that this situation is implemented on quenching, by molecular oxygen, of the excited triplet states associated with the biopolymers of tetrapyrrole molecules in aqueous (H2O and D2O) solutions.

ChemInform Abstract: Vibrational Potential Energy Surfaces of Non‐Rigid Molecules in Excited Electronic States

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Structure and dynamics of conformationally non-rigid molecules in excited electronic states: Ab initio calculations of the R 2CO (R=H, F, Cl)

Potential energy surfaces of electonically excited H 2CO (S 1, T 1), F 2CO (S 1), and Cl 2CO (S 1) molecules are investigated by means of multiconfigurational ab initio approaches. The theoretical estimations of inversion barrier heights calculated by different ab initio method and the relative energies of inversion vibrational levels are compared to the experimental data.

Resonance enhanced multiphoton ionisation (REMPI) and REMPI-photoelectron spectroscopy of carbonyl sulphide and carbon disulphide

The results of recent mass-resolved resonance enhanced multiphoton ionisation (REMPI) and REMPI-photoelectron spectroscopy (PES) studies of the sixteen valence electron molecules OCS and CS 2 are used to illustrate some of the many opportunities offered by (and a few of the possible limitations associated with) these techniques when applied to studies of the spectroscopy and the decay dynamics of small molecules in excited electronic states.

Intermolecular energy shifts between two chiral molecules in excited electronic states

Higher order multipole contributions to the energy shift between two neutral polarizable molecules in excited electronic states are calculated using molecular response theory within the formalism of non-relativistic quantum electrodynamics. The interaction between two electric-magnetic dipole polarizable molecules is discriminatory, being dependent on the chirality of each species. The energy shift between an electric dipole polarizable molecule interacting with either a magnetic dipole or electric quadrupole polarizable molecule is also evaluated. The potentials obtained apply to two randomly oriented molecules and are valid for all intermolecular separation distances outside electron overlap. The interaction energy when one or both of the pair are in ground electronic states is easily deduced from the general result for two excited molecules. The limiting behaviour of the various energy shifts at large and small separation distances is also examined.

Intermolecular energy shifts between two chiral molecules in excited electronic states

Intermolecular energy shifts between two chiral molecules in excited electronic states

Quantum Chemistry and Molecular Processes

This article gives an overview of modern electronic structure theory, which is the development of approximate quantum mechanical methods for calculating the ground and excited electronic states of molecules. A series of seven topics are discussed which have contributed to the present state of the field and that illustrate in a general way some of the essential physical models and approximations that underpin electronic structure calculations. These topics begin from the definition of theoretical model chemistries and the treatment of electron correlation by wave function-based techniques and density functional theory, for both molecular energies and a range of molecular properties. Beyond these main theoretical issues, questions of chemical interpretation, computational algorithms, and the modeling of surrounding environment are also discussed. Collectively these topics define many of the classes of chemical problems which can be reliably and feasibly solved by such calculations at present, and conversely they also serve to define a number of unresolved challenges for the future.