Magnetic resonance studies of hydrogen atom transfer

The experimental results obtained on four different types of Raman spectra: pure rotational lines, the I_VV and _VH components of the vibrational Q-branch and the vibrational rotational lines are presented for H₂, D₂, HF and N₂ dissolved at low concentration in inert solvents. The line broadening and motional narrowing due to the solvent interaction is discussed.     

Vibrational relaxation in neat crystals of naphthalene by picosecond cars

Picosecond delayed CARS experiments on totally symmetric modes in naphthalene at 1.5 K are reported. The Raman lineshape of the vibrational excitons is lorentzian and vibrational relaxation can be surprisingly slow. The Raman lineshape of the A_g exciton level of the 766 cm^?1 vibrational mode reveals that the low-temperature lorentzian lineshape occurs by motional narrowing At higher temperature the exciton is trapped by interaction with lattice phonons.     

The isotropic spectrum of the CO2 Raman 2ν3 overtone: a line-mixing band shape analysis at pressures up to several tens of atmospheres

A line-mixing shape analysis of the isotropic remnant Raman spectrum of the 2ν(3) overtone of CO(2) is reported at room temperature and for densities, ρ, rising up to tens of amagats. The analysis, experimental and theoretical, employs tools of non-resonant light scattering spectroscopy and uses the extended strong collision model (ESCM) to simulate the strong line mixing effects and to evidence motional narrowing. Excellent agreement at any pressure is observed between the calculated spectra and our experiment, which, along with the easy numerical implementation of the ESCM, makes this model stand out clearly above other semiempirical models for band shape calculations. The hitherto undefined, explicit ρ-dependence of the vibrational relaxation rate is given. Our study intends to improve the understanding of pressure-induced phenomena in a gas that is still in the forefront of the news.     

Infrared dephasing of degenerate fundamentals: The ν6 mode of liquid CD3I

A model of vibrational dephasing with motional narrowing by the librational motions of the molecules in their cages is tested on the example of the ν₆ degenerate infrared fundamental of liquid CD₃l at room temperature.     

Four-level optical line shape of a single molecule coupled to a single tunneling two-level system

We propose a density-matrix theory for the four-level system consisting of a single optical two-level system (OTLS) coupled to a single two-level system tunneling along a vibrational coordinate (VTLS). Phonons induce jumping rates of the VTLS, but coherent tunneling has to be considered explicitly as well, because the Born-Oppenheimer potential of the tunnel variable may change upon optical excitation. The OTLS is subject to spontaneous emission and driven by a laser wave with arbitrary strength. Numerical simulations for various coupling cases reproduce limiting behaviors previously discussed separately in the literature, such as motional narrowing, cross transitions, optical saturation and pumping, and nonlinear effects. Our model also perfectly fits recent measurements of the spectra of a single molecule coupled to a single tunneling system in a disordered crystal.     

A femtosecond fluorescence study of vibrational relaxation and cooling dynamics of UV dyes

We present a femtosecond broad-band fluorescence up-conversion study of the vibrational relaxation dynamics of two UV chromophores, 2,5-diphenyloxazole (PPO) and para-terphenyl (pTP), pumped with a large excess of vibrational energy (>2000 cm(-1)). The band narrowing of the transient fluorescence spectrum reflects a biphasic cooling process in a few hundreds of fs and a few ps. In the sub-ps regime, our data suggest a structural rearrangement in the excited state, followed by thermalization of the excess energy. These dynamics affect the fluorescence spectra of PPO and pTP in different ways. In PPO, the damping of a low frequency vibrational wavepacket and a significant sub-ps narrowing of the band characterize the vibrational relaxation. In pTP, the latter is faster and appears as a red shift with distortion of the band in <200 fs.     

Nuclear magnetic resonance study of 19F diffusion in lead difluoride

Fluorine spin-spin and spin-lattice relaxation time measurements as a function of temperature in pure and NaF doped samples of PbF₂ are reported. The doped samples exhibit pronounced motional narrowing at low temperatures due to the high ¹⁹F diffusivity.     

A transferable electrostatic map for solvation effects on amide I vibrations and its application to linear and two-dimensional spectroscopy

A method for modeling infrared solvent shifts using the electrostatic field generated by the solvent is presented. The method is applied to the amide I vibration of N-methyl acetamide. Using ab initio calculations the fundamental frequency, anharmonicity, and the transition dipoles between the three lowest vibrational states are parametrized in terms of the electrostatic field. The generated map, which takes into account the electric field and its gradients at four molecular positions, is tested in a number of common solvents. Agreement of solvent shift and linewidths with experimental Fourier transform infrared (FTIR) data is found to within seven and four wave numbers, respectively, for polar solvents. This shows that in these solvents electrostatic contributions dominate solvation effects and the map is transferable between these types of solvents. The effect of motional narrowing arising from the fast solvent fluctuations is found to be substantial for the FTIR spectra. Also the two-dimensional infrared (2DIR) spectra, simulated using the constructed map, reproduce experimental results very well. The effect of anharmonicity fluctuations on the 2DIR spectra was found to be negligible.     

