Compositions of gas samples are frequently studied by laser absorption spectroscopy. Sensitivity is improved by two orders of magnitude when absorption cell is placed inside an organic-dye laser cavity

Bass, A. M.

Brown, W.

Keller, R. A.

Kurylo, M. J.

Peterson, N. C.

Zalewski, E. F.

English

NASA Technical Reports Server

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round Glass 
pectrograph 
August 1972	 B72-10524 
NASA TECH BRIEF	 441 
NASA Headquarters 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
An Absorption Spectrum Amplifier for 
Determining Gas Composition 
100%	 90% 
Mirror	 Dye Cell	 Mirror
The problem: 
Compositions of gas samples are frequently studied 
by laser absorption spectroscopy. In this method, an 
absorption cell containing the sample is placed out-
side the laser cavity. Laser radiation and spectroscopy 
are then used to determine the absorption spectra of 
gas and, hence, its composition. Although reasonably 
accurate, this technique is not sensitive to very low 
sample concentrations, e.g., below iO Torr range for 
sodium. 
The solution: 
Sensitivity is improved by two orders of magnitude 
when the absorption cell is placed inside an organic-
dye laser cavity.
How it's done: 
The block diagram of the amplifier is shown in the 
figure. This system uses a xenon-flash-lamp-pumped 
rhodamine 6-G dye solution at 50 mg/1 in absolute 
ethanol which is contained in a 7.5-cm dye cell. The 
cell has windows with anti-reflection coatings and is 
positioned between the 90 and 100% reflectance mirrors. 
A 10-cm long absorption cell which contains the gas 
sample is positioned in the cavity. 
Absorption is measured with the laser beam which 
is partially reflected (2% of the beam) into the entrance 
slit of a spectrograph by reflection from an uncoated 
quartz disc positioned inside the cavity at 450 to the 
optical axis. The spectrograph has 25 pm slits with a 
grating of 600 lines/mm and uses a Polaroid 3000 ASA 
(continued overleaf) 
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film to record the spectra. The flash-lamp pulse duration 
is 20 ps and that of the laser pulse about 2 ps. By a 
change in the concentration of the rhodamine 6-G, the 
laser can emit a continuous spectrum of a 2 to 10 nm 
bandwith. 
Sodium and iodine samples were tested to compare 
this method with the conventional one. Laser emission 
was varied from 570 to 590 nm for sodium and 500 to 
620 nm for iodine. The conventional method using the 
absorption cell outside the cavity resulted in detection 
of sodium down to 2xl0 5 Torr (cell temperatures at 
205 0C), whereas the improved method with the cell 
inside the cavity detected sodium down to 3x10 7 Torr 
(cell temperature at 112°C). Tests on iodine revealed 
that the improved method showed the finer spectra 
structure more accurately. 
Additional tests with the new method were performed 
on europium-nitrate dissolved in methyl alcohol. Results 
show that optical densities as small as 0.004 were 
detected. 
Note: 
Requests for further information may be directed to:
	
is Technology Utilization Officer 
NASA Headquarters 
Code KT 
Washington, D.C. 20546 
Reference: TSP72-10524 
Patent status: 
No patent action is contemplated by NASA. 
Source: E. F. Zalewski, N. C. Peterson, 
M. J. Kurylo, A. M. Bass, W. Brown,
and R. A. Keller of
National Bureau of Standards, 
U.S. Department of Commerce
under contract to
NASA Headquarters
(HQN- 10752) 
B72-10524	 I	 Category 03


An absorption spectrum amplifier for determining gas composition

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