Fig. 1

The observed Q 1(3/2)(v = 19 261.9366 cm−1) and Q R 12 (1/2) (v = 19 261.9572 cm−1) field-free spectrum and the associated sub-Doppler I2 spectrum (middle) and transmission of a stabilized confocal etalon. The Q R 12(J) satellite branch quickly becomes weak relative to the main Q 1 (J) branch.