Dynamical effects in line shapes for coupled chromophores: time-averaging approximation

For an isolated resonance of an isolated chromophore in a condensed phase, the absorption line shape is often more sharply peaked than the distribution of transition frequencies as a result of motional narrowing. The latter arises from the time-dependent fluctuations of the transition frequencies. It is well known that one can incorporate these dynamical effects into line shape calculations within a semiclassical approach. For a system of coupled chromophores, both the transition frequencies and the interchromophore couplings fluctuate in time. In principle one can again solve this more complicated problem with a related semiclassical approach, but in practice, for large numbers of chromophores, the computational demands are prohibitive. This has led to the development of a number of approximate theoretical approaches to this problem. In this paper we develop another such approach, using a time-averaging approximation. The idea is that, for a single chromophore, a motionally narrowed line shape can be thought of as a distribution of time-averaged frequencies. This idea is developed and tested on both stochastic and more realistic models of isolated chromophores, and also on realistic models of coupled chromophores, and it is found that in all cases this approximation is quite satisfactory, without undue computational demands. This approach should find application for the vibrational spectroscopy of neat liquids, and also for proteins and other complicated multichromophore systems.     

Modeling vibrational dephasing and energy relaxation of intramolecular anharmonic modes for multidimensional infrared spectroscopies

Starting from a system-bath Hamiltonian in a molecular coordinate representation, we examine an applicability of a stochastic multilevel model for vibrational dephasing and energy relaxation in multidimensional infrared spectroscopy. We consider an intramolecular anharmonic mode nonlinearly coupled to a colored noise bath at finite temperature. The system-bath interaction is assumed linear plus square in the system coordinate, but linear in the bath coordinates. The square-linear system-bath interaction leads to dephasing due to the frequency fluctuation of system vibration, while the linear-linear interaction contributes to energy relaxation and a part of dephasing arises from anharmonicity. To clarify the role and origin of vibrational dephasing and energy relaxation in the stochastic model, the system part is then transformed into an energy eigenstate representation without using the rotating wave approximation. Two-dimensional (2D) infrared spectra are then calculated by solving a low-temperature corrected quantum Fokker-Planck (LTC-QFP) equation for a colored noise bath and by the stochastic theory. In motional narrowing regime, the spectra from the stochastic model are quite different from those from the LTC-QFP. In spectral diffusion regime, however, the 2D line shapes from the stochastic model resemble those from the LTC-QFP besides the blueshifts caused by the dissipation from the colored noise bath. The preconditions for validity of the stochastic theory for molecular vibrational motion are also discussed.     

High-pressure Raman study of vibrational relaxation in N2O

The vibrational widths of the ν₁ and ν₃ Raman bands of N₂O were determined at pressures ranging from 8 bar to 2 kbar and temperatures varied from 25 to 150°C. The different dephasing theories including motional narrowing collisional models and resonant vibrational energy transfer theory were tested. A comparison of the theoretical predictions with the experimental data indicates the resonance VV transfer represents the dominant broadening mechanism. The observed frequency shifts between isotropic and anisotropic components of the bands were interpreted in terms of dipole-dipole interactions in dense N₂O.     

Vibrational energy transfer in hydrogen liquid and its isotopes

The transfer of vibrational energy (V-V) from H₂ to isotopic impurities (HD or D₂) has been studied in the liquid state, between 15 and 30 K. The subsequent relaxation (V-T) of the excited impurity by the H₂ liquid host has also been measured and contrasted with the vibrational relaxation behaviour of pure H₂ and D₂ liquids. The isothermal density dependence of both V-V and V-T transfer has been investigated in the fluid state at 30 K. High density relaxation rates are also compared to our data in the pure gases and to other available gas phase results. Measurements in the solid, near the triple point temperature, are equally reported for each process studied.     

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.     

Photoinduced bimolecular electron transfer investigated by femtosecond time-resolved infrared spectroscopy

Ultrafast infrared transient absorption spectroscopy is used to study the photoinduced bimolecular electron transfer reaction between perylene in the first singlet excited state and 1,4-dicyanobenzene in acetonitrile and dichloromethane. Following vibrational marker modes on both donor and acceptor sides in real time provides direct insight into the structural dynamics during the reaction. A band narrowing on a time scale of a few tens of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion indicates that a substantial part of the excess energy is channeled into vibrational modes of the product, despite the fact that the reaction is weakly exergonic. An additional narrowing of the same band on a time scale of several hundreds of picoseconds observed in acetonitrile only is interpreted as a signature of the dissociation of the geminate ion pairs into free ions.     

