Observation of a strong inverse temperature dependence for the opacity of atmospheric water vapor in the MM continuum near 280 GHz

Using atmospheric opacity measurements made at 278 GHz (9.3 cm^–1) at McMurdo Station, Antarctica during the austral springs of 1986 and 1987, combined with measurements of water vapor profile and total column density from near-simultaneous balloon flights, we have determined the attenuation per mm of precipitable water vapor (pwv) at this frequency. Our data were taken at significantly lower temperatures than other measurements in the literature for which accompanying water vapor pressure and temperature data are available. The results show a strong inverse dependence with temperature: measured opacity per mm of pwv is roughly a factor of two times greater at –35°C than at –10°C and three times greater than measurements at the same wavelength at +25°C reported by Zammit and Ade. We briefly review various theories proposed to explain excess absorption in continuum regions. Our lowtemperature measurements demonstrate a significantly greater inverse temperature dependence than embodied in several formulations, theoretical or empirical, proposed to represent mm-wave attenuation as a function of temperature and water vapor. The present results are qualitatively similar to observations of strong inverse temperature dependence in the near IR, but if attributed to water vapor dimer formation, imply a greater binding energy for the dimer than generally proposed by others. There is some independent evidence for a local anomaly in temperature dependence as a function of frequency near 280 GHz. It remains to be established whether our own results are strongly frequency dependent or apply generally to the mm-wave continuum.     

Video detection of mm-waves via photon-assisted tunneling between two superconductors

Photon-assisted tunneling between two superconductors has been used for sensitive detection of mm-wave radiation with best current responsivity of about half the quantum limit e/. The best NEP referring to the whole system is some 10^–15 W and about 10^–15 W referring to the junction alone.     

Millimetre-wave investigation of the soft mode in ferroelectric PbHPO4

The complex dielectric function along the a axis of PbHPO₄ single crystals is determined from reflection measurements at frequencies between 70 and 225 GHz in the paraelectric and the ferroelectric phase near the transition temperature T_c = 310 K. The data are complemented by a remeasurement of the properties of the lowest far-infrared mode. The mm-wave data can be described by a simple Debye formula. The relaxation time exhibits critical slowing down, and the corresponding relaxation frequency varies between about 2 and 60 GHz near T_c. Proton tunneling appears to play no essential role in this dielectric mechanism related to proton ordering.     

Measurement of atmospheric opacity at 278 GHz at McMurdo Station, Antarctica in austral spring seasons, 1986 and 1987

We present a quasi-continuous record of measured atmospheric opacity obtained at 278 GHz (1.1 mm wavelength) from McMurdo Station, Antarctica during austral spring seasons in 1986 and 1987. McMurdo Station, at 78°S, 166°W, is easier to access than the Amundsen-Scott (South Pole) Station, although representing a warmer, sea level site with substantially higher typical opacity: the present record may be of interest to those contemplating mm-wave astronomical or atmospheric observations within the Antarctic region. Observations were made over a 256 MHz bandpass in 1986, during the period August 30 to October 30. In 1987, a 512 MHz bandpass was used, and observations were made during the period September 4 to October 13. All data are reduced to represent opacity in the zenith direction, and measurements were taken approximately every 20 minutes, except during storms or other periods of high opacity. The periods covered represent transitions from the polar winter towards summer conditions, and thus represent neither the best nor the worst that this site has to offer.