A theory of modulation spectroscopy with applications of higher harmonic detection

Abstract

A theoretical treatment of modulation absorption spectroscopy, applicable when amplitude and frequency modulation are present simultaneously, is given. The effects of any relative phase difference between these two types of modulation are incorporated and the theory is applicable to an experiment involving detection at any harmonic order of the modulation frequency. The results obtained are given in terms of the derivatives of the absorption lineshape function and bring out the connection to Nth-order derivative spectroscopy naturally. General expressions for any gaseous molecular absorption line are given. It is shown that certain advantages are obtained when one performs higher harmonic detection, including an increase in the wavelength resolution. Experimental results are obtained and compared with the predictions of the theory developed.