Knowledge Bank

A sample cell containing $CH3F$ at low pressure has been simultaneously irradiated with an infrared $CO2$ laser and with a RF field in the 0.5-100 MHz region. The RF field is chopped at 10 kHz; and the laser beam is monitored and processed by means of a phase-sensitive detect as at 10 kHz; the sample cell is outside the laser cavity. For several laser lines, a Doppler-free additional absorption is observed as the RF frequency is decreased to zero. The peak frequency of the effect appears to be related to the infrared Rabi frequency. The lineshape depends on the infrared and RF powers and on the sample pressure. The strongest effects have been seen with $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH3F$ and the 9P(20) $CO2$ laser, which is about 34 MHz from the Q(12, 2) transition and with $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CH3F$ and the 9P(32) laser, which is about 24.5 MHz from the R(4,3) transition; both transitions are in the $\upsilon 3$ band. Much weaker effects have been observed with other laser lines and $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH3F$. Attempts to observe the signals with $NH3$ and $SO2$ samples have failed. Evidence for these effects appear in infrared-RF double resonance spectra previously $published.1$ The lineshape has been computed by means of a theory based on the high frequency Stark $effect.2$ The RF field is treated as a rapidly oscillating Stark perturbation on a two-level system with first-order Stark effect that is exposed to infrared radiation. Comparisons of observed and theoretical lineshapes will be described.