Measurements Of Vibrational Relaxation Research Articles

Multicolor phase-correlation interferometer for shock wave refractive index measurements.

Refractive index measurements are critical for characterizing the properties of hypersonic flows, but moderate- to high-pressure experiments require alternative methods to traditional interferometric fringe counting. In this work, we introduce a novel, to the best of our knowledge, multi-wavelength phase-correlation interferometric technique to estimate the refractive index changes across nearly discrete shock wave boundaries and also simultaneously capture optical dispersion and vibrational relaxation times. By comparing the interference pattern of three or more wavelengths against each other, the refractive index can be accurately determined. To demonstrate this technique, laser diodes in two wavelength combinations are tested producing refractive index resolutions on the order of 2.65 × 10-7. Results in air across a range of initial pressure conditions (P1 = 2.66 to 5.33 kPa) and incident wave speeds (Mach 2 to 5) show density changes that agree with theoretical estimates within 2%. Single-shot dispersion and vibrational relaxation measurements with this method also illustrate good agreement with other techniques.

Non-negligible Axial Ligand Effect on Electrocatalytic CO2 Reduction with Iron Porphyrin Complexes

Iron(III) porphyrin complexes have been demonstrated as one of the efficient molecular catalysts for the electrochemical reduction of CO2. However, the role of axial ligands coordinated with a metal center in the complex on the electrochemical CO2 reduction activity has not been fully explored yet. Herein, iron(III) tetraphenylporphyrin thiocyanate (FeTPP-SCN) is synthesized from a commercially available catalyst of FeTPP-Cl by a counteranion exchanging reaction. Cyclic voltammetry measurements showed that the catalytic activity of FeTPP-SCN is noticeably suppressed in the DMF solutions. The structural dynamics of the axial ligand in FeTPP-SCN are further examined by the FTIR and ultrafast IR spectroscopies, where the SCN ligand is employed as the local vibrational probe. Vibrational relaxation measurements showed that the reorientational dynamics of SCN ligands was strongly restricted in DMF solution, suggesting that the subtle electrostatic interaction between the ligands and metal center in the complex can have a non-negligible effect on its catalytic activity.

Identical Water Dynamics in Acrylamide Hydrogels, Polymers, and Monomers in Solution: Ultrafast IR Spectroscopy and Molecular Dynamics Simulations.

The dynamics and structure of water in polyacrylamide hydrogels (PAAm-HG), polyacrylamide, and acrylamide solutions are investigated using ultrafast infrared experiments on the OD stretch of dilute HOD/H2O and molecular dynamics simulations. The amide moiety of the monomer/polymers interacts strongly with water through hydrogen bonding (H-bonding). The FT-IR spectra of the three systems indicate that the range of H-bond strengths is relatively unchanged from bulk water. Vibrational population relaxation measurements show that the amide/water H-bonds are somewhat weaker but fall within the range of water/water H-bond strengths. A previous study of water dynamics in PAAm-HG suggested that the slowing observed was due to increasing confinement with concentration. Here, for the same concentrations of the amide moiety, the experimental results demonstrate that the reorientational dynamics (infrared pump-probe experiments) and structural dynamics (two-dimensional infrared spectroscopy) are identical in the three acrylamide systems studied. Molecular dynamics simulations of the water orientational relaxation in aqueous solutions of the acrylamide monomer, trimer, and pentamer are in good agreement with the experimental results and are essentially chain length independent. The simulations show that there is a slower, low-amplitude (<7%) decay component not accessible by the experiments. The simulations examine the dynamics and structure of water H-bonded to acrylamide, in the first solvent shell, and beyond for acrylamide monomers and short chains. The experiments and simulations show that the slowing of water dynamics in PAAm-HG is not caused by confinement in the polymer network but rather by interactions with individual acrylamide moieties.

Vibrational Relaxation Dynamics of a Semiconductor Copper(I) Thiocyanate (CuSCN) Film as a Hole-Transporting Layer.

