Volume 111, Issue 13, 01 October 1999
Index of content:
 CONDENSED PHASE DYNAMICS, STRUCTURE, AND THERMODYNAMICS: SPECTROSCOPY, REACTIONS, AND RELAXATION


Reverse saturable absorption in doped porous glasses studied by single and doublepulse pump–probe experiments
View Description Hide DescriptionWe investigate the nonlinear absorption of doped porous solgel glasses by single and doublepulse pump–probe experiments. We find that the reverse saturable absorption (RSA) of these samples can be explained in the frame of a five level system as it is commonly used for solutions. We observe a strong saturation of the RSA at high fluences, especially if the molecules are prepared in the triplet state. In a doublepulse pump–probe experiment we measure the triplet quantum yield for solutions (0.8) and dopedglasses (0.25) and the singlet and triplet absorption cross sections. In the first excited singlet state lifetimes of 1 ns and 65 ps are determined in solutions and glasses, respectively. We find that the dynamics of both depopulation processes, direct relaxation, and intersystem crossing is faster in glasses than in the case where is in solution in a liquid aromatic solvent. We tentatively explain these findings by the absence of a stabilizing solvent and a perturbation of the molecular energy levels due to interaction with the solid glass matrix. We finally determine the lifetime of the triplet states in our glass samples to 2 μs.

Firstorder correction to classical nucleation theory: A density functional approach
View Description Hide DescriptionIt is shown that the classical expression for the change in grand potential of a system on formation of a cluster of radius R is modified by a factor to first order in where w is a correction due to the nonzero compressibilities of liquid and vapor (near the triple point, w is approximately equal to the product of liquid compressibility and surface tension), and is the coefficient in the expression relating the surface tension of the droplet, to the planar surface tension, i.e., An expression for is derived involving the pair and triplet correlation functions and the density profile of the planar surface. This complements the expression for involving the pair distribution function derived by Blokhuis and Bedeaux; the equivalence of the two expressions in the low density limit is demonstrated. Calculations of and w are performed using meanfielddensity functional theory for the Yukawa potential and an potential, as well as using the squaregradient approximation. is found to be negative for all conditions investigated; its magnitude depends on the potential used, and tends to increase with increasing temperature. However, the ratio is found to be relatively insensitive to potential and to temperature, being between about −1.2 and −1.5 for the conditions investigated. The effect of using a weighted density approximation in place of the local density approximation for the hardsphere part of the potential is estimated in a squaregradient approximation and found to be small.

Percolation for a model of statistically inhomogeneous random media
View Description Hide DescriptionWe study clustering and percolation phenomena for a model of statistically inhomogeneous twophase random media, including functionally graded materials. This model consists of inhomogeneous fully penetrable (Poisson distributed) disks and can be constructed for any specified variation of volume fraction. We quantify the transition zone in the model, defined by the frontier of the cluster of disks which are connected to the diskcovered portion of the model, by defining the coastline function and correlation functions for the coastline. We find that the behavior of these functions becomes largely independent of the specific choice of grade in volume fraction as the separation of length scales becomes large. We also show that the correlation function behaves in a manner similar to that of fractalBrownian motion. Finally, we study fractal characteristics of the frontier itself and compare to similar properties for twodimensional percolation on a lattice. In particular, we show that the average location of the frontier appears to be related to the percolation threshold for homogeneous fully penetrable disks.

Monte Carlo simulation study of the hightemperature phase diagram of model molecules
View Description Hide DescriptionThe constantMonte Carlo simulation is performed for model molecules interacting via the Girifalco potential and a full freeenergy analysis is made to predict the hightemperature phase diagram. The repulsive part of the potential is very steep and the attractive part is relatively shortranged. For such a system accurate computations of the virial pressure are difficult in simulations and it is argued that the discrepancies among the previous results for the phase diagram of can partly be attributed to the uncertainties of the virial pressure involved in simulations. To avoid this difficulty we take the energy route to calculate equation of state(EOS), in which the absolute (Helmholtz) free energy is obtained by performing isochoric integration of the excess internal energy. A difficulty of the energy route in the hightemperature limit is resolved by the aid of an analytic method. The exact second and third virial coefficients are also used in the analysis of the fluid EOS. The pressure route is taken to calculate the EOS of the solid phase, in which the virial pressure is numerically more stable than in the fluid phase. The resulting hightemperature phase diagram of is quite systematic and free from uncertainties, and the liquid–vapor critical point is found at and whereas the triple point at and confirming the existence of a stable liquid phase over the range of ∼100 K.

