Knowledge Bank

We have measured the dissociation threshold energy, $D0,(NO2+h\nu \to NO(2\Pi 1/2)+O(3P2))$ of the six $NO2$ isotopologues made with $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}N$ or $\mathrm{<mrow\; data-mjx-texclass="ORD">15</mrow>}N$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}O$ or $\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}O$ isotopes. These $NO2$ isotopologues are cooled in a Helium supersonic jet at $Trot\approx 2K$. For each isotopologue, the very dense set of bound $N=1K=0$ rovibronic eigenstates is readily observed by LIF up to $D0$. Above $D0$, the LIF signal disappear abruptly, within $\pm 0.03cm-1$ which is the average spacing between observed $R0$ lines just below $D0$. Note that resonances (lifetime $\approx 10-10$ sec.) located above $D0$ can be observed in absorption (by CRDS) but no fluorescence can be detected from these. The six measured $D0$ range from $25128.56cm-1$ for $\mathrm{<mrow\; data-mjx-texclass="ORD">16</mrow>}O14N16O$, noted (646), to $25171.80cm-1$ for (858). At the B.O. approximation, these six $D0$ should have a common $De$. The shifts between these six $D0$ are due to the ZPE shifts of $NO2$ 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 $NO2$ isotopologues have been determined from Dunham parameters and, for $NO2$, also by Canonical Perturbation Theory (CPT) using two PESs of $NO2$. The $NO2$ ZPE isotopologue shifts are estimated to be within $0.5cm-1$. The uncertainties on ZPE of NO are significantly smaller. The six values of $De$ are located within $0.5cm-1$ around $26051.17cm-1$, in agreement with the ZPE uncertainties.