Introduction

     A widely used technique in analytical chemistry is atomic absorption spectroscopy, which is a method of determining the concentration of a particular metal element in a solution.  This technique is heavily dependent on hollow cathode lamps as a light source.  Through the principle of quantization of energy, a hollow cathode lamp emits very specific wavelengths of light that then is transmitted through the sample of interest to determine concentration of the desired analyte.  Without the hollow cathode lamp, sharp absorption peaks due to isolated wavelengths would be much more difficult to obtain.

How Hollow Cathode Lamps Work…

… the mechanics

     The mechanism by which a hollow cathode lamp works is actually rather simple. The glass chamber is filled with an inert gas, which is ionized at low pressure by application of a very high voltage. Here, ionization is the process where heating breaks the bonds between gas molecules, turning individual atoms into plasma—charged particles composed of positive ions and negative electrons. These highly energetic charged particles smack into the cathode, causing excited metal ions to sputter off. Sputtering is a technical term for ejection of atoms from solid material. As these ions collide with each other, they will be excited to higher energy states. The relaxation back down to less energetic states causes the emission of wavelengths of light specific to the metal on the cathode. For practical purposes, there are over 50 different types of hollow cathode lamps available on the market.

…the relation to atomic line spectra

     The principle that causes such specific wavelengths of light to be emitted can be explained by atomic line spectra.  Atomic line spectra are a method of visualizing the different frequencies at which different atoms absorb and emit light. They are colored bands, representing different frequencies of light. An emission spectrum will be a dark band with a few lines of color, and an absorption spectrum will be a colored band with a few dark lines in it. An emission spectrum corresponds to the frequencies of photons that have an amount of energy that is equal to the energy spacing between atomic energy levels. When a photon drops from an excited state to a lower state, it gives off a photon with energy equal to that energy difference. Only light that has energy that is exactly equal  to these gaps will show up in the spectra. For an absorption spectra, it is the opposite: atoms will only absorb light that has energy that is equal to one of the gaps between the energy levels, so when the light that was shined on the atom is analyzed, there will be dark bands that correspond to the light that had an energy that could be absorbed by the atom. This result is again a consequence of the quantization of energy.

     The main science behind atomic line spectra lies in energy quantization. Energy quantization is a consequence of wave particle duality, which is part of the fundamental nature of electromagnetic waves. This means that the electromagnetic waves that make up light can also display properties that one would not associate with a wave, but with particles of matter. The basic particle of an electromagnetic wave is referred to as a photon; in the same way that a proton or neutron is a little packet of mass, a photon of light is a little packet of energy. The energy content of a photon is directly proportional to its frequency.  Energy quantization results from this: just like you can’t have half a proton, you can’t have half a packet of energy. Thus, you can only have integer multiples of energy, corresponding to one packet, two packets, three packets, and so on.

     Light is not the only natural phenomenon that displays energy quantization: energy levels in an atom are also quantized. This quantization has important effects on how absorption and emission of light by atoms works. Atoms can only absorb a photon that corresponds to the exact difference of energy between two energy levels. For example, if a photon that has energy corresponding to half the exact difference in energy between two levels strikes an atom, it won’t be absorbed, because it isn’t possible to push the electron to half of an energy level. A good analogy of an electron transitioning between energy levels is a turtle going up a staircase, where the height of each step in the staircase represents an energy level. If energy was not quantized, the turtle would be going up a ramp, where it could occupy any possible height.

Applicability of Hollow Cathode Lamps

     Hollow cathode lamps are widely used in analytical chemistry for atomic spectroscopy as part of machines like the one shown to the right. They are usually used to find the concentration of metals in different compounds. While this is

 useful because atomic spectroscopy using hollow cathode lamps can detect specific metal concentrations in the range of parts per million within 2% error, they are limited in that they can only detect that specific metal for any given type of cathode. Some cathodes have combinations of metals, but atomic spectroscopy using hollow cathode lamps can only assist in determining the concentration of the metal corresponding to their specific cathodes, because they only emit wavelengths of light specific to the metal they are made of (a result of the specific electronic states to which electrons in a given metal ion can be excited). Even so, they are extremely useful for finding the lead content in paint, for example, or the amount of iron in breakfast cereals.

     Besides their use in analytical chemistry, different types of hollow lamps have everyday applications.  The most common example of hollow cathode lamps is fluorescent light bulbs.  When such a light is turned on, the electric power heats up the cathode inside enough for it to emit electrons, much like the hollow cathode lamps used in atomic absorption spectroscopy. These electrons collide with and ionize noble gas atoms inside the bulb surrounding the filament to form a plasma by a process of impact ionization. Beyond this, the process is slightly more complicated because the excited atom usually emits a photon that has an ultraviolet wavelength.  These are not visible to the human eye, so they must be converted into visible light through the use of fluorescence.

     Ultraviolet photons are absorbed by electrons in the atoms of the lamp's interior fluorescent coating, causing a similar energy jump, then drop, with emissions of more photons. The photon that is emitted from this second interaction has a lower energy than the one that caused it. The chemicals that make up the coating are chosen so that these emitted photons are at wavelengths visible to the human eye. The difference in energy between the absorbed ultra-violet photon and the emitted visible light photon goes toward heating up the fluorescent coating.

     Another common example of a hollow cathode lamp are neon signs. Neon signs and lamps 

(which refer any type of gas-discharge lamp that uses a noble gas) also use a plasma and the principle of energy quantization to emit light of certain wavelengths which translates into different colors of the lamp. For example, a lamp containing neon gas emits a red-orange light due to the specific wavelengths emitted by the decay of excited neon.  Different elements can be utilized in this manner to produce lights of different colors.   

Citations:

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"Quantized Energy (Planck)." Mr.Kent's Chemistry Page. Web. 5 Nov. 2010. <http://www.kentchemistry.com/links/AtomicStructure/PlanckQuantized.htm>

"Atomic Absorption and Emission Spectra." Astronomy 162. Web. 5 Nov. 2010. <http://csep10.phys.utk.edu/astr162/lect/light/absorption.html>

Variano, John. "Natural Science 122. The Nature of light." Natural Science 122. Web. 5 Nov. 2010. <http://www.cbu.edu/~jvarrian/122/nsci122.html>

"Hollow Cathode Lamps (HCL)." New Mexico State University. Web. 5 Nov. 2010. <http://www.chemistry.nmsu.edu/Instrumentation/AAS_HCL.html>

"Amazing Neon Lights." GameTrailers. Web. 7 Nov. 2010. <http://www.gametrailers.com/users/NeonBullet/gamepad/?action=viewblog&id=454638>

"Hollow Cathode Lamps." Hamamatsu. Web. 9 Nov. 2010. <http://jp.hamamatsu.com/products/light-source/pd028/index_en.html>

"Hollow Cathode Lamps." Heraeus. Web. 9 Nov. 2010. <http://www.heraeus-noblelight.com/en/optics-analytics/products-for-optics-analytics/hollow-cathode-lamps.html>