Resonant Vibrational Energy Transfer Research Articles

Vibrational relaxation of carbon dioxide in water.

The dynamics of vibrational relaxation of carbon dioxide in water has been studied using femtosecond mid-infrared pump-probe spectroscopy with excitation of the anti-symmetric stretching (ν3) fundamental state of the solute. The relaxation dynamics were recorded at a constant pressure of 500 bars and in the temperature range between 300 and 600K, thereby covering the liquid-to-near-critical region of the solvent. The excited state of the ν3-mode is deactivated in two competing pathways: (i) direct relaxation to the ground state with resonant transfer of the excess vibrational energy into the bending-librational continuum of the water solvent and (ii) relaxation to the bending fundamental state with transfer into the intramolecular bending mode of H2O. The rate of pathway (i) decreases with increasing temperature, from ∼1/(9ps) at 300K to ∼(1/16ps) at 600K and obeys Fermi's golden rule strictly, provided that the spectral density of energy-accepting solvent states is derived from the stationary infrared absorption profile of H2O. The rate of pathway (ii) is 1/(23ps) and assumed to be temperature-independent within our data analysis. Finally, the bending fundamental of CO2 can also relax to the ground state by resonantly transferring the remaining excess energy to the librational fundamentals of the solvent.

Raman spectra and non-empirical calculations of dimethylformamide molecular clusters structure

The article presents the results of a study on the formation of molecular clusters of dimethylformamide. The intermolecular interactions of isolated molecular clusters have been simulated by B3LYP/(6-31G++(d,p) method. It is shown that hydrogen bonding and resonant transfer of vibrational energy play an important role in the formation of molecular clusters by dimethylformamide molecules. Methyl and aldehyde groups of a DMF molecule act as proton donors in the formation of molecular clusters, CO is a strong acceptor. It was shown that in solutions of DMF with water, an H-bond is formed between the oxygen of NCOH group and the hydrogen atom of water.

Accelerated Vibrational Energy Relaxation of Water in Alkaline Environments.

We observe that hydrated hydroxide ions introduce an additional relaxation channel for the vibrational relaxation of the OD vibrations of HDO molecules in aqueous NaOH solutions. This additional relaxation path involves resonant (Förster) vibrational energy transfer from the excited OD vibration to OH stretch vibrations of hydrated OH– complexes. This energy transfer constitutes an efficient mechanism for dissipation of the OD vibrational energy, as the accepting OH stretch vibrations show an extremely rapid subsequent relaxation with a time constant of <200 fs. We find that the Förster energy transfer is characterized by a Förster radius of 2.8 ± 0.2 Å.

Ultrafast energy relaxation dynamics of amide I vibrations coupled with protein-bound water molecules

The influence of hydration water on the vibrational energy relaxation in a protein holds the key to understand ultrafast protein dynamics, but its detection is a major challenge. Here, we report measurements on the ultrafast vibrational dynamics of amide I vibrations of proteins at the lipid membrane/H2O interface using femtosecond time-resolved sum frequency generation vibrational spectroscopy. We find that the relaxation time of the amide I mode shows a very strong dependence on the H2O exposure, but not on the D2O exposure. This observation indicates that the exposure of amide I bond to H2O opens up a resonant relaxation channel and facilitates direct resonant vibrational energy transfer from the amide I mode to the H2O bending mode. The protein backbone motions can thus be energetically coupled with protein-bound water molecules. Our findings highlight the influence of H2O on the ultrafast structure dynamics of proteins.

Efficient Vibrational Energy Transfer through Covalent Bond in Indigo Carmine Revealed by Nonlinear IR Spectroscopy.

