ArF EXCIMER LASER PHOTODISSOCIATION OF $NH_{3}$: INTERNAL ENERGY DISTRIBUTION IN $NH_{2}$ $A_{2}A$, AND $X_{2}B_{1}$

Abstract

$^{1}$ H. Okabe and M. Lenzi, J. Chem. Phys. 47, 5241 (1967). $^{2}$ J. Masamet, A. Gilles, and C. Vermeil, J. Photochem., 3, 417 (1974/1975). $^{3}$ K. Dressler and D. A. Ramsay, Phil. Trans. Roy. Soc. London 251, 553 (1959). $^{4}$ J.W.C. Johns, D. A. Ramsay, Can. J. phys. 54, 1804 (1976). V. M. Donnelly has been a NRC/NRL Resident Research Associate.""Author Institution: Naval Research LaboratoryLow pressure ($P < 35$ mtorr) samples of $NH_{3}$ are photolyzed with a Tachisto Model XR 150 pulsed excimer laser operating on the 193 nm line of ArF (20 nsec pulse duration, 35 mJ per pulse, $\sim 3$ Hz repetition rate). Contrary to previous $^findings^{1,2}$ (for $NH_{3}$ dissociation) in this excitation region, we find that $NH_{2} (A^{2}A_{1})$ is a major product formed by single photon, primary photolysis. Strong $NH_{2} (A^{2} A_{1} \rightarrow X^{2}B_{1})$ banded emission is observed between 620 and 1100 nm. Most of the lines are assignable to transitions catalogued in absorption by Dressier and $Ramsay^{3}$ and Johns, Ramsay, and $Ross^{4}$ The relative intensities in our emission experiments are much different than those in absorption, indicating that much of the 0.74 eV excess dissociation energy appears as bending vibrational energy and rotational excitation about the a-axis. These observations are expected, an the basis of changes in geometry in the primary photolysis process. Experiments are also under way to probe energy distributions with the $NH_{2} (X^{2}B_{1})$ primary photofragment, Using dye laser-induced fluorescence excitation spectroscopy on the $A^{2}A_{1}\leftarrow X_{2} B_{1}$ transition. In addition, $NH(A^{3}\Pi)$ is formed in a two photon $NH_{3}$ dissociation process, giving rise to a $A^{3} \Pi\rightarrow X^{3} \Sigma^{-}$ emission at 336 nm