Vibrational Energy Relaxation Research Articles

Bottlenecks to vibrational energy flow in carbonyl sulfide: Structures and mechanisms

Finding the causes for the nonstatistical vibrational energy relaxation in the planar carbonyl sulfide (OCS) molecule is a longstanding problem in chemical physics: Not only is the relaxation incomplete long past the predicted statistical relaxation time but it also consists of a sequence of abrupt transitions between long-lived regions of localized energy modes. We report on the phase space bottlenecks responsible for this slow and uneven vibrational energy flow in this Hamiltonian system with three degrees of freedom. They belong to a particular class of two-dimensional invariant tori which are organized around elliptic periodic orbits. We relate the trapping and transition mechanisms with the linear stability of these structures.

Comparison of Two Simple Models for High Frequency Friction: Exponential versus Gaussian Wings

Liquid phase vibrational energy relaxation (VER) times T1 typically depend critically on the relaxing mode's high frequency friction or wing function. The wing function may, in principle, be found from the mode's normalized force autocorrelation function (faf) C(t), since it is proportional to lim formula see text C(t)dt. However, the full form of C(t) is never available. Thus, the wing function is typically estimated from a model faf CM(t) which duplicates the known part of C(t) and which (hopefully) approximates its unknown part with enough realism to yield meaningful formula see text behavior. Unfortunately, apparently realistic CM(t)'s can predict unphysical wing functions, and T1's in error by tens of orders of magnitude. Thus, a condition is needed to discriminate between CM(t)'s which yield meaningful and unphysical forms for the high frequency friction. This condition is shown to be that model faf's CM(t) yield physical wing functions if and only if these functions derive from the short time "heads" of the faf's. This test is applied to the model faf's Cga(t) triple bond exp[-1/2(t/tau)2] and Cse(t) triple bond sech(t/tau). These faf's cannot both be physical, since they yield incompatible Gaussian and exponential wing functions. The test accepts Cga(t) as physical. It, however, rejects Cse(t), since its "tail" lim formula see text Cse(t) = 2 exp(-t/tau) (because of its long range) dominates the wing function.

Robust control by two chirped pulse trains in the presence of decoherence

We discuss a novel method to create maximum coherence in a two-level system that is selectively excited among many having similar frequencies by making use of two specifically chirped pulses in the Raman configuration. The feasibility of the method is analyzed in the presence of decoherence caused by vibrational energy relaxation and collisional dephasing. It has been demonstrated that a high value of coherence may be sustained by the implementation of two chirped pulse trains prepared in accordance with the proposed method and having the same period as the vibrational energy relaxation time.

Mode-Specific Vibrational Energy Relaxation of Amide I′ and II′ Modes in N-Methylacetamide/Water Clusters: Intra- and Intermolecular Energy Transfer Mechanisms

The mode-specific vibrational energy relaxation of the amide I' and amide II' modes in NMA-d(1)/(D(2)O)(n) (n = 0-3) clusters were studied using the time-dependent perturbation theory at the B3LYP/aug-cc-pvdz level. The amide modes were identified for each cluster based on the potential energy distribution of each mode. The vibrational population relaxation time constants were derived for the amide I' and II' modes. Results for the amide I' mode relaxation of NMA-d(1)/(D(2)O)(3) agree well with previous experimental results. The energy relaxation pathways were identified, and both intra- and intermolecular mechanisms were found to be important. The amide II' mode was identified in the energy transfer pathways from the excited amide I' mode of NMA-d(1)/(D(2)O)(n) (n = 1-3) clusters. The modes associated with methyl group deformation were found to play a role in the mechanism of energy transfer from both excited amide I' and II' modes. The kinetics of energy flow in the cluster were examined by solving a master equation describing the vibrational energy relaxation process from excited system mode as a multistep reaction with the third order Fermi resonance parameters as the reaction rate constants. The intramolecular energy transfer mechanism was found to dominate the short time energy flow dynamics, whereas the intermolecular mechanism was found to be dominant at longer times.

