Apparently, although it's not likely to occur under most conditions, you can form a monatomic carbon C²⁺ ion by shooting a laser into very high-temperature carb

Apparently, although it's not likely to occur under most conditions, you can form a monatomic carbon C²⁺ ion by shooting a laser into very high-temperature carbon: https://sciencetrends.com/the-charge-of-a-carbon-ion/ https://sciencetrends.com/the-charge-of-a-carbon-ion/ https://aip.scitation.org/doi/10.1063/1.4966987 https://aip.scitation.org/doi/10.1063/1.4966987 "Up to C⁴⁺ ions are observed." And I guess you could also write the C²⁺ ion as "C⁺⁺" if you wanted. :-)