The dynamics of the infrared multiphoton pumping of optically excited NO₂ molecules

The infrared multiphoton (IRMP) pumping of optically excited NO₂ molecules is investigated. From changes in the fluorescence emission spectrum due to the IRMP pumping, both the fraction of the NO₂ molecules that has dissociated, as well as the fraction that remains within one IR photon in energy below the dissociation limit, are deduced. This has made possible the first direct measurements of the final, dissociative step in the IRMP pumping of molecules, and of IRMP stimulated emission. The dynamics of the IRMP pumping is studied by systematically varying the optical excitation energy. The results are compared with numerical solutions to the well-known rate equation model for the IRMP pumping of polyatomic molecules. From this comparison the best fit transition cross sections connecting various energy regions in highly vibrationally excited NO₂ molecules were deduced. Except for the final, dissociative transition, they depend significantly on both laser intensity and pulse width for intensities as large as 10 GW/cm² and pulse widths as short as 500 ps. These dependences are probably due to the sparse density of vibrational states in such a small molecule as NO₂. In addition, the rotational energy present in room temperature NO₂ can enhance considerably the IRMP dissociation probability of optically excited NO₂.