Vibrational Dephasing Research Articles

Tracing the Background‐Suppressed Vibrational Decay Time of a Molecular Monolayer with Time‐Resolved Surface‐Enhanced Broadband Coherent Anti‐Stokes Raman Scattering

AbstractThe sensitivity of molecular spectroscopy based on surface‐enhanced coherent anti‐Stokes Raman scattering (SECARS) is limited by the spectrally overlapping background from nonresonant four‐wave mixing (FWM). While the SECARS signal is mediated by the molecular vibrational eigenstates and exhibits long lifetime (a few picoseconds), the FWM background stems from the instantaneous electronic polarization and decays rapidly with the vanishment of excitation. Therefore, CARS and FWM can be separated by time‐resolved CARS (trCARS) using ultrashort pulsed lasers. The broad spectral bandwidth also enables broadband CARS (BCARS) for simultaneous identification of multiple vibrational signatures. This work combines trCARS and BCARS with an optimized plasmonic system to demonstrate time‐resolved surface‐enhanced BCARS) and show its capability in obtaining background‐suppressed broadband vibrational spectra from a monolayer of 4‐Aminothiophenol (4‐ATP) molecules self‐assembled on a gold grating. The vibrational dephasing time of the ring‐breathing mode for the 4‐ATP monolayer on the gold grating (≈1.00 ± 0.17 ps) is significantly shortened compared to that for 4‐ATP powder on a glass substrate (2.10 ± 0.05 ps). This work presents an effective method to suppress FWM background and investigate the vibrational dynamics of molecules in the vicinity of plasmonic nanostructures.

Coherent Ultrafast Stimulated X-Ray Raman Spectroscopy of Dissipative Conical Intersections.

The quantum coherent dynamics of a vibronic wave packet in a molecule passing through a conical intersection can be revealed using attosecond transient coherent Raman spectroscopy. In particular, the time evolution of the electronic coherence can be monitored in the presence of vibrational dynamics. So far, the technique has been investigated without including environmental quantum noise. Here, we employ the numerically exact hierarchy equation of motion approach to show that the transient coherent Raman signals are robust and accessible on times of up to a few hundred femtoseconds with respect to electonic and vibrational dephasing.

Elucidating phonon dephasing mechanisms in layered perovskites with coherent Raman spectroscopies.

Organic-inorganic hybrid perovskite quantum wells exhibit electronic structures with properties intermediate between those of inorganic semiconductors and molecular crystals. In these systems, periodic layers of organic spacer molecules occupy the interstitial spaces between perovskite sheets, thereby confining electronic excitations to two dimensions. Here, we investigate spectroscopic line broadening mechanisms for phonons coupled to excitons in lead-iodide layered perovskites with phenyl ethyl ammonium (PEA) and azobenzene ethyl ammonium (AzoEA) spacer cations. Using a modified Elliot line shape analysis for the absorbance and photoluminescence spectra, polaron binding energies of 11.2 and 17.5meV are calculated for (PEA)2PbI4 and (AzoEA)2PbI4, respectively. To determine whether the polaron stabilization processes influence the dephasing mechanisms of coupled phonons, five-pulse coherent Raman spectroscopies are applied to the two systems under electronically resonant conditions. The prominence of inhomogeneous line broadening mechanisms detected in (AzoEA)2PbI4 suggests that thermal fluctuations involving the deformable organic phase broaden the distributions of phonon frequencies within the quantum wells. In addition, our data indicate that polaron stabilization primarily involves photoinduced reorganization of the organic phases for both systems, whereas the impulsively excited phonons represent less than 10% of the total polaron binding energy. The signal generation mechanisms associated with our fifth-order coherent Raman experiments are explored with a perturbative model in which cumulant expansions are used to account for time-coincident vibrational dephasing and polaron stabilization processes.

Accelerated molecular vibrational decay and suppressed electronic nonlinearities in plasmonic cavities through coherent Raman scattering

Molecular vibrations and their dynamics are of outstanding importance for electronic and thermal transport in nanoscale devices as well as for molecular catalysis. The vibrational dynamics of <100 molecules are studied through three-color time-resolved coherent anti-Stokes Raman spectroscopy using plasmonic nanoantennas. This isolates molecular signals from four-wave mixing (FWM) while using exceptionally low nanowatt powers to avoid molecular damage via single-photon lock-in detection. FWM is found to be strongly suppressed in nanometer-wide plasmonic gaps compared to plasmonic nanoparticles. Simultaneous time-resolved incoherent anti-Stokes Raman spectroscopy allows us to separate the contributions of vibrational population decay (T1) and dephasing (T2). With increasing illumination intensity, the ultrafast vibrational dephasing rates of biphenyl-4-thiol molecules are accelerated at least tenfold, while phonon population decay rates remain constant. The extreme plasmonic field enhancement within nanogaps opens up prospects for measuring single-molecule vibrationally coupled dynamics and diverse molecular optomechanics phenomena. Published by the American Physical Society 2024

Dynamics of CH/n hydrogen bond networks probed by time-resolved CARS spectroscopy.