Time-domain calculations of the infrared and polarized Raman spectra of tetraalanine in aqueous solution

The IR and polarized (isotropic and anisotropic) Raman spectra are calculated for the amide I band of tetraalanine ((Ala)4) in aqueous solution by using a time-domain computational method, which includes the effects of the diagonal frequency modulations (of individual peptide groups), the off-diagonal (interpeptide) vibrational couplings, and structural dynamics. It is shown that the calculated band profiles, especially the existence of a large negative noncoincidence effect (i.e., large frequency separations between the IR, isotropic Raman, and anisotropic Raman bands, with the isotropic Raman being higher in frequency), are in reasonable agreement with the experimental results. This negative noncoincidence effect derives from two conditions: the positive coupling between the amide I vibrations of peptide groups and the angle larger than 90 degrees between the transition dipoles of the coupled vibrations. This result means that the dynamically changing structures mainly in the polyproline II and beta-type conformations containing some repeated interconversions obtained from the molecular dynamics calculation are consistent with the existence of a large negative noncoincidence effect, as far as the structures satisfy the above two conditions. It is also shown that the electric fields from solvent water molecules induce larger frequency shifts than those of intrachain interactions, with rapid underdamped oscillatory modulations ( approximately 43 fs) due to the librational motions of water molecules that give rise to motional narrowing effect on the spectra. The reason for the difference from the behavior seen for the O-H stretching mode of liquid water is discussed. The time-domain analysis of the mode identity shows that the system proceeds halfway to complete mode mixing with a similar time scale ( approximately 60 fs), suggesting the importance of the nonadiabatic effect, which is included in a natural way in the present computational method.     

Vibrational dynamics of some anions in aqueous solutions from isotropic Raman scattering

From the I_VV and I_VH Raman spectra of a totally symmetric vibration, isotropic scattering profiles have been obtained for several anions in dilute aqueous solutions at various temperatures. The Raman spectrometer was connected to an A/D converter and a small computer, which allowed multiple scanning, data accumulation, and numerical processing of these profiles with good precision over a frequency range of about 20 times their full width at half peak height.The theoretical analysis of the isotropic Raman profiles was accomplished by assuming vibrational phase relaxation based on a MARKOV-GAUSS mechanism. Fitting procedures to the experimental data permitted the determination of the vibrational frequency distribution and its modulation time. Temperature effects are discussed in relation to the distribution and motional characteristics of the immediate environment of water molecules of the anions.     

~(11)B and ~7Li NMR Investigations of the Oxysulfide Glasses in Li_2S-B_2O_3 (-LiBr) Systems

Lithiumconductingborateglassesarebettersolidelectfolytesknowntoday.Thereplacementofoxygenbysulfurwhichispolarizedeasilyfurtherincreasestheionicconductivityoftheglass.Itisbelievedthattheplacementofoxygenwithsulfurgivesrisetosomemodificationofthemicrostructuresoftheglassnetworkl.Theincrementofconductivitymaymainlybecausedbythesestructuralchanges.Now,sulfideglassesareoneoftheLi ionicconductorsexhibitingthehighestconductivityandhavebeenexploredforitsapplicationsaselectrolytesinsolidstatebatteries…     

The observation of the first vibrational overtone of dihydrogen in the luminescence of zeolites at low temperatures

Dehydrated zeolites NaA, NaY, and barium-exchanged NaY luminesce when irradiated with the 1064 nm laser light of an FT-Raman instrument. When hydrogen is adsorbed in the zeolite, the luminescence is altered in several ways. Most remarkable is the appearance of "absorptions" in the positions of the first vibrational overtones of H(2) and HD. Although these features are in the expected positions with reasonable band profiles for overtone absorptions, the large extinction of the luminescent intensity requires a more efficient mechanism than simple reabsorption of emitted photons. In addition to the appearance of holes in the luminescence spectrum, other luminescent features are substantially quenched by the presence of hydrogen or, in one case, augmented.     

Solar‐Light‐Driven Pure Water Splitting with Ultrathin BiOCl Nanosheets

A suitable photocatalyst for overall water splitting has been produced by overcoming the disadvantage of the band structure in bulk BiOCl by reducing the thickness to the quantum scale. The ultrathin BiOCl nanosheets with surface/subsurface defects realized the solar‐driven pure water splitting in the absence of any co‐catalysts or sacrificial agent. These surface defects cannot only shift both the valence band and conduction band upwards for band‐gap narrowing but also promote charge‐carrier separation. The amount of defects in the outer layer surface of BiOCl results in an enhancement of carrier density and faster charge transport. First‐principles calculations provide clear evidence that the formation of surface oxygen vacancies is easier for the ultrathin BiOCl nanosheets than for its thicker counterpart. These defects can serve as active sites to effectively adsorb and dissociate H₂O molecules, resulting in a significantly improved water‐splitting performance.