Dynamics Of Polyatomic Molecules Research Articles

Cavity Control of Molecular Spectroscopy and Photophysics.

ConspectusOptical cavities have been established as a powerful platform for manipulating the spectroscopy and photophysics of molecules. Molecules placed inside an optical cavity will interact with the cavity field, even if the cavity is in the vacuum state with no photons. When the coupling strength between matter excitations, either electronic or vibrational, and a cavity photon mode surpasses all decay rates in the system, hybrid light-matter excitations known as cavity polaritons emerge. Originally studied in atomic systems, there has been growing interest in studying polaritons in molecules. Numerous studies, both experimental and theoretical, have demonstrated that the formation of molecular polaritons can significantly alter the optical, electronic, and chemical properties of molecules in a noninvasive manner.This Account focuses on novel studies that reveal how optical cavities can be employed to control electronic excitations, both valence and core, in molecules and the spectroscopic signatures of molecular polaritons. We first discuss the capacity of optical cavities to manipulate and control the intrinsic conical intersection dynamics in polyatomic molecules. Since conical intersections are responsible for a wide range of photochemical and photophysical processes such as internal conversion, photoisomerization, and singlet fission, this provides a practical strategy to control molecular photodynamics. Two examples are given for the internal conversion in pyrazine and singlet fission in a pentacene dimer. We further show how X-ray cavities can be exploited to control the core-level excitations of molecules. Core polaritons can be created from inequivalent core orbitals by exchanging X-ray cavity photons. The core polaritons can also alter the selection rules in nonlinear spectroscopy.Polaritonic states and dynamics can be monitored by nonlinear spectroscopy. Quantum light spectroscopy is a frontier in nonlinear spectroscopy that exploits the quantum-mechanical properties of light, such as entanglement and squeezing, to extract matter information inaccessible by classical light. We discuss how quantum spectroscopic techniques can be employed for probing polaritonic systems. In multimolecule polaritonic systems, there exist two-polariton states that are dark in the two-photon absorption spectrum due to destructive interference between transition pathways. We show that a time-frequency entangled photon pair can manipulate the interference between transition pathways in the two-photon absorption signal and thus capture classically dark two-polariton states. Finally, we discuss cooperative effects among molecules in spectroscopy and possibly in chemistry. When many molecules are involved in forming the polaritons, while the cooperative effects clearly manifest in the dependence of the Rabi splitting on the number of molecules, whether they can show up in chemical reactivity, which is intrinsically local, is an open question. We explore the cooperative nature of the charge migration process in a cavity and show that, unlike spectroscopy, polaritonic charge dynamics is intrinsically local and does not show collective many-molecule effects.

Strong-Field-Induced Coulomb Explosion Imaging of Tribromomethane.

The Coulomb explosion of tribromomethane (bromoform, CHBr3) induced by 28 fs near-infrared laser pulses is investigated by three-dimensional coincidence ion momentum imaging. We focus on the fragmentation into three, four, and five ionic fragments measured in coincidence and present different ways of visualizing the three-dimensional momentum correlations. We show that the experimentally observed momentum correlations for 4- and 5-fold coincidences are well reproduced by classical Coulomb explosion simulations and contain information about the structure of the parent molecule that could be used to differentiate structural isomers formed, for example, in a pump-probe experiment. Our results thus provide a clear path toward visualizing structural dynamics in polyatomic molecules by strong-field-induced Coulomb explosion imaging.

An accurate potential energy surface and ring polymer molecular dynamics study of the Cl + CH4 → HCl + CH3 reaction.