Equation of state for thermodynamic properties of chain fluids nearto and farfrom the vapor–liquid critical region
View Description Hide DescriptionUpon incorporation of contributions from longwavelength density fluctuations by a renormalizationgroup theory, a crossover equation of state is developed for describing thermodynamic properties of chain fluids. Outside the critical region, the crossover equation of state reduces to the classical equation; inside the critical region, it gives nonclassical universal critical exponents. The crossover equation of state correctly represents phase equilibria and properties of chain fluids in both regions. Good agreement is obtained upon comparisons with computer simulations for squarewell chain fluids. As obtained from experimental vapor–pressure and density data, the squarewell segment–segment parameters for nalkanes from ethane to eicosane are linear functions of molecular weight. Calculated thermodynamic properties agree well with experiment for nalkanes from methane to hexatriacontane.

Voronoi polyhedra analysis of the local structure of water from ambient to supercritical conditions
View Description Hide DescriptionThe changes of the local structure of water are investigated with increasing temperature and decreasing density from ambient conditions up to the critical point. The local structure around the molecules is analyzed in terms of the properties of the Voronoi polyhedra (VP) of the molecules. For reference, the entire analysis is also performed in liquid In this way, the effect of the hydrogen bonding on the local structure can also be studied. In determining the VP of the molecules an exact algorithm is used. In analyzing the local structure it is found that the decrease of the density not only leads to the increase of the free volume available for the molecules, but it also increases the volume of the voids present in the system. A linear relation is found between the average free volume of the molecules and the average volume of the vacancies, the latter being always about the double of the former. The comparison of the results concerning the shape of the VP of the molecules obtained for water at different thermodynamic state points and for liquid clearly reveals that even above the critical point,hydrogen bonds are still playing a very important role in forming the molecular level structure of water. It is also found that the tetrahedral arrangement of the nearestneighbor molecules, which is a characteristic feature of the water structure under ambient conditions, already disappears at temperatures well below the critical point, and the relative arrangement of the first four nearestneighbor molecules becomes rather similar to that in liquid

Structures and electronic phases of the bis(ethylenedithio)tetrathiafulvalene (BEDTTTF) clusters and κ(BEDTTTF) salts: A theoretical study based on ab initio molecular orbital methods
View Description Hide DescriptionElectronic and geometricalstructures of bis(ethylenedithio)tetrathiafulvalene (BEDTTTF) molecules are studied using ab initio molecular orbital methods. The optimized structure of a BEDTTTF monomer is close to the experimental one within errors of 0.02 Å and 0.5 deg in bond length and angle, respectively, except the ethylene group. Ab initio parameters such as transfer integrals and Coulomb interactions are determined from the BEDTTTF dimer and tetramer calculations. Using model Hamiltonians with the ab initio parameters, we investigate the electronic states based on the exact diagonalization method. The results show that the ground state has antiferromagnetic correlation which is consistent with experimental results. We study the effects of longrange Coulomb interactions employing the 2D extended Hubbard model with the Hartree–Fock approximation. It is found that the ground state shows various phases; antiferromagnetic, charge ordering, and paramagnetic ones, controlled by the longrange interactions.

HF stretching–bending combination bands for small complexes in solid argon
View Description Hide DescriptionInfrared spectra of HF (with DF substitution) in solid argon contain weak HF stretching–bending combination bands for and that are higher than the sum of fundamentals. This is in accord with stronger hydrogen bonds upon vibrational excitation of the H–F stretching mode, as observed in the gas phase. Overtone bands were also observed for DF and in solid argon.

Solvation and intramolecular reorganization in Analysis of resonance Raman excitation profiles and ab initio molecular orbital calculations
View Description Hide DescriptionThe observations of a clear solventdependence of resonance Raman intensities, but an absence of concommitant changes in absorption crosssections, are reported for the molecule (BA). Displacements obtained by analysis of the nonpolar solvent data are found to reproduce the absorption spectra recorded in all solvents studied, but not the resonance Raman intensities in polar solvents. Moreover, transform theory is found to be unsuccessful in reproducing the resonance Raman intensities in any solvent. These observations suggest that ultrafast relaxation dynamics (on the timescale probed by the resonance Raman experiment) are changing the displacements of the intramolecular vibrational modes. The changes in the displacements determined by analysis of the data are consistent with a correlation between the total reorganization energy accompanying the charge transfer and the solventdielectric properties (i.e., the modespecific reorganization is found to increase with solventdielectric properties). In effect, the immediate free energy surface “seen” by the molecule changes dramatically with time and causes significant intramolecular reorganization, at least for the initial stages of evolution of the emissive state. These findings are supported by ab initio molecular orbital (MO) geometry optimization, analytical frequencies, and excited state calculations It is shown that most of the normal modes of the state of BA are splittings of corresponding anthracene modes, however, for the relaxed geometry of BA (i.e., gas phase equilibrium geometry), the modes are calculated to be redshifted and have significantly greater splittings. Furthermore, the dipole moment of this relaxed state is calculated to be 0.099 debye in the gas phase, compared to 0.00 debye for the equilibrium ground state and the vertical, unrelaxed, state. The optimized geometry of BA is found to be a “90°” geometry (i.e., torsion angle between the anthracene ring planes), similar to that of the ground state except for subtle asymmetries in each anthracene ring which lower the symmetry from We suggest that these results provide direct evidence for the importance of solventdependent intramolecular reorganization in this molecule.