Ultrafast vibrational relaxation and structural dynamics of indigo carmine in dimethyl sulfoxide were examined using femtosecond pump-probe infrared and two-dimensional infrared (2D IR) spectroscopies. Using the intramolecularly hydrogen-bonded C═O and delocalized C═C stretching modes as infrared probes, local structural and dynamical variations of this blue dye molecule were observed. Energy relaxation of the vibrationally excited C═O stretching mode was found to occur through covalent bond to the delocalized aromatic vibrational modes on the time scale of a few picoseconds or less. Vibrational quantum beating was observed in magic-angle pump-probe, anisotropy, and 2D IR cross-peak dynamics, showing an oscillation period of ca. 1010 fs, which corresponds to the energy difference between the C═O and C═C transition frequency (33 cm-1). This confirms a resonant vibrational energy transfer happened between the two vibrators. However, a more efficient energy-accepting mode of the excited C═O stretching was believed to be a nearby combination and/or overtone mode that is more tightly connected to the C═O species. On the structural aspect, dynamical-time-dependent 2D IR spectra reveal an insignificant inhomogeneous contribution to time-correlation relaxation for both the C═O and C═C stretching modes, which is in agreement with the generally believed structural rigidity of such conjugated molecules.

Vibrational Quantum Decoherence in Liquid Water.

Traditional descriptions of vibrational energy transfer consider a quantum oscillator interacting with a classical environment. However, a major limitation of this simplified description is the neglect of quantum decoherence induced by the different interactions between two distinct quantum states and their environment, which can strongly affect the predicted energy-transfer rate and vibrational spectra. Here, we use quantum-classical molecular dynamics simulations to determine the vibrational quantum decoherence time for an OH stretch vibration in liquid heavy water. We show that coherence is lost on a sub-100 fs time scale due to the different responses of the first shell neighbors to the ground and excited OH vibrational states. This ultrafast decoherence induces a strong homogeneous contribution to the linear infrared spectrum and suggests that resonant vibrational energy transfer in H2O may be more incoherent than previously thought.

Local orientational order in liquids revealed by resonant vibrational energy transfer.

We demonstrate that local orientational ordering in a liquid can be observed in the decay of the vibrational anisotropy caused by resonant transfer of vibrational excitations between its constituent molecules. We show that the functional form of this decay is determined by the (distribution of) angles between the vibrating bonds of the molecules between which energy transfer occurs, and that the initial drop in the decay reflects the average angle between nearest neighbors. We use this effect to observe the difference in local orientational ordering in the two hydrogen-bonded liquids ethanol and N-methylacetamide.

Vibrational Energy Relaxation of Thiocyanate Ions in Liquid-to-Supercritical Light and Heavy Water. A Fermi's Golden Rule Analysis.

The vibrational relaxation dynamics following an ultrafast nitrile stretching (ν3) excitation of thiocyanate anions dissolved in light and heavy water have been studied over a wide temperature and density range corresponding to the aqueous liquid up to the supercritical phase. In both solvents, the relaxation of the ν3 = 1 state of the anion leads to a direct recovery of the vibrational ground state and involves the resonant transfer of the excess vibrational energy onto the solvent. In light water, the energy-accepting states are provided by the bending-librational combination band (νb + νL), while in heavy water, the relaxation is thermally assisted by virtual acceptor states derived from the stretching-librational/restricted translational hot band (νS - νL,T). The relaxation rate is found to strictly obey Fermi's Golden Rule when the density of resonant solvent states is estimated from the linear infrared spectra of the solute and the pure solvents.

Resonant vibrational energy transfer in ice Ih.

Fascinating anisotropy decay experiments have recently been performed on H2O ice Ih by Timmer and Bakker [R. L. A. Timmer, and H. J. Bakker, J. Phys. Chem. A 114, 4148 (2010)]. The very fast decay (on the order of 100 fs) is indicative of resonant energy transfer between OH stretches on different molecules. Isotope dilution experiments with deuterium show a dramatic dependence on the hydrogen mole fraction, which confirms the energy transfer picture. Timmer and Bakker have interpreted the experiments with a Förster incoherent hopping model, finding that energy transfer within the first solvation shell dominates the relaxation process. We have developed a microscopic theory of vibrational spectroscopy of water and ice, and herein we use this theory to calculate the anisotropy decay in ice as a function of hydrogen mole fraction. We obtain very good agreement with experiment. Interpretation of our results shows that four nearest-neighbor acceptors dominate the energy transfer, and that while the incoherent hopping picture is qualitatively correct, vibrational energy transport is partially coherent on the relevant timescale.

