Spectroscopic studies of quantum solvation in 4HeN-N2O clusters

High resolution microwave and infrared spectra of He(N)-N2O clusters were studied in the range N=3 to 12. The apparent cluster moments of inertia increase from N=3 to 6, but then decrease, showing oscillatory behavior for N=7 to 12. This provides direct experimental evidence for the decoupling of helium atoms from the rotation of the dopant molecule in this size regime, signaling the transition from a molecular complex to a quantum solvated system and directly exploring the microscopic evolution of "molecular superfluidity."     

Reweighted ensemble dynamics simulations:Theory,improvement,and application

Based on multiple parallel short molecular dynamics simulation trajectories,we designed the reweighted ensemble dynamics(RED) method to more efficiently sample complex(biopolymer) systems,and to explore their hierarchical metastable states.Here we further present an improvement to depress statistical errors of the RED and we discuss a few keys in practical application of the RED,provide schemes on selection of basis functions,and determination of the free parameter in the RED.We illustrate the application of the improvements in two toy models and in the solvated alanine dipeptide.The results show the RED enables us to capture the topology of multiple-state transition networks,to detect the diffusion-like dynamical behavior in an entropy-dominated system,and to identify solvent effects in the solvated peptides.The illustrations serve as general applications of the RED in more complex biopolymer systems.     

Transition from molecular complex to quantum solvation in 4HeNOCS

We present fully quantum calculations of the rotational energy levels and spectroscopic rotational constants of the linear OCS molecule in variable size clusters of 4He. The rotational constants of OCS are found to decrease monotonically from the gas phase value as the number of helium atoms increases to N=6, after which the average constant increases to saturation at the large droplet value by N=20. The minimum is shown to indicate a transition from a molecular complex to a quantum solvated molecule, with the former characterized by floppy but near rigid behavior, while the latter is characterized by nonzero permutation exchanges and a smaller extent of rigid coupling.     

Folding and unfolding of an elastinlike oligopeptide: "inverse temperature transition," reentrance, and hydrogen-bond dynamics

The temperature-dependent behavior of a solvated oligopeptide, GVG(VPGVG), is investigated. Spectroscopic measurements, thermodynamic measurements, and molecular dynamics simulations find that this elastinlike octapeptide behaves as a two-state system that undergoes an "inverse temperature" folding transition and reentrant unfolding close to the boiling point of water. A molecular picture of these processes is presented, emphasizing changes in the dynamics of hydrogen bonding at the protein/water interface and peptide backbone librational entropy.     

Formation of associates of solvated cations in salt solutions in polar dielectric liquids

Experimental results are presented on excitation of oscillations of polarization charges of solvated ions in an aqueous solution of a mixture of cerium and lead nitrates under an external asymmetric electric field and the directed motion of solvated cerium and lead cations, caused by this process. The applicability of the approximation of the existence of a self-consistent electric field in the solution volume to explain the observed frequency ranges of the manifestation of the effect of electroinduced selective drift of solvated cations is demonstrated. The results of a theoretical analysis of the possibility of forming associates from solvated ions are presented.     

An Error Analysis for the Finite Element Approximation to the Steady-State Poisson-Nernst-Planck Equations

Poisson-Nernst-Planck equations are a coupled system of nonlinear partial differential equations consisting of the Nernst-Planck equation and the electrostatic Poisson equation with delta distribution sources, which describe the electrodiffusion of ions in a solvated biomolecular system. In this paper, some error bounds for a piecewise finite element approximation to this problem are derived. Several numerical examples including biomolecular problems are shown to support our analysis.     

Study of electron solvation in polar solvents using path integral calculations

A path-integral Monte Carlo algorithm for the simulation of electrons solvated in polar liquids is briefly outlined, and results are presented for the electron solvated in liquid water.     

Mechanistic models of the ion-solvation shell system

The eigenfrequencies of solvation shell oscillations with respect to the ion are estimated in various approximations describing the ion–solvation shell system. The results of comparisonwith experiments suggest that the most appropriate is the model in which the solvated ion is considered to be a spherical rotator formed either by the entire solvation shell or by a layer of solvent molecules adjacent to the outer boundary of the solvation shell.     

