High-Resolution Rotational Spectroscopy of CH3I Using a Novel Doppler-Free Technique

Abstract A new experimental approach to double resonance spectroscopy is discussed in detail. The method uses collinear propagation of radiation of two frequencies to achieve velocity selection; it does not rely on nonlinear effects and it has a great sensitivity. Some examples of its application to the infrared–microwave region are described. By using a CO 2 laser and a mm-wave klystron we measured the frequency of rotational and vibrational transitions of CH 3 I with sub-Doppler resolution. We also obtained the first measurement of the electric dipole moment in the v 6 = 1 excited vibrational state and result is μ 6 = 1.6295(6) D.