M. A. Schmidt

Irregular variations in the melting point of size-selected atomic clusters

Small particles have a lower melting point than bulk material. The physical cause lies in the fact that small particles have a higher proportion of surface atoms than larger particles—surface atoms have fewer nearest neighbours and are thus more weakly bound and less constrained in their thermal motion, than atoms in the body of a material. The reduction in the melting point has been studied extensively for small particles or clusters on supporting surfaces. One typically observes a linear reduction of the melting point as a function of the inverse cluster radius,,. Recently, the melting point of a very small cluster, containing exactly 139 atoms, has been measured in a vacuum using a technique in which the cluster acts as its own nanometre-scale calorimeter,. Here we use the same technique to study ionized sodium clusters containing 70 to 200 atoms. The melting points of these clusters are on average 33% (120u2009K) lower than the bulk material; furthermore, we observe surprisingly large variations in the melting point (of ±30u2009K) with changing cluster size, rather than any gradual trend. These variations cannot yet be fully explained theoretically.

Phase transitions in clusters

Abstract The solid to liquid transition of clusters is discussed, mainly from an experimental point of view. An experiment is sketched which allows to measure the caloric curve of size selected sodium clusters. Melting temperatures, latent heats, and entropies of melting are determined in the size range between 55 and 357 atoms per cluster. The melting temperatures are about 30% less than the bulk value and fluctuate strongly, one additional atom can change it by ±10xa0K. Latent heats and entropies show axa0similar behaviour. From the change in entropy one can deduce the increase of phase space upon melting, which is about twelve orders of magnitude already for Na + 55 and increases exponentially for larger clusters. The theoretical prediction that a finite particle can have a negative heat capacity is verified experimentally, showing that their are differences between a canonical and a microcanonical description. Finally the analogue of the boiling phase transition is shown for a finite system. To cite this article: M. Schmidt, H. Haberland, C.xa0R. Physique 3 (2002) 327–340.

A double/triple time‐of‐flight mass spectrometer for the study of photoprocesses in clusters, or how to produce cluster ions with different temperatures

An apparatus is described for the measurement of photo processes in mass‐selected cluster ions. A gridless reflectron type of time‐of‐flight (TOF) mass spectrometer is used to select an ion of known mass. The selected ion is photoexcited with a pulsed dye laser. The charged fragments are mass separated in a second linear TOF. A new combination of reflectron and linear TOF is described, which allows one to measure the complete photofragment mass distribution over an arbitrary large mass range for each laser shot, which was not possible earlier. It is discussed how clusters with different temperatures can be prepared. The first measurement of a temperature‐dependent effect for mass‐selected free cluster ions is presented.

Melting of free sodium clusters

Abstract.Heat capacities have been measured across the solid to liquid phase transition for free, mass selected sodium cluster ions, Na+n, with 55≤n≤199. Results for the melting point and the latent heat of fusion are presented here. The melting points are about 30% lower than the bulk value, and show large variations (±50xa0K). They seem to be highest in a mass range, where both an icosahedral and an electronic shell closing occurs.

Photofragmentation and photoabsorption cross sections for mass selected argon cluster ions,n=3 to 108

A tandem time-of-flight mass spectrometer is used to measure photofragmentation mass spectra and optical absorption spectra of mass selected argon cluster ions in then=3 to 108 atoms per cluster range.

Search for the Production of Scalar Bottom Quarks in pp Collisions at {radical}(s)=1.96 TeV

We report on a search for direct scalar bottom quark (sbottom) pair production in pp collisions at square root(s) = 1.96 TeV, in events with large missing transverse energy and two jets of hadrons in the final state, where at least one of the jets is required to be identified as originating from a b quark. The study uses a collider detector at Fermilab Run II data sample corresponding to 2.65 fb(-1) of integrated luminosity. The data are in agreement with the standard model. In an R-parity conserving minimal supersymmetric scenario, and assuming that the sbottom decays exclusively into a bottom quark and a neutralino, 95% confidence-level upper limits on the sbottom pair production cross section of 0.1 pb are obtained. For neutralino masses below 70 GeV/c2, sbottom masses up to 230 GeV/c2 are excluded at 95% confidence level.