Transfer, breakup, and fusion reactions of 6He with 209Bi near the Coulomb barrier

The fusion of ⁶He with a ²⁰⁹Bi target displays a large enhancement at energies near to and below the Coulomb barrier. Recently, a ⁴He group of remarkable intensity, which dominates the total reaction cross-section, has also been observed in the near-barrier interaction of the same system. It is argued that this transfer/breakup channel acts as a doorway state to fusion. Received: 1 May 2001 / Accepted: 4 December 2001     

Stability of small mixed clusters of 4He and 3He atoms

The existence of small helium clusters containing a variable number of 4He and 3He atoms is studied within a variational Monte Carlo calculation employing the Aziz HFD-B(HE) pair interaction. The clusters 4He(2) support one and two 3He atoms; however, the system with three 3He atoms is metastable, and the next bound system requires at least 18 fermions. All clusters obtained by adding 3He atoms to the trimer 4He(3) and the tetramer 4He(4) are bound, but the clusters 4He(3)-3He(3,4,5) and 4He(3,4)-3He(9) are metastable. All remaining clusters with three or more bosons and any number of fermions are stable.     

Investigation of the electronic structure of Be2+He and Be+He,and static dipole polarisabilities of the helium atom

The potential energy and spectroscopic constants of the ground and many excited states of the Be⁺He van der Waals system have been investigated using a one-electron pseudo-potential approach, which is used to replace the effect of the Be²⁺ core and the electron-He interactions by effective potentials. Furthermore, the core–core interactions are incorporated. This permits the reduction of the number of active electrons of the Be⁺He van der Waals system to only one electron. Therefore, the potential energy of the ground state as well as the excited states is performed at the SCF level and considering the spin–orbit interaction. The core–core interaction for Be²⁺He ground state is included using accurate CCSD (T) calculations. Then, the spectroscopic properties of the Be⁺He electronic states are extracted and compared with the previous theoretical and experimental studies. This comparison has shown a very good agreement for the ground and the first excited states. Moreover, the transition dipole moment has been determined for a large and dense grid of internuclear distances including the spin orbit effect. In addition, a vibrational spacing analysis for the Be²⁺He and Be⁺He ground states is performed to extract the He atomic polarisability.     

Electron-Induced Breakup of 3He: Past Achievements, Ongoing Efforts, and a Wish List

The ³He nucleus is a calculable nuclear system (Carlson and Schiavilla in Rev Mod Phys 70:743, 1998) for which our theoretical understanding can be compared with data to an increasingly accurate degree. In turn, the theory faces extensive precise data sets from a series of experiments. Two laboratories are leading the way: MAMI/A1 and the Thomas Jefferson National Accelerator Facility (TJNAF). This paper reviews some of the outstanding current experimental efforts with (polarized or unpolarized) electron scattering on (polarized or unpolarized) ³He targets in these laboratories.     

Relaxation phenomena in water - n-methylmorpholin n-oxide binary mixtures

Binary mixtures composed of water and N-methylmorpholin N-oxide (NMMNO) were studied in the concentration range 0.19 m to 26.5 m with shear and longitudinal waves. The dependence on temperature and molality of 1)the ultrasonic spectrum, 2)the real part of the complex mechanical impedance and 3)the ultrasonic velocity indicates equilibrium between bulk and solvated water with increasing NMMNO concentration and organisation of the structure of the system.     

Evidence for a new low-lying resonance state in 7He

Low-lying resonance states in 7He(6He+n), formed after fragmentation reactions of a 227 MeV/nucleon 8He beam on a carbon target, have been studied. Coincidences between 6He nuclei and neutrons, corresponding to the one-neutron knockout channel in 8He, were selected. The relative energy spectrum in the 6He+n system shows a structure, which is interpreted as the 7He ( Ipi = 3/2(-)) ground state, unbound with 0.43(2) MeV relative to the 6He+n system and a width of Gamma = 0.15(8) MeV overlapping with an excited ( Ipi = 1/2(-)) state observed at 1.0(1) MeV with a width of Gamma = 0.75(8) MeV.     

