Objective:

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

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.

• Run the Ocean Optics software.  Make sure the correct spectrometer configuration is installed.

• Collect a dark spectrum and subtract it from the subsequent spectra.

• Remove the cap of the cuvette for the Eu³⁺:PDC crystal (2,6-pyridine-dicarboxylic acid) to expose the crystals.

• Place the handheld uv source over the cuvette. 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 2 s) to get good peaks.  You might 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.  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 Tb³⁺:PDC crystal.

• Identify any selection rules you find.

• During the last lab, the spectrum of the fluorescent room lights was taken.  Compare the room light spectrum to that of the crystals you have just taken.  Research the literature to find the phosphors most commonly used in fluorescent lighting.  How does a fluorescent light work?

If you have trouble reading this diagram or a printout of it, save the following .jpg on your desktop and open it, Dieke.