The reaction Cl + CH4 → HCl + CH3, a prototypical bimolecular reaction, has been established as an important proving ground for studying chemical reaction kinetics and dynamics of polyatomic molecules. In this work, a globally accurate full-dimensional potential energy surface (PES) for this reaction is developed by using the permutationally invariant polynomial neural network (PIP-NN) approach based on 74 000 points calculated at the level of the explicitly correlated unrestricted coupled cluster single, double, and perturbative triple level with the augmented correlation corrected valence triple-zeta basis set (UCCSD(T)-F12a/AVTZ). For points in the entrance channel, spin-orbit corrections stemming from Cl(2P) are determined at the level of complete active space self-consistent field (CASSCF) with the AVTZ basis set. With this PES, thermal rate coefficients and kinetic isotope effects are computed for reactions Cl + CH4 → HCl + CH3 and Cl + CD4 → DCl + CD3 using the ring polymer molecular dynamics (RPMD) method, which can provide reliable estimations for thermal rate coefficients effectively. Generally, the agreement with the scattered experimental results is reasonably satisfactory.

Communication: XFAIMS-eXternal Field Ab Initio Multiple Spawning for electron-nuclear dynamics triggered by short laser pulses.

Attoscience is an emerging field where attosecond pulses or few cycle IR pulses are used to pump and probe the correlated electron-nuclear motion of molecules. We present the trajectory-guided eXternal Field Ab Initio Multiple Spawning (XFAIMS) method that models such experiments "on-the-fly," from laser pulse excitation to fragmentation or nonadiabatic relaxation to the ground electronic state. For the photoexcitation of the LiH molecule, we show that XFAIMS gives results in close agreement with numerically exact quantum dynamics simulations, both for atto- and femtosecond laser pulses. We then show the ability of XFAIMS to model the dynamics in polyatomic molecules by studying the effect of nuclear motion on the photoexcitation of a sulfine (H2CSO).

Resolving XUV induced femtosecond and attosecond dynamics in polyatomic molecules with a compact attosecond beamline

XUV induced dynamics in molecules is still largely unexplored while experimental and theoretical tools are becoming available in several labs. In this work, we present a compact XUV beamline designed to induce and probe femtosecond and attosecond dynamics in gas samples (from atoms to complex molecular species). The induced processes are studied with time-resolved photoelectron or/and photoion imaging. The characterization of the performances of the experimental setup is presented. We show experimental results obtained in naphthalene molecule showing femtosecond and attosecond time-resolved photoelectron imaging experiments allowing electron wavepacket phase measurements.

Photoinduced Ultrafast Electronic Dynamics in Polyatomic Molecules: A time-dependent computational study

SynopsisWe report on the dynamics of photoexcitation and photoionization induced by ultrashort strong optical pulses in two medium sized polyatomic molecules, 1-azabicyclo[2,2,2]octane and caffeine. The time-dependent Schrödinger equation is solved for an electronic Hamiltonian that includes the interaction with the strong pulse. The partitioning technique is applied to include both the neutral and the ionized subspaces.

Ultrafast electron dynamics in phenylalanine initiated by attosecond pulses.

In the past decade, attosecond technology has opened up the investigation of ultrafast electronic processes in atoms, simple molecules, and solids. Here, we report the application of isolated attosecond pulses to prompt ionization of the amino acid phenylalanine and the subsequent detection of ultrafast dynamics on a sub-4.5-femtosecond temporal scale, which is shorter than the vibrational response of the molecule. The ability to initiate and observe such electronic dynamics in polyatomic molecules represents a crucial step forward in attosecond science, which is progressively moving toward the investigation of more and more complex systems.

Non-Born–Oppenheimer wavepacket dynamics in polyatomic molecules: vibrations at conical intersections in DABCO

The role of vibrational dynamics in the vicinity of conical intersections is investigated using the first two electronically excited states of 1,4-diazabicyclo[2,2,2]octane (DABCO) by combining time-resolved photoelectron spectroscopy with ab initio computation. Upon resonant excitation of the origin band of the short-lived S2 (1E') state, oscillations in the electronic population between the S2 (1E') and the S1 (1A'1) electronic states are observed with a period of -3 ps. Ab initio computations are employed to characterise these low-lying excited states, which arise from single excitations into the 3s and 3p Rydberg orbitals. Although Rydberg states are generally only weakly coupled, DABCO exhibits rapid nonadiabatic dynamics. This implies that strong coupling occurs only in the immediate vicinity of a conical intersection, enabling unique identification of those vibrations which generate the nonadiabatic transitions. To this end, seams of conical intersection are located at energetically relevant geometries, engendered by differential distortions of the S1 and S2 potentials due to vibronic coupling and a Jahn-Teller-distorted S2 minimum energy point. From an analysis of the conical intersection topography, those vibrations leading to a maximal modulation of the coupling between the electronic states are readily identified. The observed oscillation in the decay of S2 state population is thereby assigned to the beat frequency between two sets of vibronic eigenstates within the S1 manifold, coherently prepared together with another set at the S2 band origin, and whose nominal e' degeneracy is lifted due to differential coupling to the Jahn-Teller-distorted components of S2.

