Vibrational Population Relaxation Research Articles

On the role of dielectric friction in vibrational energy relaxation

The phrase “dielectric friction” tends to bring to mind the drag force exerted by a polar liquid on some translating ion or rotating dipolar molecule, but the underlying idea is far more general. Any relaxation process taking place in a polar environment, including those involving solvation and vibrational relaxation, has the potential to be strongly affected by the special dynamics associated with Coulombic forces. Indeed, there is considerable evidence that vibrational energy relaxation is noticeably accelerated in hydrogen-bonding solvents. What is less clear is precisely how electrostatic forces achieve the accelerations they do and to what extent this phenomenon relies on specifically protic solvents. We explore this issue in this paper by using classical molecular dynamics to study the vibrational population relaxation of diatomic solutes with varying levels of polarity dissolved in both dipolar and nondipolar aprotic solvents. We find that the conventional analysis based on partitioning the force autocorrelation function can be usefully extended by adapting an instantaneous perspective; distinguishing between the purely equilibrium effects of the instantaneous liquid structure surrounding a solute and the solely nonequilibrium effects of the relaxation dynamics launched from those initial conditions. Once one removes the powerful influence of electrostatic forces on the liquid structure, either by simple normalization or by looking at the “force-velocity” autocorrelation function, the subsequent dynamics (and therefore the mechanism) of the relaxation is revealed to be dominated by short-ranged repulsive forces, even under the most polar circumstances. The main rate-enhancing effect of Coulombic forces seems to be an equilibrium electrostriction: The solvent is simply ordered around the solute in such a way as to amplify the repulsive forces. At least in our examples, the slowly varying character of Colombic forces actually makes them quite ineffective at any kind of direct promotion of vibrational energy relaxation.

A Study of Vibrational Relaxation of B-State Carbon Monoxide in the Heme Pocket of Photolyzed Carboxymyoglobin

The vibrational energy relaxation of dissociated carbon monoxide in the heme pocket of sperm whale myoglobin has been studied using equilibrium molecular dynamics simulation and normal mode analysis methods. Molecular dynamics trajectories of solvated myoglobin were run at 300K for both the δ- and ∈-tautomers of the distal histidine, His64. Vibrational population relaxation times were estimated using the Landau–Teller model. For carbon monoxide (CO) in the myoglobin ∈-tautomer, for a frequency of ω0=2131cm−1 corresponding to the B1 state, T1∈(B1)=640±185 ps, and for a frequency of ω0=2119cm−1 corresponding to the B2 state, T1∈(B2)=590±175 ps. Although the CO relaxation rates in both the ∈- and δ-tautomers are similar in magnitude, the simulations predict that the vibrational relaxation of the CO is faster in the δ-tautomer. For CO in the myoglobin δ-tautomer, it was found that the relaxation times were identical within error for the two CO substate frequencies, T1δ(B1)=335±115 ps and T1δ(B2)=330±145 ps. These simulation results are in reasonable agreement with experimental results of Anfinrud and coworkers (unpublished results). Normal mode calculations were used to identify the dominant coupling between the protein and CO molecules. The calculations suggest that the residues of the myoglobin pocket, acting as a first solvation shell to the CO molecule, contribute the primary “doorway” modes in the vibrational relaxation of the oscillator.

Orientational and Vibrational Relaxation Dynamics of Perylene and 1-Methylperylene in n-Alcohols: Probing the Balance between van der Waals and Hydrogen-Bonding Interactions

We report on the rotational diffusion and vibrational population relaxation dynamics of perylene and 1-methylperylene in the primary normal alcohols methanol through n-decanol. The rotational diffusion dynamics of the two chromophores are the same to within the experimental uncertainty for n-propanol through n-decanol. We observe a double exponential decay of the induced orientational anisotropy for 1-methylperylene in all of the alcohols and for perylene in n-propanol through n-decanol. These data are consistent with previous literature reports on perylene in high-viscosity solvents and on 1-methylperylene in n-alkanes longer than n-octane. These data also reveal a substantial difference in the behavior of perylene in n-alkanes and n-alkanols. Both chromophores reorient as oblate rotors in the n-alkanols, with the aspect ratio of the oblate ellipsoid describing their motion depending on the solvent aliphatic chain length. The vibrational population relaxation dynamics of the two probe molecules differ significantly, not only because of the difference in the nature of the solvent−solute coupling but also because of subtle differences in the organization of the solvent around the two chromophores. These data reflect the importance of solvent self-association in determining the local environments of these two chromophores.

