The line shape of the green line (Hg-5461$A^o$) of the mercury arc lamps (the high pressure Hg-lamp made in Korea, and the low pressure Hg-lamp of Jarrel-Ash Co., U.S.A.) is investigated by Fourier transform spectroscopy using the Michelson interferometer.
One flat mirror of the Michelson interferometer is driven uniformly by using a stabilized A. C. motor and the Haidinger fringe formed at the focal plane is detected with a photomultiplier tube (RCA 7767, S-10 spectral response).
The signal through a linear amplifier is recorded and from the recorded graphs, the Fourier transform is obtained by a computer.
In this experiment for the high pressure Hg-lamp, the resolution used is $0.98 cm^{-1}$. The green line of high pressure Hg-lamp is observed as a single pressure broadened line of which FWHM is $7.3 cm^{-1}$.
In the experiment for low pressure Hg-lamp, the resolution used is $0.06 cm^{-1}$. However, due to the requirement of high resolution the time equired for a complete scanning is of order 3×$10^3$ minutes, which is prohibitively long, so that intermittent measurement of the envelope of the interferogram is performed and used for the determination of the line shape. The green line of low pressure Hg-lamp is observed to have fine structure consisted of more than three lines, and FWHM of the center line of this green line is $0.1 cm^{-1}$.
The spectrographic recording of the Hg-line from the high pressure Hg-lamp gives FWHM to be $6.4 cm^{-1}$ which agree within 14% with the result obtained by the present Fourier transform spectroscopy.
