Coherent state switching using vibrational polaritons in an asymmetric double-well potential.

The quantum dynamics of vibrational polaritonic states arising from the interaction of a bistable molecule with the quantized mode of a Fabry-Perot microcavity is investigated using a generic asymmetric double-well potential as a simplified one-dimensional model of a reactive molecule. After discussing the role of the light-matter coupling strength in the emergence of avoided crossings between polaritonic states, we investigate the possibility of using these crossings to trigger a dynamical switching of these states from one potential well to the other. Two schemes are proposed to achieve this coherent state switching, either by preparing the molecule in an appropriate vibrational excited state before inserting it into the cavity, or by applying a short laser pulse inside the cavity to obtain a coherent superposition of polaritonic states. The respective influences of dipole moment amplitude and potential asymmetry on the coherent switching process are also discussed.

Modeling the dynamics of quantum systems coupled to large-dimensional baths using effective energy states.

The quantum dynamics of a low-dimensional system in contact with a large but finite harmonic bath is theoretically investigated by coarse-graining the bath into a reduced set of effective energy states. In this model, the couplings between the system and the bath are obtained from statistically averaging over the discrete, degenerate effective states. Our model is aimed at intermediate bath sizes in which non-Markovian processes and energy transfer between the bath and the main system are important. The method is applied to a model system of a Morse oscillator coupled to 40 harmonic modes. The results are found to be in excellent agreement with the direct quantum dynamics simulations presented in the work of Bouakline et al. [J. Phys. Chem. A 116, 11118-11127 (2012)], but at a much lower computational cost. Extension to larger baths is discussed in comparison to the time-convolutionless method. We also extend this study to the case of a microcanonical bath with finite initial internal energies. The computational efficiency and convergence properties of the effective bath states model with respect to relevant parameters are also discussed.

Finite-temperature stability of hydrocarbons: Fullerenes vs flakes.

The effects of a finite temperature on the equilibrium structures of hydrocarbon molecules are computationally explored as a function of size and relative chemical composition in hydrogen and carbon. Using parallel tempering Monte Carlo simulations employing a reactive force field, we find that in addition to the phases already known for pure carbon, namely, cages, flakes, rings, and branched structures, strong changes due to temperature and the addition of little amounts of hydrogen are reported. Both entropy and the addition of moderate amounts of hydrogen favor planar structures such as nanoribbons over fullerenes. Accurate phase diagrams are proposed, highlighting the possible presence of multiple phase changes at finite size and composition. Astrophysical implications are also discussed.

Infrared Spectroscopy of Chemically Diverse Carbon Clusters: A Data-Driven Approach.

Carbon clusters exhibit a broad diversity of topologies and shapes, encompassing fullerene-like cages, graphene-like flakes, and more disordered pretzel-like and branched structures. Here, we examine computationally their infrared spectra in relation with these structures from a statistical perspective. Individual spectra for broad samples of isomers were determined by means of the self-consistent charge density functional-based tight-binding method, and an interpolation scheme is designed to reproduce the spectral features by regression on a much smaller subset of the sample. This interpolation proceeds by encoding the structures using appropriate descriptors and selecting them through principal component analysis, Gaussian regression or inverse distance weighting providing the nonlinear weighting functions. Metric learning is employed to reduce the global error on a preselected testing set. The interpolated spectra satisfactorily reproduce the specific spectral features and their dependence on the size and shape, enabling quantitative prediction away from the testing set. Finally, the classification of structures within the four proposed families is critically discussed through a statistical analysis of the sample based on iterative label spreading.

Isomerization kinetics of flexible molecules in the gas phase: Atomistic versus coarse-grained sampling.

Isomerization kinetics of molecules in the gas phase naturally falls on the microcanonical ensemble of statistical mechanics, which for small systems might significantly differ from the more traditional canonical ensemble. In this work, we explore the examples of cis-trans isomerization in butane and bibenzyl and to what extent the fully atomistic rate constants in isolated molecules can be reproduced by coarse-graining the system into a lower dimensional potential of mean force (PMF) along a reaction coordinate of interest, the orthogonal degrees of freedom acting as a canonical bath in a Langevin description. Time independent microcanonical rate constants can be properly defined from appropriate state residence time correlation functions; however, the resulting rate constants acquire some time dependence upon canonical averaging of initial conditions. Stationary rate constants are recovered once the molecule is placed into a real condensed environment pertaining to the canonical ensemble. The effective one-dimensional kinetics along the PMF, based on appropriately chosen inertia and damping parameters, quantitatively reproduces the atomistic rate constants at short times but deviates systematically over long times owing to the neglect of some couplings between the system and the bath that are all intrinsically present in the atomistic treatment. In bibenzyl, where stronger temperature effects are noted than in butane, the effective Langevin dynamics along the PMF still performs well at short times, indicating the potential interest of this extremely simplified approach for sampling high-dimensional energy surfaces and evaluating reaction rate constants.

Nonlinear effects in infrared action spectroscopy of silicon and vanadium oxide clusters: experiment and kinetic modeling.