Hydrogen bond (HB) networks are essential for stabilizing molecular structures in solution and govern the solubility and functionality of molecules in an aqueous environment. HBs are important in biological processes such as enzyme-substrate interactions, protein folding, and DNA replication. However, the exact role of weakly polarized C-H bonds as HB proton donors in solution, such as CH/n HBs, remains mostly unknown. Here, we employ a novel approach focusing on vibrational dephasing to investigate the coherence relaxation of induced dipoles in C-H bonds within CH/n HB networks, utilizing time-resolved coherent anti-Stokes Raman scattering (T-CARS) spectroscopy. Using a representative binary system of dimethyl sulfoxide (DMSO)-water, known for its C-H backboned HB system (i.e., C-H⋯S), we observed an increase in the dephasing time of the C-H bending mode with increasing water content until a percolation threshold at a 6 : 1 water : DMSO molar ratio, where the trend is reversed. These results provide compelling evidence for the existence of C-H⋯S structures and underscore the presence of a percolation effect, suggesting a critical threshold where long-range connectivity is disputed.

What is cooking in your kitchen: seeing "invisible" with time-resolved coherent anti-Stokes Raman spectroscopy.

Cooking oil is a critical component of human food and its main component, lipid, is influential to health, but assessing its authenticity and quality can be challenging due to its complex chemical composition. In this study, we introduce a novel application of time-resolved coherent anti-Stokes Raman scattering (T-CARS) spectroscopy for detecting adulteration and understanding the mechanisms of lipid oxidation in various cooking oils. Our research surpasses the limitations of conventional spontaneous Raman spectroscopy, demonstrating that intra-molecular interactions from unsaturated bonds in triglycerides significantly influence vibrational dephasing time. We observed that these dephasing times, although diverse initially, converge to a similar value after heating cycles. Notably, a longer vibrational dephasing of the CH2 symmetric stretching mode was found to correlate with a higher lipid oxidation rate. These findings underscore the potential of T-CARS in identifying and characterizing subtle molecular interactions, offering a transformative approach to understanding molecular dynamics. This research paves the way for broader applications of T-CARS across fields such as chemistry, biomedicine, and material science, marking a significant advancement in the development of innovative analytical techniques.

Simulating two-dimensional correlation spectroscopies with third-order infrared and fifth-order infrared-Raman processes of liquid water.

To investigate the possibility of measuring the intermolecular and intramolecular anharmonic coupling of balk water, we calculate third-order two-dimensional (2D) infrared spectra and fifth-order 2D IR-IR-Raman-Raman spectra expressed in terms of four-body correlation functions of optical observables. For this purpose, a multimode Brownian oscillator model of four interacting anharmonic oscillators strongly coupled to their respective heat baths is employed. The nonlinearity of system-bath interactions is considered to describe thermal relaxation and vibrational dephasing. The linear and nonlinear spectra are then computed in a non-Markovian and nonperturbative regime in a rigorous manner using discretized hierarchical equations of motion in mixed Liouville-Wigner space. The calculated 2D spectra for stretching-bending, bending-librational, stretching-librational, and stretching-translational modes consist of various positive and negative peaks exhibiting essential details of intermolecular and intramolecular mode-mode interactions under thermal relaxation and dephasing at finite temperature.

Enhanced Chemical Sensing with Multiorder Coherent Raman Scattering Spectroscopic Dephasing.

Molecular vibrational spectroscopy is widely used in various sensing and imaging applications, providing intrinsic information at the molecular level. Nonlinear optical interactions using ultrashort laser pulses facilitate the selective coherent excitation of molecular vibrational modes by focusing energy into specific molecular bonds, boosting the signal level for multiple orders of magnitude. The dephasing of such coherence, which is susceptible to the local molecular environment, however, is often neglected. The unique capability of vibrational dephasing dynamics to serve as a unique probe for complex molecular interactions and the effect of local nano- and microenvironments are beyond the reach of conventional, intensity-based spectroscopy. Here, we developed a novel multiorder coherent Raman spectroscopy platform with a special focus on the temporal evolution of molecular vibrational dephasing, termed as time-resolved coherent Raman scattering (T-CRS) spectroscopy. By utilizing a high dynamic range detection, molecular vibrational dynamics and the environmental effects are demonstrated with multidimensional spectroscopic sensing, which promises a new range of applications in biology, materials, and chemical sciences.

