Page 119 - Tunable Lasers Handbook

4 CO, Isotope Lasers and Their Applications 9

the variance-covariance matrix, the lower triangle of which is shown in Table
XI1 of the original paper [37], but is not reproduced here.
The horizontal lines drawn in the frequency tables denote the highest and
lowest J lines within which beat frequency measurements were used in the data
for computer fitting. As always. the frequency values outside the measured
regions should be used with the greatest caution, and the computed standard
deviations for such lines should be considered as only a rough guide.
The original paper [37] also contains the 915 beat frequency measurements
and their nominal standard deviations that constituted the input to our computa-
tions. These data, which are designated as Files 1 through 50 in Table I1 of [37],
will be useful to those who wish to derive better molecular constants and more
accurate frequency determinations as additional beat frequency measurements
and more precise intercomparisons of CO, lasing transitions with the prirnar:
frequency standard (cesium at the present) become available.
The frequencies predicted in Tables 2 through 10 show, for the most accurate
lines, standard deviations that are an order of magnitude smaller than those in [36]
and are principally limited by the uncertainty in the single absolute frequency mea-
surement. We believe that these standard deviations are reasonable estimates of the
uncertainties of their respective frequencies, and that our molecular constants and
predicted frequencies are the best currently available for the CO, isotopic species,
and are as good as any that can be extracted from the available data. In our opinion,
they are suitable (with appropriate care about sequence and hot bands [78-81.
89,90,10&103]) for use as secondarp standards at the indicated level of precision.
Higher precision (by perhaps two orders of magnitude) in the CQ, comparisons
could be attained by application of techniques developed in [76], whiih are summa-
rized in the next section. but for more precise absolute frequencies. this would need
to be accompanied by a similarly precise comparison with the cesium standard,
During the preparation of the manuscript for this chapter I became aware of
some very recent mork on CO, laser line calibration that was carried out at the
Time and Frequency Division if NIST in Boulder, Colorado. I am grateful to Dr.
K, hl. Evenson for providing me with a very recent reprint [80] and three addi-
tional manuscripts prior to their publication [38,81,88]. The outcome of this nev.
work will result in improved molecular constants and frequencies for the CQ,
laser and mill be very briefly summarized next.
In May 1994, Evenson et al. reported [80] the first observation of laser tran-
sitions in the (O0~1~--[11~0.0310],, 9-pm hot band of 12C1607. This band is iden-
tified bj an extra heavy solid arrow in the vibrational energy level diagram of
Fig. 17, nhich was reproduced from [XO]. These transitions. together with the
(001 1)--[111O. 03101, lower frequency hot band transitions that were previously
measured by Whitford et al. [78] and by Petersen et al. [79] were incorporated
into a new database by Maki et al. [38]. Altogether they included 84 hot band
transitions and also 12 higher J value regular band W1607 transitions that were
not measured by Bradley er a1 [37]. From the database provided in Bradley er o!.