High resolution vibronic state-specific dissociation of NO2+ in the 10.0-15.5 eV energy range by synchrotron double imaging photoelectron photoion coincidence

摘要

Vacuum ultraviolet (VUV) photoionization and dissociative photoionization of NO2 in the 10.0-15.5 eV energy range have been investigated in detail by using high-resolution double imaging photoelectron photoion coincidence (i(2)PEPICO) at synchrotron SOLEIL. Five low-lying electronic states of the NO2+ cation, X-1 sigma(+)(g), a(3)B(2), b(3)A(2), A(1)A(2) and (BB2)-B-1, are prepared with well-resolved vibronic structures and their state-specific dissociation mechanisms are unraveled and discussed. The present experimental results clarify that except the X-1 sigma(+)(g) ground electronic state and the first three vibrational levels of the a(3)B(2) electronic state, the other cationic states of NO2+ within the present energy range are totally dissociative towards the NO+(X-1 sigma(+)) + O(P-3) and/or NO+(X-1 sigma(+)) + O(D-1) dissociation limits. An energy barrier exists along the direct dissociation route of the a(3)B(2) state, and the b(3)A(2) electronic state is a quasi-bound state with a very shallow well, both of which adiabatically correlate to the NO+(X-1 sigma(+)) + O(P-3) dissociation limit. The a(3)B(2) state mainly with bending vibration excitations undergoes a non-adiabatic transition to the 2(3)A ''(B-3(1)) repulsive state along its bending potential energy curve and then predissociates into the NO+(X-1 sigma(+)) + O(P-3) products. Our experimental results firstly demonstrate that the NO+(X-1 sigma(+), v) fragment ions produced from individual vibronic levels of the dissociative NO2+(a(3)B(2), b(3)A(2)) states are produced at the v = 0 ground vibrational level with a high rotational population due to the excitation of the vibrational bending mode of NO2+ and the associated imparted torque upon dissociation. The slower predissociations of the A(1)A(2) and (BB2)-B-1 electronic states via their spin-orbit couplings with the repulsive 2(3)A ''(B-3(1)) state produce the NO+(X-1 sigma(+)) and O(P-3) fragments with a long vibrational progression. In addition, the (BB2)-B-1 state can also undergo a radiationless transition such as internal conversion into the hot X-1 sigma(+)(g) state and then dissociate into the second dissociation channel correlated to the NO+(X-1 sigma(+)) and O(D-1) products.