Ultrafast Vibrational Dephasing Times of Modified Graphene

We have measured the ultrafast dephasing times of vibrational quanta in functional groups covalently bound to graphene. It has been previously shown that electronic relaxation in graphene occurs on subpicosecond timescales, but this is the first venture into the vibrational dephasing times in functionalized graphene. We have employed time-resolved sum-frequency generation spectroscopy to observe the decay of the coherent signal arising due to vibrational excitation by delaying the upconversion pulse with respect to the infrared (IR) laser pulse. We observe decay times of 0.8 and 1.4 ps for vibrational stretches in phenyl groups attached to graphene or gold, respectively, but for the vibrations of the bond covalently connecting H atoms to graphene in hydrogenated graphene, we measure much faster decay times of 0.3 ps. This is likely due to coupling between the C–H stretch and either the two-dimensional (2D) mode of graphene or nonadiabatic coupling to electrons photoexcited by the upconversion laser pulse.

2DIR Spectroscopy for Studies of Molecular Structure and Dynamics on Surfaces of Noble Metals

Ultrafast studies of molecular dynamics using femtosecond two-dimensional vibrational spectroscopy (2DIR) provide unique insights on molecular interactions. Recently, such studies were extended from bulk samples to molecules on surfaces of noble metals, including disordered and engineered nanostructures. The metal nanostructures provide field enhancement allowing sensitive measurements at interfaces, but they also alter the molecular dynamics. 2DIR measurements at interfaces reveal correlations between vibrational transitions and rates of the vibrational relaxation, dephasing, spectral diffusion, and resonant energy transfer. At the surfaces, molecular dynamics and interaction with the environment can be modified by geometrical constrains associated with tethering, by the interaction between the molecule and the substrate, or by the enhanced near-fields associated with the metal. Interestingly, in many cases, these anticipated effects were found to be absent or fade away already at the microscopic distances away from the surface. Herein, we provide an overview of recent experimental studies in the field.

Vibrational dynamics of iron pentacarbonyl in cryogenic matrices.

Iron pentacarbonyl is a textbook example of fluxionality. We trap the molecule in cryogenic matrices to study the vibrational dynamics of CO stretching modes involved in the fluxional rearrangement. The infrared spectrum in Ar and N2 is composed of about ten narrow bands in the spectral range of interest, indicating the population of various lattice sites and a lowering of the molecular symmetry in the trapping sites. The vibrational dynamics is explored by means of infrared stimulated photon echoes at the femtosecond scale. Vibrational dephasing and population relaxation times are obtained. The non-linear signals exhibit strong oscillations useful to disentangle the site composition of the absorption spectrum. The population relaxation involves at least two characteristic times. An evolution of the photon echo signals with the waiting time is observed. The behavior of all the signals can be reproduced within a simple model that describes the population relaxation occurring in two steps: relaxation of v = 1 (population time T1 < 100ps) and return to v = 0 (recovery time > 1ns). These two steps explain the evolution of the oscillations with the waiting time in the photon echo signals. These results discard fluxional rearrangement on the time scale of hundreds of ps in our samples. Dephasing times are of the same order of magnitude as T1: dephasing processes due to the matrix environment are rather inefficient. The photon echo experiments also reveal that intermolecular resonant vibrational energy transfers between guest molecules occur at the hundreds of ps time scale in concentrated samples (guest/host > 104).

On the use of vibronic coherence to identify reaction coordinates for ultrafast excited-state dynamics of transition metal-based chromophores.

