The Journal of Physical Chemistry, Vol. 87, No. 27, 1983 4337
ecules depends, of course, on the details of the NE vs. E curve (Figure 5 ) , on temperature, and on the impact frequencies of adatoms. It has been done for specific astrophysical applications.
A. G. G. M. Tielens: In the problem of the migration of species on a surface, not the binding energy but the barrier against diffusion is important. This one is much lower. Basically because the species does not need to pass through the matrix molecules. Did you use this barrier against diffusion in your calculations or the binding energy? Smoluchowski: Anderson, Mott, and others have shown that there are many ways to describe electronic conduction (or diffusion of light atoms) in amorphous systems. In the scheme used here the role of the classical barrier is replaced by the rate of decay of the wave function a. Thus the barrier does not enter explicitly into the formalism. The height of the effective barriers in an amorphous system can be treated by using percolation theory but this leads to considerable mathematical complications which do not seem to be warranted a t the present time.
S. A . Rice: Do you have any feeling for how robust your conclusions are with respect to variation of the interaction potential (which is not well-known)? Smoluchowski: The absolute values on the density of states vs. binding energy diagram (Figure 5) would be undoubtedly affected by changes in the assumed potential. On the other hand, the random aspect of the correlation function is so important that the fundamental difference between shallow and deep sites and the resulting low mobility would remain unchanged. J . Klinger: I wonder about the difference you make between the low temperature and high temperature amorphous ice? I thought that there is rather a smooth transition due to rearrangements tending to an energetically more favorable state. Smoluchowski: Actually I used in my calculations the correlation function only for the low density amorphous ice. If the ice were more dense then the binding energies would be 10-15% higher but the main conclusions of the paper would remain the same.
Interactions of Cometary Ices with the Interplanetary Medium (S. Wyckoff) W. L. Brown: Have there been measurements on successive passes of a single comet that would suggest the comet is losing mass? Can it accrete? Is the optical density thick or thin for either the UV from the sun or the emitted radiation? Wyckoff: Yes. Several comets have short enough orbital periods that such measurements have been made. The shortest known period is 3 years for comet Encke. Measurements of production rates of successive passages of comet Encke around the sun indicate, marginally, a secular reduction in the total gas production rates. For most wavelength regions [for most comets (not the largest)], a t most heliocentric distances from the sun (r 1 1 AU), the atmosphere of a comet is optically thin.
J . Klinger: Just a comment concerning the decrease of the activity during successive revolutions: Yeoman’s calculated back the orbit of comet Halley until about 2000 years BC. He found that nongravitational forces did not change very much. My personal idea is that the evolution in activity may be strongly dependent on orbital parameters. P. Weissman: On the problem of near-extinct comets there are two which for both dynamical and physical reasons we believe are near exhaustion. The first, Arend-Rigaux, was extensively observed in 1977 but proved to still be active. However, useful information was obtained. The second, periodic comet Neujmin I, will return in 1984 and there are hopes that it will be even closer to extinction. R. Smoluchowski: (1)H 2 0 tail or coma can be seen at much larger distances than 3 AU when the crystallization of amorphous ice is included. Sekanine seems to have observed this. ( 2 ) Heat penetrating into the nucleus should produce changes in its porosity, density, and chemical composition. Evidence is still rather shaky.
P. Boutron: How is the diameter of the nucleus of a comet measured? Wyckoff: The only direct measurement of the diameter of a comet has been by radar techniques. Two comets have now been deteded by radar by the group at MIT using the Arecibo telescope. The radius of comet Encke was found to be 2 i= 1 km (Kamoun, P.; Pettengill, G.; Shapiro, I. In “Comets”, Wilkening, L., Ed.; University Arizona Press: Tucson, 1982).
Ice in Space (A. M. Greenberg) T. V. Johrwon: Have you made enough of the organic residue to obtain reflection spectra of it? I am sure that those of us interested in meteorite and asteroid spectra would be very interested. Greenberg: We have infrared and visible absorption spectra of a number of organic residences. We are planning to do much more with the organic refractories starting this fall when I will have one cryogenic setup devoted fully to producing it in sufficient quantities to be studied both chemically and physically.
W. L. Brown: What is the spectral character of the light flashes-explosions? Do the explosions play any role in your model in ejecting material from a grain? Greenberg: We have not analyzed the spectrum of the flashes. We do have spectra of the chemiluminescence of samples which have not flashed. J. Klinger: In your IR spectrum we see the H20libration band. As far as I know this feature has not yet been found in interstellar absorptions. May this be due to an overlap with the silicate bands? Greenberg: In our amorphous mixture, irradiated or otherwise, the libration band of H20 is both suppressed somewhat and shifted toward lower frequencies thus putting it outside of the region of the silicate band at 9.7 pm. We believe this is the reason it is not seen in space.
Possibility of Anomalous Relaxation Due to the Charge Dislocation Process (K. Itagaki) V. F. Petrenko: What is the main damping mechanism for dislocation motion in your model? Does the damping constant B increase or decrease when the temperature increases? Itagaki: I do not know or not want to speculate on the damping mechanism. But the damping constant B appears through the relaxation time, t = BL2/(a2C)and t apparently follows a thermally activated process having an activation energy about 0.6 eV within the range of 0 -70 “C.Therefore, B increases as the temperature decreases.
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Stability of the Vitreous Phase in Water-Ethylene Glycol Mixtures Studies by Internal Friction (J. Perez) J. Glen: Is there any evidence of whether the nucleation is of cubic or hexagonal ice, or perhaps of two nucleation processes? Perez: In fact in the case of our experiments, due to the low concentration in ethylene glycol, only about 1%of the specimen is vitreous: the rest of the fraction is hexagonal ice. So it is probable that there is not a problem of ice nucleation. The composition of vitreous islets being that (or not too far of that) of eutectic, one may think that some hydrate of ethylene glycol is concentrated by nucleation.
Ab Initio Molecular Orbital Estimates of Charge Partitioning between Bjerrum and Ionic Defects in Ice (S. Scheiner) J . Warman: You do not include lattice distortion around the positive ion in your calculations. Would this have an effect on the results? Scheiner: No. Fortunately, such effects are completely irrelevant because the changes that occur upon migration are entirely in the trail of reversed polarization left behind the defect and no changes take place in the local defect region. This is part of the general theory of defect changes of Onsager, Jaccard, and
