The ALICE time machine

According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. In such a state the normal nuclear matter could not exist: it is believed that a few microsecond after big-bang the matter underwent a phase transition, from a state called Quark-Gluon Plasma (QGP) to a hadron gas. Some of the unexplained features of the Universe could be explained by the QGP properties. One of the aims of the CERN LHC is to recreate (on a smaller scale) a QGP state, compressing and heating ordinary nuclear matter by means of ultrarelativistic heavy-ion collisions. The ALICE experiment at CERN is dedicated to the study of the medium produced in these collisions : in particular, the study of the heavy quarkonia suppression pattern can give a measure of the temperature reached in these collisions, helping us to understand how close we are getting to the conditions of the starting point of the Universe

Studies on tetrafluoropropene-CO2 based gas mixtures for the Resistive Plate Chambers of the ALICE Muon IDentifier

Due to their simplicity and comparatively low cost Resistive Plate Chambers are gaseous detectors widely used in high-energy and cosmic rays physics when large detection areas are needed. However, the best gaseous mixtures are currently based on tetrafluoroethane, which has the undesirable characteristic of a large Global Warming Potential (GWP) of about 1400 and because of this, it is currently being phased out from industrial use. As a possible replacement, tetrafluoropropene (which has a GWP close to 1) has been taken into account. Since tetrafluoropropene is more electronegative than tetrafluoroethane, it has to be diluted with gases with a lower attachment coefficient in order to maintain the operating voltage close to 10 kV. One of the main candidates for this role is carbon dioxide. In order to ascertain the feasibility and the performance of tetrafluoropropene-CO2 based mixtures, an R&D program is being carried out in the ALICE collaboration, which employs an array of 72 Bakelite RPCs (Muon Identifier, MID) to identify muons. Different proportions of tetrafluoropropene and CO2, with the addition of small quantities of isobutane and sulphur hexafluoride, have been tested with 50x50 cm2 RPC prototypes with 2 mm wide gas gap and 2 mm thick Bakelite electrodes. In the presentation, results from tests with cosmic rays will be presented, together with data concerning the current drawn by a RPC exposed to the gamma-ray flux of the Gamma Irradiation Facility (GIF) at CERN.Comment: 4 pages, 3 figures, proceeding of the 12th International Conference on Position Sensitive Detector

Intermolecular interactions in eumelanins: A computational bottom-up approach. I. small building blocks

The non-covalent interactions between pairs of the smallest eumelanins building blocks, 5,6-dihydroxy-indole (DHI) and its redox derivatives, are subjected to a systematic theoretical investigation, elucidating their nature and commenting on some of their possible effects on the layered structure of eumelanin. An accurate yet feasible protocol, based on second order perturbation theory, was set up and validated herein, and thereafter used to sample the intermolecular potential energy surfaces of several DHI related dimers. From the analysis of the resulting local minima, the crucial role of stacking interactions is assessed, evidencing strong effects on the geometrical arrangement of the dimer. Furthermore, the absorption spectra of the considered dimers in their most stable arrangements are computed and discussed in relation to the well known eumelanin broadband features. The present findings may help in elucidating several eumelanin features, supporting the recently proposed geometrical order/disorder model (Chen et al., Nat. Commun. 2014, 5, 3859)

Predicting light absorption properties of anthocyanidins in solution: a multi-level computational approach

A multi-level computational protocol is devised to calculate the absorption spectra in ethanol solution of a series of anthocyanidins relevant for dye-sensitized solar cells. The protocol exploits the high accuracy of second-order multi-reference perturbation theory to correct the results of the more feasible TD-DFT calculations, which were performed on hundreds of configurations sampled from molecular dynamics (MD) trajectories. The latter were purposely carried out with accurate and reliable force fields, specifically parameterized against quantum mechanical data, for each of the investigated dyes. Besides yielding maximum absorption wavelengths very close to the experimental values, the present approach was also capable of predicting reliable band shapes, even accounting for the subtle differences observed along the homolog series. Finally, the atomistic description achieved by MD simulations allowed for a deep insight into the different micro-solvation patterns around each anthocyanidin and their effects on the resulting dye’s properties. This work can be considered as a step toward the implementation of a computational protocol able to simulate the whole system formed by the organic dye and its heterogeneous embedding that constitutes dye-sensitized solar cells

Perturbative multireference configuration interaction (CI-MRPT2) calculations in a focused dynamical approach: A computational study of solvatochromism in pyrimidine

We have investigated solvatochromic effects over a solvent series of increasing polarity on the prototype molecule pyrimidine as a solute species. The line shape profiles, obtained by a time-dependent approach based on quantum mechanical calculations performed over frames sampled from classical molecular dynamics trajectories, were directly compared to the available experimental bands. The multireference configuration interaction second-order perturbation (CI-MRPT2) calculations are in quantitative agreement with the experiment. The results also confirm how nonprotic solvents can be confidently modeled by continuous solvation models as the polarizable continuum model, whereas protic solvents, as water, require the inclusion of explicit solvent molecules to account for the effects of hydrogen bonds

The phenoxyl group-modulated interplay of cation-π and σ-type interactions in the alkali metal series.

An extensive exploration of the interaction PESs of phenol and catechol complexes with alkali metal cations reveals a striking effect of –OH substitution on the balance between cation-π and σ-type noncovalent interactions