Photoionization of aqueous indole: Conduction band edge and energy gap in liquid water

Previous work on the determination of the photoionization threshold (I_sol) of tryptophan has now been extended to indole as a solute, both in tetramethylsilane (TMSi) and H₂O solvents. In TMSi, electron scavenging by N₂O or photoconductivity measurements lead to the same I_sol value: 4.95 ± 0.1 eV. In water, I_sol is found equal to 4.35 ± 0.1 eV. From these experiments, information on the ionization mechanism, on the oxidized solute and on the solvent can be gained: (i) the scavenger electron affinity does not intervene in the energy balance providing I_sol; (ii) an “effective” ionic radius of indole (1.40 Å) is estimated which suggests that the positive charge remains highly localized on the N-atom of the indole ring; (iii) a value of ?1.2 ± 0.1 eV can be deduced for V_o, the conduction band edge of water; (iiii) from the above findings, the energy gap E_G of pure water, considered as a semi-conductor, would be close to 7 eV. Such a result is discussed in terms of literature data pertaining to electron ejection in pure liquid water and X-ray photoelectron spectroscopy of amorphous ice.