Molecular Systems In Condensed Phase Research Articles

Phonon Signatures in Photon Correlations.

We show that the second-order, two-time correlation functions for phonons and photons emitted from a vibronic molecule in a thermal bath result in bunching and antibunching (a purely quantum effect), respectively. Signatures relating to phonon exchange with the environment are revealed in photon-photon correlations. We demonstrate that cross-correlation functions have a strong dependence on the order of detection giving insight into how phonon dynamics influences the emission of light. This work offers new opportunities to investigate quantum effects in condensed-phase molecular systems.

Machine learning force fields for molecular liquids: Ethylene Carbonate/Ethyl Methyl Carbonate binary solvent

Highly accurate ab initio molecular dynamics (MD) methods are the gold standard for studying molecular mechanisms in the condensed phase, however, they are too expensive to capture many key properties that converge slowly with respect to simulation length and time scales. Machine learning (ML) approaches which reach the accuracy of ab initio simulation, and which are, at the same time, sufficiently affordable hold the key to bridging this gap. In this work we present a robust ML potential for the EC:EMC binary solvent, a key component of liquid electrolytes in rechargeable Li-ion batteries. We identify the necessary ingredients needed to successfully model this liquid mixture of organic molecules. In particular, we address the challenge posed by the separation of scale between intra- and inter-molecular interactions, which is a general issue in all condensed phase molecular systems.

Anisotropic molecular coarse-graining by force and torque matching with neural networks.

We develop a machine-learning method for coarse-graining condensed-phase molecular systems using anisotropic particles. The method extends currently available high-dimensional neural network potentials by addressing molecular anisotropy. We demonstrate the flexibility of the method by parametrizing single-site coarse-grained models of a rigid small molecule (benzene) and a semi-flexible organic semiconductor (sexithiophene), attaining structural accuracy close to the all-atom models for both molecules at a considerably lower computational expense. The machine-learning method of constructing the coarse-grained potential is shown to be straightforward and sufficiently robust to capture anisotropic interactions and many-body effects. The method is validated through its ability to reproduce the structural properties of the small molecule's liquid phase and the phase transitions of the semi-flexible molecule over a wide temperature range.

Two-dimensional terahertz spectroscopy of condensed-phase molecular systems.

Nonlinear terahertz (THz) spectroscopy relies on the interaction of matter with few-cycle THz pulses of electric field amplitudes up to megavolts/centimeter (MV/cm). In condensed-phase molecular systems, both resonant interactions with elementary excitations at low frequencies such as intra- and intermolecular vibrations and nonresonant field-driven processes are relevant. Two-dimensional THz (2D-THz) spectroscopy is a key method for following nonequilibrium processes and dynamics of excitations to decipher the underlying interactions and molecular couplings. This article addresses the state of the art in 2D-THz spectroscopy by discussing the main concepts and illustrating them with recent results. The latter include the response of vibrational excitations in molecular crystals up to the nonperturbative regime of light-matter interaction and field-driven ionization processes and electron transport in liquid water.

Electric Field Effect on Condensed-Phase Molecular Systems. X. Interconversion Dynamics and Vibrational Stark Effect of Hydrogen Chloride Clusters in an Argon Matrix.

In this study, the effect of a strong (≤4 × 108 V·m-1) dc electric field on hydrogen chloride (HCl) dimers and trimers isolated in a solid argon matrix has been investigated using the ice film nanocapacitor and reflection-absorption infrared spectroscopy methods. The H-Cl vibrational bands of the HCl dimers showed a linear Stark frequency shift and an increased intensity under the applied electric field, and these changes were reversible with the electric field strength. This behavior indicated that the dimers were reoriented by the applied electric field. The reorientation occurred via tunneling inversion of individual HCl subunits of the dimer, which interconverted the proton-accepting and -donating HCl subunits, as observed for the heterodimers HCl-DCl and DCl-HCl. The interconversion of dimers could occur even at low electric field strength (∼107 V·m-1) and was almost complete above the field strength of 1.0 × 108 V·m-1. In contrast, the asymmetric H-Cl stretching bands of the HCl trimers exhibited Stark broadening under the influence of the electric field without a shift in frequency or change in intensity. This behavior indicated that the cyclic structure of the HCl trimer was stable even when subjected to a strong electric field. The Stark sensitivity factor (Δμ) of H-Cl vibrations was deduced from the Stark effect analysis of the HCl dimer and trimer bands, which gave the following: ΔμD1 = 2.3 ± 0.2 cm-1/(108 V·m-1) for the proton-acceptor subunit of the dimer, ΔμD2 = 5.1 ± 0.5 cm-1/(108 V·m-1) for the proton-donor subunit of the dimer, and ΔμT = 4.5 ± 0.5 cm-1/(108 V·m-1) for the asymmetric stretching vibration of the cyclic trimer.

