Studies of Molecular Carbon Cluster Dianions in DESIREE

Sammanfattning: Since the surprising first detection of small carbon cluster dianions by Compton et al 1990, several studies have been made investigating their structure, fragmentation pathways and stability. As other carbon anions have been found in space, it is likely that these dianions exists as well and their ability to react with other molecules is therefore of interest. Previous studies have shown that they may survive on microsecond timescales, but it is still an open question whether they are metastable or thermodynamically stable. Therefore, this study utilizes the cyrogenic storage ring DESIREE to investigate the lifetime and stability of carbon cluster dianions Cn(2-) (n=7-10) in the new domains. Dianions were produced with a cesium ion sputtering source and their spontaneous decay Cn(2-) -> Cn(-) + e- was monitored for almost 100 ms by detection of the singly charged decay products. Also, the extra electron's tunneling probability through the repulsive coloumb barrier for C8(2-) was calculated and the lifetime for different energy states was estimated. Analyzing the DESIREE data, the lifetime of the dianions could be determined to exceed the previous concluded lifetimes of 10 us by a whole order of magnitude. The only way to detect stable ions after that would be if they collided with rest gas and due to good vacuum, this eventual signal is too low. Probably due to varying structures, an alternating pattern was found, dianions with even n decaying slower than their odd neighbours. The fitting of power laws to the data is consistent with that the dianions were produced with a broad internal energy distribution. Calculations for C8(2-) indicated that its electron affinity had to be at least -1eV in order to agree with the experimental results. Continuing the investigations, future studies could, after sufficient time, use DESIREE in an alternative way, allowing detection of possible stable dianions.