Novel techniques for molecular ion spectroscopy in a discharge

Velocity modulation laser spectroscopy has proven to be a successful spectroscopic technique for the study of molecular ions; indeed, it has become the conventional technique. However, there are a number of limitations to velocity modulation in its conventional form. First, there is a background arising from r.f. pick-up or optical emission. Because the background is at the same frequency as the desired signals, the background is hard to eliminate. Fluctuations in the background level can make it difficult to detect signals. Second, in many experiments the dominant source of noise is the detector, and sensitive detectors are often inconveniently slow. Two new spectroscopic techniques appropriate for the study of molecular ions are described, which address these problems: (1) double modulation spectroscopy with background elimination, and (2) difference frequency spectroscopy. The first technique has been used to detect a transition in HeH{dollar}\sp{+}{dollar}, the v = 1 {dollar}\gets{dollar} 0 P(1) line at 2843.904 cm{dollar}\sp{-1}{dollar} using a diode laser. The second technique, difference frequency spectroscopy, allows the laser and the discharge to be modulated at frequencies very different from the detecting frequency. This has been demonstrated in principle