FD01		15min8:30

REMY JOST, ANTOINE DELON, JESPER NYGÅRD AND ADAM PASINSKI, Grenoble High Magnetic Field Laboratory, CNRS, BP 166, 38042 Grenoble Cedex 9, France.

The lowest photodissociation threshold of NO₂, located at 25128.57\pm0.05 cm^-1, is now well characterized: below the threshold, the highly excited vibronic levels of NO₂, observed by LIF in a jet, have a lifetime of a few µsec; above the threshold, the resonant levels have a lifetime shorter than 10² psec and no fluorescence can be observed, because the molecule dissociates. The products, NO and O, are formed in their ground states: NO₂(X²A₁)+h -> NO(²_1/2)+O(³P₂). We have studied by LIF the effect of a magnetic field on the dissociation threshold. The energy of the ground state of NO₂ (N=0,~K=0,~J=1/2) is lowered by -µ_BB for the M_S=1/2 component. The energy of the NO is unaffected (g~0 for the ²_1/2 state), while the energy of the oxygen atom is splitted into 5 components (J=2). The lowest component, M_J=-2, which correspond to the energetic threshold, is lowered by -3µ_BB, because the ³P₂ state has a Landé factor of 3/2. Consequently, we predict that the energy of the photodissociation threshold is lowered by -2µ_BB. We have experimentally and quantitatively observed this effect: the photodissociation threshold observed by LIF is lowered by 13.2 cm^-1 in a magnetic field of 14 Tesla! The fact that we have to consider only the energy of the upper levels (and not their quantum numbers) is a consequence of the strong mixing (chaos) within the rovibronic levels of NO₂ near the dissociation energy\footnoteR.Georges, A.Delon and R.Jost, J.Chem.Phys. 103, 1732 (1995). A.Delon, R.Georges and R.Jost, J.Chem.Phys. 103, 7740 (1995). .