Water-Assisted Vibrational Relaxation of a Metal Carbonyl Complex Studied with Ultrafast 2D-IR

Water is capable of assisting exceptionally rapid vibrational relaxation within dissolved solute species. Although ultrafast dynamics of metal carbonyl complexes have long served as models for vibrational relaxation, all reports to-date have investigated nonaqueous solutions due to the insolubility of the vast majority of metal carbonyl complexes in water. Using the water-soluble complex [RuCl(2)(CO)(3)](2), which belongs to a class known as "carbon monoxide (CO) releasing molecules" (CORM), we report the first ultrafast vibrational relaxation measurements of a metal carbonyl complex in water and compare this relaxation with polar organic solvents, namely, methanol. The vibrational relaxation, measured by two-dimensional IR (2D-IR) spectroscopy, is an order of magnitude faster in H(2)O (3.12 ± 0.29 ps) than in methanol (42.25 ± 3 ps). The accelerated relaxation times of the coupled CO units in H(2)O and D(2)O is interpreted as resulting from the enhancement of intramolecular relaxation pathways through additional coupling induced by the solvent. In addition, the vibrational lifetime shows a significant isotope dependence: in D(2)O the relaxation time is 4.27 ± 0.27 ps, a difference of roughly 30%. We interpret these measurements in terms of a nonresonant channel primarily arising from water's reorientational dynamics, which occur primarily through large angular jumps, as well as a resonant transfer of vibrational energy from the carbonyl bands to the libration-bend combination band. These measurements indicate that metal carbonyls, which are among the strongest IR transitions, are exquisitely sensitive to the presence of water and hold promise as IR analogs of EPR spin labels.

Vibrational energy transfer and anisotropy decay in liquid water: Is the Förster model valid?

Ultrafast pump-probe anisotropy experiments have been performed on liquid H(2)O and D(2)O. In both cases, the anisotropy decay is extremely fast (on the order of 100 or 200 fs) and is presumed due to resonant vibrational energy transfer. The experiments have been interpreted in terms of the Förster theory, wherein the rate constant for intermolecular hopping transport is proportional to the inverse sixth power of the distance between the vibrational chromophores. In particular, the anisotropy decay is assumed to be simply related to the survival probability as calculated with the Förster theory. While the theory fits the data well, and is a reasonable model for these systems, there are several assumptions in the theory that might be suspect for water. Using our mixed quantum/classical model for vibrational spectroscopy and dynamics in liquid water, which agrees well with anisotropy decay experiments on the pure liquids as well as H(2)O/D(2)O mixtures, we critically analyze both the survival probability and anisotropy decay, in order to assess the applicability of the Förster theory.

Hybrid quantum/classical simulation of the vibrational relaxation of the bend fundamental in liquid water

The Ehrenfest method with quantum corrections is used to describe the vibrational relaxation of the bend fundamental in liquid water. All the vibrational degrees of freedom of the water molecules are described using quantum mechanics, while the remaining translational and rotational degrees of freedom are described classically. The relaxation time obtained compares well with experiment and with relaxation times calculated using other theoretical approximations. The presence of resonant intermolecular vibrational energy (VV) transfer is established with a maximum percentage of excited molecules, different from the initial one, of 9.2%. It is found through an effective kinetic fit that two VV transfers occur before relaxation of water to the vibrational ground state.

Ultrafast intermolecular energy transfer in heavy water

We report on a study of the vibrational energy relaxation and resonant vibrational (Förster) energy transfer of the OD vibrations of D2O and mixtures of D2O and H2O using femtosecond mid-infrared spectroscopy. We observe the lifetime of the OD vibrations of bulk D2O to be 400 +/- 30 fs. The rate of the Förster energy transfer is measured via the dynamics of the anisotropy of the OD vibrational excitation. For a solution of 0.5% D2O in H2O, resonant energy transfer is negligible, and the anisotropy shows a single exponential decay with a time constant of 2.6 +/- 0.1 ps, representing the time scale of the molecular reorientation. With increasing concentration, the anisotropy decay becomes faster and non-exponential, showing the increased contribution of resonant energy transfer between the OD vibrations. We determine the Förster radius of the OD vibration of HDO in H2O to be r0 = 2.3 +/- 0.2 A.

