Vibrational Energy Relaxation Dynamics Research Articles

Energy relaxation dynamics of hydrogen-bonded OH vibration conjugated with free OH bond at an air/water interface.

Vibrational energy relaxation dynamics of the excited hydrogen-bonded (H-bonded) OH conjugated with free OH (OD) at an air/water (for both pure water and isotopically diluted water) interface are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The calculated results are compared with those of the excited H-bonded OH in bulk liquid water reported previously. In the case of pure water, the relaxation timescale (vibrational lifetime) of the excited H-bonded OH at the interface is T1 = 0.13 ps, which is slightly larger than that in the bulk (T1 = 0.11 ps). Conversely, in the case of isotopically diluted water, the relaxation timescale of T1 = 0.74 ps in the bulk decreases to T1 = 0.26 ps at the interface, suggesting that the relaxation dynamics of the H-bonded OH are strongly dependent on the surrounding H-bond environments particularly for the isotopically diluted conditions. The relaxation paths and their rates are estimated by introducing certain constraints on the vibrational modes except for the target path in the NE-AIMD simulation to decompose the total energy relaxation rate into contributions to possible relaxation pathways. It is found that the main relaxation pathway in the case of pure water is due to intermolecular OH⋯OH vibrational coupling, which is similar to the relaxation in the bulk. In the case of isotopically diluted water, the main pathway is due to intramolecular stretch and bend couplings, which show more efficient relaxation than in the bulk because of strong H-bonding interactions specific to the air/water interface.

Electron-Hole-Pair-Induced Vibrational Energy Relaxation of Rhenium Catalysts on Gold Surfaces.

A combination of time-resolved vibrational spectroscopy and density functional theory techniques have been applied to study the vibrational energy relaxation dynamics of the Re(4,4'-dicyano-2,2'-bipyridine)(CO)3Cl (Re(CO)3Cl) catalyst for CO2 to CO conversion bound to gold surfaces. The kinetics of vibrational relaxation exhibits a biexponential decay including an ultrafast initial relaxation and complete recovery of the ground vibrational state. Ab initio molecular dynamics simulations and time-dependent perturbation theory reveal the former to be due to vibrational population exchange between CO stretching modes and the latter to be a combination of intramolecular vibrational relaxation (IVR) and electron-hole pair (EHP)-induced energy transfer into the gold substrate. EHP-induced energy transfer from the Re(CO)3Cl adsorbate into the gold surface occurs on the same time scale as IVR of Re(CO)3Cl in aprotic solvents. Therefore, it is expected to be particularly relevant to understanding the reduced catalytic activity of the homogeneous catalyst when anchored to a metal surface.

Vibrational energy relaxation dynamics of diatomic molecules inside superfluid helium nanodroplets. The case of the I2 molecule.

The vibrational energy relaxation (VER) of a homonuclear diatomic molecule (X2) in a 4He superfluid nanodroplet (HeND; T = 0.37 K) was studied adapting appropriately a hybrid theoretical quantum approach recently proposed by us. In the first application the interesting case of the I2(X) molecule was examined and, as far as we know, this corresponds to the first theoretical investigation on the VER of molecules embedded in HeND. Vibrational relaxation of I2 takes place according to a cascade mechanism [sequential transitions between two consecutive vibrational levels (ν → ν - 1; ν - 1 → ν - 2; …; 2 → 1; 1 → 0), where an arbitrary relaxation, e.g., ν - 1 → ν - 2, can only occur once the previous relaxation has taken place, and so on]. The global relaxation from the initial excited state ν down to the ground state (ν = 0) happens on the nanosecond scale. Data on the VER of molecules in HeND are very scarce and a vibrational lifetime not far from the I2 one for ν = 1 has been estimated experimentally for Na3(24E') on HeND (a qualitatively similar 0 → 1 vibrational energy gap occurs in both species), but metastability has not been reported in the second case. The cascade mechanism was understood once the values of the coupling matrix elements were examined, and the time evolution of the populations of two consecutive vibrational levels was adequately described using a non-linear two-state Hamiltonian model for I2 in the HeND. According to the calculations, superfluid liquid helium is very efficient in releasing the excess of energy arising from the I2 vibrational de-excitation, as it should be. However, the number of He atoms evaporated is small compared to what is expected. We hope that this first theoretical work on the molecular VER dynamics in HeND will encourage researchers to investigate this important process about which we still know very little.

