We shall discuss most recent experiments with pure or mixtures of alkali vapors generated at high temperatures and densities in all sapphire cells. A simple absorption spectroscopy supported by the calculations of spectral profiles in which most recent ab initio potential curves were used, provide a mean of understanding of most prominent diffuse spectral features in Cs₂. Rb₂. KRb and K₂ dimers. Some of these spectral features named "diffuse bands" may be used for the detection of ultracold alkali molecules, produced in photoassociation of ultracold atom pair and spontaneous emission. The same diffuse band may be used for the detection of even colder molecules formed by a Raman process in Bose Einstein condensates of rubidium or lithium atoms.

Laser spectroscopy of ultracold and hot alkali atoms and molecules can mutually enrich our knowledge of collision processes in alkali vapors.

When alkali vapor is immersed in the atmosphere of hydrogen molecules and helium atoms some interesting spectral line broadening may occur which has a direct application to the atmospheres of cool brown dwarf stars. Future experiments in this interesting field will be discussed in view of the most recent astrophysical findings.