Determination of Absolute Recurrent Fluorescence Rate Coefficients for C₆^–

Abstract

We determined absolute rate coefficients for the recurrent fluorescence (RF) process in C 6 − anions at excitation energies above the adiabatic electron attachment energy of 4.18 eV. The experiment was performed by extracting C 6 − ions from a sputter ion source and storing them in a bent electrostatic ion beam trap. After 1 s of storage, during which the anions cooled down to temperatures close to room temperature, they were excited by a short laser pulse and the neutralization rate due to vibrational autodetachment (VAD) was measured as a function of time at several wavelengths. Due to the different energy dependence of the two competing decays via the RF and the VAD process, their contributions to the measured total decay rate coefficients could be disentangled. For excitation energies ≲4.6 eV, the decay is found to be dominated by the RF process with decay rate coefficients on the order of 5 × 10 4 s −1 . The result clearly demonstrates the presence of the RF process in C 6 − and illustrates the importance of this process in the production and cooling of isolated molecules of astrophysical interest.

Thermal radiation of gold clusters on microsecond time scales

“…This type of radiation was proposed a long time ago [32,33] but was only recently detected for several excited carbon species [19,25,34,35] and very recently also confirmed by direct detection of emitted photons for C 6 − [36].…”

Extended red emission in IC59 and IC63

“…By enabling RF on a rapid timescale (Chandrasekaran et al 2014), these systems are able to undergo highly efficient cooling after absorbing FUV photons, thus avoiding other energy-dissipating pathways such as fragmentation or ionization. Those pathways are more likely to occur in molecular systems not capable of RF.…”

Dust evolution, a global view: II. Top-down branching, nanoparticle fragmentation and the mystery of the diffuse interstellar band carriers

“…In some cases, the carrier can lose the energy by the faster process of inverse internal conversion (IIC) and then the emission of one or more optical photons, involving the process known as inverse electronic relaxation [64], inverse fluorescence [65] or Poincaré fluorescence [66]. For this fluorescent emission to be an efficient cooling mechanism the carrier must have a low-lying electronic excited state, as seen experimentally for C${\hspace{0.17em}}_6^{-}$ [65,67,68] and C 6 H − [68]. Low-lying electronically excited states are also characteristic of defective graphene, C 60 anions, PAH cations and polyenes, probably making such species resistant to photo-dissociation in the harsh environment of the diffuse ISM.…”