Vibrational Energy Relaxation: A Benchmark for Mixed Quantum-Classical Methods.

We investigate the ability of mixed quantum-classical methods to capture the dynamics of vibrational energy relaxation. Several methods, including surface hopping, and Ehrenfest and symmetrical quasiclassical (SQC) dynamics, are benchmarked for the exactly solvable model problem of a harmonic oscillator bilinearly coupled to a bath of harmonic oscillators. Results show that, very often, one can recover accurate vibrational relaxation rates and detailed balance using simple mixed quantum-classical approaches. A few anomalous results do appear, however, especially regarding Ehrenfest and SQC dynamics.

Communication: Probing non-equilibrium vibrational relaxation pathways of highly excited C≡N stretching modes following ultrafast back-electron transfer

Fifth-order nonlinear visible-infrared spectroscopy is used to probe coherent and incoherent vibrational energy relaxation dynamics of highly excited vibrational modes indirectly populated via ultrafast photoinduced back-electron transfer in a trinuclear cyano-bridged mixed-valence complex. The flow of excess energy deposited into four C≡N stretching (ν(CN)) modes of the molecule is monitored by performing an IR pump-probe experiment as a function of the photochemical reaction (τ(vis)). Our results provide experimental evidence that the nuclear motions of the molecule are both coherently and incoherently coupled to the electronic charge transfer process. We observe that intramolecular vibrational relaxation dynamics among the highly excited ν(CN) modes change significantly en route to equilibrium. The experiment also measures a 7 cm(-1) shift in the frequency of a ∼57 cm(-1) oscillation reflecting a modulation of the coupling between the probed high-frequency ν(CN) modes for τ(vis) < 500 fs.

OH and NH Stretching Vibrational Relaxation of Liquid Ethanolamine

Abstract Femtosecond mid-infrared pump-probe spectroscopy was carried out to obtain information about the dynamics of vibrational energy relaxation in liquid ethanolamine at room temperature and ambient pressure. Through partial deuteration it was possible to disentangle the dynamics resulting from the OH and the NH stretching modes that proceed independently and simultaneously in the hydrogen-bonded liquid following an ultrafast vibrational excitation by a resonant mid-infrared pulse. The OH-stretching vibrational lifetime was determined to be 450 fs while the NH-stretching lifetime was found to be 1.2 ps. This large difference in lifetimes highlights the importance of the hydrogen-donating and the hydrogen-accepting character of the vibrating groups that are engaged in hydrogen-bonding.

Vibrational relaxation of azide ions in liquid-to-supercritical water

The dynamics of vibrational energy relaxation (VER) of the aqueous azide anion was studied over a wide temperature (300 K ≤ T ≤ 663 K) and density (0.6 g cm(-3) ≤ ρ ≤ 1.0 g cm(-3)) range thereby covering the liquid and the supercritical phase of the water solvent. Femtosecond mid-infrared spectroscopy on the ν(3) band associated with the asymmetric stretching vibration of the azide anion was used to monitor the relaxation dynamics in a time-resolved fashion. The variation of the vibrational relaxation rate constant with temperature and density was found to be rather small. Surprisingly, the simple isolated binary collision model is able to fully reproduce the experimentally observed temperature and density dependence of the relaxation rate provided a local density correction around the vibrationally excited solute based on classical molecular dynamics simulations is used. The simulations further suggest that head-on collisions of the solvent with the terminal nitrogen atoms rather than side-on collisions with the central nitrogen atom of the azide govern the vibrational energy relaxation of this system. Finally, the importance of hydrogen bonding for the VER dynamics in this system is briefly discussed.

