MadSci Network: Physics

Re: Why is there a Na=D absorption line in high pressure Na vapor street light?

Date: Thu Feb 24 06:04:32 2005
Posted By: Gareth Evans, Industrial R&D practitioner and manager ( retired )
Area of science: Physics
ID: 1108089860.Ph

Hi Larry,  Thanks for your question.  You asked why there is a 
narrow “Na=D”  absorption line in high pressure Na vapor street lights.  
The emission of high pressure sodium lamps is substantially from sodium 
even though other common elements used in discharge lamps are used with 
sodium in practice, namely xenon and mercury.  Emissions from these other 
elements only contribute a small fraction of the output.  The first 
commercial sodium lamps were of the low-pressure types and these displaced 
mercury lamps a long time ago in the UK and the rest of Europe because of 
their energy efficiency.  One reason for the improved efficiency was that 
a large part of the emission ( the “D lines” at 589.0 and 589.6nm ) 
occurred at wavelengths at which the eye is most sensitive.  

The D lines are very narrow at low pressure.  The atoms in the discharge 
behave as isolated atoms. This very narrow emission however gave an un-
natural light and the high pressure lamps in turn displaced the low-
pressure types for most applications.  The high-pressure lamps made use of 
the phenomenon of “collisional broadening”.  At the high pressures in 
these discharges atoms are continually buffeted by collision and the 
chances are that in the process of emission the atoms are influenced by 
the proximity of another atom.  The resulting perturbation modifies the 
energy levels of the excited and ground states and consequently the 
emitted photon energy and wavelength is changed.  

Another way to look at it is given in this web page
as follows: “The finite duration of the radiation process of electron transition leads to a finite width of line, in accordance with Heisenberg's uncertainty principle. For a high pressure gas, radiating times can be much greater than the interval between atomic collisions, and this perturbation by colliding atoms causes the premature transition and emission of a photon. The decreased lifetime of the state creates an increased uncertainty in photon energy, broadening the emission line.” We still havn’t explained the absorption band yet. In any discharge there will be some degree of re-absorption of the emitted light by unexcited atoms before the light emerges from the lamp. The process of absorption is subject to a similar broadening effect to that for emission so it is not obvious why, if the emission is broadened, the absorption spectrum should contain an efficient narrow band resulting in a dark line in the broadened yellow/orange region. The explanation seems to be that the temperature within the lamp discharge is non-uniform. You suggested that a cool “film” of sodium was responsible but it is atomic sodium which provides the narrow D-line bands. In the lower temperature regions of the discharge near the walls of the lamp the broadening effect is reduced. The bulk of the emission though comes from the core region where the temperature is highest and the broadening greatest. The outer, cooler regions therefore provide the filter which produces the effect you observe. Your explanation was therefore close, the difference being that the sodium atoms in the cooler outer region rather than in a solid film are responsible. It should be interesting to see how the spectrum changes as the lamp warms up. This web site includes several useful if low-resolution spectra.

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