Real Time Quantification of Ultrafast Photoinduced Bimolecular Electron Transfer Rate: Direct Probing of the Transient Intermediate

Fluorescence quenching studies through steady-state and time-resolved measurements are inadequate to quantify the bimolecular electron transfer rate in bulk homogeneous solution due to constraints from diffusion. To nullify the effect of diffusion, direct evaluation of the rate of formation of a transient intermediate produced upon the electron transfer is essential. Methyl viologen, a well-known electron acceptor, produces a radical cation after accepting an electron, which has a characteristic strong and broad absorption band centered at 600 nm. Hence it is a good choice to evaluate the rate of photoinduced electron transfer reaction employing femtosecond broadband transient absorption spectroscopy. The time constant of the aforementioned process between pyrene and methyl viologen in methanol has been estimated to be 2.5 ± 0.4 ps using the same technique. The time constant for the backward reaction was found to be 14 ± 1 ps. These values did not change with variation of concentration of quencher, i.e., methyl viologen. Hence, we can infer that diffusion has no contribution in the estimation of rate constants. However, on changing the solvent from methanol to ethanol, the time constant of the electron transfer reaction has been found to increase and has accounted for the change in solvent reorganization energy

Achieving Surface Sensitivity in Ultrafast XUV Spectroscopy: M_2,3-Edge Reflection–Absorption of Transition Metal Oxides

Ultrafast extreme ultraviolet (XUV) spectroscopy is a powerful tool for probing electronic structure and charge carrier dynamics in catalytic materials because of its elemental, oxidation, coordination, and electronic spin-state sensitivity. To extend the benefits of this technique to investigating charge carrier dynamics at surfaces, we have developed near grazing-angle XUV reflection–absorption (RA) spectroscopy. Because RA spectra probe both the real (i.e., reflection) and the imaginary (i.e., attenuation) parts of the refractive index, a general method is required to analyze RA spectra. Using semiempirical calculations, we demonstrate that XUV RA spectra of first row transition metal oxides retain the element and chemical state specificity of XUV absorption spectroscopy. We find that the imaginary part of the refractive index reports on the chemical state of the metal center, while the real part is additionally sensitive to the surface morphology of the material

Highly Localized Charge Transfer Excitons in Metal Oxide Semiconductors

The ability to observe charge localization in photocatalytic materials on the ultrafast time scale promises to reveal important correlations between excited state electronic structure and photochemical energy conversion. Of particular interest is the ability to determine hole localization in the hybridized valence band of transition metal oxide semiconductors. Using femtosecond extreme ultraviolet reflection absorption (XUV-RA) spectroscopy we directly observe the formation of photoexcited electrons and holes in Fe₂O₃, Co₃O₄, and NiO occurring within the 100 fs instrument response. In each material, holes localize to the O 2p valence band states as probed at the O L₁-edge, while electrons localize to metal 3d conduction band states on this same time scale as probed at the metal M_2,3-edge. Chemical shifts at the O L₁-edge enable unambiguous comparison of metal–oxygen (M–O) bond covalency. Pump flux dependent measurements show that the exciton radius is on the order of a single M–O bond length, revealing a highly localized nature of exciton in each metal oxide studied