DISSOCIATION ENERGIES OF SIX $NO_{2}$ ISOTOPOLOGUES

We have measured the dissociation threshold energy, $D_{0}, (NO_{2} + h\\nu \\to NO(^{2}\\Pi_{1/2})+O(^{3}P_{2}))$ of the six $NO_{2}$ isotopologues made with $^{14}N$ or $^{15}N$ and $^{16}O$ or $^{18}O$ isotopes. These $NO_{2}$ isotopologues are cooled in a Helium supersonic jet at $T_{rot} \\approx 2K$. For each isotopologue, the very dense set of bound $N = 1 K = 0$ rovibronic eigenstates is readily observed by LIF up to $D_{0}$. Above $D_{0}$, the LIF signal disappear abruptly, within $\\pm 0.03 cm^{-1}$ which is the average spacing between observed $R_{0}$ lines just below $D_{0}$. Note that resonances (lifetime $\\approx 10^{-10}$ sec.) located above $D_{0}$ can be observed in absorption (by CRDS) but no fluorescence can be detected from these. The six measured $D_{0}$ range from $25128.56 cm^{-1}$ for $^{16}O^{14} N^{16}O$, noted (646), to $25171.80 cm^{-1}$ for (858). At the B.O. approximation, these six $D_{0}$ should have a common $D_{e}$. The shifts between these six $D_{0}$ are due to the ZPE shifts of $NO_{2}$ and NO. We have used and check the following relation: $$D_{0} (^{x}O^{y}N^{z}O)=D_{e}(NO_{2})+ZPE(^{y}N^{z}O)-ZPE(^{x}O^{y}N^{z}O)$$ The ZPEs of the various NO and $NO_{2}$ isotopologues have been determined from Dunham parameters and, for $NO_{2}$, also by Canonical Perturbation Theory (CPT) using two PESs of $NO_{2}$. The $NO_{2}$ ZPE isotopologue shifts are estimated to be within $0.5 cm^{-1}$. The uncertainties on ZPE of NO are significantly smaller. The six values of $D_{e}$ are located within $0.5 cm^{-1}$ around $26051.17 cm^{-1}$, in agreement with the ZPE uncertainties.