On the absorption saturation in the vibrational-rotational bands of molecules

Analytical expressions describing saturation of the stationary absorption coefficient in a vibrational-rotational band in the spectrum of a molecular gas are obtained taking into account that many other bands are involved in the absorption process as well. The formulas are derived for an arbitrary spectral composition of the pumping radiation with an allowance for a difference between the rotational and vibrational relaxation rates and between the relaxation rates of the lower and upper vibrational levels. The influence of saturation on the appearance of the partial inversion and the negative absorption effects is considered. The general formulas are simplified and the vibrational and rotational contributions to the saturation are separated within the framework of the local Elsasser model for the rotational band structure. Explicit relationships between the dimensionless parameters describing the vibrational and rotational saturation mechanisms are obtained and analyzed, and conditions under which one of these mechanisms is dominating are determined. Particular calculations were performed for the (000)-(010) and (000)-(001) absorption bands of the 03 molecule and features of the saturation effect in these bands were established. (C) 2002 MAIK "Nauka/Interperiodica".