ALBERT

Notes: The Ca(4p2 1D2) state is prepared in a two-step excitation with linearly polarized lasers. Two different angular wave functions are selected, Y2,0 or (Y2,−1−Y2,1)/, by using parallel or perpendicular laser polarizations, respectively. Subsequent collision with a rare gas atom (He, Ne, Ar, Kr, or Xe) populates the near-resonant Ca(3d4p 1F3) state. The dependence of the collisional energy transfer process is measured as a function of the alignment of the initial 1D2 state wave function with respect to the average relative velocity vector. The laser-selected Y2,0 and (Y2,−1−Y2,1)/ angular wave functions display dramatically different alignment dependences, which are understood by an analysis of the rotation properties of these wave functions. The relative contributions to the cross section of the individual 1D2 sublevels, ML=0, ±1, and ±2, are extracted, and these vary considerably depending on the rare gas. For He, the ML=±2 sublevel (asymptotic Δ molecular state) contributes the most to the total cross section, while for all the other rare gases, the ML=0, ±1 sublevels (asymptotic Σ and Π molecular states, respectively) are more important. The contribution of the ML=0 sublevel increases smoothly with increasing mass of the rare gas collision partner, becoming the largest contributor for Xe.

Notes: To gain insight into the mechanism of Na(3p)2P3/2→2P1/2 fine-structure transitions induced by collision with He, we monitor the expectation values of the orbital- and spin-angular momentum vectors, l and s, as a function of time along the trajectory, using a semiclassical formalism. In a typical collision, 〈s〉 remains nearly space-fixed while 〈l〉 precesses about the rotating internuclear axis. Thus, in the interaction region, the projection of 〈l〉 onto the internuclear axis, 〈λ〉, remains nearly constant, and the molecular alignment of the orbital is preserved. We show how equations of motion for the classical analogues of these expectation values agree qualitatively with the quantum equations of motion. A qualitative comparison is also made with the Cs–He system for which the spin–orbit coupling is much stronger. We calculate cross sections for Na(2P3/2)+He→Na(2P1/2)+He as a function of the alignment of the excitation laser polarization with respect to the asymptotic relative velocity vector. For stationary pumping of the excited F=3 hyperfine level, this calculation predicts that the perpendicular alignment gives a cross section which is larger by a factor of 1.8 than that obtained by parallel alignment.

Notes: In this paper we present results of coupled channel quantum scattering calculations of the alignment selected j=3/2→ j=1/2 fine structure changing integral cross section for Na(2P)+He. This cross section has in the past been written in terms of a coherent sum of partial wave amplitudes, but we have found that it can be expressed in terms of an incoherent sum of partial cross sections, each labeled by the total angular momentum J and by parity. It is also possible to define an alignment selected wave function for each J such that the azimuthal average of the square of this wave function projected onto each final state is proportional to the magnitude of the partial cross section into that state. This J labeled wave function is thus clearly related to the physical measurables, and we have used it to determine propensities for preservation of asymptotically prepared alignment during collisions. Using a potential surface based on Pascale's ab initio calculations, we find that the alignment ratio σ⊥/σ(parallel) is an increasing function of energy, with a value less than unity at low energy (〈0.01 eV), but increasing quickly to a value of about 2.0 at 0.04 eV and then more slowly at higher energy, up to a value of 2.7 at 0.2 eV (the highest energy considered). Above 0.02 eV, both the alignment ratio and the alignment selected integral cross sections are in good agreement with values calculated in an accompanying semiclassical study (Kovalenko, Leone, and Delos).An examination of the J labeled alignment selected scattering wave functions and of the expectation values of 〈Ω〉, 〈Λ〉, and 〈Σ〉 indicates that at low J when the initial state is prepared with (parallel) polarization, the dominant state at short range is Σ while with ⊥ polarization the dominant state is Π (i.e., asymptotic alignment is preserved). By way of contrast, this propensity for alignment preservation is not seen if fluxes or probability densities associated with alignment selected wave functions labeled by the initial orbital quantum number l (rather than J) are considered. This l labeled result is in accord with recent work by Pouilly and Alexander, but the lack of alignment preservation in this case has no relationship with the alignment cross sections, or with the alignment selected plane wave scattering wave function, since the l labeled wave functions must be coherently combined to generate this information. The orbital scrambling found for the l labeled solutions thus is not related to measurable properties, and instead the correct picture is provided by the J labeled solutions, which do show preservation of alignment. We find that even in the J labeled picture, alignment preservation does not by itself guarantee any specific trend in the alignment ratio for the fine structure transition.

