DOUBLE-MODULATION SPECTROSCOPY OF MOLECULAR-IONS - ELIMINATING THE BACKGROUND IN VELOCITY-MODULATION SPECTROSCOPY

Velocity-modulation spectroscopy is a well-established technique for performing laser absorption spectroscopy of molecular ions in a discharge. However, such experiments are often plagued by a coherent background signal arising from emission from the discharge or from electronic pickup. Fluctuations in the background can obscure the desired signal. We demonstrate a simple technique using amplitude modulation of the laser and two lock-in amplifiers in series to detect the signal. The background and background fluctuations are thereby eliminated, facilitating the detection of molecular ions.