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The motion of a spherical Brownian particle in an asymmetric periodic channel is considered. Under an external periodic stimulus, the particle switches between two states with different particle radius, every half-period. Using Brownian dynamics simulations, we show that the particle size oscillation, combined with the asymmetry of the channel, induces a drift along the channel axis, directed towards the steeper wall of the channel. The oscillation of the particle size is accompanied by a time variation of the space accessible to the particle and by an oscillation of its diffusion coefficient. The former underlies the drift inducing mechanism of purely entropic nature. The latter, combined with the former, leads to a significant amplification of the effect. The drift velocity vanishes when interconversion between the states occurs either very slow or very fast, having a maximum in between. The position and magnitude of the maximum are discussed by providing an analytical approach based on intuitively appealing assumptions. [ABSTRACT FROM AUTHOR]

The excited-state orientation hydrogen-bonding dynamics, and vibronic spectra of isoquinoline (IQ) and its cationic form IQc in water have been investigated at the time-dependent density functional theory quantum chemistry level plus Franck-Condon simulation and interpretation. The excited-state orientation hydrogen bond strengthening has been found in IQ:H2O complex due to the charge redistribution upon excitation; this is interpreted by simulated 1:1 mixed absorption spectra of free IQ and IQ:H2O complex having best agreement with experimental results. Conversely, the orientation hydrogen bond in IQc:H2O complex would be strongly weakening in the S1 state and this is interpreted by simulated absorption spectra of free IQc having best agreement with experimental results. By performing Franck-Condon simulation, it reveals that several important vibrational normal modes with frequencies about 1250 cm-1 involving the wagging motion of the hydrogen atoms are very sensitive to the formation of the orientation hydrogen bond for the IQ/IQc:H2O complex and this is confirmed by damped Franck-Condon simulation with free IQ/IQc in water. However, the emission spectra of the IQ and IQc in water have been found differently. Upon the excitation, the simulated fluorescence of IQ in water is dominated by the IQ:H2O complex; thus hydrogen bond between IQ and H2O is much easier to form in the S1 state. While the weakened hydrogen bond in IQc:H2O complex is probably cleaved upon the laser pulse because the simulated emission spectrum of the free IQc is in better agreement with the experimental results. [ABSTRACT FROM AUTHOR]

We consider a Brownian photomotor, namely, the directed motion of a nanoparticle in an asymmetric periodic potential under the action of periodic rectangular resonant laser pulses which cause charge redistribution in the particle. Based on the kinetics for the photoinduced electron redistribution between two or three energy levels of the particle, the time dependence of its potential energy is derived and the average directed velocity is calculated in the high-temperature approximation (when the spatial amplitude of potential energy fluctuations is small relative to the thermal energy). The thus developed theory of photoinduced molecular transport appears applicable not only to conventional dichotomous Brownian motors (with only two possible potential profiles) but also to a much wider variety of molecular nanomachines. The distinction between the realistic time dependence of the potential energy and that for a dichotomous process (a step function) is represented in terms of relaxation times (they can differ on the time intervals of the dichotomous process). As shown, a Brownian photomotor has the maximum average directed velocity at (i) large laser pulse intensities (resulting in short relaxation times on laser-on intervals) and (ii) excited state lifetimes long enough to permit efficient photoexcitation but still much shorter than laser-off intervals. A Brownian photomotor with optimized parameters is exemplified by a cylindrically shaped semiconductor nanocluster which moves directly along a polar substrate due to periodically photoinduced dipole moment (caused by the repetitive excited electron transitions to a non-resonant level of the nanocylinder surface impurity). [ABSTRACT FROM AUTHOR]

We consider a system of two coupled particles fluctuating between two states, with different interparticle interaction potentials and particle friction coefficients. An external action drives the interstate transitions that induces reciprocating motion along the internal coordinate x (the interparticle distance). The system moves unidirectionally due to rectification of the internal motion by asymmetric friction fluctuations and thus operates as a dimeric motor that converts input energy into net movement. We focus on how the law of interaction between the particles affects the dimer transport and, in particular, the role of thermal noise in the motion inducing mechanism. It is argued that if the interaction potential behaves at large distances as xα, depending on the value of the exponent α, the thermal noise plays a constructive (α >2), neutral (α = 2), or destructive (α <2) role. In the case of α = 1, corresponding piecewise linear potential profiles, an exact solution is obtained and discussed in detail. [ABSTRACT FROM AUTHOR]

