Page 115 - Tunable Lasers Handbook

96 Charles Freed

numbers for 12C1607, W16O 2’ 12C180,, W18O 2’ W160180, 14C1607. and 14C180,
were simultaneousl; computed from 390 beat frequency measurements between
pairs of adjacent (0001)-[ 1000, 0200],~,, band C02 laser transitions. The input
data included the 56 beat frequencies measured between adjacent 12C1601 rota-
tional lines by Petersen et a/. [93], and the absolute frequencies of the 10.18ym I-
R(30) and 9.33-ym 11-R(10) 12C160, transitions determined by Evenson et al. [94]
relative to the primary cesium standard. These initial results for the seven CO, iso-
topic species listed were published by Freed et al. in 1980 [36].
In 1983 Petersen et al. published [99] improved vibrational-rotational con-
stants and absolute frequency tables for the regular bands of 12C160,. These new
results obtained at the NBS in Boulder, Colorado, were based on new beat fre-
quency measurements, including high-J and across-the-band center measure-
ments, and yielded about a factor of 10 better frequency tables. In addition,
some specific 13C1602 lines were also measured with reduced uncertainties. The
new results of Petersen et a/. [99] yielded a more precise determination of the
absolute frequency (relative to the primary cesium standard) of the 12C160, I-
R(30) line, with an absolute uncertainty of 3.1 kHz. This uncertainty of 3.1 kHz
became the principal limit for the uncertainties in the frequency tables for the
absolute frequencies of regular band lasing transitions in nine C02 isotopic
species, published by Bradley et al. in 1986 [37]. The data and results published
in this paper represented the final phase and outcome of the isotopic CO, laser
frequency-calibration work that had begun at MIT Lincoln Laboratory more than
a decade earlier. This final CO, isotope frequency calibration work represented
significant improvement over previous results for the following reasons:

1. We have included in our database the most recent measurements on
13C160, regular band transitions that Petersen et al. published [99]; their more
precise-measurement of the I-R(30) line absolute frequency, and the beat fre-
quencies of their widely spaced lines [99] was included in our database as shown
in Table 1.
2. We have extended our previous measurements, particularly of lC1601 to
higher J values, and have made the first measurements of 12C170, - &d
13C160180.
3. We have improved our instrumentation and measurement techniques. and
thus have been able to measure pressure shifts in CO, laser lines with a more
sophisticated two-channel line-center-stabilized calibration system (which is
described in the next section).
4. We have recognized deficiencies in our earlier weighting of measure-
ments, and have become familiar with the use of resistant statistical procedures
for minimizing the effects of “outliers.”
As a result of the preceding changes, the number of beat frequency measurements
has increased to 915, the number of isotopic species has increased to nine, and
the precision of predicted frequencies has increased by an order of magnitude.