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

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

harvard.edu

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.

chemeducator.org