Internal conversion outcompetes autodetachment from resonances in the deprotonated tetracene anion continuum

Abstract

Publisher's copyright statement:Additional information: 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. Internal conversion outcompetes autodetachment from resonances in the deprotonated tetracene anion continuumJames N. Bull a , Christopher W. West a and Jan R. R. Verlet a † Photoelectron velocity-map imaging and electronic structure calculations have been used to study the temporary anion (resonance) dynamics of the closed-shell site-specific deprotonated tetracene anion (C18H11 -) in the hv = 3.26 eV (380 nm) to 4.13 eV (300 nm) range. In accord with a recent frequency-, angle-, and time-resolved photoelectron imaging study on a related but open-shell polyaromatic radical anion (Chem. Sci., 2015, 6, 1578-1589, population of π*-resonances situated in the detachment continuum efficiently recover the ground electronic state of the anion through ultrafast non-adiabatic dynamics, followed by characteristic statistical electron loss (thermionic emission). The combined electron yield of direct photodetachment and autodetachment from the optically-accessed resonances in C18H11 -is several orders of magnitude smaller than thermionic emission from the ground electronic electronic state in the photon energy range studied. This result implies a resiliance to prompt photoejection from UV radiation, and the ability of neutral PAH-like species to capture a free electron and form a long-lived molecular anion that ultimately decays by thermionic emission on a millisecond timescale. The attachment mechanism applies to polyaromatic species that cannot support dipole-bound states, and may provide an additional route to forming anions in astrochemical environments. IntroductionPolyaromatic hydrocarbon (PAH) molecules are found across a diverse range of chemical environments. For example, they form as products in combustion and anthropogenic pollution, 1 they are used in new generation semiconductors and microelectronics, 2,3 and they have been postulated to be present in astrochemical environments. 4,5 In astrochemistry, anions are recognised as important chemical species, [6][7][8][9][10][11] and have been identified as small polyynes such as C 4 H -and C 6 H -, [12][13][14][15][16][17][18] and are commonly postulated to be present as PAHs. However, the detailed formation mechanism of astrochemical anions is unclear. It is commonly postulated that stable anions may be formed via electric dipole-bound doorway states; 19,20 however, most neutral PAH molecules do not have a sufficient dipole moment to support such a mechanism.Most molecules,...

Charged Particle Imaging of the Deprotonated Octatrienoic Acid Anion: Evidence for a Photoinduced Cyclization Reaction

“…This level of theory has previously proven reliable to determine resonance energetics in a range of conjugated and aromatic anions. 14,15,17,20,31 …”

“…The photoelectron spectra are dominated by a peak at low electron kinetic energy, eKE, which is generally indicative of the indirect electron loss process of thermionic emission. 15,20,28,29 In addition to this low eKE peak, there are several minor features at higher eKE. In the hν = 4.13 eV spectrum, the low eKE peak has a shoulder that extends up to ~1.4 eV and a weak feature that peaks at eKE ~ 2.5 eV as indicated by the asterisk in Figure 1(a).…”

“…Several recent photoelectron studies on conjugated anions have shown that photoexcitation of resonances followed by ground electronic state recovery is a very efficient process. 15,20 Alternatively, dissociation can occur directly on the potential energy surface of the resonance if it is repulsive, 21 or from the ground electronic state provided there is an efficient internal conversion route and the total vibrational energy exceeds the bond 4 dissociation energy. 22 In the latter case, the dissociation products can provide insight into the ground state chemical transformations.…”

“…36 The slow electron loss shows that electrons are still "boiling off" the vibrationally-hot [C7H9-CO2] -over the gate timescale, which is consistent with the assignment that the low eKE feature in the photoelectron spectra is a result of thermionic emission (statistical electron ejection) from the anion ground electronic state. 15,20,28,29 Thus, the main product channel following interaction of [C7H9-CO2] -at hν = 4.13 eV or hν = 4.66 eV is recovery of the ground electronic state,…”

Dynamics of π*-resonances in anionic clusters of para-toluquinone

“…All images were accumulated with a 500 ns MCP gate; systematic delay of the MCP gate in 50 ns steps relative to the light pulse showed that most electron ejection occurred on a < 50 ns timescale regardless of cluster size, suggesting that only a modest amount of thermionic emission occurred. 7,10,28,29 Photoelectron image reconstructions used a polar onion peeling 30 algorithm, providing photoelectron spectra and angular distributions (see ESI). The photoelectron eKE scale was calibrated from the spectrum of I -and the velocitymapping resolution is around 5%.…”