Electric Field Effect on Condensed-Phase Molecular Systems. IX. Control of Proton Displacement in Matrix-Isolated Hydrogen Chloride–Water Complexes

This article focuses on the manipulation of molecular bond lengths by an external electric field. A uniform dc electric field with strength up to 1.5 × 108 V/m was applied to HCl–H2O and HCl–D2O complexes isolated in solid Ar matrices by using the ice film nanocapacitor method. The field-dependent vibrational spectra of the samples showed an extraordinarily large Stark shift of the proton vibration (H–Cl stretch) frequency of the HCl–water complexes in the electric field compared to that of the isolated HCl molecules. The results indicated that the applied electric field reversibly changed the equilibrium position of the acidic proton along the proton-transfer coordinate of the complexes. The proton displacement was highly asymmetric with respect to the elongation and contraction of the H–Cl bond and changed the vibrational coupling behavior between the proton motion and the normal mode of the hydrating D2O molecule.

Precise determination of excitation energies in condensed-phase molecular systems based on exciton-polariton measurements

The precise determination of the excitation energies in condensed-phase molecular systems is important for understanding system-environment interactions as well as for the prerequisite input data of theoretical models used to study the dynamics of the system. The excitation energies are usually determined by fitting of the measured optical spectra that contain broad and unresolved peaks as a result of the thermally random dynamics of the environment. Herein, we propose a method for precise energy determination by strongly coupling the molecular system to an optical cavity and measuring the energy of the resulting polariton. The effect of thermal fluctuations induced by the environment on the polariton is also investigated, from which a power scaling law relating the polariton's linewidth to the number of molecules is obtained. The power exponent gives important information about the environmental dynamics.

Electric Field Effect on Condensed-Phase Molecular Systems. VII. Vibrational Stark Sensitivity of Spatially Oriented Water Molecules in an Argon Matrix

The susceptibility of a water molecule to electric fields provides fundamental and essential information for understanding the vibrational spectra of water clusters and condensed-phase water. In th...

Energy Decomposition Analysis for Metal Surface-Adsorbate Interactions by Block Localized Wave Functions.

The energy decomposition analysis based on block localized wave functions (BLW-EDA) allows one to gain physical insight into the nature of chemical bonding, decomposing the interaction energy in (1) a "frozen" term, accounting for the attraction due to electrostatic and dispersion interactions, modulated by Pauli repulsion, (2) the variationally assessed polarization energy, and (3) the charge transfer. This method has so far been applied to gas- and condensed-phase molecular systems. However, its standard version is not compatible with fractionally occupied orbitals (i.e., electronic smearing) and, as a consequence, cannot be applied to metallic surfaces. In this work, we propose a simple and practical extension of BLW-EDA to fractionally occupied orbitals, termed Ensemble BLW-EDA. As illustrative examples, we have applied the developed method to analyze the nature of the interaction of various adsorbates on Pt(111), ranging from physisorbed water to strongly chemisorbed ethylene. Our results show that polarization and charge transfer both contribute significantly at the adsorption minimum for all studied systems. The energy decomposition analysis provides details with respect to competing adsorption sites (e.g., CO on atop vs hollow sites) and elucidates the respective importance of polarization and charge transfer for the increased adsorption energy of H2S compared to H2O. Our development will enable a deeper understanding of the impact of charge transfer on catalytic processes in general.

Modeling the effects of molecular disorder on the properties of Frenkel excitons in organic molecular semiconductors.

The Frenkel exciton model provides a convenient framework for simulating electronic excitations in organic conjugated systems that are too large to address with atomistic level electronic structure methods. Parameterization of this model is typically based on analytical expressions that incompletely describe the spatial and temporal correlations that are inherent to many condensed phase molecular systems. In this manuscript, we present a general procedure for including these correlations in the Frenkel exciton model, by mapping them directly from all-atom molecular configurations, for instance from classical molecular dynamics. Regardless of system morphology, this mapping automatically captures the spatial and temporal molecular correlations that are otherwise difficult or impossible to represent in terms of low-dimensional correlation functions. We apply this procedure to study the excited state properties of condensed phase materials made up of thiophene oligomers. We show that Frenkel model parameters can be mapped from a series of single molecule electronic structure calculations, and that for these materials efficient semi-empirical methods are sufficient to accurately reproduce experimental spectral measurements. By analyzing the statistics of model parameters derived from materials with different characteristic morphologies, we highlight failures in some assumptions that are commonly applied when generating model parameters. Finally, by simulating exciton dynamics on a mapped Frenekel exciton model, we demonstrate the ability to quantify the effect of material morphology on the dynamic properties of excitons.

Electric Field Effect on Condensed-Phase Molecular Systems. VI. Field-Driven Orientation of Hydrogen Chloride in an Argon Matrix.