Short-Time Dynamics of Vibrational Relaxation in Molecular Fluids

Modern approaches to vibrational relaxation in liquids have begun to move beyond the simple question of how fast vibrational population relaxation occurs to the more challenging question of how it occurs at all: the precise molecular mechanisms by which a solvent stimulates the loss of a vibrational quantum from a solute. We report here some progress in understanding these mechanisms based on looking at the dynamics of the initial triggering events in the vibrational relaxation seen in molecular fluids. With the aid of instantaneous-normal-mode analysis we find a remarkable similarity between vibrational relaxation and the dynamics of solvation. The key concept, in both cases, is that the polarity and general behavior of the solvent is far less important in determining the relevant mechanism than is the particular force or potential monitored by the relevant experiment (“the spectroscopic probe potential”). Vibrational population relaxation automatically accesses the force on a bond, a quantity sufficien...

The short-time intramolecular dynamics of solutes in liquids. II. Vibrational population relaxation

Events such as the vibrational relaxation of a solute are often well described by writing an effective equation of motion—a generalized Langevin equation—which expresses the surrounding medium’s influence on the intramolecular dynamics in terms of a friction and a fluctuating force acting on the solute. These quantities, though, can be obtained from the instantaneous normal modes (INMs) of the system when the relaxation takes place in a fluid, suggesting that we should be able to analyze in some detail the solvent motions driving the relaxation, at least for short times. In this paper we show that this promise can indeed be realized for the specific case of a vibrating diatomic molecule dissolved in an atomic solvent. Despite the relatively long times typical of vibrational population relaxation, it turns out that understanding the behavior of the vibrational friction at the short times appropriate to INMs (a few hundred femtoseconds) often suffices to predict T1 times. We use this observation to probe the dependence of these relaxation rates on thermodynamic conditions and to look at the molecular mechanisms underlying the process. We find that raising the temperature at any given density or raising the density at any given temperature will invariably increase the rate of energy relaxation. However, since these two trends may be in conflict in a typical constant-pressure laboratory experiment, we also find that it is possible to make sense of the “anomalous” inverted temperature dependence recently seen experimentally. We find, as well, that the INM theory—which has no explicit collisions built into it—predicts exactly the same density dependence as the venerable independent-binary-collision (IBC) theory (an intriguing result in view of recent claims that experimental observations of this kind of dependence provide support for the IBC theory). The actual mechanisms behind vibrational population relaxation are revealed by looking in detail at the vibrational friction “influence spectrum”—the spectrum of INMs weighted by how efficiently each mode acts to promote the relaxation. Through suitable projections we show that the average influence spectrum is dominated by longitudinal motion of the solvent atoms in the first solvation shell. We go further, however, and examine the nature of the instantaneous relaxation promoted by individual liquid configurations. The number of instantaneous modes that contribute significantly fluctuates strongly from configuration to configuration, and the number of solvent atoms strongly coupled to the solute has a certain amount of variation as well, but invariably each significant mode ends up promoting the relaxation by moving just one or two significant solvent atoms—a feature we explore in a companion paper.

Instantaneous perspectives on solute relaxation in fluids: The common origins of nonpolar solvation dynamics and vibrational population relaxation

The basic idea that the instantaneous normal modes of a fluid govern its short-time dynamics has recently been used to arrive at theories for solvation dynamics and for vibrational population relaxation, theories not quite as distinct as one might have guessed for such different-looking relaxation processes. Both theories, in particular, revolve around the weighted spectra of instantaneous normal modes we call the influence spectra, with the distinctions between the different problems showing up largely in the different weightings. We show in this paper that the influence spectra reveal a surprising amount of commonality in these two processes. For the models we consider, involving an atomic solvent and relatively short-ranged intermolecular forces, the two kinds of averaged influence spectra have virtually identical shapes. Moreover, examining a single configuration of the fluid at a time reveals that both spectra are strongly inhomogeneously broadened—that is, relatively few modes contribute at any instant, despite the breadth of the configurationally averaged spectra. What is apparently responsible for this common behavior is yet a deeper similarity. If one focuses specifically on the contributing modes, it becomes clear that the reason they contribute is their ability to move one or two solvent atoms in the immediate vicinity of the solute. This observation implies that it should always be possible for us to construct a set of effective modes involving motions that would be no more elaborate than few-body vibrations but that would still allow us to predict the influence spectra. We demonstrate just such predictions in this paper, using the one or two simple binary modes which vibrate the solute against its nearest-neighbor solvent atom. Binary modes as a class account for no more than the highest 10% of the instantaneous-normal-mode frequencies, yet we find that the solute–solvent binary modes are not only responsible for all of the high frequency aspects of solvation dynamics and vibrational population relaxation, they account in a quantitative sense for the majority of both influence spectra. At least in these examples, the bulk of the mechanism by which short-time relaxation takes place is evidently no more complicated than pair motions—what the rest of the solvent decides is how and when these motions take place.

