A THEORY OF MODULATION SPECTROSCOPY WITH APPLICATIONS OF HIGHER HARMONIC DETECTION

A theoretical treatment of modulation absorption spectroscopy, applica ble 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 mod ulation frequency. The results obtained are given in terms of the deri vatives of the absorption lineshape function and bring out the connect ion to Nth-order derivative spectroscopy naturally. General expression s for any gaseous molecular absorption line are given. It is shown tha t certain advantages are obtained when one performs higher harmonic de tection, including an increase in the wavelength resolution. Experimen tal results are obtained and compared with the predictions of the theo ry developed.