Reduction of Yb(III) to Yb(II) by Two-Color Two-Photon Excitation

Ytterbium 3+ ions in alcohol were found to be reduced to the corresponding 2+ ions upon laser irradiation with a stepwise two-color two-photon excitation. The infrared (975-nm) pulse with a duration of 4 ns pumps the ground state to the 4f excited state with the transition of 2F5/22F7/2, and the second photon (355-nm) generates the charge transfer (CT) state of Cl 3p to Yb 4f; the reduction then occurs. Laser energy and excitation wavelength dependencies well-explain the above mechanism. The product Yb2+ was detected by its absorption spectrum peak at 367 nm. The absorption spectrum of the intermediate in the two-photon chemistry was measured from the 4f excited state (2F5/2) to the CT state by nanosecond laser photolysis. The intermediate spectrum appears in the wavelengths shorter than 400 nm with the molar extinction coefficient on the order of (102 M–1 cm–1) at 340 nm and can be explained in terms of the CT absorption shifted by IR photon energy. A UV nanosecond laser pulse (266 nm from a YAG laser with a duration of 6 ns) can generate the reactive CT state by one-photon absorption and leads to Yb2+ formation. The reaction yields for single-photon UV excitation and the second photon in the two-photon excitation are on the order of 0.1, suggesting that the reactive states are a common CT state.