Figure 2: Calibration scheme demonstrating how Stokes and anti-Stokes lines are used to calibrate the laser frequency such that the Stokes lines are at the same shift with the opposite sign of the anti-Stokes lines. The present data show Raman lines of potassium permanganate. Typically the values are ≤ 0.20 cm−1 absolute value of each other. The excitation laser line is nominally at 9397.58 cm−1 (air) absolute frequency, but has typical daily variations of ±0.50 cm−1 or more.