K. Kobashi

Predictions for partial and monolayer coverages of O2 on graphite

Monolayer properties of O2 on graphite are calculated using a pattern recognition, optimization scheme. Equilibrium monolayers are predicted at two different densities with properties in agreement with recent x‐ray diffraction, specific heat, and neutron scattering data. Properties of the extremely low density regime are calculated using a model based upon a distribution of two‐dimensional O2 clusters. The results are consistent with experimental evidence.

Predicted properties of CO monolayers on graphite

Abstract The structure and orientations of CO monolayers deposited on graphite are calculated using a pattern recognition optimization of the energy. The results were found to depend importantly on assumptions made concerning the applicability of various substrate mediated interactions that have been proposed, and on the nature of CO-CO interaction itself. Despite these uncertainties, the calculations clearly show that the low density structures are herringbone with the molecules tilting slightly out of the substrate plane. Although the registered herringbone is energetically more favorable at ϱ/ϱ0 = 1, where ϱ0 is the density at registry, than the unregistered herringbone, its thermodynamic stability depends on the magnitude of the substrate mediated interactions incorporated into the calculations. With increasing density a minimum in the energy occurs at ϱ/ϱ 0 ⋍ 1.12 . The equilibrium structure for all densities ϱ/ϱ0 ≳ 1.12 is a four sublattice pinwheel arrangement that is thermodynamically stable. Thus, these results are generally in agreement with experiment. Contrary to experiment, however, is the prediction of orientational ordering with respect to the two dissimilar ends of the molecules, and reasons for this difference are presented. Properties of small clusters of N molecules deposited on graphite, 1 ≤ N ≤ 6, are also calculated. They register on the substrate which supports evidence that registered islands of CO form in the region 0≤ϱ/ϱ0≤1.

Raman spectra of solid CH4 under high pressure. II. New phases below 9 kbar at 4.2 K

We have studied the Raman spectra of solid CH4 at 4.2 K below 9 kbar. The vibration‐libration structures of the intramolecular ν1–ν4 spectra were essentially the same as the previous data of 1.5 kbar, so that the three‐site (S1, S2, and W) model of phase III proposed in paper I was utilized to analyze the observed data. It is noteworthy that one of the three bands in the ν4 spectrum exhibits a decrease in frequency with pressure. This ’’softening’’ is interpreted as a result of distortion of the local potential fields of methanes with pressure. On the other hand, the lattice Raman spectrum exhibits a marked change in the II–III phase transition at 0.5 kbar, and furthermore shows changes in the splittings of certain bands at 1.9 and 4.9 kbar. In the ν‐P plots of both intramolecular and lattice vibrational bands, prominent discontinuities are observed at 1.9 and 4.9 kbar, indicative of the existence of phase II (0–0.5 kbar), phase III (0.5–1.9 kbar), and new phases IV (1.9–4.9 kbar) and V (4.9 kbar), below ...

High pressure properties of solid α‐O2

The static and dynamic properties of solid oxygen are calculated vs pressure at zero temperature using a pattern recognition optimization scheme and harmonic lattice dynamics method. The lattice parameters, phonon and libron dispersion curves, acoustic sound velocities, compressibility, root mean square translational and librational fluctuations from equilibrium, and the pressure dependence of the intramolecular stretching mode are calculated. It is shown that the attractive magnetic interaction strongly influences the behavior of the solid at all pressures. A soft mode induced phase transition from the monoclinic α structure to an orthorhombic structure is predicted at zero temperature near 6 kbar. No volume change is observed at the transition.

