Optical Spectroscopy II
Rare Earth Ions in Solution
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 measure the absorption spectra of a variety of solutions of lanthanide ions. The solutions have been prepared in advance by dissolving rare earth salts (nitrates and chlorides) in water. The absorption spectra are taken using the Ocean Optics S2000 spectrometers. The allowed transitions between the rare earth energy levels will lead to peaks in the absorption spectra. Knowing the wavelength of such transitions, which represent absorptions from 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 absorbed, 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 LaCl3. 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.
Absorption of RE3+ in Ethanol Solutions
The following procedure will guide you through the measurement of the absorption of one rare earth salt solution, comparison with the Dieke diagram, and the identification of energy levels.
·Begin by running the Ocean Optics S2000 control software. Make sure the correct spectrometer is installed and set the acquisition parameters (integration time = 10 msec, average 300 readings. For the SD2000, int. time = 100ms, average 30 readings).
·Collect a dark spectrum.
·Turn on the tungsten light source and insert your reference cell (be sure to use the correct reference, water if your salt is dissolved in water, and ethanol if your salt is dissolved in ethanol). Measure the reference spectrum.
·With the reference cuvette (with ethanol or water) in place, select the transmission reading on the control software. This setting will enable the computer to automatically divide the signal by the reference hence displaying the percent of light transmitted at each wavelength. Of course, the percent transmission of the reference should be one hundred by definition. If it is not, press the reference button again and observe the signal go to one hundred percent.
·In the test cuvette place approximately 10 mg of EuCl3. And quickly cover with approximately 3.0 ml of ethanol (Hint: the cuvette holds 4 ml)
·If the salt does not thoroughly dissolve, place the cuvette in the ultrasound bath until the salt has dissolved.
·Placing the cuvette in the sample chamber, the transmission of the sample should appear on the screen. Notice the location of the peaks and store the spectrum. Converting these wavelengths to energies (cm-1) we find several well-spaced lines in the visible region of the spectrum. Upon inspection of the Dieke diagram, we can identify the absorbing transitions of the Eu3+ ions. The energy levels can be named by comparing the levels with the diagram.
·Are any of the lines in the spectra resolution-limited? (Could better results be obtained using a higher resolution spectrometer?) Our configurations for the S2000 and SD2000 have resolutions of 6.8nm and 1.0nm.
·While the intensities of transitions in rare earth ions is still very much an open question, comment on any general trends you observe in the intensities of the transitions. Specifically, are transitions with DS = 0 weaker or stronger? (Such trends are due to selection rules and can be predicted from first principle quantum mechanical calculations).
·Repeat the measurement after adding one, two, four, and eight drops of water to the solution. Comment on which transitions change more with the changes you make to the ion environment.
·Explain the origin of the broad lines, which appear on the low energy side of the sharp transition lines. Using the Boltzman distribution function, show that your explanation is feasible.
·Repeat the experiment using either Nd or Er salts as the solute. Do the trends seen here agree with those observed in the Eu salt?
If you have trouble reading this diagram or a printout of it, you may try this link, Dieke.