Q and Soret Band Photoexcitation of Isolated Palladium Porphyrin Tetraanions Leads to Delayed Emission of Nonthermal Electrons over Microsecond Time Scales

We have used both action and photoelectron spectroscopy to study the response of isolated PdII meso-tetra(4-sulfonatophenyl)porphyrin tetraanions ([PdTPPS]4–) to electronic excitation over the 2.22–2.98 eV photon energy range. The action spectrum obtained by recording the wavelength-dependent intensity of charged decay products closely resembles the absorption spectrum of PdTPPS in aqueous solution (which shows pronounced Q and Soret absorption bands). The two main decay channels observed are sulfonate group loss and, predominantly, electron emission. To better understand the electron emission channel, we have also acquired photoelectron spectra at multiple detachment photon energies covering the range probed in action spectroscopy. Upon both Q and Soret band excitation, we find that electrons are emitted in three characteristic kinetic energy ranges. The corresponding detachment processes are identified as (delayed) tunneling emission from both excited singlet and triplet states (each of which is accesse...