The semiconductor CuSCN film, which is typically used as the hole-transporting layer (HTL) in solar cell studies, has been investigated by Fourier transform infrared (FTIR) spectroscopy and ultrafast transient infrared (IR) spectroscopy. A sharp peak at 2175 cm-1 corresponding to the CN vibrational stretching mode in CuSCN was observed, and the peak frequency remained unchanged by varying the thickness of the CuSCN thin film. Vibrational relaxation measurements showed that the 0-1 and 1-2 transitions of CN stretching can be observed at 2175 and 2140 cm-1, respectively. The heat-induced absorption and bleaching peaks (2167 and 2175 cm-1) can be clearly seen at a waiting time of 40 ps. The vibrational relaxation of the CN stretching mode determined from the 1-2 transition exhibited a biexponential decay with time constants of 7.4 ± 0.5 (90%) and 158 ± 50 ps (10%). Importantly, the abnormal anisotropy decay of the CN stretching mode in the CuSCN thin film was also observed for the first time. A detailed analysis showed that the distinct anisotropy decay curve could be described using a triexponential decay function, which was explained by three different processes: resonance energy transfer (∼8 ps), a thermalization process (∼40 ps), and molecular rotation (∼150 ps). The time scale of the thermalization process caused by the vibrational relaxation in CuSCN is at a time scale of 40 ps, which is important for us to understand the thermally activated charge-transport property of the CuSCN film employed as the HTL. Further UV pump-IR probe measurement revealed that the carrier scattering and relaxation processes in the CuSCN film are strongly associated with the vibrational excitation and relaxation dynamics of the CN stretching mode. It is expected that the fundamental understanding of the vibrational relaxation dynamics of the CuSCN thin film should provide helpful insight to elucidate its role as the HTL in solar cell studies at the molecular level.

Vibrational Relaxation in EDTA Is Ion-Dependent.

Ion binding by carboxylate groups is common in biomolecules such as metalloproteins, but dynamical aspects of ion binding are not fully understood. We present ultrafast spectroscopic measurements of vibrational relaxation in the ion-coordinating carboxylate groups of EDTA, which we use as a model of carboxylate-mediated ion binding, as EDTA binds a series of divalent and trivalent metal ions with high affinity. The measurements are interpreted using a Redfield-based anharmonic model of vibrational relaxation that rationalizes trends in vibrational lifetimes in terms of vibrational energy transfer between EDTA's asymmetric carboxylate stretching vibrational modes and lower-lying modes. Results show ion-dependent changes in complex structure and dynamics well outside the temporal and spatial resolution of common structural methods and demonstrate how vibrational relaxation measurements may contribute to exploration of ion-binding dynamics on ultrashort length and time scales.

Modeling of Vibration-Vibration Energy Transfer in the DSMC Method

Modeling of vibration-vibration (V-V) energy transfer in collisions of diatomic anharmonic oscillators is discussed, applicable for the direct simulation Monte Carlo method. An attempt is made to take into account the fast vibrational relaxation of nitrogen through V-V exchange with oxygen by increasing the vibration-translation relaxation rate by a constant factor. The results have shown that such an approach still significantly overpredicts the available relaxation times. Then, a new approach to account for V-V exchanges is proposed, which incorporates the calculation of near-resonant transitions according to a temperature-dependent probability calibrated with available experimental data. That approach was found to provide quantitative agreement with shock tube measurements of vibrational relaxation in mixtures. While targeting primarily high-temperature air flows, the approach is applicable to any flow where the VV energy transfer needs to be modeled. It provides the benefits of numerical efficiency, simplicity of implementation, and acceptable physical accuracy.

2D attenuated total reflectance infrared spectroscopy reveals ultrafast vibrational dynamics of organic monolayers at metal-liquid interfaces.