The microscopic features of binary mixtures formed by an ionic liquid (EMI+TFSA- or EMI+FSA-) and a molecular liquid (acetonitrile or methanol) have been investigated by high-pressure infrared spectroscopy. On the basis of its responses to changes in pressure and concentration, the imidazolium C–H appears to exist at least in two different forms, i.e., isolated and associated structures. The weak band at approximately 3102 cm-1 should be assigned to the isolated structure. CD3CN can be added to change the structural organization of ionic liquids. The compression of an EMI+TFSA-/CD3CN mixture leads to the increase in the isolated C–H band intensity. Nevertheless, the loss in intensity of the isolated structures was observed for EMI+FSA-/CD3CN mixtures as the pressure was elevated. In other words, the associated configuration is favored with increasing pressure by debiting the isolated form for EMI+FSA-/CD3CN mixtures. The stronger C–H...F interactions in EMI+FSA- may be one of the reasons for the remarkable differences in the pressure-dependent results of EMI+TFSA- and EMI+FSA-. [ABSTRACT FROM AUTHOR]

Vibrational level structure in the S0 1Ag and S1 1B3u states of pyrene was investigated through analysis of fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation in a supersonic jet and through referring to the results of ab initio theoretical calculation. The vibrational energies are very similar in the both states. We found broad spectral feature in the dispersed fluorescence spectrum for single vibronic level excitation with an excess energy of 730 cm-1. This indicates that intramolecular vibrational redistribution efficiently occurs at small amounts of excess energy in the S1 1B3u state of pyrene. We have also observed a rotationally resolved ultrahigh-resolution spectrum of the 000 band. Rotational constants have been determined and it has been shown that the pyrene molecule is planar in both the S0 and S1 states, and that its geometrical structure does not change significantly upon electronic excitation. Broadening of rotational lines with the magnetic field by the Zeeman splitting of MJ levels was very small, indicating that intersystem crossing to the triplet state is minimal. The long fluorescence lifetime indicates that internal conversion to the S0 state is also slow. We conclude that the similarity of pyrene’s molecular structure and potential energy curve in its S0 and S1 states is the main cause of the slow radiationless transitions. [ABSTRACT FROM AUTHOR]

We have observed rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the 000 (a-type) and 000+467 cm-1 (b-type) bands of the S2 1A1←S0 1A1 transition of jet-cooled azulene. The observed linewidth is 0.0017 cm-1, which corresponds to the lifetime of 3.1 ns in the S2 state. Zeeman splitting of rotational lines is very small so that intersystem crossing to the triplet state is considered to be very slow. Inertial defect is very small and the molecule is considered to be planar in the S0 and S2 states (C2v symmetry). Rotational constants of the S2 state are almost identical to those of the S0 state, indicating that geometrical structure is similar in both electronic states. In this case, internal conversion (IC) by vibronic coupling is thought to be inactive. Therefore, the main radiationless transition process in the S2 1A1 state of azulene was identified to be IC to the S1 1B2 state. However, this S2→S1 IC is still slower than that of conventional polycyclic aromatic hydrocarbons. We consider it to be due to the shallower potential energy curve in the S1 1B2 state, which is also responsible for the extraordinarily fast S1→S0 IC in the isolated azulene molecule. [ABSTRACT FROM AUTHOR]

High-pressure infrared measurements were carried out to observe the microscopic structures of two imidazolium-based ionic liquids, i.e., 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [EMI+(CF3SO2)2N-, EMI+TFSA-] and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide [EMI+(FSO2)2N-, EMI+FSA-]. The results obtained at ambient pressure indicate that the imidazolium C–H may exist in two different forms, i.e., isolated and network structures. As the sample of pure EMI+FSA- was compressed, the network configuration is favored with increasing pressure by debiting the isolated form. For EMI+TFSA-/H2O mixtures, the imidazolium C–H peaks split into four bands at high pressures. The new spectral features at approximately 3117 and 3190 cm-1, being concentration sensitive, can be attributed to the interactions between the imidazolium C–H and water molecules. The alkyl C–H absorption exhibits a new band at approximately 3025 cm-1 under high pressures. This observation suggests the formation of a certain water structure around the alkyl C–H groups. The O–H stretching absorption reveals two types of O–H species, i.e., free O–H and bonded O–H. For EMI+TFSA-/H2O mixtures, the compression leads to a loss of the free O–H band intensities, and pressure somehow stabilizes the bonded O–H configurations. The results also suggest the non-negligible roles of weak hydrogen bonds in the structure of ionic liquids. [ABSTRACT FROM AUTHOR]

