Theoretical study of the finite temperature spectroscopy in van der Waals clusters. III Solvated Chromophore as an effective diatomics

The absorption spectroscopy of calcium-doped argon clusters is described in terms of an effective diatomics molecule Ca-(Ar_n), in the framework of semiclassical vertical transitions. We show how, upon choosing a suitable reaction coordinate, the effective finite-temperature equilibrium properties can be obtained for the ground- and excited-surfaces from the potential of mean force (PMF). An extension of the recent multiple range random-walk method is used to calculate the PMF over continuous intervals of distances. The absorption spectra calculated using this single-coordinate description are found to be in good agreement with the spectra obtained from high-statistics Monte Carlo data, in various situations. For CaAr$_{13}$, we compare the performances of two different choices of the reaction coordinate. For CaAr_37, the method is seen to be accurate enough to distinguish between different low-energy structures. Finally, the idea of casting the initial many-body problem into a single degree of freedom problem is tested on the spectroscopy of calcium in bulk solid argon.Comment: 8 pages, 9 figure

Channelized melt flow in downwelling mantle: Implications for 226Ra-210Pb disequilibria in arc magmas

We present the results of an analytical model of porous flow of viscous melt into a steadily dilating ‘‘channel’’ (defined as a cluster of smaller veins) in downwelling subarc mantle. The model predicts the pressure drop in the mantle wedge matrix surrounding the channel needed to drive melt flow as a function of position and time. Melt is sucked toward the dilatant region at a near-constant velocity (105 s1) until veins comprising the channel stop opening (t = t). Fluid elements that complete their journey within the time span t < t arrive at a channel. Our results make it possible to calculate the region of influence sampled by melt that surrounds the channel. This region is large compared to the model size of the channelized region driving flow. For a baseline dilation time of 1 year and channel half width of 2 m, melt can be sampled over an 80-m radius and has the opportunity to sample matrix material with potentially contrasting chemistry on geologically short timescales. Our mechanical results are consistent with a downgoing arc mantle wedge source region where melting and melt extraction by porous flow to a channel network are sufficiently rapid to preserve source-derived 238U-230Th-226Ra, and potentially also 226 Ra-210Pb, disequilibria, prior to magma ascent to the surface. Since this is the rate-determining step in the overall process, it allows the possibility that such short-lived disequilibria measured in arc rocks at the surface are derived from deep in the mantle wedge. Stresses due to partial melting do not appear capable of producing the desired sucking effect, while the order of magnitude rate of shear required to drive dilation of 107 s1 is much larger than values resulting from steady state subduction. We conclude that local deformation rates in excess of background plate tectonic rates are needed to ‘‘switch on’’ the dilatant channel network and to initiate the sucking effect

Theoretical study of finite temperature spectroscopy in van der Waals clusters. II Time-dependent absorption spectra

Using approximate partition functions and a master equation approach, we investigate the statistical relaxation toward equilibrium in selected CaAr$_n$ clusters. The Gaussian theory of absorption (previous article) is employed to calculate the average photoabsorption intensity associated with the 4s^2-> 4s^14p^1 transition of calcium as a function of time during relaxation. In CaAr_6 and CaAr_10 simple relaxation is observed with a single time scale. CaAr_13 exhibits much slower dynamics and the relaxation occurs over two distinct time scales. CaAr_37 shows much slower relaxation with multiple transients, reminiscent of glassy behavior due to competition between different low-energy structures. We interpret these results in terms of the underlying potential energy surfaces for these clusters.Comment: 10 pages, 9 figure