Vibrational Relaxation of Liquid Chloroform

The state of information about vibrational population relaxation in the liquid state is still rather scarce. By using incoherent anti-Stokes scattering after strong infrared excitation, it is now possible to obtain rather complete information about these relaxation processes. In this paper we will report a study on the chloroform molecule. As all vibrations of this molecule are Raman active, they can be investigated by this experimental approach. Population lifetimes between 23 and several hundred ps are found. This is, to our knowledge, the first measurement displaying simultaneously the time evolution of all fundamentals of a molecule after direct population of a vibrational state.

T1Relaxation of Perylene in Fluid Ethane: Pressure-Dependent Changes in Short-Range Organization

We report our measurements of vibrational population relaxation of the v7(1375 cm−1) mode of perylene in ethane at a series of fluid pressures. The data show that T1decreases with increasing fluid pressure and that the dependence of T1on pressure is not consistent with predictions based on simple spatial or stoichiometric considerations. The data indicate a nonrandom orientational distribution of the solvent molecules about the solute where the portion of the solvent environment sensed by the perylene v1coordinate changes from one ordered state to another ordered state with increasing pressure.

A detailed examination of stimulated pump-probe measurements of vibrational population relaxation

We have developed and demonstrated a pump-probe laser spectroscopic scheme to measure ground state vibrational population relaxation. There are two significant advantages to this technique over alternative measurement methods. The first is that the detection sensitivity of the pump-probe method is limited by the shot noise present on the probe laser and the second is that, because of the manner in which the ground state vibration(s) are populated, it is possible to interrogate T1 processes for degenerate donor–acceptor conditions. We detail in this article the form of the experimental stimulated signal, the effect of the detection scheme used, and the consequences of spontaneous (Boltzmann) population of ground state vibrations. A central conclusion of this work is that, even for small signal limit pump-probe experiments, absorption, stimulated emission, and spontaneous emission all play important roles in collectively determining the form of the experimental signal.

Solvent Methyl Group Density Dependence of Vibrational Population Relaxation in 1-Methylperylene: Evidence for Short-Range Organization in Branched Alkanes

We report on the vibrational population relaxation and rotational diffusion dynamics of 1-methylperylene in a series of branched alkanes with the common formula C7H16. Using the 1370 cm-1 ring breathing resonance of 1-methylperylene as the donor state and the 1378 cm-1 methyl rocking resonance of the solvents as the acceptor state, we have found that the T1 times of 1-methylperylene vary from 10 to 24 ps and the rotational diffusion times in these same solvents range from 10 to 23 ps, but the two relaxation time constants do not correlate directly. For these systems, the time constant for vibrational population relaxation, T1, is directly proportional to solvent CH3 group density, counter to the expected behavior for a statistical orientational distribution of solvent molecules about the solute. The experimental T1 times are also inversely proportional to the boiling point of the solvent, indicating that the ability of the solvent to form organized assemblies around the solute determines the coupling betw...

An Experimental Examination of the Competition between Polar Coupling and Local Organization in Determining Vibrational Population Relaxation

We report on the vibrational population relaxation and rotational diffusion dynamics of perylene and 1-methylperylene in benzene and toluene. For these experiments, the naphthalene-like ring distortion modes of both perylene (1375 cm-1) and 1-methylperylene (1370 cm-1) were excited selectively using a stimulated emission population scheme, and the efficiency of depopulation of these modes was measured in each solvent. For these systems, there is an IR-active acceptor vibrational mode at the same frequency and the order of the dominant intermolecular vibrational energy transfer process depends on the identity of the solute. For 1-methylperylene, dipole−dipole coupling is operative, and for perylene, quadrupole−dipole coupling mediates the transfer. The importance of solvent organization about the solute can be evaluated by comparing solute T1 times in the two solvents. In the limit of fast solvent cage exchange, the solute T1 relaxation times should increase with increasing order of the polar coupling process and decrease with increasing intermolecular alignment. Our experimental data indicate that the T1 times are similar for all systems studied, implying the importance of persistent intermolecular interactions in determining the efficiency of vibrational population exchange in solution.

Nonreactive Dynamics in Solution: The Emerging Molecular View of Solvation Dynamics and Vibrational Relaxation

Two of the more fundamental ways in which molecules change their behavior when they are dissolved are that they can begin to exchange energy with the surrounding liquid and they can induce their surroundings to rearrange so as to provide a significant stabilizing influence. The first of these is typified by the process of vibrational population relaxation of a vibrationally hot species. The second conceptcritical to solution chemistryis what is known as solvation. Both of these processes are sufficiently fundamental that one would really like to know, at the most mechanical and molecular level possible, just what events are required in order to make them happen. But how difficult is it going to be to extract such molecular detail from the complicated many-body dynamics? The most microscopic level of understanding one could ever hope to possess might seem far removed from the finely detailed dynamical information which is available routinely for individual isolated molecules and for molecule−molecule colli...

