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| {{Unreferenced|date=December 2009}}
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| [[File:Hollow Cathode Lamp.svg|thumb|300px|Basic diagram of a hollow-cathode lamp.]]
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| [[File:Hollow cathode lamp.JPG|thumb|300px|Hollow-cathode lamps from an [[atomic absorption spectrometer]]]]
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| A '''hollow-cathode lamp''' (HCL) is type of lamp used in [[physics]] and [[chemistry]] as a [[spectral line]] source (e.g. for [[atomic absorption spectrometer]]s) and as a [[frequency]] tuner for light sources such as [[laser]]s.
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| An HCL usually consists of a [[glass]] tube containing a [[cathode]], an [[anode]], and a buffer [[gas]] (usually a [[noble gas]]). A large [[voltage]] across the anode and cathode will cause the [[buffer gas]] to [[ionization|ionize]], creating a [[plasma (physics)|plasma]]. The buffer gas ions will then be accelerated into the cathode, [[sputtering]] off atoms from the cathode. Both the buffer gas and the sputtered cathode atoms will in turn be [[Excited state|excited]] by [[collision]]s with other atoms/particles in the plasma. As these excited atoms decay to lower states, they will emit [[photon]]s, which can then be detected and a spectrum can be determined. Either the spectrum from the buffer gas or the sputtered cathode material itself, or both, may be of interest.
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| An HCL can also be used to tune light sources to a specific atomic transition by making use of the [[optogalvanic effect]], which is a result of direct or indirect [[photoionization]]. By shining the light source into the HCL, one can excite or even eject [[electron]]s (directly photoionize) from the atoms inside the lamp, so long as the light source includes frequencies corresponding to the right atomic transitions. Indirect photoionization can then occur when electron collisions with the excited atom eject an atomic electron.
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| # <math>A + h \nu \rightarrow A^*</math>
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| # <math>A^* + e^- \rightarrow A^+ + 2e^-</math>
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| <math>A</math> = atom, <math>h \nu</math> = photon, <math>A^*</math> = atom in excited state, and <math>e^-</math> = electron
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| The newly created ions cause an increase in the current across the cathode/anode and a resulting change in the voltage, which can then be measured.
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| To tune the light source to a specific transition frequency, a tuning parameter (often the driving [[Electric current|current]]) of the light source is varied. By looking for a [[resonance]] on a [[data plot]] of the voltage signal versus source tuning parameter, the light source can be tuned to the desired frequency. This is often aided by use of a [[Lock-in amplifier|lock-in]] circuit.
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| ==See also==
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| *[[List of light sources]]
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| * [[Electrodynamic_tether#Hollow_cathode]]
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| {{Artificial light sources}}
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| [[Category:Atomic physics]]
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| [[Category:Optical devices]]
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| [[Category:Gas discharge lamps]]
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