Remarkable Site Difference of Vibrational Energy Relaxation in Benzene Dimer: Picosecond Time‐Resolved IR–UV Pump–Probe Spectroscopy

Benzene dimer (Bz2) is one of the most fundamental molecular systems and shows intriguing properties attributed to aromaticaromatic weak interactions. The structure of Bz2 in the ground electronic state (S0) has been extensively studied experimentally and theoretically. Several experimental studies on Bz2 such as the molecular-beam electric deflection 1(b)] and Raman-UV double resonance and UV-UV hole-burning spectroscopy strongly suggested that the two benzene (Bz) molecules are not symmetrically equivalent. In addition, recent high level ab initio calculations suggested that a floppy T-shaped structure is a global minimum, although there are several possible isomers in nearly isoenergetic region. The rotational constant obtained from the Fourier-transform microwave spectrum agreed well with the predicted value for the T-shaped form. An important subject strongly related to the structure is the vibrational dynamics. In the T-shaped Bz2, the two Bz molecules are at different symmetrical conditions; one is at the Stem-site, and the other is at the Top-site. Therefore, the rate constant as well as the process of the vibrational energy relaxation of the two molecules will be different. In this sense, the study of a site-specified vibrational energy relaxation will provide us with a more detailed picture of the structure and vibrational dynamics of Bz2. Several studies on the vibrational energy relaxation of Bz2 have been reported. Lee and co-workers estimated the lifetime of the CH stretching region to be ~3 ps based on the band-width measurement of the IR spectrum. Felker and co-workers suggested that the 2 (CH stretching, Wilson’s numbering) level of the Stem-site Bz relaxes faster than that of the Top-site Bz, according to the difference in the band-width of the Raman spectra. However, in both cases the measured band-width was not that of a single rovibrational level. It is also not clear how the band-width is related to the vibrational energy relaxation dynamics, such as intramolecular (intracluster) vibrational energy redistribution (IVR) and vibrational predissociation (VP). Moreover, there has been no report on the time-resolved spectroscopy for the site-specified Bz2. Here we study the dynamics of the vibrational energy relaxation of the T-shaped Bz2 in the CH stretching energy region by using picosecond time-resolved IR-UV pump-probe spectroscopy. It is known that for the isolated Bz monomer having a D6h symmetry the IR active CH vibration (20) with the e1u symmetry is anharmonically coupled with the 8+19 and 1+6+19 Figure 1. Energy level diagram of isotope-substituted benzene dimers [h(Stem)d(Top) and d(Stem)h(Top)] and an excitation scheme. The dimers are vibrationally excited to the CH stretching region by a picosecond IR pulse, and the time evolution is probed by a picosecond UV pulse. The h*(Stem) and h*(Top) benzene components (an asterisk represents the vibrationally excited site) show different vibronic structures. Thus, the vibrationally excited dimers can be separately monitored by a tunable picosecond UV pulse. In this figure, VER means vibrational energy relaxation.

Vibrational dynamics of metal cyanides

Time-resolved IR spectroscopy has been used to characterize vibrational and rotational relaxation dynamics of the C–N stretching bands for aqueous molecular cyanides, Au ( CN ) 2 - , Ni ( CN ) 4 2 - , Pt ( CN ) 4 2 - , Co ( CN ) 6 3 - , Mn ( CN ) 6 3 - , and Ru ( CN ) 6 4 - . The spectra and dynamics of Ru ( CN ) 6 4 - resemble those previously reported for ferrocyanide with a relatively short (<10 ps) vibrational energy relaxation (VER) time. VER times are longer (>30 ps) for Au ( CN ) 2 - , Ni ( CN ) 4 2 - , Pt ( CN ) 4 2 - and Co ( CN ) 6 3 - . Mn ( CN ) 6 3 - is an intermediate case with a VER time of 15 ps in water. These VER dynamics extend and reinforce the trends for metal cyanide CN vibrational band frequencies and intensities.

ND-stretching vibrational energy relaxation of NH2D in liquid-to-supercritical ammonia studied by femtosecond midinfrared spectroscopy

Femtosecond midinfrared pump-probe spectroscopy was carried out to explore the dynamics of vibrational energy relaxation of NH(2)D in fluid ammonia NH(3). The ND-stretching fundamental of the partially deuterated solute NH(2)D was excited by femtosecond pulses centered at 2450 cm(-1), and both the ground-state bleach and the anharmonically shifted transient absorption of the same vibration was probed. The temperature of the sample was varied between 230 and 450 K, while the pressure was tuned from 10 to 1500 bar, thereby entering both the liquid and the supercritical phase of the fluid solution. The density and temperature dependence of the ND-stretching lifetime suggests that hydrogen bonding is of negligible importance for vibrational energy relaxation. Rather, the energy transfer dynamics can be understood qualitatively in terms of a simple Landau-Teller description for vibrational energy relaxation using molecular dynamics simulations to estimate the spectral density of the fluctuating forces exerted by a weakly interacting Lennard-Jones solvent (NH(3)) onto the vibrationally excited solute (NH(2)D).

