Fluorescence Spectroscopy of Minerals: 1—Using a Spectrometer Robert Fosbury, FMS #1852 Munich, Germany
Total Page：16
File Type：pdf, Size：1020Kb
Fluorescence spectroscopy of minerals: 1—Using a spectrometer Robert Fosbury, FMS #1852 Munich, Germany AbstrAct In the broadest sense, this can be described as the measure- ment of the color of light where color means the distribution Light interacts strongly with matter with the result that we of energy with wavelength of electromagnetic radiation see a world rich in color. In the case of minerals and gemstones, and the word light is used as a proxy for the entire electro- the color is both beautiful and informative. Shining (invisible) magnetic spectrum, from gamma-rays through to the long ultraviolet light on minerals can produce a spectacular palette wavelength radio waves used to communicate with nuclear of colors. But what does this tell us and how can we investigate submarines. This proxy is useful for us not only because it is a it beyond the stage of being filled with a sense of delight and short and familiar word but also because the visible spectrum wonder? The science of spectroscopy allows us to travel deep is generally the place to look when we study the fluorescence into the structure of rocks. Here is the first part of an article of minerals and gemstones. that explains how to do it. In the second part, we hope to be able to survey in more depth the causes and mechanisms of Spectroscopy is so important for a fundamental reason mineral-light interactions with a greater range of examples of that lies at the heart of the physics of matter and energy. The fluorescence spectroscopy. relationship between photons (quanta of light) and matter (atoms and their constituents) is an intimate one. Photons 1. INTROdUCTION are the carriers of the electromagnetic force that helps hold atoms together and directs their structure and chemistry. The matter that we see, feel and smell—and indeed are In the language of physics: Photons mediate the force of made of—can trace its origins back to the first few minutes electromagnetism. The consequence of this is that light and of the Big Bang some 13.8 billion years ago. However, as matter interact very strongly with one another and, following the primordial fireball expanded and cooled, there was only such an interaction, the light carries an imprint of what time to form the nuclei of hydrogen, helium and a trace of happened. Reading this imprint and understanding what it lithium before these initial products were frozen in and there means is what spectroscopy is all about. remained insufficient energy (the temperature and density were too low) for further nuclear reactions. The remaining When a sample of a mineral is bathed in light, a rich elements that fill the Periodic Table had to be constructed assembly of physical processes will take place depending through nuclear reactions that took place slowly and gently on the mineral and the range of colors included in the light in the cores of stars, more rapidly during their violent deaths impinging upon it. Some of the light will be reflected from in supernovae and, in a few cases, in the space between the the surface. If the mineral is opaque, there may be some stars as a result of high energy cosmic rays colliding with interaction with a thin surface layer with the processed light atoms in interstellar gas and dust. This process has been going emerging from the surface along with the specular reflection. on throughout the Universe for almost its entire history and If the sample is transparent or translucent, some of the light a fraction of the resulting products have ended up in planets will traverse it and emerge carrying the imprint of absorption orbiting stars, some of these like our Sun. The most massive and scattering processes that have occurred in its interior. If elements—the crust on the Periodic Table—have extremely certain colors are present in the light source, usually those of short lifetimes and, although they may have appeared shorter wavelength containing the more energetic photons, fleetingly in dying stars, today we have to make them to there may be evidence of fluorescence where light of one order in particle accelerators. The somewhat hackneyed, but color is absorbed by components of the mineral and re- nonetheless true, conclusion is that we are stardust. emitted with another color, usually, but not exclusively, with a longer (redder, lower energy) wavelength. So, how do we know so much of this remarkable story? The dominant technique that has enabled us to understand The phenomenon of fluorescence was studied intensively the origin of the chemical elements, the structure of atoms in the mid-nineteenth century, notably by the Irish physicist and molecules and a great deal more besides, is spectroscopy. Sir George Stokes (President of the Royal Society from The Fluorescent Mineral Society 5 1885–1890). In a wonderfully descriptive (100 page!) paper the removal of the excitation can be used to follow different published by the Royal Society in 1852 (Stokes, G. G. 1852. paths of de-excitation and it has proved very valuable for On the Change of Refrangibility of Light, Phil. Trans. R. Soc. identifying the fluorescent activator responsible for a given Lond. 142, 463–562), Stokes explored the behavior of many emission. The useful timescales are often short: from nano- different materials—including fluorite—and his work led him, seconds to milliseconds, but systems using lasers can easily in a following shorter paper, to coin the word fluorescence for be configured to do this and there is much literature on the the phenomenon he was seeing. These papers are freely subject (Gaft, Reisfeld, & Panczer, 2005, Modern Luminescence available from the Royal Society website and are a joy to read. Spectroscopy of Minerals and Materials, Springer-Verlag, His work was carried out half a century before a revolution in Berlin, Heidelberg). Longer timescale phenomena are usually atomic physics gave birth to the quantum mechanics that was referred to as phosphorescence. necessary for fluorescence to be properly understood. It is fascinating to read how close Stokes was able to come to this The infrared is the realm of the molecule. These longer understanding so long before the theoretical infrastructure wavelength photons do not often carry sufficient energy to was in place. excite electrons directly to higher atomic levels. They can and do, however, incite the molecules to vibrate and rotate, rather Stokes made his phenomenological observations and his like loud rock music influencing the inmates of a disco. The measurements using his eyes and so, by definition, he was so-called vibrational spectra of molecules tend to be excited restricted to the visible spectrum which is at wavelengths of by light from the red end of the visible spectrum to what is approximately 400–700nm (a nanometer, nm, is a convenient called the mid-infrared which is up to around 100,000nm (or unit for visible spectroscopy and there are a thousand million 100 microns, µm). Rotational states generally have a lower of them in a meter: 550nm is a good, bright green to most energy still and so transitions fall at longer wavelengths people). While the visible spectrum is particularly useful for into the far-infrared and microwave range. Rotational and our field of interest—electrons in atoms march obediently to vibrational states are seen in combination and result in the orders given to them by ultraviolet and visible photons— extremely complex spectra that can contain millions of other wavebands certainly have their uses. More than that, transitions (e.g., in water). fluorescence is not excited only by light. Minerals can be blasted by other sorts of energy and so be made to glow. As a final topic in our limited tour of the range of spectro- Beams of electrons are frequently used to encourage atoms scopy, we mention the influence of the states of spin of to emit light in a technique called cathodoluminescence, electrons and nuclei on the spectra of atoms and molecules. familiar from the old TV tube. Mechanical and chemical While a change in spin state (direction) produces a very small energy can also be used for certain purposes. change in the overall energy, it has proved to be extremely important in several branches of science by producing fine- However, returning to the electromagnetic spectrum and and hyper-fine structure in spectra. Perhaps the most widely starting at the high energy end, X-rays can burrow deep into known example of hyperfine structure is in the hydrogen the electron cloud around an atom and reach the ones that atom that can exist with the single electron spin either longer wavelengths cannot, often exciting fluorescence best parallel or anti-parallel to the spin of the proton nucleus. The studied at X-ray or ultraviolet wavelengths. They are also transition between these two quantum states results in the strongly influenced by the regular arrays of atoms packed absorption or emission of radiation with a wavelength of closely (around a nm or less) together in a crystal where a 21cm. Such radiation is used by astronomers to map atomic beam will produce Bragg diffraction patterns that can be (neutral, i.e, with its electron in place) hydrogen throughout read to determine the crystal structure and atomic spacings. the Universe. All atoms will absorb ultraviolet light, resulting in the Particularly in mineralogy, where we are usually dealing excitation of electrons to higher energy levels or even to with crystalline matter, polarimetry—the measurement of their ejection (ionization). The fluorescence observed in the state of linear or circular polarization of light—is often minerals illuminated with ultraviolet lamps originates from combined with spectrometry into what is called spectro- the atoms readjusting to these excitations/ionizations.