Line-center stabilized CO2lasers as secondary frequency standards: Determination of pressure shifts and other errors

The 4.3 μm CO
2
fluorescence stabilization method is reviewed and a two-channel heterodyne system with a fractional frequency stability of
\sigma_{y}(\tau) < 2 \times 10^{-12}
is described. Frequency reproducibility and causes of frequency shifts and errors in the saturation resonance are discussed. A new technique is employed to eliminate frequency offset errors caused by the nonzero slope of the power versus frequency characteristics of the lasers over the frequency range of the nonlinear resonance dip. Pressure shifts of the standing wave saturated resonance have been measured in the 9 and 10 μm
P
and
R
lasing transitions of CO
2
. At low pressures (≤ 100 mTorr) the measured shifts for four different isotopes are all blue, instead of red as predicted by semiclassical theory. Measurements at higher (> 1 Torr) pressures reveal red shifts. Perturber gas data show blue shifts for heavier perturber atoms or molecules, red shifts for He and H
2
.