The orientation state of hydrogen chloride (HCl) molecules in a solid argon matrix was reversibly controlled by applying an external electric field of up to 4 × 108 V·m-1 using the ice film capacitor method. The rovibrational transitions of the field-oriented HCl were measured by reflection absorption infrared spectroscopy with p-polarized light. Upon application of the external field, free rotation of HCl inside the matrix gradually changed to perturbed rotation and then to a pendular state harmonically bound in the Stark potential well. Further increase in the field strength increased the degree of dipole alignment along the field direction, approaching an asymptotically perfect orientation of the molecules with an average tilt angle of <30° at a field strength above 1 × 108 V·m-1.

Control of Excitation Energy Transfer in Condensed Phase Molecular Systems by Floquet Engineering.

Excitation energy transfer (EET) is one of the most important processes in both natural and artificial chemical systems including, for example, photosynthetic complexes and organic solar cells. The EET rate, however, is strongly suppressed when there is a large difference in the excitation energy between the donor and acceptor molecules. Here, we demonstrate both analytically and numerically that the EET rate can be greatly enhanced by periodically modulating the excitation energy difference. The enhancement of EET by using this Floquet engineering, in which the system's Hamiltonian is made periodically time-dependent, turns out to be efficient even in the presence of strong fluctuations and dissipations induced by the coupling with a huge number of dynamic degrees of freedom in the surrounding molecular environments. As an effect of the environment on the Floquet engineering of EET, the optimal driving frequency is found to depend on the relative magnitudes of the system and environment's characteristic time scales with an observed frequency shift when moving from the limit of slow environmental fluctuations (inhomogeneous broadening limit) to that of fast fluctuations (homogeneous broadening limit).

Electric Field Effect on Condensed-Phase Molecular Systems: V. Acid–Base Proton Transfer at the Interface of Molecular Films

In this work, we studied the effect of an applied electric field on the dissociation of acetic acid (AA) at the interface of the acid and ammonia molecular layers using the ice film capacitor method. The dissociation of AA was monitored by reflection absorption infrared spectroscopy, which measured the changes in the intensities of the vibrational modes of AA and the acetate anion. The extent of field-induced dissociation was found to be dependent on the direction of the applied electric field. Interestingly, the extent of acid dissociation was greater when the field was applied in the opposite direction to that of the proton-transfer motion. This suggested that the reaction was aided by the reorientation of the reagent molecules rather than by electrostatic stabilization of the charge-transfer energy. The effect of molecular size was studied by comparing the results for AA with those of formic acid and propionic acid. The dissociation of smaller acids was more strongly enhanced than that of larger acids, supporting the interpretation that the field-induced dissociation occurred via molecular reorientation.

Directing the path of light-induced electron transfer at a molecular fork using vibrational excitation.

Ultrafast electron transfer in condensed-phase molecular systems is often strongly coupled to intramolecular vibrations that can promote, suppress and direct electronic processes. Recent experiments exploring this phenomenon proved that light-induced electron transfer can be strongly modulated by vibrational excitation, suggesting a new avenue for active control over molecular function. Here, we achieve the first example of such explicit vibrational control through judicious design of a Pt(II)-acetylide charge-transfer donor-bridge-acceptor-bridge-donor 'fork' system: asymmetric 13C isotopic labelling of one of the two -C≡C- bridges makes the two parallel and otherwise identical donor→acceptor electron-transfer pathways structurally distinct, enabling independent vibrational perturbation of either. Applying an ultrafast UVpump(excitation)-IRpump(perturbation)-IRprobe(monitoring) pulse sequence, we show that the pathway that is vibrationally perturbed during UV-induced electron transfer is dramatically slowed down compared to its unperturbed counterpart. One can thus choose the dominant electron transfer pathway. The findings deliver a new opportunity for precise perturbative control of electronic energy propagation in molecular devices.

Electric Field Effect on Condensed-Phase Molecular Systems. III. The Origin of the Field-Induced Change in the Vibrational Frequency of Adsorbed CO on Pt(111)

The effect of an external electric field on the C–O stretch frequency, ν(C–O), of carbon monoxide was studied for CO in different environments of condensed molecular films: (I) chemisorbed CO on Pt(111) covered with amorphous solid water (ASW), (II) CO trapped in an ASW matrix, (III) chemisorbed CO on Pt(111) covered with solid Ar, and (IV) CO trapped in a solid Ar matrix. Changes in ν(C–O) of these samples under an electric field were measured to investigate the Stark frequency shift and the effect of metal–adsorbate charge transfer on the frequency change. The electric field was applied up to 4.3 × 108 V·m–1 using the ice film capacitor method. Reflection absorption infrared spectroscopy was used to monitor the spectral changes of the ν(C–O) band. The Stark shift was measured from the ν(C–O) change of isolated CO in the ASW matrix under the field. The effect of metal–adsorbate charge transfer was estimated for chemisorbed CO by measuring the ν(C–O) shift under the field and subtracting the electrostatic...

