Determining excitation temperature of fragmented C60via momentum distributions of fragments

Abstract

Hot C(60) molecules under nanosecond laser excitation decay by a variety of fragmentation channels. An experimental search has been made to determine the excitation temperature of fragmented C(60)via analyzing the momentum distributions of the prompt ionic fragments C(n)(+) (n ≤ 58). It was found that all the C(60) precursors appearing as these ionic fragments have almost the same temperature and the temperature shows little variation with the laser fluences in our limited range. The results provide a clear evidence that a first-order phase transition in the fragmented C(60) is occurring at this temperature. The value of phase transition temperature is found to be about 6050 ± 250 K, which is in a good agreement with the most recent estimations based on the molecular dynamics simulation. This approach offers an experimental opportunity for studying the fragmentation thermodynamics of more complex polyatomic molecules under excitation temperature determined conditions.

New insight into power-law behavior of fragment size distributions in the C60 multifragmentation regime

“…In each mass spectrum shown in Fig. 1, the large-mass fullerene IFs C 2n + (32 ≤ 2n ≤ 58), as described by previous studies, 9,18 are produced in the evaporation regime of C 60 + precursors located in the lower energy region. Temporarily, one cannot conclude whether the smalland intermediate-mass IFs C n + (n = 1-20) are produced in the multifragmentation regime or in a mixed regime of multifragmentation and complete disintegration of C 60 + precursors located in the higher energy region.…”

“…Temporarily, one cannot conclude whether the smalland intermediate-mass IFs C n + (n = 1-20) are produced in the multifragmentation regime or in a mixed regime of multifragmentation and complete disintegration of C 60 + precursors located in the higher energy region. Considering the fact that the C 60 + evaporation processes are characterized by the emission of small-or intermediate-mass NFs in correlation with large-mass fullerene IFs C 2n + (32 ≤ 2n ≤ 58), 9,11,18,28 there is no doubt that the IFs C n + (n = 1-20) are not contaminated by the evaporation processes because all NFs remain undetected with the present experimental apparatus. To clarify the origin of IFs C n + (n = 1-20), using a recent developed molecular thermometer, 18 we have estimated the temperature of C 60 + precursors appearing as all the IFs C n + (n = 1-20) shown in Fig.…”

“…The almost constant temperature corresponds to a plateau in the caloric curve of C 60 + , providing the typical prerequisites of a first-order phase transition. 2,3,18 It implies that the absorbed photon energies for each precursor have never overstepped the plateau region of caloric curve. In other words, the C 60 + precursors have never reached the conditions for occurring complete disintegration because the temperature of precursors suffering com-plete disintegration will exhibit sensitive dependence on the excitation energy.…”

“…The apparatus and experimental procedure have been described in detail in our previous papers, 18,28 and only a brief outline is given. The C 60 molecular beam was produced by heating a high-purity (>99.9%) C 60 powder at 450 • in a resistively heated oven.…”

“…Actually, the C 60 fragmentation has been frequently discussed in terms of equilibrium statistical mechanics, and of its possible association with a phase transition. [17][18][19][20][21] Specifically, Campbell et al 17 and Hussien et al 20 presented two completely different theoretical models treating the C 60 fragmentation as a phase transition and predicted that the critical point occurs at almost constant temperature covering a wide excitation energy region. Subsequently, our group 18 experimentally investigated the relation between the temperature and the excitation energy (caloric curve) of a fragmenting C 60 system.…”