Transition light wave-molecule

In both cases (i. e. emission or absorption saturation) the halfwidth of this/Lamb dip is slightly dependent on laser power but mainly determined by the interaction time of the individual molecules with the standing light wave in the cavity. This time may be limited by the finite lifetimes rb of upper or lower states, by the average time l/aup between two disturbing collisions, or by the transit time Tt of the gas molecules across the laser beam. This last limitation becomes important at low pressures of the absorbing gas and for transitions between long-lived states (see Section IV.3). 

The dipole moment induced by the light wave is a transitory dipole moment and is not related to the permament dipole moment of the molecule. It is known as transition moment. 

Atoms and molecules change their energy levels and emit or absorb radiation. All transitions between states, even though the states themselves are possible, are not necessarily legitimate. The restrictions imposed upon transfers are known as sdeclion ndes. Rather closely related to them are the prescriptions which define the state of polarization of an emitted light wave, such a formulation amounting to a selection rule applicable to particular components of the wave. 

For a molecule moving with a velocity v, the probability A,/ for a two-photon transition between the ground state Et and an excited state E/ induced by the photons fuoi and fuo2 from two light waves with the wave vectors k, k2, the polariza-... 

If the two photons are absorbed out of two light waves with equal frequencies coi = C02 = CO, which travel in opposite directions, we obtain k = —k2 and (2.68) shows that the Doppler shift of the two-photon transition becomes zero. This means that all molecules, independent of their velocities, absorb at the same sum frequency coi- -(02 = 2co. 

At a density A, of molecules in level i) and a negligible population Nk in the upper level of a transition the power absorbed from the incident light wave within the volume element AV is... 

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