Direct evidence for mode-specific vibrational energy relaxation from quantum time-dependent perturbation theory. II. The ν4 and ν7 modes of iron-protoporphyrin IX and iron porphine

The mode-specific vibrational energy relaxation (VER) of the iron-protoporphyrin IX (the heme) was studied using a non-Markovian time-dependent perturbation theory at the UB3LYP/6-31G(d) level. The derived VER time constants of the excited nu(4) and nu(7) modes, 1.2+/-0.1 and 2.1+/-0.1 ps, respectively, agree well with previous experimental results for MbCO (1.1+/-0.6 ps for the nu(4) mode and 1.9+/-0.6 ps for the nu(7) mode). The energy transfer pathways from the excited nu(4) and nu(7) modes were identified. The different symmetries of the nu(4) and nu(7) modes are reflected in distinct relaxation pathways. No direct energy transfer between the nu(4) and nu(7) modes was observed. The overtone of the approximately 350 cm(-1) iron out-of-plane gamma(7) mode was observed to be strongly coupled to the nu(7) mode and essential to its relaxation. The two isopropionate side chains of the heme were found to play an essential role in the VER mechanism for both nu(4) and nu(7) modes, providing the mode-specific level explanation to previous observations. Comparison of the results for VER in iron porphine (FeP) to results for the imidazole-ligated porphine model (FeP-Im) demonstrates that the axial Im ligand has little effect on the nu(4) or nu(7) mode relaxation processes. By considering the VER process as a multistep reaction and the third order Fermi resonance parameters the reaction rate constants, the VER kinetics of FeP was examined by solving the master equation.

Study on the Structure and Vibrational Dynamics of Functional Molecules and Molecular Clusters by Double Resonance Vibrational Spectroscopy

Abstract Structure and dynamics of gas-phase molecules and their clusters formed in a supersonic jet has been a great interest to many researchers. Studies on these simple systems provide us with the fundamental concept of many physical and chemical phenomena observed in bulk systems. These studies are also important for the verification of theoretical prediction and quantum chemical calculations. Especially molecular clusters have been thought to be a good model for molecular level understanding of intermolecular interaction and relaxation in condensed phase. However, until the beginning of 1990, studies of molecular clusters mainly concentrated on electronic spectroscopy and very few studies were reported for vibrational spectroscopy. On the other hand, vibrational spectroscopy is essential in condensed phase, and there has been a gap between cluster science and condensed phase science. In 1993 we first reported infrared (IR)–ultraviolet (UV) double resonance spectroscopy of the OH stretching vibration of phenol and its hydrated clusters in a supersonic jet. The observed vibrational spectra showed a characteristic feature of the H-bonding, that is a larger frequency shift as well as broadening of the OH stretching band, and the structures were unambiguously determined with the aid of quantum chemical calculation. It is not too much to say that the combination of double resonance vibrational spectroscopy and quantum chemical calculation has become a very popular method in cluster science. In this paper, we describe various double resonance vibrational spectroscopies in frequency domain. Typical examples of application are shown; determination of H-bonded clusters of benzonitrile, conformer selective vibrational spectroscopy for l-phenylalanine and its hydrated cluster, and study of the encapsulation structure of guest atom and molecules by functional molecules, calixarene and crown ether. Finally, time-domain studies for the dynamics of the vibrations are described. Picosecond IR–UV pump–probe spectroscopy is applied to study the vibrational energy relaxation (VER) of the OH stretching vibration of isolated aromatic molecules and the H-bonded clusters.

Direct evidence for mode-specific vibrational energy relaxation from quantum time-dependent perturbation theory. I. Five-coordinate ferrous iron porphyrin model

The time scales and mechanisms of mode-specific vibrational energy relaxation in imidazole ligated ferrous iron porphine were studied using a non-Markovian time-dependent perturbation theory and density functional theory calculation. Seven normal modes, including nu(4), nu(7), and five Fe out-of-plane modes (Fe-oop), were treated as the relaxing system mode coupled to all other modes forming the bath. The derived cooling time constants for the nu(4) and nu(7) modes agree well with the results of previous experimental studies. The pathways for energy transfer from each system mode were identified. The gamma(7) mode, associated with Fe-oop motion with frequency approximately 350 cm(-1), was observed to couple strongly through its overtone with the nu(7) porphine in-plane vibration. This suggests a possible mechanism for the excitation of the nu(7) mode, which is distinct from the direct excitation together with Fe-oop motion of the nu(4) mode. Four other Fe-oop motions were observed to couple to low frequency modes including those involving significant imidazole ligand motions. Through these couplings, excitation following ligand photodissociation may be efficiently transferred from the heme doming mode to the protein backbone motions essential to conformational changes associated with the protein's function.