The question of whether one can use information from quantum coherence as a means of identifying vibrational degrees of freedom that are active along an excited-state reaction coordinate is discussed. Specifically, we are exploring the notion of whether quantum oscillations observed in single-wavelength kinetics data exhibiting coherence dephasing times that are intermediate between that expected for either pure electronic or pure vibrational dephasing are vibronic in nature and therefore may be coupled to electronic state-to-state evolution. In the case of a previously published Fe(II) polypyridyl complex, coherences observed subsequent to 1A1 → 1MLCT excitation were linked to large-amplitude motion of a portion of the ligand framework; dephasing times on the order of 200-300 fs suggested that these degrees of freedom could be associated with ultrafast (∼100 fs) conversion from the initially formed MLCT excited state to lower-energy, metal-centered ligand-field excited state(s) of the compound. Incorporation of an electronically benign but sterically restrictive Cu(I) ion into the superstructure designed to interfere with this motion yielded a compound exhibiting a ∼25-fold increase in the compound's MLCT lifetime, a result that was interpreted as confirmation of the initial hypothesis. However, new data acquired on a different chemical system - Cr(acac')3 (where acac' represents various derivatives of acetylacetonate) - yielded results that call into question this same hypothesis. Coherences observed subsequent to 4A2 → 4T2 ligand-field excitation on a series of molecules implicated similar vibrational degrees of freedom across the series, but exhibited dephasing times ranging from 340 fs to 2.5 ps without any clear correlation to the dynamics of excited-state evolution in the system. Taken together, the results obtained on both of these chemical platforms suggest that while identification of coherences can indeed point to degrees of freedom that should be considered as candidate modes for defining reaction trajectories, our understanding of the factors that determine the interplay across coherences, dephasing times, and electronic and geometric structure is insufficient at the present time to view this parameter as a robust metric for differentiating active versus spectator modes for ultrafast dynamics.

Vibrational relaxation of monomeric and self-associated acrylonitrile species in solutions

The dynamic interactions between acrylonitrile and carbon tetrachloride in solutions were investigated by means of Raman spectroscopy in a wide concentration range. We progress on the comprehensive understanding of vibrational frequency modulation and dephasing by analyzing the spectra and the time correlation functions of vibrational relaxation. Unlike to the usual Raman line profile analysis, we used an alternative approach to extract valuable parameters related to vibrational short-time dynamics of the system by applying a fitting procedure of the spectra directly in the frequency domain without prejudging the Lorentzian or Gaussian character of the analytical function. We focused on the CN symmetric stretching mode of acrylonitrile, which was used as a probe of the dynamics of these solutions. This relatively complex mode appears as an asymmetric line profile that was decomposed into two individual components assigned to monomeric and self-associated species. Systematic analysis of the dynamic parameters revealed a strong dependence of vibrational relaxation on the local environment around the acrylonitrile species in solutions with carbon tetrachloride. The vibrational relaxation time τV and the frequency modulation time τω increase from dilute to dense solutions implying modification of interactions upon dilution. The variations observed in the second moments of vibrations imply an interplay between repulsive and attractive forces acting on monomeric and self-associated species in solution environment. The relative contribution of the homogeneous and inhomogeneous broadening to the isotropic Raman line profile of the CN symmetric stretching mode of acrylonitrile has been analyzed and discussed in view of the recent phenomenological status of the field.

Vibrational Dephasing along the Reaction Coordinate of an Electron Transfer Reaction.

The role of molecular vibration in photoinduced electron transfer (ET) reactions has been extensively debated in recent years. In this study, we investigated vibrational wavepacket dynamics in a model ET system consisting of an organic dye molecule as an electron acceptor dissolved in various electron donating solvents. By using broad band pump-probe (BBPP) spectroscopy with visible laser pulses of sub-10 fs duration, coherent vibrational wavepackets of naphthacene dye with frequencies spanning 170-1600 cm-1 were observed in the time domain. The coherence properties of 11 vibrational modes were analyzed by an inverse Fourier filtering procedure, and we discovered that the dephasing times of some vibrational coherences are reduced with increasing ET rates. Density functional theory calculations indicated that the corresponding vibrational modes have a large Huang-Rhys factor between the reactant and the product states, supporting the hypothesis that the loss of phase coherence along certain vibrational modes elucidates that those vibrations are coupled to the reaction coordinate of an ET reaction.

Nonradiative Decay and Vibrational Dephasing of Malachite Green in Trehalose Glass Monitored by Femtosecond Time-Resolved Transient Absorption Spectroscopy

Nonradiative Decay and Vibrational Dephasing of Malachite Green in Trehalose Glass Monitored by Femtosecond Time-Resolved Transient Absorption Spectroscopy

Raman investigation of vibrational energy interactions of liquids in nanoscale confinement

The Raman scattering parameters of linewidth and peak energy of liquids that are undergoing local nanoscale spatial confinement are investigated. This is done using several polar and nonpolar liquid molecules of varying size, weight, shape, and polarity that are placed inside different nanometer-sized silica glass pores. Nanoscale confinement is shown to produce changes in the Raman spectra that are related to different quantum effects. Raman spectra are collected from filled porous glass samples with average pore sizes of 2.5, 5, 10, and 20 nm. The resulting spectra are then compared to bulk liquid samples. The most salient spectral changes of individual vibrational modes, overtone modes, and combinations modes of methanol, deuterated methanol, acetone, benzene, carbon disulfide, carbon tetrachloride, and beta-carotene are graphed and tabularized. Results illustrate how different types of molecular vibrations respond to confinement, and how molecular geometrical restrictions affect Raman active modes based on the ratio of pore size to molecular size. Nano-confinement allows for the direct measurement of how intra-molecular and inter-molecular forces affect the expression of vibrational resonant peak energies as well as vibrational dephasing times. The outcome demonstrates a preference for polar molecules, specific vibrational types, and ratios of molecular size to cavity size.