Investigation of the Influence of Solute−Solvent Interactions on the Vibrational Energy Relaxation Dynamics of Large Molecules in Liquids

The influence of solute-solvent interactions on the vibrational energy relaxation dynamics of perylene and substituted perylenes in the first singlet excited-state upon excitation with moderate (<0.4 eV) excess energy has been investigated by monitoring the early narrowing of their fluorescence spectrum. This narrowing was found to occur on timescales ranging from a few hundreds of femtoseconds to a few picoseconds. Other processes, such as a partial decay of the fluorescence anisotropy and the damping of a low-frequency oscillation due to the propagation of a vibrational wavepacket, were found to take place on a very similar time scale. No significant relationship between the strength of nonspecific solute-solvent interactions and the vibrational energy relaxation dynamics of the solutes could be evidenced. On the other hand, in alcohols the spectral narrowing is faster with a solute having H-bonding sites, indicating that this specific interaction tends to favor vibrational energy relaxation. No relationship between the dynamics of spectral narrowing and macroscopic solvent properties, such as the thermal diffusivity, could be found. On the other hand, a correlation between this narrowing dynamics and the number of low-frequency modes of the solvent molecules was evidenced. All these observations cannot be discussed with a model where vibrational energy relaxation occurs via two consecutive and dynamically well-separated steps, namely ultrafast intramolecular vibrational redistribution followed by slower vibrational cooling. On the contrary, the results indicate that both intra- and intermolecular vibrational energy redistribution processes are closely entangled and occur, at least partially, on similar timescales.

Energy relaxation versus spectral diffusion of the OH-stretching vibration of HOD in liquid-to-supercritical deuterated water

The dynamics of vibrational energy relaxation (VER) of the OH-stretching vibration of HOD in liquid-to-supercritical heavy water is studied as a function of temperature and solvent density by femtosecond mid-infrared spectroscopy. Using the dielectric constant of the fluid both, the OH-stretching absorption frequency and the VER rate, can be correlated phenomenologically with the average hydrogen-bond connectivity within the random D2O network. This correlation enables the identification of thermodynamic conditions under which spectral diffusion due to hydrogen-bond breakage/formation is much faster than VER.

Surfactant Charge Effects on the Location, Vibrational Spectra, and Relaxation Dynamics of Cyanoferrates in Reverse Micelles

Ultrafast infrared spectroscopy has been used to measure vibrational energy relaxation (VER) and reorientation (Tr) times for the high frequency CN stretches of potassium ferrocyanide and ferricyanide and the NO stretch of sodium nitroprusside (SNP) in several reverse micelle (RM) systems using cationic, anionic, and nonionic surfactants. The confinement effects on anion vibrational spectra and dynamics in aqueous RMs depend on the charge of the surfactant that is used to form the RMs. Spectra and VER dynamics of ferrocyanide are not significantly altered in the limited number of RMs in which it could be solubilized. The static spectra of ferricyanide suggest an environment that is most bulklike in anionic RMs and least bulklike in cationic RMs. The dynamics of ferricyanide are slower in cationic RMs and indistinguishable from the bulk in nonionic RMs. The VER dynamics and static spectra of SNP are indistinguishable from the bulk in anionic RMs, but much slower in cationic RMs. This suggests a strong surfactant-solute repulsion in the former and an attraction in the latter. Broad static spectra and probe frequency dependent dynamics are seen for SNP in nonionic RMs, indicating an inhomogeneous distribution of environments. Similar measurements were carried out for SNP in mixtures of water and a model compound containing only the hydrophilic portion of the nonionic surfactants in which RMs are not formed. The results closely resemble those observed for SNP in nonionic RMs and provide evidence that in the latter water penetrates the interface and hydrates the ethylene oxide groups before forming a water pool. The results are consistent with the explanation that Coulombic forces determine the anion location. The anions are repelled to the interior of the water pool, which has a bulklike environment in anionic RMs, and are attracted to the interface in cationic RMs, resulting in a strong interaction with the surfactant. The solute location in the nonionic RMs depends on the hydrophilic nature of the probe, with ferrocyanide and ferricyanide being more hydrophilic than SNP. These results and the dependence on surfactant charge are similar to those reported for azide.