Intermolecular Interaction between W(CO)6 and Alkane Molecules Probed by Ultrafast Vibrational Energy Relaxation: Anomalously Strong Interaction between W(CO)6 and Decane

The rate of vibrational energy relaxation (VER) is a valuable probe of microscopic intermolecular interaction. In the present article, the VER rate of the T(1u) CO stretch of W(CO)(6) is measured in binary solutions composed of an alkane (hexane, decane, tetradecane) and cyclohexane or CCl(4), with seven mixing volume ratios. The volume ratio dependence indicates that, among the three alkane-cyclohexane solutions, only decane shows an exceptionally strong contribution to the VER process. It is strongly suggested that decane forms a specific solute-solvent complex with W(CO)(6). The "V-shaped" dependence of the VER rate on the number of carbons in alkane solvents (Banno, M.; Sato, S.; Iwata, K.; Hamaguchi, H. Chem. Phys. Lett. 2005, 412, 464.) is explained by the solute-solvent complex model. There are no experimental results that suggest the formation of the specific solute-solvent complex in alkane-CCl(4) solutions.

Quantum and classical vibrational relaxation dynamics of N‐methylacetamide on ab initio potential energy surfaces

AbstractEmploying extensive quantum‐chemical calculations at the DFT/B3LYP and MP2 level, quartic force fields of isolated N‐methylacetamide are constructed. Taking into account 24 vibrational degrees of freedom, the model is employed to perform numerically exact vibrational configuration interaction calculations of the vibrational energy relaxation of the amide I mode. It is found that the energy transfer pathways may sensitively depend on details of the theoretical description. Moreover, the exact reference calculations were used to study the applicability and accuracy of (i) the quasiclassical trajectory method, (ii) time‐dependent second‐order perturbation theory, and (iii) the instantaneous normal mode description of frequency fluctuations. Based on the results, several strategies to describe vibrational energy relaxation in biomolecular systems are discussed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

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.

Equilibrium and mid-infrared driven vibrational dynamics of artificial hydrogen-bonded networks

Stereo-selectively synthesized 1,3-poly-alcohols are introduced as low-dimensional spectroscopic model systems for molecular dynamics in hydrogen-bonded networks. The molecular and vibrational structures of the artificial networks are studied by means of density functional theory. The flexibility of the networks and the time scales associated with hydrogen-bond breakage and formation are investigated through Langevin dynamics simulations. Experimentally, the dynamics of the polyols are explored by femtosecond mid-infrared spectroscopy in the OH-stretching spectral region. Polyols with their hydroxyl groups distributed along the hydrocarbon backbone in an all-syn configuration are highly rigid and form an extended quasi 1-dimensional hydrogen-bond wire that is stable for tens of picoseconds. The mid-infrared pump-probe data on these rigid networks exhibit biexponential kinetics. This finding supports a mechanism for vibrational energy relaxation in all-syn polyols that is mediated by hydrogen-bond dissociation within 850 fs. The hydrogen-bond wire is subsequently re-established on a time scale of about 14 ps. In contrast, poly-alcohols with their OH groups in an all-anti configuration are highly flexible and display hydrogen-bond breakage-formation on a 100 fs time scale already at thermal equilibrium. As a result the pump-probe data are mono-exponential and can be understood in terms of pure intramolecular vibrational relaxation occurring with a time constant of 1.3 ps.

