Extension of Submillimeter Wave Spectroscopy below a Half-Millimeter Wavelength

Abstract

The methods of high-resolution microwave spectroscopy have been extended to $\ensuremath{\lambda}=0.43$ mm. The J=5\ensuremath{\rightarrow}6 rotational line of ${\mathrm{C}}^{12}$${\mathrm{O}}^{16}$ has been measured to be 691 472.60\ifmmode\pm\else\textpm\fi{}0.60 Mc/sec. With other measured lower frequency transitions, this value leads to a centrifugal stretching constant ${D}_{0}=0.18390\ifmmode\pm\else\textpm\fi{}0.00014$ Mc/sec and to ${B}_{0}=57635.970\ifmmode\pm\else\textpm\fi{}0.003$ Mc/sec for ${\mathrm{C}}^{12}$${\mathrm{O}}^{16}$. Measurements of the three hyperfine components of the $J=0\ensuremath{\rightarrow}1$ transition of H${\mathrm{Cl}}^{35}$ at $\ensuremath{\lambda}=0.48$ mm yield for this molecule ${\ensuremath{\nu}}_{0}=625919.24\ifmmode\pm\else\textpm\fi{}0.52$ Mc/sec and ${B}_{0}=312991.30\ifmmode\pm\else\textpm\fi{}0.26$ Mc/sec. With the infrared value of ${B}_{0}$ in wavelength units measured with great accuracy by Rank and his associates, our ${B}_{0}$ value yields the spectral value $c=299792.8\ifmmode\pm\else\textpm\fi{}0.4 \mathrm{km}/sec$ for the velocity of light. This value is in agreement with, and is of comparable accuracy to, the best values obtained by other more direct methods for the measurement of $c$.