Sub-Doppler measurements of the rotational spectrum of (CO)-C-13-O-16

The five lowest J rotational transitions of (CO)-C-13-O-16 have been measur ed by saturation-dip spectroscopy to an accuracy of about 2 kHz, employing phase-stabilized backward-wave oscillators (BWOs). These highly precise mea surements cover the transitions from J = 2 <-- 1 to J = 6 <-- 5 with freque ncies ranging from 210 to 661 GHz. For each of the five observed rotational transitions, the narrow linewidths of the saturation dips (about 20 kHz) p ermitted the resolution of the hyperfine splitting for the first time. This splitting is caused by the C-13-nuclear spin-rotation interaction yielding a value for the nuclear spin-rotation coupling constant of C-1((CO)-C-13-O -16). If combined with the beam measurements (C-1((CO)-C-13-O-16) = 32.63(1 0) kHz), a slight J-dependence of the spin-rotation coupling constant can b e determined (C-J = 30 +/- 13 Hz). In addition, we have measured in the Dop pler-limited mode several higher J rotational line positions of (CO)-C-13-O -16 UP to 991 GHz with an accuracy of 5 kHz. The two line positions (J = 12 <-- 11 and J = 14 <-- 13) were recorded by multiplying BWO frequency with an accuracy of 100 kHz. The rotational transitions J = 17 <-- 16 and J = 18 <-- 17 were measured with an accuracy between 15 and 25 kHz by using the C ologne sideband spectrometer for terahertz applications COSSTA. (C) 2000 Ac ademic Press.