The saturation or nonlinear absorption of laser radiation by a molecular transition burns a hole in the molecular velocity distribution. Examples of hole-burning spectroscopy are described. The interaction of molecules with a standing-wave field is calculated to exhibit the inverted Lamb dip. Experimental results on I2, CH4, SF6, NH2D, and H2COare reviewed.
Double resonance spectroscopy of molecules has recently been performed with laser radiations. Well-resolved signals of infrared-microwave double resonance have been observed in 14NH3 and in 15NH3 under resonant pumping with an N2O laser. A Zeeman-tuned He-Xe laser has been used to observe infrared-microwave double resonance in H2CO. Pressure dependence of double resonance in H2CO showing higher-order effects due to strong infrared and strong microwave radiations is observed and explained. Double resonance effects on the inverted Lamb dip of H2CO under microwave perturbation are also shown.
Other types of nonlinear spectroscopy and their applications are briefly discussed.
