Cryogenic photoelectron and photodetachment spectroscopy of complex anions

While supersonic cooling revolutionized spectroscopic studies of neutral molecules, cooling molecular ions remains far more challenging, especially for ions generated from electrospray ionization (ESI). Cryogenic cooling has been transformative, particularly for ESI-produced ions, by enabling intrinsically cold spectroscopic interrogation of solution-phase species transferred into the gas phase. This perspective focuses on cryogenic photoelectron spectroscopy (PES) and photodetachment spectroscopy (PDS) of complex anions produced by ESI and cooled in a 3D Paul trap, a platform that has been widely adopted because of its relative simplicity and robust performance. We discuss the technical evolution from the initial ESI-PES for solution species to cryogenic ESI-PES with a magnetic-bottle analyzer, and the current cryogenic PDS and high-resolution photoelectron imaging. We emphasize significant advances enabled by coupling the cryogenic 3D Paul trap first to ESI sources-highlighting studies of temperature-dependent phenomena, solution-phase chemistry in the gas phase, nonvalence excited states, vibrationally induced autodetachment, and resonant PES-and more recently to a laser-vaporization cluster source, demonstrating more effective vibrational cooling for cluster ions than supersonic expansion.