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Vibrational relaxation of the free terminal hydroxyl stretch in methanol oligomers: Indirect pathway to hydrogen bond breaking (2001)

Levinger, Nancy E. ; Davis, Paul H. ; Fayer, M. D.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 115 (2001), S. 9352-9360

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Vibrational relaxation of methanol-d (MeOD) in carbon tetrachloride has been investigated via ultrafast infrared pump–probe experiments. Exciting at 2690 cm−1, only the free O-D (where the D is not H-bonded) stretching mode is initially populated. For MeOD mole fractions ≤0.025, a 2.15 ps single exponential decay is observed. At mole fractions ≥0.0375, the signal decays (2.15 ps decay time) below zero (increased absorption) and then recovers on time scales of 22 ps and (very-much-greater-than)300 ps. The increased absorption indicates the formation of additional free ODs caused by the breaking of H-bonds that are not directly coupled to the initially excited vibration. The two-time scale recovery of this signal arises from geminate and nongeminate recombination. The data are fit with a set of kinetic equations that accurately reproduce the data. The results suggest that vibrational relaxation of the initially excited free OD stretch into intramolecular modes of the methanol leads to H-bond breaking. This contrasts studies that suggest direct relaxation of a vibrationally excited OH stretch into an H-bond stretch is responsible for H-bond breaking. © 2001 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1415447

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Two-dimensional ultrafast infrared vibrational echo studies of solute–solvent interactions and dynamics (2001)

Thompson, David E. ; Merchant, K. A. ; Fayer, M. D.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 115 (2001), S. 317-330

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Two-dimensional spectrally resolved ultrafast infrared vibrational echo experiments were used to investigate the nature of solute–solvent interactions in solution. The experiments were performed on (acetylacetonato)dicarbonylrhodium(I) in dibutylphthalate at 150 K. The 2D spectra display features that reflect the 0–1 and 1–2 transitions and the combination band transition of the symmetric (S) and antisymmetric (A) CO stretching modes. Three oscillations in the data arise from the frequency difference between the S and A modes (quantum beats) and the S and A anharmonicities. The novel mechanism that gives rise to the anharmonic oscillations, which is distinct from that of a conventional quantum beat, is described. The frequency of the S/A mode-splitting quantum beats varies for different observation wavelengths across the 0–1 inhomogeneous lines. For either the S or A lines, as the wavelength of observation of the vibrational echo is moved to higher energy, the quantum beat frequency decreases. The change in frequency is related to the nature of the solute–solvent interactions (inhomogeneous broadening) of the S and A transitions. A simple analytical approach is used to determine how a solute vibrational oscillator is influenced by the solvent. Three models of solute–solvent interactions are considered in terms of CO local mode energies and coupling. In one, the transition energies in the S and A lines are anticorrelated either because the inhomogeneous broadening arises from variations in the local mode coupling or the local mode energies are anticorrelated. In the other two, the local mode energies are either correlated or uncorrelated. The results of the model calculations indicate that interactions with the solvent result in local mode frequencies that are strongly correlated. © 2001 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1376423

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Temperature-dependent vibrational dephasing: Comparison of liquid and glassy solvents using frequency-selected vibrational echoes (2002)

Xu, Qing-Hua ; Fayer, M. D.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 117 (2002), S. 2732-2740

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Frequency-selected vibrational echo experiments were used to investigate the temperature dependences of vibrational dephasing associated with the 0-1 transition of the CO stretching mode of RuTPPCOPy (TPP=5,10,15,20-tetraphenylporphyrin, Py=pyridine) in two solvents: polymethylmethacrylate (PMMA) and 2-methyltetrahydrofuran (2-MTHF). In PMMA, a glass, the echo decay is exponential at all the temperatures studied, and the dephasing rate increases linearly with increasing temperature. In 2-MTHF, there is a change in the functional form of the temperature dependence when the solvent goes through the glass transition temperature (Tg). Below Tg, the dephasing rate increases linearly with temperature, while above Tg, it rises very steeply in a nonlinear manner. In the liquid at higher temperatures, the vibrational echo decays are nonexponential. A model frequency-frequency correlation function (FFCF) is proposed in which the FFCF differs for a glass and a liquid because of the intrinsic differences in the nature of the dynamics. At least two motions, inertial and diffusive, contribute to the vibrational dephasing in the liquids. The different temperature dependences of inertial and diffusive motions are discussed. Comparison of the model calculations of the vibrational echo temperature dependence and the data show reasonable, but not quantitative agreement. © 2002 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1492280