Vibrational energy relaxation of aqueous azide ion confined in reverse micelles

Vibrational energy relaxation (VER) times have been measured by ultrafast infrared spectroscopy near 2000 cm−1 for the antisymmetric stretching ν3 band of azide ion in water pools of nonionic reverse micelles (RM). The water pool radii were varied in the 1–3 nm range by adjusting the water-to-surfactant molar ratio, ω=[H2O]/[surfactant]. Compared to the value measured in this work for bulk water (0.81±0.06 ps), the VER decay times are about three times longer (2.5±0.2 ps) for the smallest RM studied (ω=1) and become shorter with increasing ω and RM size but do not reach the bulk value at the largest ω studied. Solvent shifts of the azide vibrational band in RMs have been previously reported [Langmuir 18, 7401 (2002)], and in a manner similar to the VER rates, tend toward the bulk water value with increasing ω. Studies of the VER dynamics of azide ion in RMs are used to investigate confinement effects on solvation and to explore the effects of continuously modifying the solute–solvent interaction by varying ω in the RMs.

Microscopic description of vibrational energy relaxation in supercritical fluids: On the dominance of binary solute–solvent contributions

The representation of the frequency dependent friction coefficient ia a four-particle two-time correlation function is used to analyze the applicability of collisional and hydrodynamical models of vibrational energy relaxation (VER). The solute–solvent binary dynamics is separated from collective equilibrium correlations by means of Green's functions. The collective contributions manifest themselves mainly ia the solute–solvent radial distribution function (RDF), which reflects peculiarities of the particular solvent thermodynamical (e.g., supercritical) state. The binary dynamics is also closely related to many-body equilibrium correlations, as initial conditions sample microscopic system states in the vicinity of the solute which are the most important for VER. VER rates along a close to critical isotherm are calculated on the basis of the breathing sphere model and the Douglas approximation for force–time correlation functions, while Monte Carlo simulations are used for calculating RDFs. The results are compared with molecular dynamics simulations at low, intermediate and high densities. It is shown that at near-critical conditions as well as far from the critical point the key contribution to VER comes from the short and intermediate time behavior of the force–time correlation function. In configuration space only short range binary solute–solvent correlations are important. Analytical estimations, Monte Carlo and molecular dynamics simulations clearly show that the dynamics of VER can only be understood on the basis of a detailed description of local solute–solvent interactions and correlations.

Three-Dimensional Spectroscopy of Vibrational Energy Relaxation in Liquid Methanol

Vibrational energy relaxation (VER) of neat methanol at ambient temperature is studied with a three-dimensional vibrational spectroscopy technique. The first two dimensions are represented by a time series of incoherent anti-Stokes Raman spectra at a given mid-IR pump frequency. The third dimension involves changing the mid-IR pump frequency within the manifold of CH and OH stretching transitions ν(CH) and ν(OH). Each 2D representation shows the VER dynamics occurring as a result of pumping a specific vibrational transition. The decay of the pumped transition into first-generation daughters, and the subsequent decay into second-, and even third-generation daughters can be monitored. The third dimension shows how VER depends on the nature of the pumped transition. Additional information about the buildup of excitation in the bath of lower-energy collective excitations (phonons) is obtained by monitoring the heating of CCl4 spectators in methanol−CCl4 mixtures. Three distinct stages are seen in VER of ν(CH). The pumped ν(CH) creates two quanta of bending vibrations δ(CH) or δ(OH) (0−1 ps), the δ(CH) vibrations preferentially create CO stretch ν(CO), and the δ(OH) create methyl rock ρ(CH3) (2−5 ps), and then ρ(CH3) and ν(CO) decay into phonons (5−15 ps). Three stages are also seen in ν(OH) VER except that the first stage involves exciting every other vibration to some extent. About 10% of ν(OH) decay occurs in four stages, where the first stage is ν(OH) → ν(CH). The IR−Raman 3D technique is compared to other multidimensional vibrational spectroscopies. This 3D method is presently the most powerful technique for studying VER in condensed phases. It is sensitive to overtone and combination transitions buried under more intense fundamental absorption spectra. In addition it is sensitive to very weak cascaded anharmonic couplings responsible for the excitation of successive generations of daughter vibrations.

