spectrum and the high signal-to-noise ratio. The $\{ 3-2 \}$ interval is twice the center frequency (because the AOM is double passed). For technical reasons, there is an additional AOM with a fixed frequency in the path of the probe, and this offset has to be added to obtain the interval.

The average value from 14 individual measurements is listed in Table \[t1\]. The standard deviation of the set is 32 kHz, which means the expected error in the mean is $32/\sqrt{14} = 8.6$ kHz, less than our estimated error of 20 kHz. This value is compared to other values reported in the literature. The two most accurate measurements [@DAN08; @YSJ96] have uncertainties below 10 kHz and have overlapping error bars. The more recent measurement [@DAN08] is also from our laboratory and used an AOM to measure the interval, but the AOM was locked to the peak. The current measurement obtained by measuring the entire spectrum is consistent with this value, thus giving confidence in the current technique. The only slightly discrepant measurement is from the work in Ref.[@BGR91], which is $1.7 \sigma$ away.

  $\{ 3-2 \} $ Interval    Reference
  ----------------------- -----------
  266.653(20)              This work
  266.657(8)               [@DAN08]
  266.650(9)               [@YSJ96]
  266.503(84)              