Infrared-to-visible upconversion emission intensities are investigated in Li+ and Er3+ codoped ZnO nanocrystals. Li+ ions doped in ZnO/Er3+ nanocrystals can greatly enhance the upconversion emission intensity of Er3+ ions. The extended X-ray absorption fine structure spectroscopy data show that both the Er−O bond length and coordination number of the Er−Er bond have been altered by introducing Li+ ions in the ZnO/Er3+ nanocrystals. The variation of Er−O bond length leads to the change in the local asymmetry around Er3+ ions. Meanwhile, the greater coordination number of the Er−Er bond causes stronger interaction between neighboring Er3+ ions and, hence, strengthens the reaction Er3+ (4F7/2) + Er3+ (4I11/2) → 2Er3+ (4F9/2). In this process, the 4F9/2 state is a metastable state that could be excited to the 2H9/2 state by absorbing one photon at high excitation power. Li+ ions also can reduce the OH groups in specimen, which is the other reason for enhancing the upconversion emission intensities.
Centrosymmetric furan-containing cyclophandienes 3 and 4, synthesized by our furan annulation protocol, have been shown to exhibit extraordinarily large Stokes shifts and second-order nonlinear optical beta values. The beta values for 3 and 4 measured at 1.32 mum are 208 and 530x10(-30) esu, respectively. The beta values of 3 and 4 are similar to those of respective cyclophenes 1 a and 7 in which strong hyperpoarizable interactions between two twisted pi-systems (oligoaryl and bridging double bond) might take place. Symmetry breaking due to the resonance contribution (cf. 2) and the unique structural features of 3 and 4 has been used to account for this unusual photophysical behavior.
The low-frequency Raman-active longitudinal acoustical mode (LAM) has been observed in high-pressure crystallized polyethylene (HPCPE), which exhibits an extended-chain morphology. Through the use of a multipass Fabry-Perot interferometer a band at 0.5 cm"1 has been observed in the low-frequency spectral region and tentatively assigned to LAM. After melting and recrystallization this band shifted to 12.8 cm"1 and was then observed with standard Raman instrumentation, with its position reflecting a significant reduction in lamellar thickness.
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