Photoelectrons in Rb/THF solution: Spectral dependence of photodetachment cross-section

Photoconductivity method, combined with laser pulse excitation, was employed to obtain the spectral dependence of electron photodetachment in rubidium-tetrahydrofuran (Rb/THF) solution at room temperature. Wavelength dependence of photodetachment cross section, σ_pd, qualitatively retraces that of the optical absorption cross section, aa. Moreover, a wavelength dependence of the quantum yield, κ = σ_pd/σ_a, was also found, where κ ≈ 0.03 is held constant for the wavelengths region λ ≥ 680 nm (hv ≤ 1.8 eV). Also, in the short wavelengths region, λ ≤ 510 nm (hv ≥ 2.4 eV), κ is constant, but almost three times larger (κ ≈ 0.085). The inflection region is located approximately at hv = 2.2 eV, that coincides with the energy of Rb^- ground state relative to the lower edge of the conduction band of THF. Mechanism of Charge-Transfer-To-Solvent (CTTS) with a subsequent release of a solvated electron is suggested to be valid below the conduction band of Rb/THF. It implies that κ = k_s(k_s + k_c) ≈ 0.03, and consequently, k_c = 30k_s, where k_c is the rate constant of the radiationless interconversion, and k_s is the rate constants of the isomerization of CTTS excited state with the formation of the solvated electron. When photon energy becomes higher than the energetic gap between the CTTS ground state and the edge of the THF conduction band, photoinduced autoionization transitions into the continuum generate quasi-free electrons.