Vibrational Relaxation and Dynamical Transitions in Atactic Polystyrene

Infrared bands and Raman lines recorded in the frequency domain have a counterpart in the time domain in the form of time-correlation functions, which are sensitive to molecular dynamics on the picosecond time scale. Time correlation functions and their variation with temperature have been calculated for the modes observed near 1601 and 1583 cm−1 in the infrared spectrum of atactic polystyrene. The correlation functions can be modeled by assuming that there is a fast relaxation process characterized by a single relaxation time that is inhomogeneously broadened by a slower process, also characterized by a single relaxation time. The fast modulation is of the order of 0.014 ps for both modes. Librational motions of the phenyl ring occur on the 0.01 ps time scale, suggesting that this mode of relaxation involves a rapid dephasing process. One cannot draw firm conclusions, however, because dephasing and vibrational energy relaxation mechanisms cannot be separated by studies of simple band shapes alone. A slower process, with a relaxation time of the order of 1−10 ps, inhomogeneously broadens both modes. The slower process for the 1601 cm−1 mode is relatively insensitive to temperature and mirrors the so-called “fast” transition seen in neutron diffraction studies of this polymer. The second fundamental mode, near 1583 cm−1, is also inhomogeneously broadened, but the relaxation time calculated for this mode is far more sensitive to temperature as a result of anharmonic coupling to a combination mode that is probably mediated by lattice or bath modes. This provides a path for vibrational energy relaxation and sensitivity to dynamical transitions. A change in the modulation of the 1583 cm−1 band becomes apparent about 10−20 °C below the thermally measured Tg. Relaxation times at first increase (from about 2 ps to near 10 ps) then decreases and becomes negligible at temperatures near 180 °C. These results are consistent with theories of the glass transition, which predict a dynamical transition near 1.2Tg and a crossover region between this temperature and the laboratory Tg.

Optimal coherence via chirped pulse adiabatic passage in the presence of dephasing

We propose the adiabatic passage control scheme to maximize coherence in a predetermined molecular vibrational mode via two-photon Raman transitions. We investigate vibrational energy relaxation and collisional dephasing as factors of coherence loss, and demonstrate the possibility for sustaining high value of coherence in the presence of dephasing by two chirped pulse trains. The proposed method may be used to advance imaging techniques currently applied in the coherent anti-stokes Raman scattering (CARS) microscopy.

Intramolecular vibrational energy relaxation in nitrous acid (HONO)

Intramolecular vibrational energy relaxation (IVR) in nitrous acid (HONO) is studied with the help of ultrafast two-color pump-probe spectroscopy. In a previous paper [V. Botan et al., J. Chem. Phys. 124, 234511 (2006)], it has been observed that trans-HONO cools through a cascade of overtones of one specific mode after pumping the OH stretch vibration. We had suggested that this cooling mode is the ONO bend vibration. Furthermore, molecules that have initially been excited by the OH stretch vibration of cis-HONO and then underwent isomerization follow the same relaxation pathway. In the present study, we extend the investigation of IVR of cis- and trans-HONO to the N=O stretch and HON bend spectral regions, finding further evidence that the bottleneck of trans cooling is indeed the ONO bend vibration. In combination with information on the anharmonic coupling constants of different modes, the energy relaxation dynamics preceding this cooling cascade can also be followed in unprecedented detail.

Dynamic treatment of vibrational energy relaxation in a heterogeneous and fluctuating environment

A computational approach to describe the energy relaxation of a high-frequency vibrational mode in a fluctuating heterogeneous environment is outlined. Extending previous work [H. Fujisaki, Y. Zhang, and J. E. Straub, J. Chem. Phys. 124, 144910 (2006)], second-order time-dependent perturbation theory is employed which includes the fluctuations of the parameters in the Hamiltonian within the vibrational adiabatic approximation. This means that the time-dependent vibrational frequencies along a molecular dynamics trajectory are obtained via a partial geometry optimization of the solute with fixed solvent and a subsequent normal mode calculation. Adopting the amide I mode of N-methylacetamide in heavy water as a test problem, it is shown that the inclusion of dynamic fluctuations may significantly change the vibrational energy relaxation. In particular, it is found that relaxation occurs in two phases, because for short times (approximately < 200 fs) the spectral density appears continuous due to the frequency-time uncertainty relation, while at longer times the discrete nature of the bath becomes apparent. Considering the excellent agreement between theory and experiment, it is speculated if this behavior can explain the experimentally obtained biphasic relaxation the amide I mode of N-methylacetamide.