We present two-dimensional infrared (2D IR) spectra of organic monolayers immobilized on thin metallic films at the solid liquid interface. The experiments are acquired under Attenuated Total Reflectance (ATR) conditions which allow a surface-sensitive measurement of spectral diffusion, sample inhomogeneity, and vibrational relaxation of the monolayers. Terminal azide functional groups are used as local probes of the environment and structural dynamics of the samples. Specifically, we investigate the influence of different alkyl chain-lengths on the ultrafast dynamics of the monolayer, revealing a smaller initial inhomogeneity and faster spectral diffusion with increasing chain-length. Furthermore, by varying the environment (i.e., in different solvents or as bare sample), we conclude that the most significant contribution to spectral diffusion stems from intra- and intermolecular dynamics within the monolayer. The obtained results demonstrate that 2D ATR IR spectroscopy is a versatile tool for measuring interfacial dynamics of adsorbed molecules.

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 overtone excitation in electron mediated energy transfer at metal surfaces

Vibrational overtone excitation is, in general, inefficiently stimulated by photons, but can under some circumstances be efficiently stimulated by electrons. Here, we demonstrate electron mediated vibrational overtone excitation in molecular collisions with a metal surface. Specifically, we report absolute vibrational excitation probabilities to ν = 1 and 2 for collisions of NO(ν = 0) with a Au(111) surface as a function of surface temperature from 300 to 985 K. In all cases, the observed populations of vibrationally excited NO are near those expected for complete thermalization with the surface, despite the fact that the scattering occurs through a direct “single bounce” mechanism of sub-ps duration. We present a state-to-state kinetic model, which accurately describes the case of near complete thermalization (a regime we call the strong coupling case) and use this model to extract state-to-state rate constants. This analysis unambiguously shows that direct vibrational overtone excitation dominates the production of ν = 2 and that, within the context of our model, the intrinsic strength of the overtone transition is of the same order as the single quantum transition, suggesting a possible way to circumvent optical selection rules in vibrational pumping of molecules. This result also suggests that previous measurements of vibrational relaxation of highly vibrationally excited NO exhibiting highly efficient multi-quantum jumps (Δν ∼ −8) are mechanistically similar to vibrational excitation of NO(ν = 0).

Shock tube study of dissociation and relaxation in 1,1-difluoroethane and vinyl fluoride

This paper reports measurements of the thermal dissociation of 1,1-difluoroethane in the shock tube. The experiments employ laser-schlieren measurements of rate for the dominant HF elimination using 10% 1,1-difluoroethane in Kr over 1500-2000 K and 43 < P < 424 torr. The vinyl fluoride product of this process then dissociates affecting the late observations. We thus include a laser schlieren study (1717-2332 K, 75 < P < 482 torr in 10 and 4% vinyl fluoride in Kr) of this dissociation. This latter work also includes a set of experiments using shock-tube time-of-flight mass spectrometry (4% vinyl fluoride in neon, 1500-1980 K, 500 < P < 1300 torr). These time-of-flight experiments confirm the theoretical expectation that the only reaction in vinyl fluoride is HF elimination. The dissociation experiments are augmented by laser schlieren measurements of vibrational relaxation (1-20% C(2)H(3)F in Kr, 415-1975 K, 5 < P < 50 torr, and 2 and 5% C(2)H(4)F(2) in Kr, 700-1350 K, 6 < P < 22 torr). These experiments exhibit very rapid relaxation, and incubation delays should be negligible in dissociation. An RRKM model of dissociation in 1,1-difluoroethane based on a G3B3 calculation of barrier and other properties fits the experiments but requires a very large DeltaE(down) of 1600 cm(-1), similar to that found in a previous examination of 1,1,1-trifluoroethane. Dissociation of vinyl fluoride is complicated by the presence of two parallel HF eliminations, both three-center and four-center. Structure calculations find nearly equal barriers for these, and TST calculations show almost identical k(infinity). An RRKM fit to the observed falloff again requires an unusually large DeltaE(down) and the experiments actually support a slightly reduced barrier. These large energy-transfer parameters now seem routine in these large fluorinated species. It is perhaps a surprising result for which there is as yet no explanation.

