The hydroperoxyl radical, HO2, was identified by
Parise et al. toward the
SM1 core of
ρ Oph A in 2012 using the
IRAM 30 m telescope as well as the
Atacama Pathfinder EXperiment (APEX) telescope. Five
rotational lines from 130 to 260 GHz were identified. The hydroperoxyl radical is thought to be formed on
grain mantles and to be relevant to the
formation of water and hydrogen peroxide in the interstellar medium.
The HO2 radical forms when one H is removed from
hydrogen peroxide. It posseses two low-lying
electronic states: the X2A" ground state, where the singly occupied p-like orbital on oxygen is out of
the molecular plane, and the A2A' excited state, where the singly occupied orbital is in the molecular
plane. Bonding diagrams for the two states are shown below (the singly occupied orbitals are yellow).