Measurement of the Ratio {sigma}{sub tt}/{sigma}{sub Z/{gamma}}{sup *}{sub {yields}ll} and Precise Extraction of the tt Cross Section

We report a measurement of the ratio of the t (t) over bar to Z/gamma* production cross sections in root s = 1.96 TeV p (p) over bar collisions using data corresponding to an integrated luminosity of up to 4.6 fb(-1), collected by the CDF II detector. The t (t) over bar cross section ratio is measured using two complementary methods, a b-jet tagging measurement and a topological approach. By multiplying the ratios by the well-known theoretical Z/gamma* -> ll cross section predicted by the standard model, the extracted t (t) over bar cross sections are effectively insensitive to the uncertainty on luminosity. A best linear unbiased estimate is used to combine both measurements with the result sigma(t (t) over bar) = 7.70 +/- 0.52 pb, for a top-quark mass of 172.5 GeV/c(2).We report a measurement of the ratio of the tt to Z/{gamma}* production cross sections in {radical}(s)=1.96 TeV pp collisions using data corresponding to an integrated luminosity of up to 4.6 fb{sup -1}, collected by the CDF II detector. The tt cross section ratio is measured using two complementary methods, a b-jet tagging measurement and a topological approach. By multiplying the ratios by the well-known theoretical Z/{gamma}{sup *{yields}}ll cross section predicted by the standard model, the extracted tt cross sections are effectively insensitive to the uncertainty on luminosity. A best linear unbiased estimate is used to combine both measurements with the result {sigma}{sub tt}=7.70{+-}0.52 pb, for a top-quark mass of 172.5 GeV/c{sup 2}.

Measurements of branching fraction ratios and CP asymmetries in B{sup {+-}{yields}D}{sub CP}K{sup {+-}}decays in hadron collisions

9 paginas, 3 figuras, 2 tablas.-- PACS numbers: 13.25.Hw, 11.30.Er, 14.40.Nd.--CDF Collaboration: et al.

Search for the Decays B{sub (s)}{sup 0}{yields}e{sup +}{mu}{sup -} and B{sub (s)}{sup 0}{yields}e{sup +}e{sup -} in CDF Run II

We report results from a search for the lepton flavor violating decays B{sub s}{sup 0}{yields}e{sup +}{mu}{sup -} and B{sup 0}{yields}e{sup +}{mu}{sup -}, and the flavor-changing neutral-current decays B{sub s}{sup 0}{yields}e{sup +}e{sup -} and B{sup 0}{yields}e{sup +}e{sup -}. The analysis uses data corresponding to 2 fb{sup -1} of integrated luminosity of pp collisions at {radical}(s)=1.96 TeV collected with the upgraded Collider Detector (CDF II) at the Fermilab Tevatron. The observed number of B{sup 0} and B{sub s}{sup 0} candidates is consistent with background expectations. The resulting Bayesian upper limits on the branching ratios at 90% credibility level are B(B{sub s}{sup 0}{yields}e{sup +}{mu}{sup -}) 47.8 TeV/c{sup 2}, and M{sub LQ}(B{sup 0}{yields}e{sup +}{mu}{sup -})>59.3 TeV/c{sup 2}, at 90% credibility level.

First Measurement of the tt Differential Cross Section d{sigma}/dM{sub tt} in pp Collisions at {radical}(s)=1.96 TeV

We present a measurement of the tt[over ] differential cross section with respect to the tt[over ] invariant mass, dsigma/dM_{tt[over ]}, in pp[over ] collisions at sqrt[s]=1.96 TeV using an integrated luminosity of 2.7 fb;{-1} collected by the CDF II experiment. The tt[over ] invariant mass spectrum is sensitive to a variety of exotic particles decaying into tt[over ] pairs. The result is consistent with the standard model expectation, as modeled by PYTHIA with CTEQ5L parton distribution functions.