Gobetet al.Reply

Abstract

Gobet et al. ReplyThis Reply aims to clarify a key argument in two recent publications [1,2] which has been criticized in the preceding Comment by Chabot and Wohrer [3]. The Letters in question feature a novel method to derive a caloric curve from event-by-event ion decay data measured using multicoincidence techniques after high-energy collisions (60 keV=amu) between mass selected hydrogen cluster ions [H 3 H 2 n , n 6-14] and a helium target. Reactions corresponding to a given deposited energy E d are selected and grouped into statistical subensembles. A caloric curve can then be constructed by deriving corresponding temperatures for these microcanonical cluster ensembles. It is this method which is criticized by Chabot and Wohrer.The essence of their criticism is expressed in the following extract from the Comment: ''. . .The authors assume that the internal energy E is the energy deposited in the cluster by the collision, E d , . . .The deposited energy E d is the sum of the energy due to electronic excitation and, as a dominant contribution in their systems, to ionization. But the ionization energy should not be included in E .'' Apparently, Chabot and Wohrer have misinterpreted the work, arguing along a single event consideration and not taking into account the statistical ensemble-type situation which applies.As explained in detail in [1,2], the temperature derived from the data is that of a statistical ensemble comprising a large number of decaying H 3 H 2 n ions. Each subensemble is characterized by the deposited energy and includes all of the processes induced by the high-energy collision (ionization, excitation, etc.). This energy is deposited during a very short time (0.1 fs) and the system (n protons and n ÿ 1 electrons) is left isolated immediately after the collision. Therefore, the subsequent statistical analysis is carried out in the microcanonical frame [4,5]. The results obtained are averages for each statistical ensemble comprising a large number of cluster ions with the same total amount of energy deposited, albeit distributed in different channels. The total energy (in the frame of reference of a single cluster ion) is equal to the sum of the internal energy before the collision and the energy deposited by the collision. It can be assumed that the internal energy before the collision is low in comparison with the deposited energy.The temperature is derived from one observable: the size distribution of the largest fragment among residual cluster ions in the statistical ensemble. The residual cluster sizes are measured not only after the ejection of electrons (ionization) but also following the ejection of ions, atoms, and molecules. The correlation between the temperature and the size distribution of the largest fragment has been previously demonstrated both in cluster physics [6] and in nuclear physics [7,8]. Moreover, regarding the recent work of Thirring et al. [9] (triggered by [1,2]), it is worth noting that our measurements and

Remarks on Negative Heat Capacities of Clusters

“…[16][17][18] It is likely that the effective temperatures inferred for these ensembles are the kinetic tempera-tures, T 2 . There are now many persuasive results from simulations and analytic theory to give credence to the negative heat capacities of clusters of adequate size.…”

Blocking temperature in magnetic nanoclusters

“…In order to distinguish between the choice of the initial density an index will be introduced: we will use LS for an initial uniform density (21) and MS for the binomial density (22).…”

“…In the same line of thought and more important for the approach we will use here, is the observation by Gulminelli [16] about the equivalence of the location of the zeroes of the partition function in the complex temperature plane and the modes of the probability density function (PDF) of the order parameter. Investigations along this line include: metallic clusters [17], pairing in nuclei [18,19], and clusters with a negative heat capacity [20,21]. It was first shown that for non-extensive systems the location of Yang Lee zeroes reveals the transition from one phase to another [22,23,24].…”

Extended Gibbs ensembles with flow

“…Going down to the femto-scale, the thermodynamic properties of nuclear systems can only be accessed through collisions [9]. In the cluster world the dynamical evaporation may still be associated to the thermodynamics of the liquid-vapor phase transition [1,2,5] making use of a time-dependent temperature within the concept of an evaporative ensemble [7,8]; however in the nuclear collisions case the time scales can be so short that the reaction and decay channels cannot be decoupled, collective flows appear, and the statistical equipartition hypothesis breaks down [10]. If in the Fermi energy regime and in the associated multi-fragmentation phase transition these collective flows may be only a perturbation in the global energetics, this is not true at SIS energies where they are likely to influence light cluster formation by coalescence [11].…”

“…In condensed matter physics, clusters dissociation induced by photo-ionization [1,2,3,4] or charge transfer collisions [5,6], cannot be studied without properly accounting for the time window of the experiment [1]. Going down to the femto-scale, the thermodynamic properties of nuclear systems can only be accessed through collisions [9].…”