Nonequilibrium Fermi's Golden Rule Charge Transfer Rates via the Linearized Semiclassical Method.

Nonequilibrium Fermi's golden rule (NE-FGR) describes the transition between a photoexcited bright donor electronic state and a dark acceptor electronic state when the nuclear degrees of freedom start out in a nonequilibrium state. In this article, we derive a new expression for NE-FGR within the framework of the linearized semiclassical approximation. The new expression opens the door for applications of NE-FGR in complex condensed-phase molecular systems described in terms of anharmonic force fields. We show that the linearized semiclassical expression for NE-FGR yields the exact fully quantum-mechanical result for the canonical Marcus model, where the coupling between donor and acceptor is assumed constant (the Condon approximation) and the donor and acceptor potential energy surfaces are parabolic and identical except for a shift in the equilibrium energy and geometry. For this model, we also present a comprehensive comparison between the linearized semiclassical expression and a hierarchy of more approximate expressions, in both normal and inverted regions and over a wide range of initial nonequilibrium states, temperatures, and frictions.

Two-Dimensional Infrared Study of (13)C-Natural Abundant Vibrational Transition Reveals Intramolecular Vibrational Redistribution Rather than Fluxional Exchange in Mn(CO)5Br.

In this work, molecular-symmetry enhanced (13)CO natural abundant isotopic infrared transition was identified in Mn(CO)5Br dissolved in CCl4 by FTIR spectroscopy. Diagonal and associated off-diagonal two-dimensional IR (2D IR) peaks of the (13)CO-species were found to be spectrally separated from the all-(12)CO species, allowing a direct probe of the (13)C natural abundant ensemble. Temperature-dependent FTIR experiment showed no evidence of ligand exchange in the metal carbonyl complex. Intramolecular vibrational redistribution dynamics among the CO stretching vibrational states were extracted using population-time dependent 2D IR diagonal and off-diagonal peaks for both radial mono-(13)CO and all-(12)CO isotopomers. This work demonstrates the potential use of natural abundant isotopic molecular species as a probe for revealing equilibrium and nonequilibrium structural dynamics in condensed-phase molecular systems.

Nonlinear terahertz coherent excitation of vibrational modes of liquids

We report the first coherent excitation of intramolecular vibrational modes via the nonlinear interaction of a TeraHertz (THz) light field with molecular liquids. A terahertz-terahertz-Raman pulse sequence prepares the coherences with a broadband, high-energy, (sub)picosecond terahertz pulse, that are then measured in a terahertz Kerr effect spectrometer via phase-sensitive, heterodyne detection with an optical pulse. The spectrometer reported here has broader terahertz frequency coverage, and an increased sensitivity relative to previously reported terahertz Kerr effect experiments. Vibrational coherences are observed in liquid diiodomethane at 3.66 THz (122 cm(-1)), and in carbon tetrachloride at 6.50 THz (217 cm(-1)), in exact agreement with literature values of those intramolecular modes. This work opens the door to 2D spectroscopies, nonlinear in terahertz field, that can study the dynamics of condensed-phase molecular systems, as well as coherent control at terahertz frequencies.

Effect of Electric Field on Condensed-Phase Molecular Systems. I. Dipolar Polarization of Amorphous Solid Acetone

We investigated the dipolar reorientation of acetone molecules in amorphous solids under the influence of externally applied electric fields in the range of (0–4.3) × 108 V·m–1. The electric field ...

Effect of Electric Field on Condensed-Phase Molecular Systems. II. Stark Effect on the Hydroxyl Stretch Vibration of Ice

We studied the Stark effect on the hydroxyl stretching vibration of water molecules in ice under the influence of an external electric field. Electric fields with strengths in the range from 6.4 × 107 to 2.3 × 108 V·m–1 were applied to an ice sample using the ice film capacitor method. Reflection absorption infrared spectroscopy was used to monitor the field-induced spectral changes of vibrationally decoupled O–H and O–D bands of dilute HOD in D2O and H2O–ice, respectively. The spectral changes of the hydroxyl bands under applied field were analyzed using a model that simulates the absorption of a collection of Stark-shifted oscillators. The analysis shows that the Stark tuning rate of ν(O–D) is 6.4–12 cm–1/(MV·cm–1) at a field strength from 1.8 × 108 to 6.4 × 107 V·m–1, and the Stark tuning rate of ν(O–H) is 10–16 cm–1/(MV·cm–1) at a field strength from 2.3 × 108 to 9.2 × 107 V·m–1. These values are uniquely large compared to the Stark tuning rates of carbonyl or nitrile vibrations in other frozen molecu...