Time-resolved photoelectron spectroscopy: from wavepackets to observables

Time-resolved photoelectron spectroscopy (TRPES) is a powerful tool for the study of intramolecular dynamics, particularly excited state non-adiabatic dynamics in polyatomic molecules. Depending on the problem at hand, different levels of TRPES measurements can be performed: time-resolved photoelectron yield; time- and energy-resolved photoelectron yield; time-, energy-, and angle-resolved photoelectron yield. In this pedagogical overview, a conceptual framework for time-resolved photoionization measurements is presented, together with discussion of relevant theory for the different aspects of TRPES. Simple models are used to illustrate the theory, and key concepts are further amplified by experimental examples. These examples are chosen to show the application of TRPES to the investigation of a range of problems in the excited state dynamics of molecules: from the simplest vibrational wavepacket on a single potential energy surface; to disentangling intrinsically coupled electronic and nuclear motions; to identifying the electronic character of the intermediate states involved in non-adiabatic dynamics by angle-resolved measurements in the molecular frame, the most complete measurement.

Semiclassical Model for Vibrational Dynamics in Polyatomic Molecules: Investigation of Internal Vibrational Relaxation

We propose a semiclassical model describing vibrational relaxation and intramolecular vibrational energy redistribution (IVR) in polyatomic molecules. Transition rates between different harmonic states are found using the generalized self-consistent Marcus−Levich−Jortner approach, treating low-frequency modes as reaction coordinates. We applied our method to the 1,4-acetylbenzonitrile molecule (AcPhCN) in order to study internal vibrational dynamics. Our analysis of internal vibrational dynamics following photoexcitations of CN and CO stretch modes is consistent with recent experimental data.

Time-Resolved Dynamics in N 2 O 4 Probed Using High Harmonic Generation

The attosecond time-scale electron-recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. We showed that high harmonic generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in dinitrogen tetraoxide, we showed that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration but populates the first excited state at the inner turning point. This state-switching mechanism is manifested as bursts of high harmonic light that is emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.

Intramolecular dynamics from frequency domain spectroscopy

Vibrationally mediated photodissociation is emerging as a useful tool for studies of intramolecular dynamics in polyatomic molecules since it is a sensitive indicator of both the vibrational and electronic dynamics. The method involves energy deposition in skeletal motions of molecules, their subsequent promotion to excited electronic states and dissociation. Jet-cooled action spectra, reflecting the yield of the ensuing atomic fragments as a function of vibrational excitation, are compared to simultaneously measured room temperature photoacoustic spectra and simulated spectra. The narrowed action spectra expose resonance splitting and enable estimating the homogeneous broadening and thus time scales for intramolecular vibrational energy redistribution to close resonating states and to bath states. Overtone spectra of the acetylenic and alkyl C H stretches of the acetylene homologues, propyne and 1-butyne, demonstrate how the nature of rovibrationally excited states is highlighted by photolysis of pre-excited molecules.

Nonadiabatic chemical dynamics in an intense laser field: Electronic wave packet coupled with classical nuclear motions

Dynamics of molecules in an intense laser field is studied in terms of the quantum electronic wave packet coupled with classical nuclear motions. The equations of motion are derived taking a proper account of molecular interactions with the vector potential of a classical electromagnetic field, along with the nonadiabatic interaction due to the breakdown of the Born-Oppenheimer approximation. With the aid of electronic structure calculations, the present method enables us to track, in an ab initio manner, the dynamics of polyatomic molecules in an intense field. Preliminary calculations are carried out for the vibrational state of LiF and a collision of Li+F under an intense laser pulse, which are limited to the domain of no ionization.

