special efforts were taken to ensure that frequency shifts, which can appear The six lowest rotational transitions of CO have been measured to an from optical misalignment or other sources such as baseline problems, accuracy of {500 Hz. Highly accurate rotational transition frequencies of were inside the frequency reproducibility of each measured transition. The CO have always been in demand since they serve in the laboratory as an experimental uncertainties for the Lamb-dip frequencies were then estieasy but important secondary calibration standard, covering the millimeter mated to be not larger than 500 Hz. to the far infrared region. Astrophysically, CO is a ubiquitous interstellar
The present measurements are probably the most precise absolute line molecule, and also an important trace constituent of planetary atmospheres.
frequency determinations achievable to date in the millimeter-and submilli-For both cases, precise laboratory rest frequencies are required for accurate meter-wave region. It is therefore desirable to have the line center positions velocity determinations.
of the lower J rotational transitions of CO brought up to the level of The literature on laboratory rest frequency determinations of CO has accuracy obtainable by saturation-dip spectroscopy. Accordingly, we have always reflected the most recent advances in technological developments remeasured these transitions and studied the J Γ
5 R 4 and J Γ
6 R 5 concerning measurement accuracy. This is illustrated by recent work from transitions. Tables 1 and2 summarize both the far-infrared data of Evenson two groups on this topic. One line of work, by Varberg and Evenson ( 1),
(2) and the CO transitions measured with the Cologne terahertz spectromeconcerns the determination of accurate far-infrared rotational frequencies ter operating in the saturation mode up to 800 GHz and in the conventional of CO employing the laser sideband technique and its most recent revision Doppler-limited mode up to 1.3 THz. The line center frequencies were (2) using improved CO 2 frequencies (3). The accuracy achieved for the derived from the experimental data points by fitting them to a parabolar individual frequencies ranges between 10 and 43 kHz in the frequency function. For these measurements the pressure was maintained near 1 mbar region 600-4340 GHz. A second line of work concerns the precise Dopplerin a 3.9-m free space absorption cell. The highest frequency Lamb-dip limited measurements of the three CO rotational transitions J Γ
2 R 1, spectrum recorded for CO is displayed in Fig. 2. The accuracy of the Lamb-3 R 2, and 4 R 3 by Belov et al. (4) using the Nizhnii Novgorod based dip measurements is estimated to be within {500 Hz as already mentioned RAD-2 spectrometer. This technique achieved an accuracy of 1-2 kHz (1s above, whereas the Doppler-limited transition frequencies of the Cologne uncertainty) on the appropriate line center positions, when pressure shift spectrometer can be trusted to {5 kHz. These levels of accuracy are substaneffects were taken into account.
tiated not only by the quality of the fit of these lines, but also by internal The purpose of the present Letter is to report Lamb-dip measurements consistency with previous microwave measurements. The internal consiscarried out on CO with the Cologne terahertz spectrometer ( 5) operating tency checks include the following: in the saturation mode (6-8). This spectrometer has been used recently for sub-Doppler resolution measurements on a variety of molecules. Among 1. The four lowest J rotational transitions have been measured to rather the molecular spectra studied is the rotation-inversion spectrum of ammohigh precision by several laboratories with different types of spectrometers. nia in its ground (6) and first excited vibrational states ( 7), with the aim All previous results are, within their quoted uncertainty, in excellent agreeof resolving its nuclear hyperfine structure. For the ground state of NH 3 ment with our present Lamb-dip data (Table 1). In particular, our recent we have resolved for the first time the hyperfine structure of the ( J, K) Γ
Doppler-limited submillimeter CO measurements (12) agree to within 1 s(1, 0) 0 a(0, 0) rotation-inversion transition and determined its unsplit kHz with our new sub-Doppler results. line center frequency to be 572 498.163 MHz with an accuracy of 10 kHz 2. The statistics on the calculated J Γ
1 R 0 line position using different (8). The quoted uncertainty is not determined by the ultimately reachable data sets can be used to gain some insight into the residual systematic error measurement accuracy of the spectrometer, but rather reflects possible presand thus into the internal consistency of the measurements. For example, sure shift contributions. For CO, it has been shown that self-pressure shift our fit of the new data together with far infrared data from ( 2) (Table 2) effects are negligibly small not only for the J Γ
2 R 1 and 3 R 2 transitions yielded 115271.20202 (6) MHz for the calculated frequency of the J Γ
1 (4), but even for the 1 R 0 line, as has been confirmed recently by Ma Β¨der R 0 transition both when the measured frequency of this transition was et al. (9). In the case of negligible contributions from pressure shiftsincluded in the fit and when it was not included. A similar result was and this should normally be so for a number of molecules-the expected obtained when the frequencies of the first four transitions in Table 2 were accuracy of the Lamb-dip measurements is estimated to be about 1 kHz replaced in the fit by the data reported by Winnewisser et al. (10). 1 In this for isolated strong lines. In favorable cases the achieved measurement precilatter case, n calc Γ
115271.20200(7) MHz when we kept the uncertainty sion of a transition frequency can even be better.
of the remaining two Lamb-dip measurements. When their uncertainty was Figure 1 shows the Lamb-dip of the J Γ
4 R 3 transition of CO recorded in the second derivative form. The full linewidth of the Lamb-dip is close to 40 kHz and consists of about 30 measured points. The frequency step 1 In addition to 12 C 16 O, the lowest rotational transitions of the following can be as small as 100 Hz multiplied by the number of harmonics ( n) of CO isotopomers have been measured (10): 12 C 17 O, 12 C 18 O, 13 C 16 O, 13 C 17 O, and 13 C 18 O. the KVARZ synthesizer (n Γ
4 or 5 in this case). During the measurements, 468