Role of weak transitions in multiphoton excitation of molecules by IR laser radiation

Simultaneous excitation of a considerable part of molecules from many rotational levels of the ground state to higher vibrational states by IR laser radiation can be explained by considering weak transitions in a rotational band structure as it is shown at the example of SF₆ molecule. Very accurate compensation of anharmonicity in relatively wide spectral interval at comparatively low intensity of laser radiation can be explained on this basis. The considered scheme can be applied to the molecules of various symmetry with arbitrary anharmonicity.