Formalism for the T(d,n)4He and 3He(d,p)4He reactions

The formalism required to describe the T(d, n)⁴He or ³He(d, p)⁴He reaction when all particles may be polarized is discussed. The relation among the various sets of spin-1 tensors is stated explicitly, so that the formulas applying to a particular experiment may be written easily in terms of any desired system.     

Influence of Solvents on the Spectral and Luminescent Properties of Phenol

The features of the absorption and fluorescence spectra of phenol in homogeneous and binary solvents have been investigated. The spectral characteristics of normal-hydrogen-bonded phenol–solvent complexes are compared to those of solvated anions. To explain the extinction of the fluorescence of the solvated anion, results of quantum-chemical calculations are involved.     

Instability of the uniform distribution of electrons solvated in ammonia to transition to a 2D periodic state

The possibility of transition of uniformly distributed solvated electrons in metal-ammonia solutions to a 2D periodic state is considered. The temperature and concentration criteria for such a transition are established by solving nonlinear integral equations for distribution functions. The concentration dependence of the constant periodic component is determined. It is found that at a temperature below the critical point, the 2D distribution of solvated electrons is more efficient than the uniform distribution from the energy’s point of view.     

Infrared laser spectroscopy of diacetylene and cyclopropane in helium nanodroplets

High resolution infrared laser spectroscopy has been used to obtain rotationally resolved vibrational spectra of diactyelene and cyclopropane in helium nanodroplets. In part, this work was motivated by the need for a large database of rotational constants for molecules solvated in liquid helium. Data of this type provides benchmarks with which to test the theoretical methods that are currently being developed for calculating the effect of the helium on the rotational dynamics of these solvated molecules. In general, the correlated motion of the helium and the molecule results in an effective moment of inertia that is considerably larger than that of the gas phase molecule.     

Magnetic trapping of silver and copper, and anomalous spin relaxation in the ag-he system

We have trapped large numbers of copper (Cu) and silver (Ag) atoms using buffer-gas cooling. Up to 3 x 10{12} Cu atoms and 4 x 10{13} Ag atoms are trapped. Lifetimes are as long as 5 s, limited by collisions with the buffer gas. Ratios of elastic to inelastic collision rates with He are >or=10{6}, suggesting Cu and Ag are favorable for use in ultracold applications. The temperature dependence of the Ag-3He collision rate varies as T;{5.8+/-0.4}. We find that this temperature dependence is inconsistent with the behavior predicted for relaxation arising from the spin-rotation interaction, and conclude that the Ag-3He system displays anomalous collisional behavior in the multiple-partial wave regime. Gold (Au) was ablated into 3He buffer gas, however, atomic Au lifetimes were observed to be too short to permit trapping.     

An open-access, very-low-field MRI system for posture-dependent 3He human lung imaging

We describe the design and operation of an open-access, very-low-field, magnetic resonance imaging (MRI) system for in vivo hyperpolarized ³He imaging of the human lungs. This system permits the study of lung function in both horizontal and upright postures, a capability with important implications in pulmonary physiology and clinical medicine, including asthma and obesity. The imager uses a bi-planar B₀ coil design that produces an optimized 65 G (6.5 mT) magnetic field for ³He MRI at 210 kHz. Three sets of bi-planar coils produce the x, y, and z magnetic field gradients while providing a 79-cm inter-coil gap for the imaging subject. We use solenoidal Q-spoiled RF coils for operation at low frequencies, and are able to exploit insignificant sample loading to allow for pre-tuning/matching schemes and for accurate pre-calibration of flip angles. We obtain sufficient SNR to acquire 2D ³He images with up to 2.8 mm resolution, and present initial 2D and 3D ³He images of human lungs in both supine and upright orientations. ¹H MRI can also be performed for diagnostic and calibration reasons.