For structural assignment of gas phase compounds, infrared action spectra are usually compared to computed linear absorption spectra. However, action spectroscopy is highly nonlinear owing to the necessary transfer of the excitation energy and its subsequent redistribution leading to statistical ionization or dissociation. Here, we examine by joint experiment and dedicated modeling how such nonlinear effects affect the spectroscopic features in the case of selected inorganic clusters. Vibrational spectra of neutral silicon clusters are recorded by tunable IR-UV two-color ionization while IR spectra for cationic vanadium oxide clusters are obtained by IR multiphoton absorption followed by dissociation of the bare cluster or of its complex with Xe. Our kinetic modeling accounts for vibrational anharmonicities, for the laser interaction through photon absorption and stimulated emission rates, as well as for the relevant ionization or dissociation rates, all based on input parameters from quantum chemical calculations. Comparison of the measured and calculated spectra indicates an overall agreement as far as trends are concerned, except for the photodissociation of the V3O7(+)-Xe messenger complex, for which anharmonicities are too large and poorly captured by the perturbative anharmonic model. In all systems studied, nonlinear effects are essentially manifested by variations in the intensities as well as spectral broadenings. Differences in some band positions originate from inaccuracies of the quantum chemical data rather than specific nonlinear effects. The simulations further yield information on the average number of photons absorbed, which is otherwise unaccessible information: several to several tens of photons need to be absorbed to observe a band through dissociation, while three to five photons can be sufficient for detection of a band via IR-UV ionization.

Photoionization of cold gas phase coronene and its clusters: autoionization resonances in monomer, dimer, and trimer and electronic structure of monomer cation.

Polycyclic aromatic hydrocarbons (PAHs) are key species encountered in a large variety of environments such as the Interstellar Medium (ISM) and in combustion media. Their UV spectroscopy and photodynamics in neutral and cationic forms are important to investigate in order to learn about their structure, formation mechanisms, and reactivity. Here, we report an experimental photoelectron-photoion coincidence study of a prototypical PAH molecule, coronene, and its small clusters, in a molecular beam using the vacuum ultraviolet (VUV) photons provided by the SOLEIL synchrotron facility. Mass-selected high resolution threshold photoelectron (TPES) and total ion yield spectra were obtained and analyzed in detail. Intense series of autoionizing resonances have been characterized as originating from the monomer, dimer, and trimer neutral species, which may be used as spectral fingerprints for their detection in the ISM by VUV absorption spectroscopy. Finally, a full description of the electronic structure of the monomer cation was made and discussed in detail in relation to previous spectroscopic optical absorption data. Tentative vibrational assignments in the near-threshold TPES spectrum of the monomer have been made with the support of a theoretical approach based on density functional theory.

Visible photodissociation spectra of the 1- and 2-methylnaphthalene cations: laser spectroscopy and theoretical simulations.

The electronic absorption spectra of the two methyl derivatives of the naphthalene cation were measured using an argon tagging technique. In both cases, a band system was observed in the visible range and assigned to the D2 ← D0 electronic transition. The 1-methylnaphthalene(+) absorption bands revealed a red shift of 808 cm(-1), relative to those of the naphthalene cation (14,906 cm(-1)), whereas for 2-methylnaphthalene(+) a blue shift of 226 cm(-1) appeared. A short vibrational progression, similar to the naphthalene cation, was also observed for both isomers and found to involve similar aromatic ring skeleton vibrations. Moreover, insights into the internal rotation motion of the methyl group were inferred, although the spectral resolution was not sufficient to fully resolve the substructure. These measurements were supported by detailed quantum chemical calculations. They allowed exploration of the potential energy curves along this internal coordinate, along with a complete simulation of the harmonic Franck-Condon factors using the cumulant Gaussian fluctuations formalism extended to include the internal rotation.

Effects of hydrogen dissociation on the infrared emission spectra of naphthalene: theoretical modeling.

The IR emission spectroscopy of naphthalene and its singly- and doubly-dehydrogenated radicals has been modeled using kinetic Monte Carlo simulations, taking into account the various relaxation pathways of radiative emission and hydrogen loss. Our modeling relies on quantum chemistry ingredients that were obtained from dedicated calculations based on density functional theory, including explicitly anharmonicity contributions. Our results show that the fragmentation products significantly contribute to the overall IR emission spectrum, especially to the intensity ratios between bands. Owing to the likely presence of polycyclic aromatic hydrocarbons in the interstellar medium, these findings are particularly relevant in the astrophysical context.

Accurate modeling of infrared multiple photon dissociation spectra: the dynamical role of anharmonicities.

The dynamical response of a molecular system to a macropulse typically produced by a free-electron laser is theoretically modeled over experimentally long times, within a realistic kinetic Monte Carlo framework that incorporates absorption, stimulated emission, spontaneous emission, and dissociation events. The simulation relies on an anharmonic potential energy surface obtained from quantum chemistry calculations. Application to cationic naphthalene yields a better agreement with measurements than the anharmonic linear absorption spectrum, thus emphasizing the importance of specific dynamical effects on the spectral properties.

A simple but accurate potential for the naphthalene-argon complex: applications to collisional energy transfer and matrix isolated IR spectroscopy.