The structural organization in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4])/water or methanol was studied by infrared spectroscopy. No drastic change in the concentration dependence of the alkyl C–H band frequency was observed at high concentration of the ionic liquid. This behavior indicates a clustering of the ionic liquid in alkyl regions. Nevertheless, the presence of methanol significantly perturbs the ionic liquid–ionic liquid associations in the imidazolium region. On the basis of the responses to change in pressure and concentration, two different types of O–H species, i.e., free O–H and bonded O–H, were observed in the O–H stretching region. For [bmim][BF4]/water mixtures, the compression leads to loss of the free O–H band intensity. It is likely that free O–H is switched to bonded O–H as high pressures are applied. For [bmim][BF4]/methanol mixtures, the free O–H is still stable under high pressures. [ABSTRACT FROM AUTHOR]

Unified semiclasical solution for general nonadiabatic tunneling between two adiabatic potential energy surfaces is established by employing unified semiclassical solution for pure nonadiabatic transition [C. Zhu, J. Chem. Phys. 105, 4159 (1996)] with the certain symmetry transformation. This symmetry comes from a detailed analysis of the reduced scattering matrix for Landau-Zener type of crossing as a special case of nonadiabatic transition and nonadiabatic tunneling. Traditional classification of crossing and noncrossing types of nonadiabatic transition can be quantitatively defined by the rotation angle of adiabatic-to-diabatic transformation, and this rotational angle enters the analytical solution for general nonadiabatic tunneling. The certain two-state exponential potential models are employed for numerical tests, and the calculations from the present general nonadiabatic tunneling formula are demonstrated in very good agreement with the results from exact quantum mechanical calculations. The present general nonadiabatic tunneling formula can be incorporated with various mixed quantum-classical methods for modeling electronically nonadiabatic processes in photochemistry. [ABSTRACT FROM AUTHOR]

Photodissociation of pyrimidine at 193 and 248 nm was investigated separately using vacuum ultraviolet photoionization at 118.4 and 88.6 nm and multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C4N2H4→C4N2H3+H and five ring opening dissociation channels, C4N2H4→C3NH3+HCN, C4N2H4→2C2NH2, C4N2H4→CH3N+C3NH, C4N2H4→C4NH2+NH2, and C4N2H4→CH2N+C3NH2. Only the first four channels were observed at 248 nm. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion at both wavelengths. The dissociation rates were found to be >5×107 and 1×106 s-1 at 193 and 248 nm, respectively. Comparison with the potential energies from ab initio calculations have been made. [ABSTRACT FROM AUTHOR]

We have calculated the state-to-state integral cross sections and rate constants for the rotationally inelastic collision of CH(B 2Σ-, ν=0, N→N′) with Ar using the quantum coupled-state and close-coupling methods on an ab initio potential-energy surface constructed by Alexander et al. [J. Chem. Phys. 101, 4547 (1994)]. Overall the calculated rate constants are in good agreements with the three available experimental results. The rate constants are comparable to the usual gas kinetic and decrease with increasing N and ΔN. For the multiquantum transition cases, the theory underestimates the experiment. We discuss some possible causes to the discrepancies among the theory and the experiments. [ABSTRACT FROM AUTHOR]

Photodissociation of pyridine, 2,6-d2-pyridine, and d5-pyridine at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C5NH5→C5NH4+H (10%) and five ring opening dissociation channels, C5NH5→C4H4+HCN, C5NH5→C3H3+C2NH2, C5NH5→C2H4+C3NH, C5NH5→C4NH2+CH3 (14%), and C5NH5→C2H2+C3NH3. Extensive H and D atom exchanges of 2,6-d2-pyridine prior to dissociation were observed. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion. The dissociation rate of pyridine excited by 248-nm photons was too slow to be measured, and the upper limit of the dissociation rate was estimated to be 2×103 s-1. Comparisons with potential energies obtained from ab initio calculations and dissociation rates obtained from the Rice-Ramsperger-Kassel-Marcus theory have been made. [ABSTRACT FROM AUTHOR]