Ultrafast measurements of vibrational relaxation in the conjugated polymer poly(9,9-dioctylfluorene)

We report the resonant pump–probe measurements used to study the dynamics of molecular vibrations in the conjugated polymer poly(9,9-dioctlyfluorene) (PFO). Free-electron-laser excited, pump–probe measurements on a drop-cast polymer film yield the lifetime of a series of different infrared active, high frequency vibrational modes at 5K. A general trend of decreasing lifetime with increasing frequency of the vibrational mode seems consistent with an enhanced density of accepting states for high frequency modes. Our measurements provide an insight into the dissipation of energy in conjugated polymers, and have implications for exciton generation and dissociation mechanisms in organic optoelectronic devices.

Orientational and Vibrational Relaxation Dynamics of Perylene and 1-Methylperylene in Aldehydes and Ketones

We report on the rotational diffusion and vibrational population relaxation dynamics of the probe molecules perylene and 1-methylperylene in selected aliphatic aldehyde and ketone solvents. The reorientation data demonstrate that, for both solvent systems, there is a change in the nature of the solvent−solute interactions with both probe molecules as the hydrodynamic volume of the solvent molecules approaches that of the probes. The solvent-dependent change in behavior occurs for different solvent aliphatic chain lengths in the two systems. Vibrational population relaxation measurements for two probe molecule vibrational modes reveal efficient intermolecular coupling in all cases. The data point to the role of the solvent aldehyde proton in mediating solvent−solvent and solvent−solute interactions, leading to chromophore dynamics reminiscent of those seen in the n-alcohols. Reorientation of the two probes in the ketones yields results that are more consistent with those seen in the alkanes, suggesting the...

Temperature dependent vibrational lifetimes in supercritical fluids near the critical point

Vibrational relaxation measurements on the CO asymmetric stretching mode (∼1980 cm−1) of tungsten hexacarbonyl (W(CO)6) as a function of temperature at constant density in several supercritical solvents in the vicinity of the critical point are presented. In supercritical ethane, at the critical density, there is a region above the critical temperature (Tc) in which the lifetime increases with increasing temperature. When the temperature is raised sufficiently (∼Tc+70 °C), the lifetime decreases with further increase in temperature. A recent hydrodynamic/thermodynamic theory of vibrational relaxation in supercritical fluids reproduces this behavior semiquantitatively. The temperature dependent data for fixed densities somewhat above and below the critical density is in better agreement with the theory. In fluoroform solvent at the critical density, the vibrational lifetime also initially increases with increasing temperature. However, in supercritical CO2 at the critical density, the temperature dependent vibrational lifetime decreases approximately linearly with temperature beginning almost immediately above Tc. The theory does not reproduce this behavior. A comparison between the absolute lifetimes in the three solvents and the temperature trends is made.

Separability of Intra- and Intermolecular Vibrational Relaxation Processes in the S1 State of trans-Stilbene

Detailed time-resolved measurements of anti-Stokes intensities of several high-frequency modes of electronically excited trans-stilbene in hexane and methanol are reported. The most striking changes are observed in the ethylenic band at 1570 cm-1. Nonequilibrium population in the v = 1 level of this mode is found to exhibit a biphasic decay, which is interpreted as a rapid energy redistribution followed by a slightly slower process that reflects the transfer of excess energy to the solvent. Comparison of the time-dependent intensity changes of several high-frequency bands with those from a model that assumes instantaneous thermalization of the excited state suggests that internal energy randomization is incomplete on the time scale of solute−solvent thermal equilibration process. No evidence of mode specificity in the energy redistribution process is observed. The results show us to place a value of ∼2 ps on the lifetime of the v = 1 level of the ethylenic stretching mode of trans-stilbene in both hexane and methanol. Conclusions from this work are discussed in the context of other recent time-resolved Raman measurements of vibrational relaxation in this system in both the gas phase and condensed phase.

