Ralph Allen Hewes

4f7 -4f7 emission from Eu2+ in the system MF2 · AlF3

Abstract The emission from Eu 2+ , which is isoelectronic to Gd 3+ , in all previously investigated systems consists of a broad (R~ 2000 cm -1 band, which at low temperatures may be accompanied by a zero-phonon line. We have discovered a group of compounds including SrAlF 5 , BaAlF 5 , and Ba 3 Al 2 F 12 in which the emission consists of a number of lines of narrow (less than 5 cm -1 ) width, at higher energies than the Eu 2+ bands previously seen. On the basis of the similarity of these spectra to that of Gd 3+ and of the luminescence decay times (τ ≈ 10 -3 sec, compared to τ ≈ 10 -6 sec for the usual case), the emission is identified as resulting from transitions within the 4f 7 configuration, rather than between the 4f 6 5d and 4f 7 configurations as in other materials doped with Eu 2+ . The excitation is seen to be broad band, indicating that the 5d configuration lies only slightly above the 4f levels. Investigation of the excitation spectra for the individual emission lines and decay measurements on these lines show that in the mixed fluorides there are several inequivalent sites with little interaction between ions on the different sites, as the number of lines attributed to the 6 P 7 2 to 8 S 7 2 transition is several times the number predictable on the basis of a single site.

Multiphoton excitation and efficiency in the Yb3+-R.E.3+ (Ho3+, Er3+, Tm3+) systems

Abstract The investigation of the production of visible luminescence from infrared radiation in the systems LnF 3 : Yb 3+ , R.E. 3+ , where Ln is La, Y, Gd, or Lu, and R.E. is Er, Ho or Tm, revealed that the excitation process involves absorption by Yb 3+ ions followed by two or more consecutive energy transfers to R.E. 3+ , and is not achieved by cooperative sensitization. The dependence of luminescence intensity on excitation intensity and the dependence of luminescence on sensitizer concentration are given as evidence. The steady state solutions for resonant and nonresonant two photon excitation are compared and it is shown that highest efficiency will result for activator absorption at slightly less energy than sensitizer emission, so that energy transfer from activator to sensitizer does not occur. The energy efficiency for LaF 3 : Yb 0.12 Er 0.02 , which has two resonant transfers, and GdF 3 : Yb 0.4 Tm 0.0015 which has at least one phonon assisted transfer, were measured. The quantum efficiency for the Tm 3+ material was found to be 1.8% at 0.36 mW cm -2 nm -1 , 80 times that of the Er 3+ material. The intensity dependence of the green Er 3+ emission was quadratic in agreement with the theoretical prediction, while that of the 0.8 μ Tm 3+ emission was subquadratic, indicating ground state depletion. Yb 3+ lifetimes were measured and used to obtain the energy transfer coefficients for the nonresonant transfer. Estimates of lifetimes from the literature and measured parameters were used to calculate the emission rate ratio of the Tm 3+ material to the Er 3+ material and agreement was satisfactory.

High Resolution Laser Ultrasound Detection of Metal Defects

The standard for sensitive detection and resolution of defects in metal components is scanned focused immersion inspection to produce C-scans. Laser ultrasound, although successfully applied to composite inspection, has previously not produced comparable results in this arena.

Broadband Acoustic Microscopy: Scanned Images with Amplitude and Velocity Information

Since the introduction of the mechanically scanned acoustic microscope by Lemons and Quate,1 a world-wide effort has taken place to make this device2 one of the most widely used tools for materials characterization and development. With the exception of work by Tsai,2,3 and the pulse compression acoustic microscopy by Ni-koonahad, Yue, and Ash2 most studies have utilized acoustic pulses containing many wavelengths, resulting in narrow bandwidth systems. Calculations for material properties therefore require amplitude and phase measurements at different heights of the acoustic transducer above the sample. Algorithms for the use of these V(z) data have become highly sophisticated as reported in the work of Weglein, Kushibiki, Chubachi, Bertoni, Kino, Laing, Kuri-Yakub, Ash, Wickramasinghe, and others.2 This work continues to develop methods for time and frequency domain observations of broadband acoustic pulses. These observations permit material properties to be determined with the acoustic transducer at a constant height above the sample. The necessary data can be acquired during uninterrupted mechanical scanning by digitizing the reflected waveforms from the sample. One of the advantages of this approach is that scanned broadband systems are, and historically have been, widely used for industrial quality control.