Multipole moments of water molecules in clusters and ice Ih from first principles calculations
View Description Hide DescriptionWe have calculated molecular multipole moments for water molecules in clusters and in ice Ih by partitioning the charge density obtained from first principles calculations. Various schemes for dividing the electronic charge density among the water molecules were used. They include Bader’s zero flux surfaces and Voronoi partitioning schemes. A comparison was also made with an induction model including dipole, dipolequadrupole, quadrupolequadrupole polarizability and first hyperpolarizability as well as fixed octopole and hexadecapole moments. We have found that the different density partitioning schemes lead to widely different values for the molecular multipoles, illustrating how poorly defined molecular multipoles are in clusters and condensed environments. For instance, the magnitude of the molecular dipole moment in ice Ih ranges between 2.3 D and 3.1 D depending on the partitioning scheme used. Within each scheme, though, the value for the molecular dipole moment in ice is larger than in the hexamer. The magnitude of the molecular dipole moment in the clusters shows a monotonic increase from the gas phase value to the one in ice Ih, with the molecular dipole moment in the water ring hexamer being smaller than the one in ice Ih for all the partitioning schemes used.

Electric field impact on solvated electron reactions: Trapping of randomly walking electron
View Description Hide DescriptionThe impact of an external electric field on electron seizure by neutral traps was studied within the Torrey model of a solvated electronrandom walk. The step length of the random motion, determined by the free diffusion of a temporarily escaped electron, increases with electric field strength, changing the reaction mechanism from a diffusional to a hopping one. The field dependence of the rate of the hoppingreaction essentially differs from that obtained for diffusional trapping. Such a difference may serve for experimental discrimination between the hopping and diffusional mechanisms of this reaction.

Orbitalfree molecular dynamics simulations of melting in and Melting in steps
View Description Hide DescriptionThe meltinglike transitions of and are investigated by ab initio constant energy molecular dynamics simulations using a variant of the Car–Parrinello method which employs an explicit electronic kinetic energy functional of the density, thus avoiding the use of oneparticle orbitals. Several melting indicators are evaluated in order to determine the nature of the various transitions, and are compared with other simulations. Both and melt over a wide temperature range. For a transition is observed to begin at ∼110 K, between a rigid phase and a phase involving isomerizations among the different permutational isomers of the ground state structure. The “liquid” phase is completely established at ∼220 K. For two transitions are observed: the first, at ∼110 K, is associated with isomerizationtransitions among those permutational isomers of the ground state structure which are obtained by interchanging the positions of the surfacelike atoms; the second, at ∼160 K, involves a structural transition from the ground state isomer to a new set of isomers with the surface molten. The cluster is completely liquid at ∼220 K.

Infrared spectra of and isomers trapped in solid neon
View Description Hide DescriptionLaserablation of six different metal targets with concurrent deposition of neon/nitric oxide samples at 4 K produces metal independent absorptions that can be assigned to charged species. The sharp 1227.3 cm^{−1} band and weaker 1225.1 cm^{−1} absorption show the different isotopic frequency ratios observed for the argon matrix counterparts at 1221.0 and 1222.7 cm^{−1} for trans and cis respectively. Strong 1619.0 and weaker 1424.1 cm^{−1} absorptions, also observed in previous experiments with discharged neon, are enhanced with doping while the bands are diminished, which supports cation identifications. These cation bands reverse absorbance with yellow and nearinfrared photolysis. We suggested that the 1619.0 cm^{−1} band corresponds to the isomer made by direct photoionization of cis The 1424.1 cm^{−1} absorption shares the extreme red photosensitivity found for thermal ions. Accordingly, the 1424.1 cm^{−1} absorption is probably due to the trans isomer, more stable in the gas phase, based on high level calculations, and the 1619.0 cm^{−1} band arises from the more stable matrixisolated cis isomer with a dipole moment, which is produced by the more favorable Franck–Condon photoionization of the cis precursor.
