Electronic Raman Effect in Metal Oxides with substitutional 3d-transition metal ions:

1. Electronic Raman Effect in Metal Oxides with substitutional 3d-transition metal ions: Cr3+ in MgO and the nature of the red emission line. Anant K. Ramdas, Purdue University, DMR 0705793 The extremely sharp red emission lines (R) in corundum (Al2O3) doped with Cr replacing Al3+,viz. ruby, has been recently investigated by us in the context of the Raman Electron Paramagnetic Resonance [Phys. Rev. B 79, 235204 (2009)]. The electronic configuration of Cr3+, its site symmetry in ruby, crystal field and spin-orbit interaction, on the one hand, and the microscopic mechanism which allows large resonance tunable resonances in the Raman-EPR, on the other hand, have been reported by us in the above reference. In the present highlight we have focused on MgO:Cr3+ in which Cr3+ occurs either in the cubic site replacing Mg and the associated Mg vacancy far away or with non-cubic site symmetry because the vacancy is close by Cr3+. The electronic Raman Electronic Paramagnetic Resonance (Raman-EPR) and the resonance enhancement of the Raman EPR lines in Al2O3, successfully observed and explained in terms of symmetry of the Cr3+ centers, and the microscopic mechanism underlying the Raman effect, are now being investigated in MgO. Fig 1 displays the single photoluminescence line PL1 of Cr3+ in the cubic site with the associated vacancy far away. Its Zeeman splitting into and testifies to the cubic symmetry of the center. Fig 2 (upper) exhibits the PL spectrum of a Cr3+ center with non-cubic symmetry and its Zeeman splitting. The more complex nature of the spectra, both with and without magnetic field, related to a symmetry lower than cubic, are clearly evident. Fig 2 (lower) shows the Stokes and the anti-Stokes Raman-EPR spectrum of Cr3+ with non-cubic site symmetry. The larger intensity of many of the lines in the anti-Stokes Raman spectrum, is a clear indication that resonant enhancements have occurred by tuning the laser energy to be in resonance with an electronic transition of the PL. Fig 1

2. Electronic Raman Effect in Metal Oxides with substitutional 3d-transition metal ions: Cr3+ in MgO and the nature of the red emission line. Anant K. Ramdas, Purdue University, DMR 0705793 Future Plans:The novel results featured in this ‘highlight’ exploiting PL and Raman spectroscopy promise to be fruitful for a larger class of oxides e.g. Mg2Al2O4 (a spinel) and other TMIs like Mn2+, Ti2+…. They allow the discovery, delineation, and characterization of the nature of impurities and the defect complexes, on the one hand, and the understanding of the microscopic mechanisms governing electronic Raman Effect, on the other. Education/Broader Impact: The research reported here is the outcome of a close collaboration between the two PIs Ramdas (Experimental), Rodriguez (Theory); Dr. Irek Miotkowski (Senior Scientist in charge of crystal growth facility); X. Lu (Graduate Student); Professor S. Venugopalan (SUNY, Binghamton); Professor Hyunjung Kim (Sogang University, Korea, and Dr. Marcos Grimsditch (ANL). X. Lu and G. Chen from our group received their Ph.Ds during 2007-2008; J. Bhosale and R. Garrelts are the current graduate research students. John Wright, is a REU student on this program, learning Raman and Fourier Transform Spectroscopy. Collaborations with Professor H. Alawadhi (Univ. of Shiraz, UAE), Professor Eugene Haller (UCB), Dr. Thomas Anthony (formerly of GE) and Professor L.R. Ram-Mohan (WPI) are further examples of the interactive and intellectually stimulating ambience in which our graduate students and undergraduates carry out their research; this experience prepares them for an exciting future in universities and national and industrial laboratories. Fig 2