Prevention of decoherence by two femtosecond chirped pulse trains

We study the vibrational energy relaxation (VER) and collisional dephasing as channels of coherence loss in a vibrational mode that is selectively excited using chirped pulse adiabatic passage in the Raman configuration. Based on the dressed state picture analysis we propose a method to reduce decoherence using femtosecond chirped pulse trains. When applied with a period close to the VER time, the pulse trains allow one to sustain high coherence in the selected vibrational mode.

Nitrous Oxide Vibrational Energy Relaxation Is a Probe of Interfacial Water in Lipid Bilayers

Ultrafast infrared spectroscopy of N 2O is shown to be a sensitive probe of hydrophobic and aqueous sites in lipid bilayers. Distinct rates of VER of the nu 3 antisymmetric stretching mode of N 2O can be distinguished for N 2O solvated in the acyl tail, interfacial water, and bulk water regions of hydrated dioleoylphosphatidylcholine (DOPC) bilayers. The lifetime of the interfacial N 2O population is hydration-dependent. This effect is attributed to changes in the density of intermolecular states resonant with the nu 3 band ( approximately 2230 cm (-1)) resulting from oriented interfacial water molecules near the lipid phosphate. Thus, the N 2O VER rate becomes a novel and experimentally convenient tool for reporting on the structure and dynamics of interfacial water in lipids and, potentially, in other biological systems.

Relaxation dynamics of NH stretching vibrations of 2-aminopyridine and its dimer in a supersonic beam

Picosecond time-resolved IR-UV pump-probe spectroscopy has been carried out to elucidate vibrational energy relaxation (VER) of the NH stretching vibrations of 2-aminopyridine monomer (2AP) and dimer [(2AP)(2)] in supersonic beams. In bare 2AP, intramolecular vibrational energy redistribution (IVR) of the NH vibrations is described by the two-bath mode model, in which the initial vibrational energy flows to the doorway states rapidly (6.5 ps) and then dissipates into the dense bath states with a time constant of approximately 20 ps. No clear difference was observed in the IVR lifetime between the symmetric and asymmetric NH(2) stretch modes. In (2AP)(2), IVR and vibrational predissociation (VP) were involved in VER. It was found that the rate constants of both IVR and VP of the hydrogen-bonded NH stretching vibration are larger than those of the free NH.

Ultrafast vibrational dynamics and solvation complexes of methyl acetate in methanol studied by sub‐picosecond infrared spectroscopy

AbstractThe vibrational dynamics of the CO stretching mode of methyl acetate (MA) in methanol was studied by time‐resolved infrared (IR) pump‐probe spectroscopy. The vibrational energy relaxation (VER) process includes two components with time constants of 1.3 ± 0.1 and 4.0 ± 0.2 ps. These components result from the vibrational excitations of the hydrogen‐bonding complex of MA with one methanol and the MA monomer, respectively. The difference in the VER time is explained by the increase of the vibrational density of states (VDOS) by the intermolecular hydrogen bond. The time constants and the decay‐associated spectra are almost identical between the CH3OH and the CH3OD solutions. The vibrational modes localized in the intramolecular OH/OD bond of the solvent methanol have little effect on the vibrational dynamics of MA. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Vibrational relaxation of adsorbates at semiconductor surfaces: H on Ge(100)

Infrared pump-sum frequency generation (IR pump-SFG) probe spectroscopyis used to study the vibrational energy relaxation of hydrogen adsorbed ona Ge(100) surface. We found that the symmetric Ge–H stretch mode at1992 cm−1 (υs = 1) for theGe(100)-(2 × 1):H surface has a population relaxation time(T1)of 0.70 ± 0.07 ns at a substrate temperature of 323 K which is of the same order asSi(100)-(2 × 1):H. The population of the stretch mode decays exponentially in the temperature rangetested from 273 to 400 K. The observed decay constants decrease from a value of the orderof one nanosecond to a value of the order of 100 picoseconds, when the substratetemperature is increased from 273 to 400 K. This strong temperature dependence isconsistent with a decay via a multiphonon process, which may be facilitated by the rapidincrease of the density of states associated with the low-frequency TA mode.From the evaluation of the temperature dependence we suggest that the Ge–Hstretch mode decays by simultaneously exciting three bending mode quanta at530 cm−1 and more than two bulk Ge phonons.