Multiply-charged anions are common in the condensed phase, but are rare in the gas phase because of their low stability as a result of strong intramolecular Coulomb repulsion. The Coulomb repulsion prevents the formation of multiply-charged anions via sequential electron attachment, but provides dynamic stability for metastable multiply-charged anions. We developed an experimental apparatus integrating an electrospray ionization source with photoelectron spectroscopy, which allowed negative ions from solution samples to be studied in the gas phase, including multiply-charged anions, solvated anions, and biological molecules. In this talk, I will first review our early effort in the study of multiply-charged anions, including the observation of the repulsive Coulomb barrier, metastable anions with negative electron binding energies, and the effects of the Coulomb barrier on the outgoing electrons. We have subsequently developed a cryogenically-cooled Paul trap to create cold anions from electrospray. This is a critical advance, not only allowing high-resolution photoelectron spectra to be obtained for anions from electrospray, but also opening opportunities to study gaseous molecular ions from solution samples by other spectroscopic techniques. Our recent effort is focused on the development of resonant photoelectron spectroscopy using high-resolution photoelectron imaging of cryogenically-cooled anions. Negative ions do not possess Rydberg states. However, polar anions may have diffuse dipole-bound states just below the detachment threshold, analogous to Rydberg states. Excitation to vibrational levels of the dipole-bound state can induce autodetachment via vibronic coupling. The resulting resonant photoelectron spectrum is highly non-Franck-Condon and yields much richer vibrational information than conventional photoelectron spectroscopy. I will present our efforts to search for dipole-bound states using photodetachment spectroscopy and to use resonant photoelectron spectroscopy to obtain vibrational information of polar molecular and radical species with a sufficient dipole moment.

Electrospray photoelectron spectroscopy: From multiply-charged anions to cryogenically-cooled anions

ACS Spring Meeting 2021

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