Notes: We developed a self-consistent three-dimensional reference interaction site model integral equation theory with the molecular hypernetted chain closure (SC-3D-RISM/HNC) for studying thermochemistry of solvation of ionic solutes in a polar molecular solvent. It is free from the inconsistency in the positions of the ion–solvent site distribution peaks, peculiar to the conventional RISM/HNC approach and improves the predictions for the solvation thermodynamics. The SC-3D-RISM treatment can be readily generalized to the case of finite ionic concentrations, including the consistent dielectric corrections to provide a consistent description of the dielectric properties of ion–molecular solution. The proposed theory is tested for hydration of the Na+ and Cl− ions in ambient water at infinite dilution. An improved agreement of the ion hydration structure and thermodynamics with molecular simulation results is found as compared to the conventional RISM/HNC treatment. © 2000 American Institute of Physics.

Notes: We study the hydration structure and free energy of several conformations of Met-enkephalin in ambient water by employing the one-dimensional (1D) as well as three-dimensional (3D) reference interaction site model (RISM) integral equation theories, complemented by the hypernetted chain (HNC) closure with the repulsive bridge correction (RBC). The RBC contribution to the excess chemical potential of solvation is calculated by means of the thermodynamic perturbation theory (TPT), which crucially reduces computational burden and thus is especially important for a hybrid algorithm of the RISM with molecular simulation. The 3D-RISM/HNC+RBC-TPT approach provides improved prediction of the solvation thermodynamics and gives a detailed description of the solvation structure of a biomolecule. The results obtained are discussed and compared to those following from the 1D-RISM/HNC theory. The latter yields physically reasonable results for the conformational stability of biomolecules in solution, which is further improved by adding the 1D-RBC. The modified, 1D-RISM/HNC+RBC-TPT integral equation theory combined with the simulated annealing or generalized-ensemble Monte Carlo simulation methods is capable of reliable prediction of conformations of biomolecules in solution with due account for the solvent effect at the microscopic level. © 2000 American Institute of Physics.

Notes: We have developed a self-consistent description of an interface between a metal and a molecular liquid by combination of the density functional theory in the Kohn–Sham formulation (KS DFT) for the electronic structure, and the three-dimensional generalization of the reference interaction site model (3D RISM) for the classical site distribution profiles of liquid. The electron and classical subsystems are coupled in the mean field approximation. The procedure takes account of many-body effects of dense fluid on the metal–liquid interactions by averaging the pseudopotentials of liquid molecules over the classical distributions of the liquid. The proposed approach is substantially less time-consuming as compared to a Car–Parrinello-type simulation since it replaces molecular dynamics with the integral equation theory of molecular liquids. The calculation has been performed for pure water at normal conditions in contact with the (100) face cubic centered (fcc) surface of a metal roughly modeled after copper. The results are in good agreement with the Car–Parrinello simulation for the same metal model. The shift of the Fermi level due to the presence of water conforms with experiment. The electron distribution near an adsorbed water molecule is affected by dense water, and so the metal–water attraction follows the shapes of the metal effective electrostatic potential. For the metal model employed, it is strongest at the hollow site adsorption positions, and water molecules are adsorbed mainly at the hollow and bridge site positions rather than over metal atoms. Layering of water molecules near the metal surface is found. In the first hydration layer, adsorbed water molecules are oriented in parallel to the surface or tilted with hydrogens mainly outwards the metal. This orientation at the potential of zero charge agrees with experiment. © 1999 American Institute of Physics.

