MEASUREMENT OF RELATIVE INTENSITIES OF MICROWAVE ABSORPTION $LINES^{*}$

Abstract

$^{*}$The research was made possible by support extended Harvard University by the Office of Naval Research under ONR Contract Nonr 1866, Task Order XIV.Author Institution: Mallinckrodt Chemical Laboratory, Harvard UniversityA technique has been developed which allows a conventional Stark-modulated spectrometer to be utilized for the measurement of the relative intensities of absorption lines. It has long been known that reflections from various waveguide components set up standing waves within the absorption cell resulting in effective path lengths that are functions of frequency. This effect is largely responsible for the errors of 20\% or more usually encountered in measuring intensity ratios. By careful choice and design of various waveguide elements and by the use of ferrite isolators, particularly ahead of the crystal holder, which was found to reflect very strongly, the problem of reflections has been satisfactorily eliminated. Also, non-linearities in the detection system were neutralized by the use of a calibrated attenuator inserted between the preamplifier and the detector. Finally, a number of other phenomena including power saturation, interference of nearby lines and lobes, and the presence of stray signals at the modulation frequency (pickup) have been investigated. Measurements of the relative intensities of lines due to different isotopes, to various quadrupole components, and to molecules in different vibrational states have been made. The deviation between the values obtained and those predicted by other means has been between 1 and 2\% for 7 such measurements