Sensitive spectroscopy of absorbing rare earth compounds

The author has developed a sensitive optical spectrometer system capable of yielding very high signal-to-noise ratios throughout the visible spectrum and the near infrared. The system employs a one-meter Czerny-Turner grating spectrometer coupled to a linear photodiode array. With appropriate experimental techniques and suitable postprocessing, signal-to-noise ratios greater than 105 can be routinely obtained. The sensitive spectrometer has been successfully applied in two areas. The first is the observation of sharp optical absorption lines in the high Tc superconductor RBa2Cu3O7-x where R = Er, Ho, and Yb. These lines are due to very small concentrations of the impurity phase R2BaCuO5, concentrations difficult to detect by x-ray methods. Expected strong magnetic dipole transitions in the superconductor were not observed, implying oscillator strengths less than 10-8, approximately twenty times smaller than predicted. The second application of the system is the investigation of very thin epitaxial films of NdF3. Sharp absorption lines were observed in films as thin as 10 angstrom(1.5 monolayers). The thinnest films exhibited lines much weaker and broader than projected from thicker films, demonstrating that significant crystal field inhomogeneities are present in very thin films. In addition, lines in NdF3 on GaAs are systematically shifted by several angstroms with respect to their positions in NdF3 on LaF3, indicating the differing strain borne by NdF3 in the two situations. The spectrometer system thus provides an extremely sensitive probe of the atomic environment at the film boundaries