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

Abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACTA photoelectron imaging study of the gas-phase dianion of pyrromethene-556 is presented. The photoelectron spectra and angular distributions following resonant excitation of the S 1 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 S 1 ← S 0 transition which leads to large amounts of internal energy and enables electron detachment directly above the RCB. Fragmentation and detachment from the monoanion is 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.3

Effects of resonant excitation, pulse duration and intensity on photoelectron imaging of a dianion

“…1), using a polarised femtosecond pulse. This is consistent with previous observations 20,31 and suggests that the RCB has a determining impact on the PAD of the outgoing PE. In order to correlate the observed PAD to the RCB shape, however, the correlation between molecular and 85 laboratory frame must be established.…”

“…28 Hence, electron emission from this leaves a positively charged hole in the chromophore, while the two SO 3 groups remain negatively charged and provide a repulsive force on the emitted PE. 20,31 Because the RCB is highly anisotropic, one might intuitively anticipate that the PAD will also be highly 20 anisotropic, as long as there exists some correlation between the laboratory and molecular frames of reference. This can either be attained through resonant excitation leading to an aligned sample or by virtue of an anisotropic differential photodetachment cross section.…”

Excited states of multiply-charged anions probed by photoelectron imaging: riding the repulsive Coulomb barrier

“…Qualitatively similar PE spectra were observed at a range of photon energies resonant with the S 1 ← S 0 transition. 68 The differences between the PE spectra arise because the 85 timescale of the experiments are different. Specifically, the internal conversion rate from S 1 back to S 0 , which was measured to be ~120 ps, 68 exceeds the tunnelling rate and is much shorter than a nanosecond pulse but longer than a femtosecond pulse.…”

“…68 The differences between the PE spectra arise because the 85 timescale of the experiments are different. Specifically, the internal conversion rate from S 1 back to S 0 , which was measured to be ~120 ps, 68 exceeds the tunnelling rate and is much shorter than a nanosecond pulse but longer than a femtosecond pulse. Consequently, during the nanosecond pulse, population returns to 90 S 0 by converting the electronic energy into vibrational energy (or heat) as the intramolecular vibration redistribution rate is also faster.…”

Perspective: Electrospray photoelectron spectroscopy: From multiply-charged anions to ultracold anions

“…As shown below, Verlet and co-workers have also observed perpendicular PE angular distributions for several dianions with similar structures and charge carriers. [94][95][96] …”