Notes: A novel broadband femtosecond version of the stimulated emission pumping (SEP) technique is demonstrated. A nonstationary ground state of a molecular sample in the condensed phase is prepared by two optical pulses. The first picosecond PUMP pulse resonantly excites the sample. The second femtosecond DUMP pulse, which is tuned to the molecular fluorescence band, is applied after relaxation in the excited state and creates a "particle" in the ground state and a "hole" in the excited state. The relaxation of this system is probed by a femtosecond supercontinuum. An advantage of the proposed scheme is that the hole contribution is constant for certain conditions, and hence, the transient absorption spectrum of the particle may be isolated. As an application of the technique, the ground-state evolution of coumarin 102 in acetonitrile is studied. Intramolecular vibrational redistribution (IVR), with a characteristic time τIVR∼10 fs, is observed in the frequency domain. Subsequently, the absorption band shifts to the blue and shows isosbestic points in the course of relaxation. © 1998 American Institute of Physics.

Notes: Transient absorption measurements of aminonitrofluorene in acetonitrile reveal for the first time an oscillatory behavior in the dynamic Stokes shift of stimulated emission. The measured relaxation curve for the maximum of the stimulated emission band is in excellent agreement with the solvation correlation function C(t) obtained from the simple continuum theory of dipolar solvation. © 1998 American Institute of Physics.

Notes: Transient absorption and gain spectra of the styryl dye LDS-750 in solution have been studied by the pump/supercontinuum probe (PSCP) technique with excitation at 530 nm. The pump/probe intensity correlation width was 70 fs, providing a time resolution of 40 fs. Spectra were detected in the range 400–800 nm with 1.5 nm resolution. Before 70 fs, prominent spectral structure is observed due to resonant Raman scattering from a 1500 cm−1 active mode of the chromophore. At later time, the gain spectrum undergoes an ultrafast redshift and change of shape, with time constants of ∼200 and ∼600 fs for acetonitrile and chloroform solutions, respectively. At high pumping energy (1.2 μJ), the final emitting state is reached by internal conversion from higher electronic states without a further essential Stokes shift. The emitting state is assigned to an excited isomeric form of the molecule. At low pumping energy (0.3 μJ), the first excited electronic state isomerizes in an ultrafast process followed by a slower process, the dynamics of which is controlled by the solvent. The geometrical and electronic nature of these processes and their coupling to the solvent needs further clarification. © 1997 American Institute of Physics.

Notes: We modify the site–site as well as three-dimensional (3D) versions of the reference interaction site model (RISM) integral equations with the hypernetted chain (HNC) closures by adding a repulsive bridge correction (RBC). The RBC treats the overestimation of water ordering around a hydrophobic solute in the RISM/HNC approximation, and thus refines the entropic component in the hydration free energy. We build up the bridge functions on r−12 repulsive core potentials, and propose RBC expressions for both the site–site and 3D-RISM approaches. To provide fast calculation, we obtain the excess chemical potential of hydration by using the thermodynamic perturbation theory (TPT). The site–site RISM/HNC+RBC as well as 3D-RISM/HNC+RBC approaches are applied to calculate the structure and thermodynamics of hydration of rare gases and alkanes in ambient water. For both approaches, the RBC drastically improves the agreement of the hydration chemical potential with simulation data and provides its correct dependence on the solute size. For solutes of a nonspherical form, the 3D treatment yields the hydration structure in detail and better fits simulation results, whereas the site–site approach is essentially faster. The TPT approximation gives the hydration thermodynamics in good qualitative agreement with the exact results of the thermodynamic integration, and substantially reduces computational burden. The RBC–TPT approximation can improve the predictive capability of the hybrid algorithm of a generalized-ensemble Monte Carlo simulation combined with the site–site RISM theory, used to describe protein folding with due account for the water effect at the microscopic level. The RBC can be optimized for better fit to reference simulation data, and can be generalized for solute molecules with charged groups. © 2000 American Institute of Physics.