Our research area covers high-resolution spectroscopy of unstable molecular species such as free radicals and ions, in particular, of astrophysical interest. We use submillimeter-wave and laser spectroscopic technique to observe such species in the laboratory.

Submillimeter-wave Spectroscopy
Our submillimeter-wave spectrometer employs three backward-wave oscillaotors(BWO). which cover the freuency range of 280-860 GHz. Our highest priority is to detect and identify ionic species. To achieve this goal, an extended negative glow discharge cell was built. Unlike infrared spectroscopy of ions, sample pressure in microwave spectroscopy is relatively low, about several milli-Torr, while in infrared observation it is about several Torr. Diameter of a cell employed in most cases for microwave spectroscopy cannot be proportionately large to compensate ambipolar diffusion loss. Therefore ion loss by ambipolar diffusion process is a very severe problem to maintain sufficient ion concentration for microwave observations. De Lucia and his co-workers discovered that an axial magnetic field of about 200 Gauss applied to a discharge cell of about 2.5 cm bore enhanced abundances of ionic species by about two orders of magnitude in their millimeter wave observations of HCO+ and other ions.

[Sub-mm spectrometer at Ibaraki University]
[Sub-mm spectrometer at University of Waterloo]

The extended negative glow absorption cell was designed after the original work by De Lucia. A double jacketed Pyrex tube of 38 mm inner diameter and of 1.8 m in length is accommodated inside a stainless steel tube of 10 cm in diameter and of 1.5 m in length. The inside of this stainless steel tube can be evacuated, which serves as a heat insulator. Through the outer jacket of the Pyrex tubing cooled methanol can be flowed to cool the discharge. Two electrodes are fitted to both ends of the Pyrex tube. The electrodes are placed inside the glass envelope in concentric configuration and the tips of the electrodes are just stick into the entrance of the solenoid. Three layers of solenoid coil are wound around the stainless steel tubing. A DC current of 8A induces axial magnetic field of about 250 Gauss.

Traditionally, in sub-millimeter wave region, frequency modulation technique is utilized to enhance the detection sensitivity. Stark modulation is out of scope due to inevitable difficulty of using a Stark cell for higher frequency region. Frequency modulation, however, causes a severe baseline distortion due to source power variation, which limits the detection sensitivity. The background can be subtracted and corrected numerically. The procedure, however, is often subjective and very dubious. A double modulation method will enable us to do such correction automatically and more effectively. Double modulation scheme employed in this work is a combination of frequency modulation and either magnetic field modulation or discharge modulation.