We have probed under high pressure the C–H hydrogen bonds formed by N,N′-disubstituted imidazolium ions having PF6- and Br- counterions. High-pressure infrared spectral profiles, x-ray crystallographic analysis, and ab initio calculations allow us to make a vibrational assignment of these compounds. The appearance of a signal for the free-NH unit (or weakly bonded N–H...F unit) in the infrared spectrum of the PF6- salt indicates that conventional N–H...O and N–H...N hydrogen bonds do not fully dominate the packing. It is likely that the charge-enhanced C2–H...F interactions, combined with other weak hydrogen bonds, disturb the formation of N–H hydrogen bonds in the PF6- salt. This finding is consistent with the pressure-dependent results, which reveal that the C2–H...F interaction is enhanced upon increasing the pressure. In contrast to the PF6- salt, the imidazolium C–H bonds of the Br- salt have low sensitivity to high pressure. This finding suggests that the hydrogen bonding patterns are determined by the relative hydrogen bond acceptor strengths of the Br- and PF6- ions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Charge-enhanced C–H–O interactions are investigated for imidazole hydrochloride/D[sub 2]O mixtures. Based on the concentration-dependence results, the infrared spectra reveal a strong C–H band and a weak shoulder at ∼3065 cm-1. The pressure-dependence measurements also exhibit considerable spectral changes as the mixtures transform to high-pressure ices. The absorption intensity of the weak shoulder, located in the region of 3060–3090 cm-1, drastically increases as the pressure is elevated. Observation of this low frequency band provides the experimental evidence of C[sup 2]–H–O hydrogen bonds in the aqueous imidazolium solutions. Ab initio calculation results, predicting the frequency shift of the C–H stretching vibrations as C–H–O is interacting via hydrogen bonding, are discussed. Structural identification of the hydrophobic isomers in infrared spectra seems to be complicated by the presence of more than one stable isomeric form. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electron transfer processes involving a multimode mixing mechanism (the Duschinsky rotation) are systematically examined. Such processes can be analyzed with a very general “spin–boson” model with N displaced and quadratically coupled harmonic potentials. The very general Franck–Condon factor obtained here is applicable to the studies of electron transfer as well as energy transfer processes, where frequency shifts and the Duschinsky rotation are involved. Although there are several numerical studies of such a mechanism, the derivation of an analytical expression for the rate constant is presented here and the temperature dependence is examined. As, in general, at very low temperatures where the thermal energy is smaller, the electron transfer rate becomes temperature independent due to nuclear quantum tunneling. However, in the presence of Duschinsky rotation, the pre-exponential factor in the rate constant can deviate from the characteristic 1/[Square_Root]T dependence of the Marcus theory. For processes with no or a small activation energy, the rate can be dominated by the pre-exponential factor and becomes 1/T dependence in the high-temperature regime. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses the role of trap clustering in the trapping kinetics of Brownian particles. Occurrence of traps in spherical clusters; Use of an effective medium treatment to handle trap-correlation effects; Analysis of how to trap clustering.

Investigates the dependence of electron transfer rates on temperatures and energy gaps using a spectral density for solvent fluctuation, such as ohmic, sub-ohmic, super-ohmic dissipation and the Debye models. Analysis of the temperature and energy-gap dependence covering the quantum-tunneling, classical Marcus and intermediate crossover regimes.

Discusses the potential energy surfaces for the first singlet and triplet excited states of methane using multireference configuration interaction and equation-of-motion coupled cluster ab initio molecular orbital calculations. Vertical excitation energies for the excited states; Dissociation pathways leading to various photofragmentation products.

Explains the Smoluchowski-Collins-Kimball theory of stochastically gated diffusion-influenced reactions. Study of kinetics in both cases of gated ligands and protein; Partial mean-field approximation; Explicit analytical solution for a particular two state model of gating and one-dimensional diffusion of the ligands.

The proton-exchanged OH- absorption spectra of congruent and 8.1 mol % Zn-doped LiNbO3 single crystals with and without polarization inversion are presented. The variation in the proton-exchanged OH- spectra for the congruent sample is not affected by polarization inversion. This result confirms the Li-vacancy model. However, the highly doped sample shows stronger increase in intensities with polarization inversion. In our investigation, highly negative charge vacancy model confirms with proton-exchanged OH- absorption spectra and domain inversion etching experiment. Finally, we demonstrated the polarization inversion mechanism with these two samples based on extended x-ray-absorption fine structure experimental results. [ABSTRACT FROM AUTHOR]

Reports that the measurements of photoluminescence (PL) from composites of silica nanoparticles show that the PL results from hydrogen-related species and thermally produced structural defects. Surface condition of the particles as a function of heat pretreatment temperature; Process of interfacial water formation.