High sensitivity of 17O NMR to p-d hybridization in transition metal perovskites: first principles calculations of large anisotropic chemical shielding

A first principles embedded cluster approach is used to calculate O chemical shielding tensors, sigma, in prototypical transition metal oxide ABO_3 perovskite crystals. Our principal findings are 1) a large anisotropy of sigma between deshielded sigma_x ~ sigma_y and shielded sigma_z components (z along the Ti-O bond); 2) a nearly linear variation, across all the systems studied, of the isotropic sigma_iso and uniaxial sigma_ax components, as a function of the B-O-B bond asymmetry. We show that the anisotropy and linear variation arise from large paramagnetic contributions to sigma_x and sigma_y due to virtual transitions between O(2p) and unoccupied B(nd) states. The calculated isotropic delta_iso and uniaxial delta_ax chemical shifts are in good agreement with recent BaTiO_3 and SrTiO_3 single crystal 17O NMR measurements. In PbTiO_3 and PbZrO_3, calculated delta_iso are also in good agreement with NMR powder spectrum measurements. In PbZrO_3, delta_iso calculations of the five chemically distinct sites indicate a correction of the experimental assignments. The strong dependence of sigma on covalent O(2p)-B(nd) interactions seen in our calculations indicates that 17O NMR spectroscopy, coupled with first principles calculations, can be an especially useful tool to study the local structure in complex perovskite alloys.Comment: 12 pages, 3 figures, and 3 Table

Enhancement of the formation of ultracold 85^{85}Rb2_2 molecules due to resonant coupling

We have studied the effect of resonant electronic state coupling on the formation of ultracold ground-state $^{85}$Rb$_2$. Ultracold Rb$_2$ molecules are formed by photoassociation (PA) to a coupled pair of $0_u^+$ states, $0_u^+(P_{1/2})$ and $0_u^+(P_{3/2})$, in the region below the $5S+5P_{1/2}$ limit. Subsequent radiative decay produces high vibrational levels of the ground state, $X ^1\Sigma_g^+$. The population distribution of these $X$ state vibrational levels is monitored by resonance-enhanced two-photon ionization through the $2 ^1\Sigma_u^+$ state. We find that the populations of vibrational levels $v''$=112$-$116 are far larger than can be accounted for by the Franck-Condon factors for $0_u^+(P_{1/2}) \to X ^1\Sigma_g^+$ transitions with the $0_u^+(P_{1/2})$ state treated as a single channel. Further, the ground-state molecule population exhibits oscillatory behavior as the PA laser is tuned through a succession of $0_u^+$ state vibrational levels. Both of these effects are explained by a new calculation of transition amplitudes that includes the resonant character of the spin-orbit coupling of the two $0_u^+$ states. The resulting enhancement of more deeply bound ground-state molecule formation will be useful for future experiments on ultracold molecules.Comment: 6 pages, 5 figures; corrected author lis

PROGRESS TOWARDS FORMATION AND SPECTROSCOPY OF ULTRACOLD GROUND-STATE 85^{85}Rb2_2 MOLECULES IN AN OPTICAL TRAP

Y. Huang, J. Qi, H. K. Pechkis, D. Wang, E. E. Eyler, P. L. Gould, and W. C. Stwalley, J. Phys. B 39Author Institution: Physics Department, University of Connecticut, Storrs, CT 06269-3046We have very recently completed construction of an apparatus designed to efficiently produce ultracold Rb$_2$ molecules in a quasi-electrostatic optical trap (QUEST) by photoassociation (PA). The QUEST is loaded from a magneto-optical trap (MOT), with additional cooling and compression stages to optimize the density and temperature. The trapped atom cloud is detected by absorption imaging. Molecules will be formed from the optically trapped atoms by PA into levels bound by $\simeq 1-100$ cm$^{-1}$, followed by radiative decay. Employing the QUEST will allow optical trapping of Rb$_2$ in the singlet X\,^1\Sigma_g^+ state, as well as enhancing greatly the PA rates relative to previous work in an ordinary MOT.}, S857 (2006).} We will describe in detail our progress in forming and detecting ultracold molecules in the dipole trap. We will also describe progress on experiments using state-selective detection to investigate collisions involving the trapped molecules. This work is supported by the National Science Foundation

RESONANCE-ENHANCED PHOTOASSOCIATIVE FORMATION OF GROUND-STATE Rb2_2 AND SPECTROSCOPY OF MIXED-CHARACTER EXCITED STATES

Author Institution: Physics Department, University of Connecticut, Storrs, CT 06269; Institut fur Theoretische Physik, Freie Universitat Berlin, 14195 Berlin, GermanyWe describe experimental and theoretical studies of the effects of resonant electronic state coupling on the formation of ultracold ground-state $^{85}$Rb$_2$. The molecules are formed by photoassociation of ultracold atoms in a MOT into the 0$_u^+$ state converging to the $5S+5P_{1/2}$ limit, followed by radiative decay into high vibrational levels of the ground electronic state, $X\ ^1\Sigma_g^+$. The populations of these high-$v$ ground-state levels are monitored by resonance-enhanced two-photon ionization (R2PI) through the $2\ ^1\Sigma_u^+$ state. We find that the populations of vibrational levels $v''$=112-116 are far larger than can be accounted for by the Franck-Condon factors for $0_u^+ \leftarrow X\ ^1\Sigma_g^+$ transitions. Further, the total number of ground-state molecules formed by this process exhibits oscillatory behavior as the PA laser is tuned through a succession of $0_u^+$ state vibrational levels. Both of these effects are explained by a new calculation of transition amplitudes that includes the resonant character of the spin-orbit coupling between the two $0_u^+$ states converging to the $5P_{1/2}$ and $5P_{3/2}$ limits. The resulting enhancement of more deeply bound ground-state molecule formation will be useful for future experiments on ultracold molecules. \vspace{12 pt} We also describe evidence from our R2PI spectra for extensive singlet-triplet mixing between excited states of Rb$_2$ at intermediate internuclear separations, apparently also induced by spin-orbit interactions. In particular, the $3\ ^1\Sigma_g^+$ and $1\ ^1\Delta_g$ states converging to $5s+4d$ have been observed in excitation from the $a\ ^3\Sigma_u^+$ state,}, 261 (2006).} and the $2\ ^3\Pi_u$ state has been observed in excitation from the $X\ ^1\Sigma_g^+$ state

Detection by two-photon ionization and magnetic trapping of cold Rb

We present detailed experimental spectra and accurate theoretical interpretation of resonance-enhanced two-photon ionization of ultracold rubidium molecules in the 14000–17000 cm-1 transition energy range. The dimers are formed in a magneto-optical trap by photoassociation followed by radiative decay into the a 3Σu+ lowest triplet state. The theoretical treatment of the process, which reproduces the main features of the spectra, takes into account the photoassociation and decay steps as well as the resonant ionization through the manifold of intermediate gerade states correlated to the 5S + 4D limit. In particular, the energy of the v=1 level of the $(2)\ ^{3}\Sigma_{g}^{+}$ potential well has been determined for the first time. In addition, a tight constraint has been put on the position of the a 3Σu+ repulsive wall. Finally, magnetic trapping of rubidium molecules in the a 3Σu+ state is demonstrated