Accurate Measurement and Assignment of High Rotational Energy Levels of the 3ν3 ← ν3 Band of Methane

We use optical-optical double-resonance spectroscopy with a high-power continuous wave pump and a cavity-enhanced comb probe to expand sub-Doppler measurements of the 3ν
3
←ν
3
band of CH
4
to higher rotational levels. We assign the final states using combination differences, i.e., by reaching the same state using different pump/probe combinations.