Atom-wall interactions and their role in the spectroscopy of spatially constrained atomic vapors

Atom-wall interactions play an unexpectedly important role in the atomic spectroscopy. J.L. Cojan was the first who observed and then interpreted the effects of the atom-wall interactions on the reflection spectra in the vicinity of the atomic spectral line. His observation was made on the mercury vapors of such a low concentration that the Doppler width was much larger than the homogeneous width of the atomic transition. Surprisingly, the width of the spectral line he observed in reflection was much smaller than the Doppler width. He pointed out that the atoms those leave the window posses a transient rather than the stationary polarization. This is the reason why their contribution to the reflected field differs from what was expected. M. Ducloy employed the tiny distortions of these narrow resonances in reflection spectra to measure for the first time the van der Waals constants in the excited atomic states. In our work we considered reflection from a narrow slice of atomic vapors and found a manifold of spectral line shapes depending on the width of the vapor slice that have nothing in common with the Fabri-Perot resonances. It was not until the invention of an Extremely Thin Cell (ETC) by D. Sarkisyan that the observation of these effects becomes possible in the optical domain. In the subsequent years ETC proved to be a very powerful tool of modern spectroscopy.