Vibrational Population Relaxation of Tetracene in n-Alkanes. Evidence for Short-Range Molecular Alignment

Vibrational Population Relaxation of Tetracene in n-Alkanes. Evidence for Short-Range Molecular Alignment

Vibrational Population and Orientational Relaxation Dynamics of 1-Methylperylene in n-Alkanes. The Effective Range of Dipolar Energy Relaxation in Solution

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTVibrational Population and Orientational Relaxation Dynamics of 1-Methylperylene in n-Alkanes. The Effective Range of Dipolar Energy Relaxation in SolutionY. Jiang and G. J. BlanchardCite this: J. Phys. Chem. 1995, 99, 20, 7904–7912Publication Date (Print):May 1, 1995Publication History Published online1 May 2002Published inissue 1 May 1995https://doi.org/10.1021/j100020a012RIGHTS & PERMISSIONSArticle Views92Altmetric-Citations35LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts

Vibrational spectral diffusion and population dynamics in a glass-forming liquid: Variable bandwidth picosecond infrared spectroscopy

The temperature-dependent vibrational population dynamics and spectral diffusion of the CO stretching mode of tungsten hexacarbonyl in 2-methylpentane are observed from the room temperature liquid to the low temperature glass using picosecond infrared transient grating and pump–probe experiments. These experiments were performed between 10 and 300 K on the triply degenerate T1u asymmetric CO stretching mode at 1984 cm−1 using pulses with bandwidths narrower and wider than the absorption bandwidth of the transition. The rate of vibrational population relaxation (100≤T1<150 ps) is observed to decrease with increasing temperature. The orientational dynamics for this transition are observed on a faster time scale than the population relaxation. Although the liquid viscosity changes over 14 orders of magnitude, the orientational relaxation rate slows by less than one order of magnitude over the full temperature range. By comparing polarization-dependent experiments performed with both narrow and broad bandwidth transform-limited pulses, it is possible to measure temperature-dependent spectral diffusion in both the liquid and the glass. The spectral diffusion and the orientational relaxation are shown to be intimately related. It is proposed that both arise from the time evolution of the superposition of the three degenerate states created by the excitation pulse.

Vibrational Population Relaxation of Perylene in Its Ground and Excited Electronic States

Vibrational Population Relaxation of Perylene in Its Ground and Excited Electronic States

Vibrational Population Relaxation of Perylene in n-Alkanes. The Role of Solvent Local Structure in Long-Range Vibrational Energy Transfer

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTVibrational Population Relaxation of Perylene in n-Alkanes. The Role of Solvent Local Structure in Long-Range Vibrational Energy TransferY. Jiang and G. J. BlanchardCite this: J. Phys. Chem. 1994, 98, 38, 9411–9416Publication Date (Print):September 1, 1994Publication History Published online1 May 2002Published inissue 1 September 1994https://doi.org/10.1021/j100089a010RIGHTS & PERMISSIONSArticle Views166Altmetric-Citations36LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (782 KB) Get e-Alerts

ChemInform Abstract: Ultrafast Stimulated Emission Spectroscopy of Perylene in Dilute Solution: Measurement of Ground State Vibrational Population Relaxation.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Picosecond infrared vibrational photon echoes in a liquid and glass using a free electron laser.

The first infrared vibrational photon echo experiments conducted in a liquid and a glass are reported. The experiments were performed on the CO stretching mode of tungsten hexacarbonyl at 5.1 \ensuremath{\mu}m (1960 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) in 2-methyltetrahydrofuran over the temperature range 300 to 16 K using picosecond pulses from the free electron laser at Stanford University. In addition, the first vibrational population relaxation measurements spanning a temperature range that takes a system from a liquid to a supercooled liquid to a glass are reported.

Ultrafast stimulated emission spectroscopy of perylene in dilute solution: Measurement of ground state vibrational population relaxation

We report ultrafast stimulated emission measurements of perylene in a series of polar and nonpolar solvents. In all solvents the perylene stimulated emission spectra evolve in time. We observe individual features corresponding to distinct vibronic resonances in the stimulated emission spectra. The intensities of these features increase subsequent to excitation and persist for hundreds of picoseconds. The fast build-up seen at short delay times is related directly to the vibrational population relaxation time, T1 , of the ground vibrational state that is the lower energy state of the stimulated transition. The measured T1 times for perylene vary with both ground vibrational state and solvent. The slow decay rates for these data, the sum of the stimulated and spontaneous decay rates for the particular transition, depend critically on the particular transition that is resonant with the probe laser electric field.