Observation of a core-excited dipole-bound state ∼1 eV above the electron detachment threshold in cryogenically cooled acetylacetonate

Abstract

Dipole-bound states in anions exist when a polar neutral core binds an electron in a diffuse orbital through charge–dipole interaction. Electronicallyexcited polar neutral cores can also bind an electron in a diffuse orbital to form Core-Excited Dipole-Bound States (CE-DBSs), whichare difficult to observe because they usually lie above the electron detachment threshold, leading to very short lifetimes and, thus, unstructuredtransitions. We report here the photodetachment spectroscopy of cryogenically cooled acetylacetonate anion (C5H7O2−) recorded bydetecting the neutral radical produced upon photodetachment and the infrared spectroscopy in He-nanodroplets. Two DBSs were identifiedin this anion. One of them lies close to the electron detachment threshold (∼2.74 eV) and is associated with the ground state of theradical (D0-DBS). Surprisingly, the other DBS appears as resonant transitions at 3.69 eV and is assigned to the CE-DBS associated with thefirst excited state of the radical (D1-DBS). It is proposed that the resonant transitions of the D1-DBS are observed ∼1 eV above the detachmentthreshold because its lifetime is determined by the internal conversion to the D0-DBS, after which the fast electron detachment takesplace.