Optical Spectroscopy of
Rare Earth Ions in Yttrium Aluminum Garnet
In the following exercises we will gain first hand experience with the discrete energy levels and the transitions between them in rare earth ions. We will also learn how to interpret the quantum numbers provided by spectroscopic notation.
We will measure the fluorescence spectra of a variety of crystals containing lanthanide ions. The crystals are grown from a solution by dissolving rare earth salts (nitrates and chlorides) in the chelating agent PDC. The fluorescence spectra are taken using the Ocean Optics S2000 spectrometers. The allowed transitions between the rare earth energy levels will lead to peaks in the fluorescence spectra. Knowing the wavelength of such transitions, which represent emissions to the ion ground states, we can determine the associated energies using the Einstein-Plank equation
E = hc/l,
where h is Plank’s constant, c is the speed of light, l is the wavelength of the light emitted, and E is the energy of the separation between the ground and excited states. Notice that the energy of a transition is inversely proportional to the wavelength. A convenient unit for energy is the wave number (cm-1). In this unit system one simply writes the wavelength of the transition in centimeters and reciprocates it to get the energy (e.g., the energy of a 500nm photon is 20,000 cm-1).
Professor Dieke’s research group at Johns Hopkins (1960’s) compiled a table of energy levels for the trivalent rare earths in crystals. Since the optically active electrons in rare earths are well shielded from the local Coulombic environment, the energy levels remain fairly constant when comparing the levels in different hosts. In the diagram, pendant semicircles indicate luminescent levels. The relative width of the levels in the diagram represent the splitting of degenerate levels produced by the Coulombic environment. The levels are labeled using L-S coupling term notation since these are still fairly good quantum numbers when the levels are well separated.
Fluorescence from RE3+ Ions
Run the Ocean Optics software. Make sure the correct spectrometer configuration is installed.
Collect a dark spectrum and subtract it from the subsequent spectra.
Place the 4% Eu3+:YAG microcrystalline sample in the holder.
Turn on the mercury lamp and use the short wavelength setting. Caution: Wear uv-protection glasses!!!
Collect the fluorescence spectrum. You will need to use a longer integration time (up to 10 s) to take full advantage of the maximum counts allowed (4000), but remember to obtain a new dark spectrum each time you change the integration time. You may want to set the average and boxcar to higher numbers to get better signal to noise resolution.
Identify the lines from the scattered mercury light source using the Hg-spectrum. These are to be ignored in further analysis. (Hint: Looking at the linewidth may help you determine the source of a line.)
Convert the wavelengths of the Eu3+ fluorescence lines to wavenumbers.
Use the Dieke diagram to identify the transitions that produce the fluorescence. You may need to change the limits on the y-axis to see weaker intensities.
Repeat the above procedure for the 4% Tb3+:YAG microcrystalline sample.
Using the conventions of spectroscopic notation, determine the angular momentum quantum numbers of each level that participates in an observed transition. For each level, show that the numbers are consistent with quantum mechanical angular momentum addition.
How do the angular momentum quantum numbers change in the transitions that you observe? What can you say about any selection rules that may be operating?
Repeat the above procedure for the 0.45% Tb3+:YAG microcrystalline sample.
If you have trouble reading this diagram or a printout of it, save the following .jpg on your desktop and open it, Dieke.