Detection of Optogalvanic Spectra Using Driven Quasi-Periodic Oscillator Dynamics

The narrowband light from a scannable, single-mode dye laser influences the electrical properties of gas discharges. The variation in these properties as the laser wavelength λ is scanned yields the optogalvanic spectrum of the discharge (i.e., electrical conductivity vs. frequency). By connecting a neon lamp, capacitor, and power supply in parallel, an undriven relaxation oscillator is formed whose natural frequency f0 is affected by neon-resonant laser light and this λ-dependence of the relaxation oscillator frequency f0 yields a variant optogalvanic spectrum (i.e., f0 vs. frequency). In this paper, a driving force is effectively applied to an otherwise undriven oscillator when the incident light is chopped periodically at fd. For fd ≈ f0 and a sufficiently large driving force amplitude (laser intensity and the degree of neon resonance), the relaxation oscillator can be entrained so that f0 is locked on fd and is independent of λ. For the new chopped-light technique described here, fd is adjusted to be the subthreshold of the entrainment range, where the λ-dependence of f0 is advantageously exaggerated by periodic pulling, and the beat frequency |fd − f0| vs. λ provides an optogalvanic spectrum with appealingly amplified signal-to-noise qualities. Beat frequency neon spectra are reported for the cases fd < f0 and fd > f0 and are compared with spectra obtained using the unchopped-light (i.e., undriven) method.