Energetics and spin- and Λ-doublet selectivity in the infrared multiphoton dissociation DN3→DN(X 3Σ−, a 1Δ)+N2(X 1Σ+g): Experiment

Abstract

Multiphoton vibrational excitation of deuterated hydrazoic acid, DN3, by a CO2 laser (I=10 GW/cm2) leads to dissociation forming DN in both X 3Σ− (spin forbidden) and a 1Δ (spin allowed) electronic states. Under collisionless conditions, the nascent DN fragments were probed via laser induced fluorescence, to determine initial product state distributions. The DN(X 3Σ−) molecules are formed predominantly in the symmetric F1 and F3 spin–rotation states with little population (≤6%) in the antisymmetric F2 levels. There is no significant population (<3%) in excited DN(3Σ−) vibrational levels. The distribution of rotational states is Boltzmann-like, characterized by a rotational ‘‘temperature’’ of about 920 K for the F1, F3 states and 500 K for F2 levels. Doppler profiles showed a large kinetic energy release of about 10 100 cm−1 total in the triplet channel. The DN(1Δ) products are formed preferentially in the symmetric Δ(A′), e-labeled lambda doublet levels: Δ(A′)/Δ(A″)=1.44. The DN(1Δ) is formed with no vibrational excitation (<2%); the rotational states are populated Boltzmann-like with a rotational ‘‘temperature’’ of 425 K. Doppler profiles give a total kinetic energy of about 1500 cm−1 in this channel. These observations give information about the distribution of energy in the reactant, the location of the barriers to dissociation, and the geometry of the transition states. Alexander, Werner, and Dagdigian (accompanying article) show that the observed DN(3Σ−) spin- and DN(1Δ) Λ-doublet selectivities reflect the symmetry properties of a planar transition state and that the low degree of DN(3Σ−) rotational and vibrational excitation is also expected from the transition state geometry.