Raman noncoincidence effect on OH stretching profiles in liquid alcohols

A detailed analysis of the temperature evolution of polarized and depolarized Raman spectra in the OH stretching region of pure liquid alcohols has been performed in order to gain more insights on the nature of the noncoincidence effect (NCE) in inhomogeneously broadened OH profiles. For 1-octanol temperature-induced difference spectra, wavenumber-dependent depolarization ratios and H-bond energy dispersion have been evaluated. The results allow for a consistent interpretation of the overall NCE in terms of distinct spectral components. In practice, it results that different classes of oscillators can be singularly characterized by specific NCEs. The major contribution to the overall NCE is experimentally separated and arises from OH oscillators embedded within the H-bonded chain structures characterized by an average H-bonding enthalpy of ca 20 kJ/mol. The study has been extended to Raman OH profiles of liquid ethanol, 2-octanol and tert-butanol to understand the effects of the hydrophilic and hydrophobic molecular moieties in determining the strength on the vibrational coupling, responsible for the resonant transfer of vibrational energy (RET), in these liquid systems. Copyright © 2005 John Wiley & Sons, Ltd.

Classical and quantum mechanical infrared echoes from resonantly coupled molecular vibrations

The nonlinear response function associated with the infrared vibrational echo is calculated for a quantum mechanical model of resonantly coupled, anharmonic oscillators at zero temperature. The classical mechanical response function is determined from the quantum response function by setting variant Planck's over 2pi-->0, permitting the comparison of the effects of resonant vibrational coupling among an arbitrary number of anharmonic oscillators on quantum and classical vibrational echoes. The quantum response function displays a time dependence that reflects both anharmonicity and resonant coupling, while the classical response function depends on anharmonicity only through a time-independent amplitude, and shows a time dependence controlled only by the resonant coupling. In addition, the classical response function grows without bound in time, a phenomenon associated with the nonlinearity of classical mechanics, and absent in quantum mechanics. This unbounded growth was previously identified in the response function for a system without resonant vibrational energy transfer, and is observed to persist in the presence of resonant coupling among vibrations. Quantitative agreement between classical and quantum response functions is limited to a time scale of duration inversely proportional to the anharmonicity.

Near-infrared spectroscopy of ethylene and ethylene dimer in superfluid helium droplets

The nu(5)+nu(9) spectra of ethylene, C(2)H(4), and its dimer, solvated in helium nanodroplets, have been recorded in the wavelength region near 1.6 microm. The monomer transitions show homogeneous broadening of approximately 0.5 cm(-1), which is interpreted as due to an upper state vibrational relaxation lifetime of approximately 10 ps. Nearly resonant vibrational energy transfer (nu(5)+nu(9)-->2nu(5)) is proposed as the relaxation pathway. The dimer gives a single unresolved absorption feature located 4 cm(-1) to the red of the monomer band origin. The scaling of moments of inertia upon solvation in helium is 1.18 for the monomer and >2.5 for the dimer. In terms of the adiabatic following approximation, this classifies the monomer as a fast rotor and the dimer as a slow rotor.

Collision-Partner Dependence of Energy Transfer between the CH A2Δ and B2Σ- States

We have investigated experimentally the collision-induced electronic energy transfer between the CH A(2)Delta and B(2)Sigma(-) states with the series of partners He, Ar, H(2), N(2), CO, and CO(2). Single rovibronic states of either of the near-degenerate levels A(2)Delta, v = 1, or B(2)Sigma(-), v = 0, were prepared by laser excitation. Collisional transfer processes were monitored by detecting dispersed, time-resolved fluorescence from the initial and product states. The microscopic rate constants for vibronically resolved transfer between the A(2)Delta and B(2)Sigma(-) states, vibrational relaxation within the A state, and total removal to unobserved final products were determined for each partner. In line with previous work, we find that only CO and H(2) are efficient at total removal of CH A(2)Delta and B(2)Sigma(-), most probably through chemical reaction. CO(2) is notably effective at A(2)Delta state vibrational relaxation, possibly through resonant vibrational energy transfer. All the partners cause transfer between CH A(2)Delta and B(2)Sigma(-). An important new observation is that their efficiencies are well correlated with the strength of long-range attractive forces, as revealed through a positive correlation of the Parmenter-Seaver type. The vibronic branching to A(2)Delta, v = 0 and 1 from B(2)Sigma(-), v = 0 is found to be significantly collision-partner-dependent and not well predicted by energy gap scaling laws. We do not find any enhanced effectiveness in B(2)Sigma(-) to A(2)Delta coupling for those partners which form strongly bound intermediates, suggesting that this specific electronic channel is controlled by different regions of the potential energy surfaces.

