Base-line suppression in microwave spectroscopy using frequency modulation and harmonic detection

In a microwave spectrometer, systematic variations of the rf source amplitude with frequency can cause a spurious signal (base line) which severely limits the attainable sensitivity. Normally, the base line is eliminated by using Stark or Zeeman modulation. However, these modulation methods cannot be used in an experiment on nonparamagnetic molecular ions contained in a glow discharge. This paper discusses a technique which we have used to reduce the base line by a factor of about 10(-5) and which can be used to study the spectra of species contained in a glow discharge. The scheme uses wideband sinusoidal frequency modulation of the microwave source combined with synchronous detection at the nth harmonic of the modulation frequency. The system produces a signal proportional to the nth derivative with respect to frequency of the base line. However, the signal due to an absorption line is only slightly smaller than that produced by other modulation methods. The system may be considered to behave like a high-pass filter which discriminates against features that vary slowly with frequency.