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.

Quasiclassical trajectory study of NO vibrational relaxation by collisions with atomic oxygen

Room-temperature and temperature-dependent thermal rate constants are calculated for the state-to-state vibrational relaxation of NO(v ⩽ 9) by atomic oxygen using the quasiclassical trajectory method and limited ab initio information on the two lowest O + NO potential-energy surfaces which are responsible for efficient vibrational relaxation. Comparisons of the theoretical results with the available experimental measurements indicate reasonable agreement for the deactivation of NO(v = 2, 3) at 300 K and NO(v = 1) at 2700 K, although the calculated relaxation rate constant for NO(v = 1) at 300 K is approximately a factor of two below the measured value. The state-to-state relaxation rate coefficients involve the formation of long-lived collision complexes and indicate the importance of multiquantum vibrational relaxation consistent with statistical behaviour in O + NO collisions. The present results, combined with recent measurements of vibrational relaxation for NO(v = 2, 3), suggest that the current atmospheric models of NO cooling rates require higher atmospheric temperatures and/or an increase in the NO/O number densities.

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.

Measurement of rotational and vibrational relaxation in gases by photoacoustic resonance: Application to SF6

The photoacoustic resonance method has been applied to study molecular relaxation in a cylindrical cavity. The profile of the acoustic resonance, excited by the vibrational mode υ3 of gas sample SF6 with a CO2 laser, was measured as a function of pressure between 1 and 800 Torr. A general theoretical treatment of vibrational and rotational relaxation on the acoustically resonant characteristics, such as the resonant frequency and the broadening of half-width, taking into account the contributions of surface and volume losses, and the nonideality of gas, was worked out in detail and was used to frame the discussion of the relaxation processes following the vibrational excitation. The analysis of the experimental data yields a value of vibrational relaxation time (pτ)V-T=0.21±0.01 μs atm, and a value of (pτ)R-T=0.09±0.06 ns atm for the rotational relaxation time of SF6 at T=295 K.

Vibrational relaxation and sound absorption and dispersion in binary mixtures of gases

Reaction kinetics is used in concert with the usual gas species continuity and momentum equations to derive a pure wave equation for a single parameter (pressure or density). Solutions are obtained for sound absorption and dispersion in a binary mixture of gases which are equivalent to those derived from the phenomenological theory of irreversible thermodynamics. The methods are illustrated by comparison with acoustic measurements of vibrational relaxation in nitrogen–water vapor mixtures near room temperature.

Vibrational Dynamics of Carbon Monoxide at the Active Sites of Mutant Heme Proteins

Picosecond mid-IR pump-probe measurements of vibrational relaxation (VR) of CO bound to the active sites of wild-type and mutant myoglobins (Mb) reveal that an approximately linear relationship exists between the protein matrix-induced CO frequency shift and the VR rate. This relation parallels a similar linear relationship seen in a series of heme model compounds where Fe was replaced by Ru and Os. The VR rate of CO in the Mb is sensitive only to the magnitude of protein-induced carbonyl frequency shifts and apparently is not sensitive to the specific details of how the shift is induced, e.g., hydrogen bonding to CO or electrostatic interactions in the heme pocket. CO VR is insensitive to substantial changes in protein structure that do not affect the CO vibrational frequency. These observations suggest that the mechanism of carbon monoxide VR in heme proteins such as Mb occurs by through-﷿-bond anharmonic coupling, which, as shown in prior work, is also the dominant coupling in the model compounds. The experiments indicate that differing protein structures influence VR of CO bound at the active site not by opening and closing channels for vibrational energy flow from CO to the protein but by affecting the rate of energy flow from CO to heme. The rates are determined by the extent of back-bonding, which determines the magnitude of through-﷿-bond anharmonic coupling between CO and heme. The back-bonding and, therefore, the extent of anharmonic coupling are influenced by the electric fields in the heme pocket, which likely differ in the proteins studied here.