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Vibrational dynamics of large hot molecules in the collisionless gas phase (2002)

Stromberg, C. ; Myers, D. J. ; Fayer, M. D.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 116 (2002), S. 3540-3553

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Infrared ps pump-probe experiments are presented for the P, Q, and R rotational branches of the asymmetric CO stretching mode of tungsten hexacarbonyl (1997 cm−1) in the collisionless gas phase. The pump-probe decays are tri-exponentials (140 ps, 1.3 ns, and 〉100 ns) in contrast to single exponential decays observed in supercritical fluids and liquid solvents. The 1.3 ns decay component is the vibrational energy relaxation (VER) time. The long component occurs following intramolecular VER into a distribution of low-frequency modes. After VER is complete, the R signal is 48%, the Q signal is 29%, and the P signal is −10% (absorption increase) compared to the t=0 signal. These long-lived signals result from an increase in the occupation numbers of low-frequency modes (internal heating) that causes a shift of the vibrational spectrum. The fastest decay is produced by spectral diffusion. The spectrally narrow pump pulse burns a hole in the inhomogeneous ground state spectrum and generates a narrow spectral population in the excited state (excited state peak). The inhomogeneity arises from the distribution of occupation numbers of the low-frequency modes that produces different combination band spectral shifts. Spectral diffusion is caused by the time evolution of the complex low-frequency thermal vibrational wave packet. Two possible models for the spectral diffusion are evaluated, one in which spectral diffusion reduces the pump-probe signal by hole filling and broadening of the excited state peak, and one in which the spectral diffusion only broadens the excited state peak. © 2002 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1446850

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Solute–solute spatial distribution in hydrogen bonding liquids probed with time-dependent intermolecular electron transfer (2001)

Tavernier, H. L. ; Fayer, M. D.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 114 (2001), S. 4552-4564

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Solute–solute spatial distribution in strongly hydrogen bonding solvents is investigated using photoinduced electron transfer dynamics between rhodamine 3B (R3B) and N,N-dimethylaniline (DMA) in a series of monoalcohols, polyalcohols, and alcohol mixtures. Fluorescence up-conversion data are presented on electron transfer in ethylene glycol and are compared to data characterizing electron transfer in seven other solvents. The data are analyzed with a detailed statistical mechanical theory that includes a distance-dependent Marcus rate constant, diffusion with the hydrodynamic effect, and solute–solute radial distribution functions. When the standard assumption is made that for low concentration solutes the solute–solute spatial distribution follows that of the solvent's radial distribution function, a single parameter fit to the electronic coupling matrix element results in the same value, independent of solvent, for data from five solvents. However, it is impossible to fit the data from the solvent ethylene glycol using the model based on the solvent radial distribution function. When the assumption that the solute–solute spatial distribution tracks the single molecule solvent radial distribution function is relaxed by using a large "effective" solvent diameter to establish the donor–acceptor distance distribution and hydrodynamic effect, excellent fits to the electron transfer data are obtained. The fits give the same parameters for ethylene glycol and two other solvents with high OH/C ratios as the five "normal" solvents. The results suggest that the solute–solute (donor–acceptor) spatial distributions in the high OH/C ratio solvents are determined by multiple hydrogen bond solvent "aggregates" that inhibit solute molecules from distributing freely among solvent molecules. © 2001 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1349705

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Orientational dynamics of the glass forming liquid, dibutylphthalate: Time domain experiments and comparison to mode coupling theory (2002)

Brace, David D. ; Gottke, S. D. ; Cang, H. ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 116 (2002), S. 1598-1606

