Abstract
In this dissertation, we establish a measurement setup for saturation absorption spectroscopy of HeH+. It consists a discharge tube which can produce HeH+ molecular ion, and an optical parametric oscillator (OPO). Because of the simple structure of HeH+, its transition frequency can be calculated accurately by quantum mechanics. At present, the theoretical accuracy in ro-vibrational transitions is about 30 MHz, and the experimental accuracy is 7 MHz which is limited by the large Doppler line width (~ 600 MHz). We expect to improve the accuracy using the saturation absorption spectroscopy. However, there are difficulties from low ion concentration in the discharge and low power of our DFG light source. In order to increase the ion concentration of HeH+, we have constructed an extended negative glow discharge tube, following the design of Professor T. Amano at University of Waterloo. In the negative glow region, positive ions have the highest concentration. However, this region is relatively short in the normal glow discharge. The negative glow region can be greatly extended by using a longitudinal magnetic field of up to 300 Gauss on a discharge tube with 4 cm inner diameter and 160 cm in length. We use a 99:1 He-H2 gas mixture at 80 mTorr, and 5 k□ ballast resistor for maintaining the abnormal discharge mode. When the discharge voltage rises to 2.2 kV, we can obtain stable extended negative glow discharge with 6 mA discharge current. For the light source, we are constructing a singly resonant optical parametric oscillator (OPO). Compare to our DFG, OPO has stable high output power over the mid-IR wavelength, it’s an ideal light source for probing the saturation absorption spectroscopy signal of HeH+. The OPO is a bow-tied ring cavity configuration, based on a MgO:PPLN crystal, and pumping by an □-DFB laser. The pump laser can be continuously tuned over 30~40 GHz without mode hop, and maintains single mode. We expect that the OPO provides over 200 mW idler power, and the idler wavelength can be tuned by tuning the wavelength of pump. In the near future, we plan to search the saturation absorption spectrum of HeH+ at R(1) transition, by using the extended negative glow discharge tube and OPO.