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Laser-induced fluorescence and dispersed-fluorescence spectroscopy of the ?2E?X?2A1 transition of jet-cooled calcium methoxide (CaOCH <sub/>3</sub> ) radicals
摘要： Laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of the ?A2E ? ?X2A1 electronic transition of the calcium methoxide (CaOCH3) radical have been obtained under jet-cooled conditions. Complete active space self-consistent field and coupled-cluster calculations on the free radical were performed to aid the assignment of vibronic transitions observed in the LIF/DF spectra. In addition to dominant spectral features that are well reproduced by vibrational frequencies and Franck-Condon (FC) factors calculated ab initio, the FC matrix for the ?A2E ? ?X2A1 electronic transition contains considerable off-diagonal elements that connect (i) the CaO-stretch (ν4) mode and non-CaO stretch modes and (ii) the asymmetric CaOC stretch (ν3) and the CaOC bending (ν8) modes. The Jahn-Teller and pseudo-Jahn-Teller interactions involving the ?2E state as well as the spin-orbit interaction induce additional vibronic transitions that are not allowed under the Born-Oppenheimer approximation. Additionally, anharmonic vibrational terms in the ground state induce transitions that are forbidden in the harmonic-oscillator approximation. Spin-orbit splitting has been observed for several vibrational levels of the ?2E state, and an essentially constant value was measured at all levels accessed in the LIF experiment. Implications of the present spectroscopic investigation to the proposed schemes of laser-cooling MOCH3 (M = alkaline earth metals) molecules and detection of time-reversal-symmetry-violating interactions are discussed.
关键词： calcium methoxide,laser cooling,spin-orbit interaction,Jahn-Teller effect,Franck-Condon factors,vibronic transitions,CaOCH3,Laser-induced fluorescence,pseudo-Jahn-Teller effect,dispersed-fluorescence spectroscopy
更新于2025-09-19 17:13:59
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Origin of polar nanoregions and relaxor properties of ferroelectrics
摘要： In spite of more than 60 years of research, hundreds of publications, and dramatic influence on properties, the driving force of the formation of the polar nanoregion (PNR) in the paraelectric phase of perovskite ferroelectrics and consequent relaxor properties remains uncertain. We show that these peculiar features follow directly from the vibronic, pseudo-Jahn-Teller (PJT) theory of ferroelectricity. Due to the higher disorder (and entropy) in the paraelectric phase (created by the local PJT dynamics), as compared with the polarized phase (where the PJT dynamics is partially quenched), a small PNR of the latter with n unit cells is formed in a dipole-alignment self-assembly process. It emerges encapsulated by a border layer with intermedium ordering that produces “surface tension” and limits its size. The thermodynamic equilibrium between the PNR and the bulk cubic phase at temperatures Tn, well above the phase transition TC, is reached by compensation of the excessive entropy contribution Tn(cid:2)S with ordering energy and the work against this surface tension. The calculations based on the vibronic theory, including the PJT induced local dipolar dynamics and intercell interactions, yield the size of PNR as a function of the temperature increments Tn ? TC and some crystal parameters. In accordance with experimental data, the size of the emerging PNR decreases with temperature, n ～ (Tn ? TC)?3, becoming undetectable at the Burns temperature TB. At temperatures Tf , nearer to the phase transition, the sizes of PNRs grow rapidly, and their interaction leads to the formation of the nonergodic (glasslike) phase.
关键词： polar nanoregions,pseudo-Jahn-Teller effect,relaxor properties,ferroelectrics,vibronic theory
更新于2025-09-09 09:28:46