An explicit polarizable potential for the naphthalene-argon complex has been derived assuming only atomic contributions, aiming at large scale simulations of naphthalene under argon environment. The potential was parametrized from dedicated quantum chemical calculations at the CCSD(T) level, and satisfactorily reproduces available structural and energetic properties. Combining this potential with a tight-binding model for naphthalene, collisional energy transfer is studied by means of dedicated molecular dynamics simulations, nuclear quantum effects being accounted for in the path-integral framework. Except at low target temperature, nuclear quantum effects do not alter the average energies transferred by the collision or the collision duration. However, the distribution of energy transferred is much broader in the quantum case due to the significant zero-point energy and the higher density of states. Using an ab initio potential for the Ar-Ar interaction, the IR absorption spectrum of naphthalene solvated by argon clusters or an entire Ar matrix is computed via classical and centroid molecular dynamics. The classical spectra exhibit variations with growing argon environment that are absent from quantum spectra. This is interpreted by the greater fluxional character experienced by the argon atoms due to vibrational delocalization.

Probing the spin multiplicity of gas-phase polycyclic aromatic hydrocarbons through their infrared emission spectrum: a theoretical study.

The anharmonic infrared emission spectrum following an optical excitation has been calculated for a variety of polycyclic aromatic hydrocarbon molecules in their ground singlet electronic state or in their triplet state. The computational protocol relies on second-order perturbation theory and involves a quartic vibrational Hamiltonian, the vibrational quantum numbers being sampled according to a Monte Carlo procedure. In the case of neutral naphthalene, the IR spectrum obtained in the (ground) singlet state differs significantly from the spectrum in the triplet state, especially for out-of-plane CH bending modes. Although not as prominent, spectral differences in larger molecules are still observable.

Vibrational spectra of polyatomic molecules assisted by quantum thermal baths.

We assess the performance of colored-noise thermostats to generate quantum mechanical initial conditions for molecular dynamics simulations, in the context of infrared spectra of large polyatomic molecules. Comparison with centroid molecular dynamics simulations taken as reference shows that the method is accurate in predicting line shifts and band widths in the ionic cluster (NaCl)(32) and in the naphthalene molecule. As illustrated on much larger polycyclic aromatic hydrocarbons, the method also allows fundamental spectra to be evaluated in the limit of T = 0, taking into account anharmonicities and vibrational delocalization.

Improving anharmonic infrared spectra using semiclassically prepared molecular dynamics simulations.

Classical molecular dynamics is a convenient method for computing anharmonic infrared spectra of polyatomic molecules and condensed phase systems. However it does not perform well for predicting accurate intensities and it lacks nuclear quantization, two deficiencies that are usually accounted for by empirical scaling factors. In this paper we show on the examples of the trans isomer of nitrous acid and naphthalene that both issues can be alleviated by preparing the initial conditions according to semiclassical quantization based on a normal mode representation. The method correctly reproduces fundamental frequencies obtained with quantum mechanical methods. At increasing temperatures, the effective frequencies are found to follow the same trends as path-integral based methods. In the low-temperature limit, the band intensities predicted by the method are also found to agree with quantum mechanical considerations.

Vibrations and thermodynamics of clusters of polycyclic aromatic hydrocarbon molecules: the role of internal modes.

The vibrational spectra of clusters of coronene molecules are theoretically calculated using a mixed quantum/classical scheme, each molecule being described by a tight-binding Hamiltonian, the intermolecular forces being provided by explicit Lennard-Jones and point charge sites. The normal modes of vibrations are shown to exhibit significant variations upon clustering. In particular, for large clusters intra- and intermolecular modes tend to mix and fill the mid-infrared range. We also calculate the heat capacity of the (C24H12)8 cluster as a function of temperature, emphasizing the isomerizations that take place during melting. Quantum delocalization effects, as obtained from the Pitzer-Gwinn semiclassical approximation, are important enough to wash out all signatures of the structural transitions on the caloric curve. On the basis of a simple two-state model we estimate that clusters containing about 300 molecules are required for melting to be detected on the caloric curve.

Far-infrared spectroscopy of small polycyclic aromatic hydrocarbons.

Both experimental and theoretical spectroscopic studies on small gas phase polycyclic aromatic hydrocarbons in the far-infrared spectral region are reported. The experimental set-up based on thermal emission and Fourier transform far infrared analysis led to the detection of relatively broad vibrational bands, unresolved in rotation, representative of each molecule. Detailed theoretical investigations were performed, including both ab initio calculations and spectral simulations. For the majority of the samples, this study provides the first detection of the vibrational modes associated with the skeleton motions.

Intramolecular anharmonicity in the naphthalene molecule: thermodynamics and spectral effects.

The anharmonicity of the ground electronic state of the naphthalene molecule has been investigated using a tight-binding potential energy surface model. The vibrational density of states is calculated from a mixed classical quantum theoretical approach. The primary assumption of this model is that the evolution of the mean anharmonic quantum energy as a function of temperature follows the classical evolution. From the thermal expansion of the classical free energy obtained using a reversible-scaling method, the mean vibrational frequency has also been determined as a function of temperature.