Spectrometer Using superconductor MIxer Receiver (SUMIRE) for laboratory submillimeter spectroscopy

Recent spectroscopic observations by sensitive radio telescopes require accurate molecular spectral line frequencies to identify molecular species in a forest of lines detected. To measure rest frequencies of molecular spectral lines in the laboratory, an emission-type millimeter and submillimeter-wave spectrometer utilizing state-of-the-art radio-astronomical technologies is developed. The spectrometer is equipped with a 200 cm glass cylinder cell, a two-sideband (2SB) superconductor-insulator-superconductor (SIS) receiver in the 230 GHz band, and wide-band auto-correlation digital spectrometers. By using the four 2.5 GHz digital spectrometers, a total instantaneous bandwidth of the 2SB SIS receiver of 8 GHz can be covered with a frequency resolution of 88.5 kHz. Spectroscopic measurements of CH3CN and HDO are carried out in the 230 GHz band so as to examine the frequency accuracy, stability, sensitivity, as well as the intensity calibration accuracy of our system. As for the result of CH3CN, we confirm that the frequency accuracy for lines detected with sufficient signal-to-noise ratio is better than 1 kHz, when the high-resolution spectrometer having a channel resolution of 17.7 kHz is used. In addition, we demonstrate the capability of this system by spectral scan measurement of CH3OH from 216 GHz to 264 GHz. We assign 242 transitions of CH3OH, 51 transitions of 13CH3OH, and 21 unidentified emission lines for 295 detected lines. Consequently, our spectrometer demonstrates sufficient sensitivity, spectral resolution, and frequency accuracy for in-situ experimental-based rest frequency measurements of spectral lines for various molecular species.