Proton tunneling in a two-dimensional potential energy surface with a non-linear system-bath interaction: Thermal suppression of reaction rate.

We consider a proton transfer (PT) system described by a proton transfer reaction (PTR) coordinate and a rate promoting vibrational (RPV) coordinate interacting with a non-Markovian heat bath. While dynamics of PT processes has been widely discussed using two-dimensional potential energy surfaces, the role of the heat bath, in particular, in a realistic form of the system-bath interaction has not been well explored. Previous studies are largely based on a one-dimensional model and linear-linear system-bath interaction. In the present study, we introduce an exponential-linear (EL) system-bath interaction, which is derived from the analysis of a PTR-RPV system in a realistic situation. This interaction mainly causes vibrational dephasing in the PTR mode and population relaxation in the RPV mode. Numerical simulations were carried out using the hierarchical equations of motion approach. We analyze the role of the heat bath interaction in the chemical reaction rate as a function of the system-bath coupling strength at different temperatures and for different values of the bath correlation time. A prominent feature of the present result is that while the reaction rate predicted from classical and quantum Kramers theory increases as the temperature increases, the present EL interaction model exhibits opposite temperature dependence. The Kramers turn-over profile of the reaction rate as a function of the system-bath coupling is also suppressed in the present EL model, turning into a plateau-like curve for larger system-bath interaction strength. Such features arise from the interplay of the vibrational dephasing process in the PTR mode and the population relaxation process in the RPV mode.

Generalizing the Marcus equation

The Marcus equation for the rate of an electron-transfer reaction can be generalized to cover larger electronic-interaction matrix elements, irregular free-energy surfaces, and coupling to multiple vibrational modes and to recognize the different effects of vibrational relaxations and pure dephasing. Almost all the information needed to calculate the rate constant can be obtained from a quantum-classical molecular dynamics simulation of the system in the reactant state. Because the final expression for the rate constant does not depend on the reorganization energy, it is insensitive to slow relaxations that follow the reaction.

Vibrational Relaxation of Functional Groups in dAMP Molecules Probed with UV Resonance Raman Spectroscopy

Dynamical properties of functional groups in 2�-deoxyadenosine-5�-monophosphate (dAMP) compound, were identified by UV resonance Raman spectroscopy (UVRR), upon varying nucleotide concentration in aqueous solution (200-600 μM). The studied full-widths at half-maximum (fwhm�s) were found between 13 - 21 cm-1 and the corresponding global relaxation times were faster than 0.817 ps and slower than 0.506 ps. Also, the band around 1430 cm-1 (C4N9-δC8H) in the UV resonance Raman spectrum of dAMP molecule at 400 μM concentration in aqueous solution, was selected for vibrational band shape analysis through time correlation function (CF) concept. Current theories developed for vibrational dephasing (Kubo-Rothschild and Oxtoby) have been applied to this profile and relevant relaxation parameters have been obtained and discussed. The best fit parameters for this dissipation channel of the vibrational excitation energy were established. To our knowledge this is the first UVRR study on nucleotide vibrational band shape analysis through time correlation function concept.

Purcell-Enhanced Spontaneous Emission of Molecular Vibrations.

Infrared (IR) spectroscopy of molecular vibrations provides insight into molecular structure, coupling, and dynamics. However, picosecond scale intermolecular and intramolecular many-body interactions, nonradiative relaxation, absorption, and thermalization typically dominate over IR spontaneous emission. We demonstrate how coupling to a resonant IR antenna can enhance spontaneous emission of molecular vibrations. Using time-domain nanoprobe spectroscopy we observe an up to 50% decrease in vibrational dephasing time T_{2,vib}, based on the coupling-induced population decay with T_{κ}≃550 fs and an associated Purcell factor of >10^{6}. This rate enhancement of the spontaneous emission of antenna-coupled molecular vibrations opens new avenues for IR coherent control, quantum information processing, and quantum chemistry.