Vibrational energy relaxation dynamics of SH stretching modes on stepped H/Si(111)1x1 surfaces

The vibrational energy relaxation rates of excited SiH stretching modes on the monohydride step of miscut H/Si(111)1x1 surfaces are calculated using Bloch-Redfield theory combined with classical molecular dynamics (MD) simulation. The structure and vibrational frequencies of the surface are first investigated using the Car-Parrinello ab initio MD method. The calculated SiSiH bending frequencies and relaxed structures are then used to refine the empirical potential for the classical MD simulations. The lifetime of the excited SiH stretching mode at the step is found to be shorter than the modes on the rerrace. Both the magnitude and the trend of the calculated results agree well with the experimental measurement on the 9° monohydride stepped surface. The vibrational relaxation rate of the SiH stretching modes on the 15° monohydride stepped surface are also calculated and predicted to have a slightly shorter lifetime than for the 9° surface.

Vibrational energy relaxation dynamics of SiH stretching modes on stepped H/Si(111) 1×1 surfaces

Abstract The vibrational energy relaxation rates of excited SiH stretching modes on the monohydride steps of miscut H/Si(111) 1×1 surfaces are calculated using Bloch-Redfield theory combined with classical molecular dynamics (MD) simulation. The structure and vibrational frequencies of the surface are first investigated using the Car-Parrinello ab initio MD method. The calculated SiSiH bending frequencies and relaxed structures are then used to refine the empirical potential for the classical MD simulations. The lifetime of the excited SiH stretching mode at the step is found to be shorter than the modes on the terrace. Both the magnitude and the trend of the calculated results agree well with the experimental measurement on the 9° monohydride stepped surface. The vibrational relaxation rate of the SiH stretching modes on the 15° monohydride stepped surface are also calculated and predicted to have a slightly shorter lifetime than for the 9° surface.

Vibrational energy relaxation dynamics of C–H stretching modes on the hydrogen-terminated H/C(111)1×1 surface

The vibrational energy relaxation rate of an excited C–H stretching mode on the hydrogen-terminated H/C(111)1×1 surface is calculated using Bloch–Redfield theory combined with classical molecular dynamics. The lifetime of an excited state is determined by the strength of the power spectrum of the force on the stretching mode at the resonance frequency. The lifetime of the first excited state is found to be 60 ps at 300 K which is shorter than the Si–H stretching mode lifetime on the H/Si(111)1×1 surface. The lifetime of the v=2 first overtone state is found to be 200 times shorter (0.30 ps). Analysis of the power spectrum of the fluctuating force along the C–H bond suggests that the mechanism of the energy relaxation for the v=1 stretching state on the H/C(111)1×1 surface is due to lower-order interactions than on the H/Si(111)1×1 surface. The predicted fast relaxation of the overtone state may cast some doubt on the observability of that state.

Dynamics of liquid state chemical reactions: Vibrational energy relaxation of molecular iodine in liquid solution

The dynamics of vibrational energy relaxation of highly excited molecular iodine in three monatomic solvents is studied via stochastic classical trajectory simulations based on the molecular timescale generalized Langevin equation (MTGLE) of motion for liquid state chemical reactions [S. A. Adelman, J. Chem. Phys. 73, 3145 (1980)]. Also presented for comparison purposes are parallel studies based on a matrix Langevin equation of motion characterized by friction coefficients which depend on the instantaneous I2 internuclear separation R. The qualitative features of the energy relaxation may be interpreted as effects arising from modifications of the solute dynamics due to molecular timescale correlations between its motion and that of its solvation shells. Such dynamical solvent effects are realistically described by the MTGLE equation of motion but not by the Langevin equation. Thus, for example, the marked slowdown of the rate of I2 energy relaxtion in simple solvents when the I2 vibrational quantum number drops below a solvent-dependent critical value, earlier predicted by Nesbitt and Hynes, is predicted by MTGLE dynamics but not by Langevin dynamics. Finally, practical algorithms for numerically constructing the MTGLE and Langevin equations for specific solute–solvent systems are presented.