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Heavy-element chemistry at Los Alamos

The fluorescence observed following the interaction of light with actinide oxide species has fascinated people for millennia, in large part because of its visually striking effect. An artifact of yellow-colored, uranium-doped glass was found near Naples, Italy, and determined to have been made in 79 A.D. Early 19th century European glassmakers added small amounts of uranium to their glass formulations as a coloring agent to make Canary-glass objects in which the appearance is clearly enhanced by the characteristic yellow-green fluorescence. As recently as the late 1930s, uranium oxides were added to the glazes employed to decorate the brilliant red dinnerware developed by Fiestaware. Although the production of uranium glass in the United States was halted in 1942, the marvelous colors of these uranium-containing objets d’art continue to appeal to enthusiasts today.

Early 19th century European glassmakers added small amounts of uranium to their glass formulations as a coloring agent to make Canary-glass objects. A uranium-glass candy dish is shown before illumination (top left) and after illumination (top right) with ultraviolet light. The Canary-glass Bohemian perfume bottle (bottom left) dates from the mid-19th century. As recently as the 1930s, uranium oxides were added to the glazes used to create the brilliant red color of dinnerware developed by Fiestaware, such as the cup and saucer shown below.

Photos by Mick Greenbank

Intrigue

The historical importance of uranyl fluorescence is not only artistic, but also scientific as well. The Canary-glass objects popular in the 19th century attracted the interest of the Scottish scholar Sir David Brewster, who mentioned the first scientific observation of visually appealing green fluorescence of uranium-containing glass in 1849. The term for this fundamental phenomenon, fluorescence, was later introduced by Sir George Gabriel Stokes in 1852 in his paper titled "The Change in the Refrangibility (wavelength) of Light." In this historic work, he described fluorescence from uranium oxide analytes:

"The intervals between the absorption bands of green uranite were nearly equal to the intervals between the bright bands of which the derived spectrum consisted in the case of yellow uranite. After having seen both systems, I could not fail to be impressed with the conviction of a most intimate connexion [sic] between the causes of the two phenomena, unconnected as at first sight they might appear. The more I examined the compounds of uranium, the more this conviction was strengthened in my mind."

This work contributed to the rule that emitted light is always of longer wavelength than that of the exciting light, a principle that is now called Stokes' Law. In modern years, fluorescence from compounds containing uranyl ions has attracted widespread attention as a tool for understanding the complex electronic structure of actinide molecules, characterizing speciation of uranium materials, and detecting the presence of uranium. Although investigations of fluorescence from other actinide species are not as extensive, reports from Russian scientists suggest that fluorescence from neptunium analytes is quite sensitive. However, the molecular composition of their neptunium samples is unknown.

Laser-induced fluorescence spectroscopy is used to probe the electronic structure of neptunyl compounds. John Berg collects fluorescence data from a sample containing Cs2U(Np)O2Cl4 while Marianne Wilkerson adjusts the sample.

Discovery

Researchers at Los Alamos have discovered the first example of fluorescence from an actinyl species other than uranyl. A team led by Marianne Wilkerson and Harry Dewey of the Chemistry Division and John Berg of the Nuclear Materials Technology Division has established a capability at the Integrated Spectroscopy Laboratory to detect near-infrared photons emitted by actinide compounds following visible laser excitation. The spectra arise from a 5f-5f transition, opening up the ability to explore the electronic structure of actinide compounds in the near-infrared region of the electromagnetic spectrum by using sensitive and selective spectroscopic capabilities. This research is part of a Basic Energy Sciences (DOE BES) program to investigate chemical behavior of heavy elements.

Actinide challenges and limitations

Many innate chemical properties of molecules are a consequence of electronic structure, or the manner in which electrons are arranged in orbitals around bonding atoms, and the electronic structures of molecules containing actinide elements are especially intriguing and challenging to understand. The large radii and lack of strongly directional bonding of actinide atoms contribute to a large variety of molecular arrangements. Only actinide-containing molecules can house electrons in the 5f group of orbitals, and the large number of orbitals in this 5f group can interact with other available molecular orbitals, generating intricate electronic structures. Optical spectroscopy is a powerful tool for probing electronic structures of molecules, but the spectral signatures of actinide compounds are complicated due to extensive numbers of spectral lines. As a result, correlations between the molecular structures and optical spectra of actinide molecules remain difficult to predict or even describe.

