Department of Physics, Middlebury College 1992-93
Modern Physics Laboratory


XIII. Laser Induced Fluorescence of the Uranyl Ion in Autunite


Discussion

The uranyl ion UO22+ is present in many naturally occurring minerals such as autunite, liebigite, andersonite, and opal. The uranyl ion is responsible for the bright green fluorescence of these minerals when exposed to ultraviolet radiation. The characteristic green fluorescence of these minerals is often used to identify and assay uraniferous minerals in uranium ore deposits.1

Fig. 1 shows a fluorescence spectrum of autunite [Ca(UO2)2(PO4)2*10-12 H2O] induced by 3371 Å ultraviolet radiation from a N2 laser.2 From Fig. 1(a), one can see that the characteristic green fluorescence results from a series of broad spectral lines in the range from 4900 to 6200 Å. A similar fluorescence spectrum is seen for most solids and liquids containing appreciable concentrations of the uranyl ion.

The simple structure of the uranyl ion fluorescence spectrum suggests an interpretation based on the quantum mechanical, one-dimensional harmonic oscillator model. It has been determined in other experiments that the uranyl ion occurs as the linear molecule O-U-O, as depicted in Fig. 1(b). The action of the 106 binding electrons in the O-U-O system can be crudely approximated by a spring, of spring constant k, that stretches between the two outer oxygen atoms. One can then imagine that the two oxygen atoms undergo one-dimensional, symmetric harmonic motion as in Fig. 1(b). The massive uranium atom does not participate in this motion, but provides a fixed point about which the oxygen oscillations occur.

In the above model, the vibrational energy levels of the uranyl ion are given by3,4

En = ( n + 1/2 )    n= 0, 1, 2, 3, ....        (1)

where = (2k/mO)1/2 and mO is the mass of a single oxygen atom. The unfamiliar factor of 2 arises because two  bodies of mO are taking part in the symmetric motion restrained by the single spring of spring constant k. The vibrational energy levels (of the electronic ground state) of the uranyl ion are shown in the lower part of Fig. 2. At room temperature the uranyl ion is in its lowest vibrational state n = 0. When excited by ultraviolet light of frequency , a uranyl ion absorbs a photon of energy h and the electronic cloud binding the ion is altered, leaving the ion in an excited electronic  state. In the excited electronic state, the effect of the 106 binding electrons must be characterized by a slightly different spring constant k' and the oxygen atoms execute small vibrations with frequency    ' = (2k'/mO)1/2, in a manner similar to the electronic ground state vibrations at frequency .

A uranyl ion excited to one of the vibrational levels of the excited electronic state will gradually decay by two mechanisms.

(1) Phonon emission. An ion in the vibrational level n' is very unlikely to make a transition to the n' - 1 vibrational level by emission of a photon of energy ' because of the very small quantum mechanical transition probability for such a low energy process. Also, there is a negligible probability for a photon transition to one of the vibrational levels n associated with the electronic ground state.

However, there is a high probability of transfering the energy ' to vibrational motion of other nearby atoms in the autunite crystal lattice. This transfer of energy is termed "phonon emission". By slowly transferring phonons of energy ' to the surrounding crystal lattice, the ion decays to the n' = 0 vibrational level labeled F (for Fluorescence) in Fig. 2. The process of emitting tens of phonons typically occurs within 10 - 100 ps following excitation.

(2) Photon emission. Once in the n' = 0 excited level (F level) in Fig. 2, phonon emission to the vibrational levels of the electronic ground state is highly improbable because the surrounding crystal lattice cannot absorb that great a phonon energy in a single process. Instead, photon emission occurs from the n' = 0 excited level (F level) to various vibrational levels n of the electronic ground state as shown in Fig. 2. These transitions make up the spectral lines seen in the autunite fluorescence spectrum of Fig. 1(a). The fluorescence lifetime for the n' = 0 --> n transitions is approximately 100 Ás.

The UO22+ fluorescence lines observed in this experiment are more than one hundred times broader than the Hg and H emission lines you observed in previous experiments. This breadth is typical of optical spectra of impurity atoms and constituent molecular ions in solid lattices.2,5 The breadth originates from the strong interaction of the UO22+ ion with the thermal vibrations of the surrounding crystal lattice. Fluorescence measurements of autunite at 4.2 K reveal substantially reduced linewidths and direct evidence for the phonon modes discussed above.6

One can use the fluorescence transition wavelengths to determine the energies of the vibrational levels of the electronic ground state and thus verify the assumptions of this simple model for the UO22+ ion.

Procedure

Place an autunite sample near the entrance slit of the SPEX 1704 spectrometer and irradiate it with 3371 Å ultraviolet light from the PRA LN-1000 N2 laser. Be sure that the bright green fluorescent spot occurs at the correct vertical and horizontal position for acceptance by the spectrometer. Entrance and exit slits must be opened to at least 50 - 200 Ám depending on the strength of the fluorescence source. Take spectra of the autunite fluorescence in the range from 4500 - 6500 Å with sufficient quality that you can clearly see at least five of the fluorescence transitions.

Take the following precautions.

(1) Use disposable plastic gloves to handle the autunite mineral sample. If you touch the sample, immediately wash your hands so that you reduce the chance that you might accidentally ingest any material containing uranium. In any event, be sure to wash your hands after you have completed taking data.

(2) Always wear the UV protective goggles when adjusting optical components or changing sample position. Do not look at the autunite fluorescence without goggle protection.

Analysis

(1) Determine the centroid and full width at half maximum (FWHM) of each peak in your fluorescence spectrum. Include an uncertainty estimate in your determination of the centroids. Place these results in a table entitled "Raw Experimental Data". Assume that the strong transition near 5020 Å corresponds to the n' = 0 --> n = 0 transition and label your entries in the above table according to what you should expect from Fig. 2.

(2) From your data construct an energy level diagram similar to Fig. 2, but with the energy of every level (including the n' = 0 level) given in units of eV and cm-1. Choose the state n = 0 to have zero energy. Indicate your observed transitions and label them by wavelengths given in Å. This figure should be entitled "Energy Level Diagram for Uranyl Ion in Autunite". (Note: 1 eV = 8065.479 cm-1 in spectroscopy units).

(3) Look up the mass of the oxygen atom and use your spectrum to determine the spring constant k associated with vibrations of the electronic ground state of the uranyl ion.

Reference
1. J.P. deNeufville, A. Kasden, and R.J. L. Chimenti, Applied Optics, Vol. 20, pp. 1279-1307, 1981.
2. D.L.Vehse, Laser Induced Fluorescence of the Uranyl Ion in Uraniferous Minerals, B.A. thesis, Middlebury College, 1986.
3. R. Eisberg and R. Resnick, Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles  (John Wiley & Sons, New York, 1985), pp. 221-225.
4. P.A. Tipler, Modern Physics  (Worth Publishers, New York, 1969), pp. 222-225.
5. R.C. Belanger, Laser Induced Luminescence of Manganese Impurities in Carbonate Minerals, B.A. thesis, Middlebury College, 1986.
6. R.S. Tucker, Laser Induced Fluorescence of the Uranyl Ion in Autunite at Liquid Nitrogen and Liquid Helium Temperatures, B.A. thesis, Middlebury College, 1987.

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