Relaxation processes of the liquid crystal ME6N in the isotropic phase studied by Raman scattering experiments.

We have investigated the Raman profiles of the nu(C[Triple Bond]N) and nu(C=O) vibrational modes of the nematic liquid crystal ME6N (4-cyanophenyl-4(')-hexylbenzoate) in the isotropic phase at different temperatures and used them as probes of the dynamics and structural organization of this liquid. The vibrational time correlation functions of the nu(C[Triple Bond]N) mode, rather adequately interpreted within the assumption of exponential modulation function (the Kubo-Rothschild theory), indicate that the system experiences an intermediate dynamical regime that gets only slightly faster with increasing temperature. However, this theory fails in predicting the non-exponential behavior that the time correlation functions manifest in the long time range (t>3 ps). For this reason we have additionally approached the interpretation of vibrational correlation functions in terms of the theory formulated by Rothschild and co-workers for locally structured liquids. The application of this theory reveals that the molecular dynamics in this liquid crystal in the isotropic phase is that deriving from a distribution of differently sized clusters, which narrows as the temperature increases. Even at the highest temperature reached in this study (87 degrees C above the nematic-isotropic transition), the liquid has not yet achieved the structure of the simple liquid and the dynamics has not reached the limit of the single channel process. The vibrational and orientational relaxations occur in very different time scales. The temperature independence of the orientational dynamics in the whole range from 55 degrees C to 135 degrees C has been referred to the nonhydrodynamic behavior of the system, arising when local pseudonematic structures persist for times longer than the orientational relaxation. The occurrence of the process of resonant vibrational energy transfer between the C=O groups of adjacent molecules has been revealed in the isotropic phase by a slightly positive Raman noncoincidence effect in the band associated with the nu(C=O) mode. A qualitative interpretation is tentatively given in terms of partial cancellation of contributions deriving from structures having opposite orientations of their C=O groups.

Intermolecular exchange of vibrational population in a binary liquid mixture

Using a picosecond infrared pump–probe technique, we have studied the decay of transient excess population of the CH stretch vibrational modes in binary liquid mixtures of CHBr3 and CHCl3. Analyzing the concentration dependence of the observed decay curves on the basis of a simple rate equation model gives strong evidence for an effective resonant transfer of vibrational energy between the two different types of molecules. A value of κ=[(22±5) ps]−1 is determined for the rate constant κ describing this process. This value is larger than the total relaxation rates of both neat bromoform and chloroform indicating that even there CH stretching quanta can be transferred from molecule to molecule several times before they are relaxing.

Wave Packet Study of Ultrafast Relaxation in Ice Ih and Liquid Water. Resonant Intermolecular Vibrational Energy Transfer

By using quantum chemistry, the fundamental normal modes and the frequencies of ice Ih and room-temperature water clusters containing up to 15 water molecules are constructed. These normal modes are then used on the basis of assumed harmonic dynamics for analyzing the survival probability and energy decay of wave packets which reflect symmetric and asymmetric stretch excitations in single water molecules. Following symmetric stretch excitation, it is found that the wave packet survival probability and the OH stretch mode energy both decay on a sub-100-fs time scale in both phases. For asymmetric stretch excited states, the characteristic relaxation time is below 100 fs in ice Ih, but it is slower in liquid water. In both cases, it is found that the dynamics are truly many-body in character, since clusters of size ∼15 are required to converge the early time behavior of the OH mode relaxation processes. The results support the suggestion of Woutersen and Bakker (Nature 1999, 402, 507−509) that, in liquid wa...