Fluorescence from a sample containing the neptunyl tetrachloride ion can be induced using excitation from a simple helium neon laser source.

The neptunyl tetrachloride ion

The neptunium compound chosen to initiate this fluorescence investigation is Cs2NpO2Cl4. The structure of this relatively simple molecule consists of a neptunium atom coordinated in a pseudo-octrahedral fashion by two apical oxo groups and four equatorial chloride ligands such that the NpO2Cl42- anion is centrosymmetric and has approximate D4h symmetry. The charge of the dianion is balanced by two cesium cations. This class of molecular solids is particularly amenable to fluorescence studies because the preparation of Cs2NpO2Cl4 is easily carried out in air, the simple chloride ligands lack any intraligand vibrational modes that could give rise to complicating vibronic structure or fluorescence quenching, and the lack of water in the crystalline lattice eliminates another potential source of fluorescence quenching.

Dilution, or doping, of a fluorescent ion or molecule into a host crystal is often necessary to minimize self-quenching and to offer some degree of control over the local environment. Model host material should have sites of appropriate size for accommodating the dopant, be chemically unreactive with the dopant, and lack any high-frequency vibrational modes that could serve as acceptors for radiationless deactivation. Cs2UO2Cl4 is an ideal host for inclusion of Cs2NpO2Cl4 because it is isostructural and only slightly larger in size, and its spectroscopic signature offers a large optical window throughout the near-infrared into most of the visible spectrum. Furthermore, there is a fortuitous wealth of supporting information on both Cs2UO2Cl4 and Cs2NpO2Cl4 obtained from complementary spectroscopic methods.

Researchers used the neptunium compound Cs2NpO2Cl4 in their fluorescence investigation. Neptunium is green, oxygen is red, chlorine is black, and cesium is blue.

Opportunities

To date, a large number of the spectroscopic studies of actinide compounds have focused on understanding the electronic structure and optical spectra of uranyl, a ubiquitous anion consisting of a metal that is coordinated by two oxygen ligands in a trans fashion. The electronic spectra of uranyl compounds are characterized by a particular type of interaction in which an electron is excited from an orbital that is mostly associated with the oxygen atoms to an orbital that is more representative of the central metal atom. These so-called ligand-to-metal charge-transfer (LMCT) transitions are readily detected using fluorescence, which in some cases is visible to the unaided eye in the mesmerizing yellow and green colors of uranium oxide compounds.

Efforts toward understanding the electronic structures of all other actinide species, however, are much less common. One complicating factor is that there is an additional class of transitions available to these other species: transitions that correspond to promotion of an electron from one state of predominantly 5f metal orbital character to another state of primarily 5f metal orbital character. As suggested earlier, the number of possible excitations involving 5f electrons complicates theoretical treatment. Furthermore, fluorescence transitions involving electrons in 5f orbitals will not be found in the easily detected visible region of the optical spectrum, but rather in the near-infrared region, where detection technology has been much less advanced until recently.

Many spectroscopic studies of actinide compounds have focused on understanding the electronic structure and optical spectra of uranyl, an anion consisting of a metal that is coordinated by two oxygen ligands in a trans fashion. This is the structure of an actinyl ion (M = uranium, neptunium, plutonium, or americium). The electronic spectra of uranyl compounds, which are characterized by so-called ligand-to-metal charge-transfer transitions, are detected using fluorescence, but some of them are visible to the unaided eye, as seen in the yellow and green colors of uranium glass.

The discovery that fluorescence from the neptunyl ion is observable, particularly near-infrared fluorescence, opens uncharted scientific territory for the study of a significant class of transitions available to other stable forms of uranium, neptunium, plutonium, and americium. The signatures mapped out by 5f-5f transitions are relatively simpler than those of LMCT transitions with regard to the numbers of electronic states and potential relaxation pathways. The ability to probe these transitions with molecular fluorescence will enrich our understanding of the fundamental properties of 5f orbitals, such as their role in actinide bonding, transition probabilities as a function of temperature, and the lifetimes of excited states.

