There exists a surprising theoretical prediction for a small system: its microcanonical heat capacity can become negative. An increase of energy can-under certain conditions-lead to a lower temperature. Here we present experimental evidence that a cluster containing exactly 147 sodium atoms does indeed have a negative microcanonical heat capacity near its solid to liquid transition.
Melting temperatures of Na clusters show size-dependent fluctuations that have resisted interpretation so far. Here we discuss that these temperatures, in fact, cannot be expected to exhibit an easily understandable behavior. The energy and entropy differences between the liquid and the solid clusters turn out to be much more relevant parameters. They exhibit pronounced maxima that correlate well with geometrical shell closings, demonstrating the importance of geometric structure for the melting process. Icosahedral symmetry dominates, a conclusion corroborated by new photoelectron spectra measured on cold cluster anions. In the vicinity of the geometrical shell closings the measured entropy change upon melting is in good agreement with a simple combinatorial model.
Energetic and entropic influences on the melting temperatures of size selected sodium clusters are experimentally separated. It is shown that the energetic difference between solid and liquid is the leading influence for the still puzzling features in the size dependence of sodium melting points. Additionally, this energy difference decreases towards smaller cluster sizes and causes steplike melting phase transitions to vanish. The entropy difference between solid and liquid has been found to be strongly correlated with the energy and causes a pronounced damping of the energetic influences.
The dynamics of vibrational wave packets in triplet states of rubidium dimers (Rb2) formed on helium nanodroplets are studied using femtosecond pump-probe photoionization spectroscopy. Due to fast desorption of the excited Rb2 molecules off the droplets and due to their low internal temperature, wave packet oscillations can be followed up to very long pump-probe delay times 1.5 ns. In the first excited triplet state (1) 3 Σ + g , full and fractional revivals are observed with high contrast. Fourier analysis provides high-resolution vibrational spectra which are in excellent agreement with ab initio calculations.
The caloric curve for Na(139)(+) has been measured from 100 K up to the temperature where the clusters are boiling hot and spontaneously emit atoms. In this limit the clusters form an evaporative ensemble, the temperature and energy of which have been determined. As the caloric curve of an atomic gas with a finite number of atoms is known, one can construct the caloric curve for this finite system below and above the boiling point. A conjecture is made on how to link the evaporative ensemble temperature of the free cluster in vacuum to the boiling temperature of a finite system at a given pressure. This allows one to determine the enthalpy of vaporization at the phase transition of the finite system.
Dilatometric and X-ray scattering experiments of the crystallization kinetics of a sample of poly(ethylene-co-octene) show pronounced melt memory effects, i.e., the shapes of isotherms and characteristic times vary systematically with the temperature of the melt prior to cooling to the crystallization temperature. The temperature range of the effect is limited; crystallization kinetics remains constant below a melt temperature T(m)l and above a melt temperature T(m)h and varies only in-between. Analysis shows that the melt memory effect is caused by a variation of the characteristic time of a first order crystallization process. The process can be assigned to the in-filling of crystallites into objects of a previously generated precursor structure.
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