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Orientation dynamics of the glass forming liquid, dibutylphthalate (DBP), were studied using optical heterodyne detected optical Kerr effect (OHD-OKE) techniques. A combination of experimental methods permitted acquisition of data over 6 decades of time and 5 decades in signal amplitude. Data collected from several hundred fs to several hundred ns, cover the full range of orientational dynamics. The data are compared to the predictions of ideal mode-coupling theory (MCT). Two of the MCT scaling law predictions yield an average value of 227 K for the MCT critical temperature, Tc. Measurements were made at temperatures below Tc. The data were found to agree well with some of the predictions of MCT. The long time scale structural relaxation (α relaxation) obeys the MCT scaling law, and the slowest time scale power law (the von Schweidler power law) is also in accord with MCT predictions. However, the "master curves" predicted by MCT fit the data well only at higher temperatures. The master curves show increasingly significant deviations at shorter times (〈100 ps) as the temperature is decreased, in contrast to previous OHD-OKE studies on salol [G. Hinze, D. D. Brace, S. D. Gottke, and M. D. Fayer, J. Chem. Phys. 113, 3723 (2000)] and ortho-terphenyl [S. D. Gottke, G. Hinze, D. D. Brace, and M. D. Fayer, J. Phys. Chem. B 105, 238 (2000)] in which the agreement with ideal MCT is far superior. The DBP data show no discontinuity in the trends as the temperature is reduced below Tc, but an increasingly prominent peak appears in the data at ∼2.5 ps. The peak may be related to the boson peak. © 2002 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1427711

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A detailed test of mode-coupling theory on all time scales: Time domain studies of structural relaxation in a supercooled liquid (2000)

Hinze, G. ; Brace, David D. ; Gottke, S. D. ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 113 (2000), S. 3723-3733

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The dynamics of supercooled salol (phenyl salicylate) was measured in the time domain using optical Kerr effect techniques. By combining several experimental setups, data spanning more than six decades in amplitude and time (∼100 fs to ∼1 μs) were observed. The data have a complex shape, ranging from high-frequency intramolecular oscillations at short times, to nearly exponential relaxation at long times. As predicted by mode-coupling theory (MCT), the data for some ranges of time appear as power laws. The slowest power law, the von Schweidler power law, has an almost constant exponent of ∼0.59 over the entire temperature range studied (247–340 K). Above the MCT Tc (T〉∼1.17 Tg, where Tg is the laboratory glass transition temperature) for t〉∼1 ps, the decays are shown to be in excellent agreement with the master curve predicted by ideal MCT when higher order terms are included. However, the data do not display the plateau predicted by ideal MCT. To discuss the data at all temperatures, the intermediate time scale portion of the data, 2〈t〈10 to 500 ps (depending on the temperature), is modeled as a power law that falls between the critical decay and the von Schweidler power law. This intermediate power law shows significant temperature dependence with an exponent that decreases to a value of ∼−1 below Tc. Calculations using extended MCT, for a full range of hopping times, demonstrate that the temperature dependence of the intermediate time scale data near and below Tc cannot be explained by extended MCT. © 2000 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1287595

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Photoinduced intermolecular electron transfer in complex liquids: Experiment and theory (2000)

Tavernier, H. L. ; Kalashnikov, M. M. ; Fayer, M. D.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 113 (2000), S. 10191-10201

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Photoinduced intermolecular electron transfer between Rhodamine 3B and N,N-dimethylaniline has been studied in a series of seven liquids: acetonitrile, ethanol, propylene glycol, and mixtures of ethanol, 2-butanol, ethylene glycol, propylene glycol, and glycerol. In each liquid, the donor and acceptors have different diffusion constants and experience distinct dielectric properties. Ps time-dependent fluorescence measurements and steady-state fluorescence yield measurements were made and analyzed using a detailed statistical mechanical theory that includes a distance-dependent Marcus rate constant, diffusion with the hydrodynamic effect, and solvent structure. All solvent-dependent parameters necessary for calculations were measured, including dielectric constants, diffusion constants, and redox potentials, leaving the electronic coupling unknown. Taking the distance-dependence of the coupling to be β=1 Å−1, data were fit to a single parameter, the coupling matrix element at contact, J0. The theory is able to reproduce both the functional form of the time-dependence and the concentration-dependence of the data in all seven liquids by fitting only J0. Despite the substantial differences in the properties of the experimental systems studied, fits to the data are very good and the values for J0 are very similar for all solvents. © 2000 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1323505

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