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We have locked the frequency of a 3 THz quantum cascade laser (QCL) to that of a far-infrared gas laser with a tunable microwave offset frequency. The locked QCL line shape is essentially Gaussian, with linewidths of 65 and 141 kHz at the -3 and -10 dB levels, respectively. The lock condition can be maintained indefinitely, without requiring temperature or bias current regulation of the QCL other than that provided by the lock error signal. The result demonstrates that a terahertz QCL can be frequency controlled with 1-part-in-10(8) accuracy, which is a factor of 100 better than that needed for a local oscillator in a heterodyne receiver for atmospheric and astronomic spectroscopy.

We have observed the 2P3/2-2P1/2 fine-structure line of C II at 1900 GHz in five sources with ionization fronts nearly perpendicular to the plane of the sky. The LSR velocity of the C II emission is generally in good agreement with that observed for molecular species such as CO. However, the observed line widths of 3-14 km s-1 are typically wider than those of molecular lines and often show rapid spatial variations in the regions observed. In some sources this may indicate that part of the C II emanates from an ionized gas component, while for others it suggests an association between C II emission and an outflow. The C II brightness temperatures are typically equal to or slighty higher than the dust temperature at all locations observed. In the optically thin approximation, C II excitation temperatures are > or = 100 K and column densities are < or = 10(18) cm-2 for all sources except M17, which has a more intense and complicated line profile with a larger spatial extent than any other source observed. The quoted column density estimates derived in the optically thin limit appear to be somewhat lower than those predicted by models of photodissociation regions for sources with a side-illuminated geometry, but uncertainties in the UV flux and geometry of the ionization front preclude a definitive comparison. The estimated column densities would be higher if the C II emission were somewhat optically thick, in which case the ionized carbon would be more in equilibrium with the dust at temperatures lower than predicted by current models.

The design, construction, and performance of a combined solar telescope and Michelson interferometer for balloon-borne stratospheric far-IR absorption studies is discussed in terms of the relevant parameters and precautions necessary to obtain high resolution radiometrically calibrated spectra of the stratosphere between 20 and 100 cm(-1). Preliminary data are presented to indicate that the system performed as designed and provided constituent concentration values for H(2)O and O(3) from measurements of strong absorption lines in this spectrum.

Absorption and complete multipath interference have been included in the calculation of the modulation efficiency of a far i.r. Michelson interferometer with 25-mum and 100-mum Mylar beam splitters over the range of wavenumbers between 20 cm(-1) and 125 cm(-1) for angles of incidence between 45 degrees and 80 degrees . These results show that optimum performance of an interferometer in terms of highest and most uniform modulation efficiency for unpolarized radiation will be obtained by selecting the beam-splitter thickness to cover the wavenumber range of interest within the first interference lobe and by utilizing higher angles of incidence than the mechanically convenient 45 degrees . Practical constraints will usually limit this angle to a value somewhat less than the optimum of about 75 degrees for Mylar beam splitters.

We have obtained high-resolution spectra of the 153 micrometers J = 17-16 CO line in the BN-KL region of Orion using a laser heterodyne spectrometer. The line shows broad wings (30 km s-1 FWHM at BN) characteristic of the plateau emission as well as a narrower component probably associated with the quiescent gas in the molecular ridge. From an analysis of the plateau emission together with that observed in lower J CO transitions, we derive an excitation temperature of 180 +/- 50 K and minimum column density of 1 x 10(18) cm-2 for CO in this component, which constitutes 80% of the total integrated intensity of the J = 17-16 line near BN. The peak intensity of the narrower component observed at 0.8 km s-1 resolution increases relative to that of the plateau component toward theta 1C and away from BN, while the width decreases from 10 to 4 km s-1 (FWHM).

The fundamental OH rotational transition at 2514 GHz (119.2 micrometers) has been observed in the Orion IRc2 region at a spectral resolution of 0.6 km s-1. The emission is spatially compact (< 25" FWHM) and centered near IRc2. A comparison of the observed profile with spectra of other species known to exist in the region suggests that the entire blueshifted side of the OH profile has been "self-absorbed," leaving only a redshifted emission component. In addition, there is pronounced absorption near 5 km s-1 VLSR indicative of the "hot core" source. The presence of the 119 micrometers absorption provides a straightforward explanation for the high ratio of 163/119 micrometers integrated line intensities which was previously considered anomalous.

We have observed the J = 22-21 line of 12CO at 2528 GHz (118.8 micrometers) in the IRc2 region of Orion. The spectra at 0.6 km s-1 resolution show both plateau emission with FWHM approximately 35 km s-1 and a narrower component with FWHM approximately 8 km s-1. Comparison with heterodyne data of similar quality on the J = 17-16 line indicates that the broad and narrow components both originate in gas with an excitation temperature Tex approximately 600 K. The emission is consistent with the predictions of shock models in which the wide component arises from the heated outflow gas and postshock molecular material, while the narrow component comes from ambient material near the leading edge of the shock front where temperatures are high but significant acceleration has not yet occurred.

We have obtained velocity-resolved spectra of the 12C II 157.8 microns 2P3/2-2P1/2 fine structure line in the M42 region of Orion. Observations at 0.8 km s-1 resolution with a laser heterodyne spectrometer show multiple velocity components in some locations, with typical linewidths of 3-5 km s-1. Spectra of theta 1C and BN-KL also show weak emission from the F = 2-1 hyperfine component of the equivalent 13C II line. From the observed 12C II/13C II line intensity ratios, we deduce that the 12C II emission is optically thick with tau approximately 5 at both positions. Excitation temperatures of 128 K and 90 K, together with column densities of approximately 1 x 10(19) cm-2 and approximately 4 x 10(18) cm-2, are derived for theta 1C and BN-KL, respectively.

A new type of corner-reflector mixer mount, which has the advantages of ease of fabrication and assembly as well as frequency versatility, has been designed and constructed. The mixer works with arbitrary antenna lengths > or = 4 lambda with the reflector to antenna spacing adjusted to give a strong and symmetric central lobe. The predicted response patterns have been experimentally verified for various antenna lengths and operating frequencies between 800 and 2000 GHz. An important design feature is the incorporation of a microstrip matching network which eliminates IF impedance mismatch and provides mechanical isolation of the whisker antenna.