Dianion diagnostics in DESIREE: High-sensitivity detection of $\text{C}_{n}^{2-}$ from a sputter ion source

TL;DR

This study demonstrates high-sensitivity detection of carbon cluster dianions using a cryogenic ion-beam storage ring, revealing their production, decay, and isotopic distributions, with implications for dianion stability and detection limits.

Contribution

The paper introduces a novel high-sensitivity detection method for dianions in a cryogenic storage ring, providing new insights into their production, decay, and isotopic characteristics.

Findings

Detected $ ext{C}_{n}^{2-}$ dianions with $n=7-24$

No conclusive evidence for $ ext{C}_{6}^{2-}$

Established upper detection limits for $ ext{C}_{6}^{2-}$

Abstract

A sputter ion source with a solid graphite target has been used to produce dianions with a focus on carbon cluster dianions, $\text{C}_{n}^{2-}$, with $n=7-24$. Singly and doubly charged anions from the source were accelerated together to kinetic energies of 10 keV per atomic unit of charge and injected into one of the cryogenic (13 K) ion-beam storage rings of the Double ElectroStatic Ion Ring Experiment facility at Stockholm University. Spontaneous decay of internally hot $\text{C}_{n}^{2-}$ dianions injected into the ring yielded $\text{C}^{-}$ anions with kinetic energies of 20 keV, which were counted with a microchannel plate detector. Mass spectra produced by scanning the magnetic field of a $90^{\circ}$ analyzing magnet on the ion injection line reflect the production of internally hot $\text{C}_{7}^{2-}-\text{C}_{24}^{2-}$ dianions with lifetimes in the range of tens of microseconds to milliseconds. In spite of the high sensitivity of this method, no conclusive evidence of $\text{C}_{6}^{2-}$ was found while there was a clear $\text{C}_{7}^{2-}$ signal with the expect isotopic distribution. An upper limit is deduced for a $\text{C}_{6}^{2-}$ signal that is two orders-of-magnitue smaller than that for $\text{C}_{7}^{2-}$. In addition, $\text{C}_{n}\text{O}^{2-}$ and $\text{CsCu}^{2-}$ dianions were detected.