Direct MD Simulations of Terahertz Absorption and 2D Spectroscopy Applied to Explosive Crystals.

A direct molecular dynamics simulation of the THz spectrum of a molecular crystal is presented. A time-dependent electric field is added to a molecular dynamics simulation of a crystal slab. The absorption spectrum is composed from the energy dissipated calculated from a series of applied pulses characterized by a carrier frequency. The spectrum of crystalline cyclotrimethylenetrinitramine (RDX) and triacetone triperoxide (TATP) were simulated with the ReaxFF force field. The proposed direct method avoids the linear response and harmonic approximations. A multidimensional extension of the spectroscopy is suggested and simulated based on the nonlinear response to a single polarized pulse of radiation in the perpendicular polarization direction.

Trace element concentration in hair samples as an indicator of exposure of population in the Negev, Israel.

The concentration of the toxic elements Ag, As, Cd, Co, Mn, Mo, Pb, Se, and U and the elements Al, Mg, Cu, Fe, and Zn in human hair samples of population living in the north of the Negev Desert in Israel was determined. The study population consisted of three subgroups: Jewish urban population, Bedouin urban population, and Bedouins living in unrecognized villages (the "dispersion"). The main focus is on the differences between these subgroups in an attempt to explore factors responsible for the variation in trace metal contents in hair samples. The results show that the level of several elements, particularly Ag, Mn, and Pb, in the female Bedouin group significantly differed from the other groups in the study. Exploring the reasons for these differences, we concluded that the lifestyle of those women is the main cause. The female Bedouin subgroup is exposed to heavy metals from kitchen utensils, jewelry, and makeup. Therefore, differences in the heavy metal concentration in the hair samples of this group were attributed to the traditional unique lifestyle and social behavior of the females in the Bedouin society.

Intralines of quasi-conical intersections on torsion planes: methylamine as a case study.

Recently we reported on a novel feature associated with the intersection of the two lowest states (1)A' and (1)A'' of the methylamine (J. Chem. Phys. 2008, 128, 244302). We established the existence of a finite (closed) line of conical intersections (ci), namely, a finite seam, located in the HC-NHH symmetry plane, a line that is formed by moving a single hydrogen on that plane while locking the positions of the (six) other atoms. In the present article, this study is extended to the corresponding torsion planes formed by rotating the methyl group around the CN axis. The torsion planes, in contrast with the HC-NHH symmetry plane, do not satisfy the symmetry feature that enables the seam just mentioned. Nevertheless, the calculated nonadiabatic coupling terms (NACTs) resemble features similar to those encountered in the HC-NHH symmetry plane. Following a tedious numerical study supported by a theoretical model (Section III), it was verified that these NACTs may become similar to those on the symmetry plane, sometimes even to the level of almost no distinction, but lack one basic feature; namely, they are not singular and therefore do not form topological effects.

An intraline of conical intersections for methylamine.

In this article are considered the conical intersections (ci's) related to the N-H bond in the methylamine, CH(3)NH(2), molecule. The novel feature that was revealed is that the two lowest states 1A(') and 1A(") are coupled by a line of cis located in HC-NHH plane-a line that is formed by moving a single hydrogen on that plane while fixing the (six) other atoms. The validity of this line was proven first by studying the singularities of the (angular) nonadiabatic coupling terms and then by revealing the degeneracy points formed by the two interacting adiabatic potential energy surfaces (PESs). A theoretical analysis indicated that the line has to be a finite closed line. We also calculated the Berry phase for a contour that surrounds this line and found it to be 3.127 rad, namely, a value reasonably close to pi. The existence of such lines of cis-instead of isolated cis (as exhibited by other n-atomic (n>3) molecules such as HNCO or C(2)H(2))-may enhance significantly the transition rate from an upper adiabatic state to a lower one. There are also numerical advantages in such situations, that is, if such a line is properly placed in that plane (like in the present case) the wave-packet treatment of the nuclei can be carried out employing a single diabatic PES instead of having to consider two coupled PESs.

Dynamics of charge transfer states on metal surfaces: the competition between reactivity and quenching.

The dynamics of excited states of adsorbates on surfaces caused by charge transfer is studied. Both negative and positive charge transfer processes are possible. In particular we are interested in positive charge transfer from a metal surface to molecular or atomic oxygen adsorbed on the surface. Once the negatively charged oxygen on the surface loses an electron it becomes chemically activated. The ability of this species to react depends on the quenching time or back transfer. The analysis of these processes is based on a set of diabatic potential energy surfaces each representing a different charged oxygen species. The dynamics is followed by solving the multichannel time-dependent Schrödinger equation or Liouville von Neumann equation. Due to the nonadiabatic character of these reactions large isotope effects are predicted.

Charge-transfer reactions in atom scattering from ionic surfaces: a time-dependent wavepacket approach.

A diabetic description of charge transfer between atoms and ionic surfaces is presented, specifically examining the F/LiF(100) and F/KI(100) systems for which experiment shows ion formation to be very efficient. Potential energy surfaces describing the energetics for these systems have been generated with a semi-empirical scheme. At the site of charge exchange, there is a curve-crossing between the ground state and the state representing charge capture by the projectile. Quantum dynamics calculations with time-dependent wavepacket methods give an initial ion-formation probability of unity for all cases considered. At lowest energies, the ions cannot escape the surface, giving an effective threshold for negative-ion production very close to that observed in experiment. Re-neutralization by charge transfer back to the conduction band of the solid is also examined.