Use of a Fabry–Perot interferometer to isolate pure rotational Raman spectra of diatomic molecules

Abstract

We propose to use a Fabry-Perot interferometer (FPI) as a comb frequency filter to isolate pure rotational Raman spectra (PRRS) of nitrogen molecules. In making the FPI's free spectral range equal to the spectral spacing between the lines of nitrogen PRRS, which are practically equidistant, one obtains a device with a comb transmission function with the same period. However, to match the FPI transmission comb completely with the comb of nitrogen PRRS lines one should tune the wavelength of the radiation used to excite the PRRS of nitrogen exactly to the position of any minimum in the FPI transmission comb. Thus to achieve this task for the case of nitrogen PRRS one must take the FPI's free spectral range Dnu(f)= 4B(N(2)) and the wavelength of the exciting radiation such that (1/lambda(exc)) = 4B(N(2))(k + 1/2), where B(N(2)) is the rotational constant of the nitrogen molecule and k is an arbitrary integer number. In this case all (odd and even) pure rotational Raman lines of nitrogen will pass through the FPI while the line of exciting radiation is being suppressed. Additionally, a FPI cuts out the spectrally continuous sky background light from the spectral gaps between the PRRS lines.