Applications

Researchers are attempting to develop near-infrared fluorescence to complement a variety of other techniques for a Laboratory Directed Research and Development-Directed Research (LDRD-DR) project aimed at a fundamental scientific understanding of the interaction of actinide molecules with oxide minerals within the solid (mineral)/water interface. By coupling these experiments with transport models and other techniques, researchers hope to develop a better understanding of interfacial reactions of actinides in the environment.

Collective results suggest that neptunium fluorescence may have practical application as a detection capability. Russian researchers claimed more than ten years ago that a neptunium detection limit as low as a few picograms could be achieved through fluorescence detection in the red region of the visible spectrum. New data suggests that this region may not be where even the most intense signal should occur.

Understanding the electronic structures of actinide species other than uranyl is complicated by the fact that there is an additional class of transition--the 5f-5f transition--available to these other species. In this type of transition, an electron is promoted from one state of predominantly 5f metal orbital character to another state of primarily 5f metal orbital character. This figure shows the transitions available to the neptunyl ion.

The Los Alamos team has discovered that the neptunyl ion fluoresces with reasonable efficiency in the near-infrared not only at liquid-nitrogen temperature, but more significantly at room temperature. Furthermore, fluorescence is readily achieved using excitation by red light of a simple helium neon laser or diode lasers common in consumer electronic devices. Specific optical transitions also can be identified by their characteristic energy and by an additional identifying tag, the length of time that an electron resides in an excited state before the excitation decays in a fluorescence transition. Measurements by Los Alamos researchers reveal that low-lying excited states of neptunyl may have reasonably long lifetimes, comparable to those of uranyl species.

How does a molecule interact with a photon of light?

Every molecule has a characteristic set of electronic states, each of which is defined by a particular arrangement of the molecule's electrons into molecular orbitals. These electronic states lie at different energies. Most of the time a molecule resides in the state of the lowest energy, known as its ground state, but there are accessible higher-energy states known as excited states. One way that a molecule may be transformed into an excited state is by absorbing a photon of light whose energy matches the difference in energy between the ground state and an excited state of the molecule. When this absorption occurs, the electrons rearrange to form the excited state. A plot of the tendency to absorb light versus the photon energy is known as an absorption spectrum and exhibits peaks that correspond to energy differences between the ground and excited states.

Because a molecule is inherently unstable in an excited state, it tends to return to the ground state by any of several pathways. The length of time that the molecule resides in an excited state before it decays to a lower-energy state is termed the excited state lifetime. One of the decay pathways is by emission of a photon in a process known as luminescence, a word coined from the Latin word for light--lumen. Luminescence is a relatively rare decay pathway for excited states of most molecules, but if it occurs, it greatly eases the detection and study of excited states, making its discovery in a neptunium-containing molecule particularly interesting.

The characteristic vibrations of the molecule may also be excited by either absorption or luminescent processes. Much of the complex structure in an absorption spectrum is due to excitation of one or more quanta of vibrational energy in addition to the change of electronic states.

The absorption versus luminescent process is defined in this illustration. The red curve represents the potential energy of a lower-lying electronic state such as the ground state and the blue curve represents the potential energy of an excited electronic state. The black levels on each curve symbolize the energy of levels of a vibrational mode. When an absorption process occurs, a photon of appropriate energy is absorbed by a molecule, promoting the molecule to a vibrational level of a higher-lying electronic state. Conversely, when a luminescent process occurs, a photon is emitted by a molecule in an excited state, allowing it to relax to a vibrational level of a lower-lying state.

Future

It had been presumed that fluorescence, the workhorse technique for understanding uranyl LMCT transitions, was unemployable for observing the 5f excitations that are much more common in all other dioxo actinide species. The team's discovery of 5f-5f fluorescence from the neptunyl ion has revealed a new and exciting tool for future fundamental studies of electronic structure and applied development of detection technologies.

Recent advances in detection technology have made it possible to observe fluorescence transitions in the near-infrared region, as shown in this fluorescence spectrum of Cs2NpO2Cl4, the compound used by Los Alamos researchers in the early part of their investigation.

This article was contributed by Los Alamos researchers Marianne Wilderson and Harry J. Dewey of the Chemistry Division and John Berg of the Nuclear Materials Technology Division.


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