Response to the Letter to the Editor

This paper has attracted interest around the world from the media (both TV and newspapers). In addition, we have received letters, emails and telephone calls. One of our favorites was a voicemail message asking us to return a call to Australia at which point we would learn who really killed JFK. We welcome the opportunity to respond to the letter to the editor from Mr. Fiorentino. Mr. Fiorentino claims that our ``statement relating to the likelihood of a second assassin based on the premise of three or more separate bullets is demonstrably false.'' In response we would like to simply quote from page 327 of Gerald Posner's book Case Closed, one of the most well known works supporting the single assassin theory: ``If Connally was hit by another bullet, it had to be fired from a second shooter, since the Warren Commission's own reconstructions showed that Oswald could not have operated the bolt and refired in 1.4 seconds.'' Mr. Fiorentino also claims that the ``second fatal flaw is the use of a rather uncomplicated formula based on Bayes Theorem.'' Let $E$ denote the evidence and $T$ denote the theory that there were just two bullets (and hence a single shooter). We used Bayes Theorem to hypothetically calculate $P(T|E)$ from $P(E|T)$ and the prior probability $P(T)$. In order to make $P(T|E)$ ten times more likely than $P(\bar{T}|E)$, the ratio of the prior probabilities [i.e., $P(T) / P(\bar{T})$] would have to be greater than 15. Thus, we again conclude that this casts serious doubt on Dr. Guinn's conclusion that the evidence supported just two bullets. Sadly, this is far from the first time that probability has been misunderstood and/or misapplied in a case of public interest. A notable British example is the Clark case. See Nobles and Schiff (2005) for details. Finally, we welcome and, in fact, encourage members of the scientific community to provide alternative analyses of the data.Comment: Published in at http://dx.doi.org/10.1214/07-AOAS154 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org

Chemical and forensic analysis of JFK assassination bullet lots: Is a second shooter possible?

The assassination of President John Fitzgerald Kennedy (JFK) traumatized the nation. In this paper we show that evidence used to rule out a second assassin is fundamentally flawed. This paper discusses new compositional analyses of bullets reportedly to have been derived from the same batch as those used in the assassination. The new analyses show that the bullet fragments involved in the assassination are not nearly as rare as previously reported. In particular, the new test results are compared to key bullet composition testimony presented before the House Select Committee on Assassinations (HSCA). Matches of bullets within the same box of bullets are shown to be much more likely than indicated in the House Select Committee on Assassinations' testimony. Additionally, we show that one of the ten test bullets is considered a match to one or more assassination fragments. This finding means that the bullet fragments from the assassination that match could have come from three or more separate bullets. Finally, this paper presents a case for reanalyzing the assassination bullet fragments and conducting the necessary supporting scientific studies. These analyses will shed light on whether the five bullet fragments constitute three or more separate bullets. If the assassination fragments are derived from three or more separate bullets, then a second assassin is likely, as the additional bullet would not easily be attributable to the main suspect, Mr. Oswald, under widely accepted shooting scenarios [see Posner (1993), Case Closed, Bantam, New York].Comment: Published in at http://dx.doi.org/10.1214/07-AOAS119 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org

Size effect in the ionization energy of PAH clusters

We report the first experimental measurement of the near-threshold photo-ionization spectra of polycyclic aromatic hydrocarbon clusters made of pyrene C16H10 and coronene C24H12, obtained using imaging photoelectron photoion coincidence spectrometry with a VUV synchrotron beamline. The experimental results of the ionization energy are confronted to calculated ones obtained from simulations using dedicated electronic structure treatment for large ionized molecular clusters. Experiment and theory consistently find a decrease of the ionization energy with cluster size. The inclusion of temperature effects in the simulations leads to a lowering of this energy and to a quantitative agreement with the experiment. In the case of pyrene, both theory and experiment show a discontinuity in the IE trend for the hexamer

Wetting to Non-wetting Transition in Sodium-Coated C_60

Based on ab initi and density-functional theory calculations, an empirical potential is proposed to model the interaction between a fullerene molecule and many sodium atoms. This model predicts homogeneous coverage of C_60 below 8 Na atoms, and a progressive droplet formation above this size. The effects of ionization, temperature, and external electric field indicate that the various, and apparently contradictory, experimental results can indeed be put into agreement.Comment: 4 pages, 4 postscript figure