5min:

REMY JOST, M. JOYEUX AND D. SUGNY, Laboratoire de Spectrométrie Physique,
Université J. Fourier--Grenoble I, B.P. 87 -- 38402 Saint Martin d'Hères Cedex, FRANCE; G. MICHALSKI AND M. THIEMENS, University of California, San Diego La Jolla Ca USA.

We have measured the dissociation threshold energy, D₀, (NO₂ + h NO(² _1/2) + O(³P₂)) of the six NO₂ isotopologues made with ¹⁴N or ¹⁵N and ¹⁶O or ¹⁸O isotopes. These NO₂ isotopologues are cooled in a Helium supersonic jet at T_rot~ 2K. For each isotopologue, the very dense set of bound N=1\:K=0 rovibronic eigenstates is readily observed by LIF up to D₀. Above D₀, the LIF signal disappear abruptly, within \pm 0.03 cm^-1 which is the average spacing between observed R₀ lines just below D₀. Note that resonances (lifetime ~ 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 25128.56 cm^-1 for ¹⁶O¹⁴N¹⁶O, noted (646), to 25171.80 cm^-1 for (858). At the B.O. approximation, these six D₀ should have a common D_e. The shifts between these six D₀ are due to the ZPE shifts of NO₂ and NO. We have used and check the following relation:

D₀(^xO^yN^zO)=D_e(NO₂)+ZPE (^yN^zO)-ZPE(^xO^yN^zO)

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 0.5 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.