Douglas T. Petkie

THE MILLIMETER- AND SUBMILLIMETER-WAVE SPECTRUM OF GLYCOLALDEHYDE (CH 2 OHCHO)

The simplest monosaccharide, glycolaldehyde (CH2OHCHO), has recently been detected toward the Galactic center in the source Sgr B2(N) at five frequencies from 71-104 GHz. None of the individual lines used in the detection had been measured previously in the laboratory; rather, their frequencies were predicted based on lower frequency measurements. We have now recorded and analyzed many new rotational transitions of glycolaldehyde through 354 GHz using two spectrometers. Lines through 48 GHz in frequency were measured with a spectrometer that uses Stark modulation, while the higher frequency transitions were measured with a FASSST (Fast Scan Submillimeter Spectroscopic Technique) apparatus. Analysis of the data has allowed us to confirm the interstellar identifications and to predict the frequencies of many additional lines not measured in the laboratory.

MEMS-Based Terahertz Photoacoustic Chemical Sensing System

Advancements in microelectromechanical system (MEMS) technology over the last several decades has been a driving force behind miniaturizing and improving sensor designs. In this work, a specialized cantilever pressure sensor was designed, modeled, and fabricated to investigate the photoacoustic (PA) response of gases to terahertz (THz) radiation under low-vacuum conditions associated with high-resolution spectroscopy. Microfabricated cantilever devices made using silicon-on-insulator (SOI) wafers were tested in a custom-built test chamber in this first ever demonstration of a cantileverbased PA chemical sensor and spectroscopy system in the THz frequency regime. The THz radiation source was amplitude modulated to excite acoustic waves in the chamber, and PA molecular spectroscopy of a gas species was performed. An optical measure‐ ment technique was used to evaluate the PA effect on the cantilever sensor; a laser beam was reflected off the cantilever tip and through an iris to a photodiode. As the cantile‐ ver movement deflected the laser beam, the beam was clipped by an iris and generated the PA signal. Experimental data indicated a predominantly linear response in signal amplitude from the photodiode measurement technique, which directly correlated to measured cantilever deflections. Using the custom-designed PA chamber and MEMS cantilever sensor, excellent low-pressure PA spectral data of methyl cyanide (CH3CN) at 2 to 40 mTorr range has been obtained. At low chamber pressures, the sensitivity of our system was 1.97 × 10−5 cm−1 and had an excellent normalized noise equivalent absorp‐ tion (NNEA) coefficient of 1.39 × 10−9 cm−1 W Hz-1⁄2 using a 0.5 s signal averaging time.