Non-adiabatic intramolecular and photodissociation dynamics studied by femtosecond time-resolved photoelectron and coincidence imaging spectroscopy

Time-resolved photoelectron spectroscopy (TRPES) is emerging as a useful tool for the study of non-adiabatic dynamics in isolated polyatomic molecules and clusters due to its sensitivity to both electronic and vibrational dynamics. A powerful extension of TRPES, coincidence imaging spectroscopy (CIS), based upon femtosecond time-resolved 3D momentum vector imaging of both photoions and photoelectrons in coincidence, is a new technique for the study of complex dissociative processes. Here we show how these spectroscopies can be used to study both non-adiabatic intramolecular and photodissociation dynamics in polyatomic molecules. Intramolecular dynamics in the alpha, beta-enones acrolein, crotonaldehyde and methyl vinyl ketone are studied using both TRPES and laser-induced fluorescence of HCO(X) product yields. The location of the methyl group is seen to have very dramatic effects on the relative electronic relaxation rates and the HCO(X) yield. Applying both TRPES and CIS to the 200 nm and 209 nm photodissociation of the nitric oxide dimer, (NO)2, we observe the fs time-scale evolution of the excited parent neutral via its photoelectron spectrum and the emergence of the NO(A) photofragment including its energy and angular distributions.

Time-resolved photoelectron spectroscopy: Non-adiabatic dynamics in polyatomic molecules

Femtosecond time-resolved photoelectron spectroscopy is emerging as an important technique for investigating photodynamics in polyatomic molecules. Photoelectron spectroscopy is sensitive to both electronic configurations and vibrational dynamics and is therefore well suited to the study of ultrafast non-adiabatic processes. We emphasize the conceptual interpretation of wavepacket dynamics experiments, particularly stressing the critical role of the final state. We discuss the advantages of the molecular ionization continuum as the final state in polyatomic wavepacket experiments and show how the electronic structure of the continuum can be used to help to disentangle electronic from vibrational dynamics. We illustrate these methods with examples from internal conversion in polyenes and polyaromatic hydrocarbons, electronic relaxation in substituted benzenes, excited state intramolecular proton transfer, azobenzene photoiosomerization dynamics and non-adiabatic photodissociation dynamics.

Nonadiabatic Multielectron Dynamics in Strong Field Molecular Ionization.

We report the observation of a general strong field ionization mechanism due to highly nonadiabatic multielectron excitation dynamics in polyatomic molecules. We observe that such excitation mechanisms greatly affect molecular ionization, fragmentation, and energetics. We characterized this phenomenon as a function of optical frequency, intensity, and molecular properties.

