Measurement of the 2₀₂ ← 1₀₁ rotational ground state transition with the IR-mm-wave double resonance technique at 11 K. The energy level diagram (inset) explains the double resonance method: with the IR light held in resonance (OPO; long green arrow) at 3029.6352 cm^-1, more than 5100 CH signal counts are obtained. When the submm-wave source (THz, small red arrow) is tuned into resonance, additional CH₂D⁺ ions are pumped into the 2₀₂ rotational level, leading to an increase of the signal by about 500 counts. In this example, the submm-wave source has been tuned up and down six times, stepping 10 kHz after each 1 s trapping cycle. A Gaussian fit of the seen signal gives a centre frequency of 534 280.0217(24) MHz and a kinetic temperature of about 30 K. Eight such measurements lead to the final value given in Table 1.