Excitation of vibrations of homonuclear molecules by infrared radiation

The adiabatic approximation is used to show that the quadratic Stark effect of an electronic term of a molecule in an external field of frequency ω modulates the molecular vibrations at a frequency 2ω. The threshold external field for the parametric excitation of vibrations is about 1 MV/cm for H₂. A calculation is also made of the probability of two-photon transitions between neighboring vibrational levels which occur when the pump frequency is equal to half the vibration frequency.