Collisional transfer of vibrational energy in vapors of polyatomic molecules

Thc collisional transfer of vibrational energy has been the subject of much investigation l 1, 2 ], the results of which are of fundamental importance for the molecular physics of the gas phase and have numerous practical applications. However, the laws governing intermolecular vibrational relaxation in the quasi-continuum of vibrational states, which is characteristic of polyalomic molecules even at comparatively low levcls of vibrational excitation, have not rcceived sufficient study. This results from the limited number of direct expcrimenlal methods and from the absence of corresponding theoretical models. Experimental difficulties are responsiblc for the differences in the quantitative characteristics of collisional transfer as well as ira the laws governing their variation with an increase in the internal energy of the molecule. The diversity of the postulated means of deactivation of excited molecules and the complexity' of the energy spectrum and of the motion dynamics of polyatomic molecules make simulation of the process difficult and require new, original experimental works along these lines. As early as in the late fifties, B. S. Neporent and N. A. Borisevich [3 I demonstrated lhe high cfficiencv of luminescence methods as applied to the study of collisional transfer of vibrational energy. A new direct method of obsem'ing the process of collisional transfer and exchange of vibrational energy from the change in the rates of decay of the delayed fluorescence activated by the nonequilibrium vibrational laser excitation of triplet molecules was suggested in I4 1. In subsequent works 15-71 il was shown that the use of multiphoion excitalion (MPE) of triplet molecules by CO2-taser radiation permits onc to investigate the process at both low and high vibrational energies for molecules of differcnl degrees of complexity. In the present work the method is used Io investigatc the energy dependences of the characteristics of the collisional transfer of vibrational energy ira vapors of diacclvl and benzophenone and their mixtures with foreign gases. As noted earlier, the choice of objects for investigalion is based on the spectroscopic charactcristics of the molecules, which in the gas phase possess long-lived triplcl states, a low barrier to the activation of delayed fluorescence, resulting from efficicnt Tl----" SI interconversion, and the ability to absorb thc IR radiation of a CO2-1aser in the electron ground and tripleT, states. The characteristics of the collisional transfer of vibrational energy are estimated using dcpendences of {he intensities and rates of decay Kdc c of the fast, microsecond component of activated fluorescence on the pressures of the vapors and foreign gases. The pressure dependence of Kde c varies in the investigated range of pressures and energies of excitation. At a constant density of CO2-1ascr energy, the rate of decay Kde c increases linearly with the pressures of the vapors and foreign gases only in a limited range of pressures and then begins to decrease. It should be noted that such dependences in mixtures with foreign gases are obscrvcd when there is a noticeable increase in the range of the values of ECO 2 used to the higher side. Anomalous dependences manifesting themselves in a decrease of Kde c with pressure begin to dominate at low laser energy densities. The dependences of the instantaneous values of lhe relative intensity l(v, 0/lo (v, t), where l(v, 0 and lo(v, t) are the intensities of luminescence at the maximum of the pulse of the activated fluorescence and of the

A test of an adiabatic treatment of rotation for vibration/rotation energies of polyatomic molecules

A test of a quantum adiabatic treatment of rotational energy for the calculation of vibrational/rotational energies and dynamics of polyatomic molecules is presented. The approach is given in detail for triatomic systems, where it is shown, in the symmetric-top limit, to be the quantum analog of an earlier semiclassical Hamiltonian of McCurdy and Miller. A test of the adiabatic and standard centrifugal sudden decoupling methods is made for all the bound states of HCO for total angular momentum of one. The adiabatic approach is shown to be significantly more accurate that the centrifugal sudden approximation.

Dynamics of Polyatomic Molecules in Porous Silica

AbstractThe liquid-solid phase transition of cyclohexane in small pores of sol-gel glass is studied by computer simulation. A cavity model is obtained by placing silica clusters at the corners of a cubic box. A molecule inside the cavity is approximated by an assembly of six Lennard-Jones (LJ) potentials. Translational and rotational motions of C6H12 are studied in the temperature range from 190 K to 333 K. Supercooling is observed and the freezing temperature is depressed in comparison to the bulk phase.

Femtosecond time-resolved ionization spectroscopy of ultrafast internal-conversion dynamics in polyatomic molecules: Theory and computational studies

A framework for the theoretical description of two-pulse time-resolved ionization spectroscopy of ultrafast excited-state dynamics of polyatomic molecules is developed. The radiation–matter interaction as well as intramolecular couplings in the excited-state manifold are treated nonperturbatively by solving the time-dependent Schrödinger equation. The numerical solution is based on a discretization of the ionization continua which becomes particularly efficient for ultrashort laser pulses. With this method converged computations of ionization signals become possible even for complex molecular systems. Computer simulations are performed for a model system representing three-dimensional non-Born–Oppenheimer excited-state dynamics on conically intersecting potential-energy surfaces (the S1 and S2 surfaces of pyrazine). The dependence of the observable time-resolved ionization signals (total ion yield as well as photoelectron spectrum) on the properties of the laser pulses (carrier frequency and pulse duration) is explored. It is demonstrated that ultrafast electronic decay processes as well as coherent vibrational motion in excited states can be monitored by pump–probe ionization with suitable pulses. The dependence of the time-resolved ionization signals on properties of the cation (ionization potentials and potential-energy surfaces) is also discussed.