High Resolution Rotational Spectroscopy of CH3I using a Novel Doppler-free Technique

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 CO2 laser and a mm-wave klystron we measured the frequency of rotational and vibrational transitions of CH3I with sub-Doppler resolution. We also obtained the first measurement of the electric dipole moment in the v(6) = 1 excited vibrational state acid result is mu(6) = 1.6295(6) D. (C) 1996 Academic Press, Inc.