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 SPECTRASUITE software and verify that the spectrometer is working by observing the response when the fiber probe is directed toward the room lights.  Click on the ADC1000-USB "+" symbol to view its contents, expand the acquisition folder to view the input channels, and expand each channel to view the acquisition properties.  If the spectrometer has two inputs (master and slave), you can remove the slave by right-clicking on the properties of Channel 1 and terminating the acquisition.  You can adjust the acquisition settings of Channel 0 by highlighting its acquisition parameters and using the controls along the top of the window.

• Tilt the probe station upward by supporting one end on top of a book or block to obtain a strong room light signal and save the spectrum for future reference.

• Now put the probe station flat on the table, collect a dark spectrum, and subtract it from the subsequent spectra (see the Semiconductor QW Lab if you don't remember how to do this).

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

• Place the handheld UV source on the 2 support stands so that it illuminates the cuvette. Use the short wavelength setting.  Caution:  Wear UV-protection glasses!!!  Slide the cuvette up and down in the holder while observing the spectrum on the computer.  Since you want the cuvette to be positioned where the signal is strongest, you may want to use the lid to support the cuvette at a higher position. 

• Collect the fluorescence spectrum.  You may need to use a longer integration time (up to 2 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.

• Identify the lines from the scattered mercury light source by removing the sample, directing the lamp into the other optical port on the probe station, and obtaining the source spectrum.  These lines 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.)  Justify each assignment by citing the theoretical transition energy from the Dieke diagram and comparing it with the measured energy.

• Repeat the above procedure for the Tb³⁺:PDC crystal.

• 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?

• Compare the room light spectrum to that of the crystals you have just taken.  Do any of the lines match?  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.