Influence of the repulsive Coulomb barrier on photoelectron spectra and angular distributions in a resonantly excited dianion

A photoelectron imaging study of the gas-phase dianion of pyrromethene-556 is presented. The photoelectron spectra and angular distributions following resonant excitation of the S₁ excited state with nanosecond and femtosecond laser pulses are compared, and the influence of the repulsive Coulomb barrier (RCB) in both cases evaluated. Photoelectron angular distributions show the effect of molecular alignment due to an allowed electronic excitation and can be understood qualitatively based on the calculated RCB surface using the Local Static Approximation. Comparison between femtosecond and nanosecond excitation reveals marked differences in the photoelectron spectra. While femtosecond experiments confirm that tunneling through the RCB is adiabatic, nanosecond experiments show a broad photoelectron feature peaking near the RCB maximum. This is explained in terms of the lifetime of internal conversion, which has been determined by time-resolved photoelectron spectroscopy to be ∼120 ps: as this is faster than the nanosecond laser pulses, multiple photons can be absorbed through the S₁ ← S₀ transition which leads to large amounts of internal energy and enables electron detachment directly above the RCB. Fragmentation and detachment from the monoanion are also inferred by the presence of photoelectrons emitted at very low kinetic energy. Our results highlight the difficulty in interpreting photoelectron spectra of polyanions in which a resonant state is excited.