Lattice dynamics of solid I2 under high pressure

A calculation of the lattice dynamics of solid I2 is made on the basis of a semiphenomenological potential with terms describing the intramolecular interaction, orientation‐dependent charge transfer interactions, and nonbonding interactions. In addition, two terms describing a coupling between intramolecular vibrations of different molecules and a vibrational coupling between charge transferred bonds are included. At normal pressure, calculated vibrational frequencies are in good agreement with Raman and neutron scattering data. Agreement with experiment at high pressures requires that some potential parameters be volume dependent, and the implication of this result to the pressure induced change in the electronic states of solid I2 is discussed. The merging of the intramolecular Ag and B3g Raman bands upon compression is explained quantitatively. The pressure dependence of several lattice vibrational mode frequencies are predicted but the observed softening of the librational Ag mode is not reproduced. T...

Raman spectra of solid CH4 under high pressure. III. New high pressure phases in solid CH4 and CD4

We have observed the Raman spectra of the intramolecular vibrations in solid CH4 and CD4 at T=4.2 K under high pressure. For solid CH4, the spectra changed markedly at P≂9 kbar, indicative of a new phase VI above this pressure. For solid CD4, an analogous change was observed at P≂5.4 kbar, indicating a phase transition from phase III to a new phase IV. The spectra of these new phases, CH4 VI and CD4 IV, are very similar so that it is concluded that the crystal structures of the phases are isomorphous. The observed spectra seem to indicate that the molecular orientations of methane molecules in the new high pressure phases are fully ordered.

Raman spectra of solid benzene under high pressure

Abstract Raman spectra of solid benzene have been measured at room temperature up to about 140 kbar, using the diamond anvil cell. Effort has been focused upon the lattice vibration spectra at pressures above that of phase II. It is found that a change in slopes occurs in the frequency-pressure curves at about 40 kbar. Furthermore, a new band appears above 90 kbar. These features probably correspond respectively to the II–III phase transition, which has been reported previously, and a III–IV phase transition, reported here for the first time.

A mean field calculation of the librational properties of δ phase O2 monolayers on graphite

A mean field approximation is used to calculate the librational properties of δ‐phase molecular oxygen monolayers on graphite at temperatures 0≤T≤20 K. The frequencies of the two libron modes are found to occur at 35.07 and 60.28 cm−1. The libron specific heat and the rms orientational fluctuations of the O2 molecules about equilibrium are also determined. It is found that the contribution of zero‐point orientational fluctuations to librational behavior is large. The molecules exhibit well defined small angle librational motion at low temperatures and there is no evidence of an orientational order–disorder phase transition prior to melting at 25.6 K.

Theoretical study of the surface structures and vibrations of solid α‐N2 a)

The structures and vibrations of the (001), (110), and (111) surfaces of solid α‐N2 at zero temperature have been studied using the modified Thiery–Chandrasekharan potential model. The equilibrium structures of the semi‐infinite surfaces are determined using a static energy minimization scheme, and the Einstein frequencies for both translational vibrations and librations of the molecules near the surfaces, and their root mean square (rms) oscillation amplitudes, are calculated. Based upon the equilibrium surface structures, the lattice dynamical calculations for finite thickness slabs [20 layers for the (001) and (110) surfaces and 10 layers for the (111) surfaces] have also been made for wave vectors along certain paths in the two‐dimensional Brillouin zones. It is found that the molecular centers of mass positions, orientations, and bond lengths for the molecules in the vicinity of the surfaces, as well as the Einstein frequencies and rms oscillation amplitudes, are significantly different from their bu...

High pressure effects on the Raman spectra of solid C6H6

Raman spectra of solid benzene have been investigated under high pressures up to 15 GPa at room temperature using a diamond anvil cell. The pressure dependences of the intramolecular and lattice vibration modes were used to identify different solid phases of C6H6. It is found that discontinuities and changes in slopes occur in the ν(P) curves at about 4 GPa. This probably corresponds to the II–III phase transition which has been reported previously. Moreover, a new band appears in the lattice vibration spectra, above 9 GPa, indicative of a new phase (IV). The controversy over the location of triple points previously observed is discussed with respect to our results. A schematic P-T phase diagram, where speculative phase boundaries II–III and III–IV are drawn, is suggested.