Isotope-selective infrared laser dissociation of molecules with a small isotopic shift in a gas-dynamically cooled molecular flow interacting with a solid surface

A method for isotope-selective infrared laser photodissociation of molecules characterized by a small (less than $0.25$ cm$^{-1}$) isotopic shift in the infrared absorption spectra is demonstrated by the example of the CF$_3$Br molecule. The method is based on infrared laser excitation of molecules in a pulsed gas-dynamically cooled molecular flow interacting with a solid surface. The dissociation of molecules has been studied under three different conditions of their optical excitation: (i) in an undisturbed molecular flow, (ii) in a flow incident on the surface, and (iii) in the shock wave region formed in front of the surface upon its interaction of the incident molecular flow. It is shown that the dissociation yield upon the excitation of molecules in the shock wave and in the flow incident on the surface is $5{-}10$ times higher than the dissociation yield upon excitation in the undisturbed flow, whereas the dissociation threshold in the former case is $3{-}5$ times lower than that in the latter case. This allows the observation of the isotope selective dissociation at low energy densities of the exciting laser pulse ($\Phi\leqslant 1.5{-}2.0$ J/cm$^2$). The enrichment factor Kenr($^{79}$Br/$^{81}$Br) for the dissociation product Br$_2$ is $0.85 \pm 0.07$ and $1.20 \pm 0.09$ for excitation of molecules in the incident flow and in the shock wave, respectively.