Measuring the Spectral Line Profile and Area Using Frequency Modulation

1 Measuring the Spectral Line Profile and Area Using Frequency Modulation 

Once the sample pressure has become sufficiently high that Doppler broadening may be neglected, the peak signal from an unsaturated spectral line becomes independent of the pressure for a particular sample composition (Section 1.2). In this regime Omax depends on frequency, temperature, linewidth and the molecular concentration of the state under study. As an isotopomer, stereoisomer or vibrational excimer, the species concentration will be less than that of its parent. 

The signal after detection is passed through a phase-coherent detector tuned to twice the modulation frequency o . The signal at 2o) is the appropriate Fourier component of Equation 4.1 and has been derived for the case of sinusoidal modulation by Baker et aV This signal is not of pure derivative form, but possesses a finite integral (Chapter 6). 

For a single-pass FM spectrometer working at 150 GHz the best operating condition is to minimise background, by controlling power reflection within and at each end of a long-path absorption cell, and to apply a modulation depth of 240 kHz p-p at a sample pressure 8-13 Pa. By contrast in a cavity spectrometer, the optimum modulation depth is governed by the cavity width, rather than the sample linewidth (Section 2.4). 

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