Imaging the rotationally state-selected NO(A,n) product from the predissociation of the A state of the NO-Ar van der Waals cluster.

The origin of the resonant structures in the spectrum of the predissociative part of the A state in the NO-Ar van der Waals cluster has been investigated. We have employed direct excitation to the predissociative part of the NO-Ar A state followed by rotational state selective ionization of the NO fragment. Velocity map imaging of the NO ion yields the recoil energy of the rotational state-selected fragment. A substantial contribution of rotational hotbands to the resonant structures is observed. Our data indicate that a centrifugal barrier as the origin of these resonances can be ruled out. We hypothesize that after the NO-Ar cluster is excited to the A state sufficient mixing within the rotating cluster takes place as it changes geometry from being T shaped in the NO(X)-Ar state to linear in the NO(A)-Ar state. This mixing allows the low energy and high angular momentum (J approximately = 4.5) tumbling motion of the initially populated hotbands in the ground state NO(X)-Ar complex to be converted into NO(A,n = 2) spinning rotation in the A state of the complex. The electronically excited spinning complex falls apart adiabatically producing rotationally excited NO(A,n = 2) at the energetic threshold. This interpretation indicates that the resonances can be attributed to some type of vibrational Feshbach resonance. The appearance energy for the formation of NO(A,n = 0)+Ar is found to be 44294.3+/-1.4 cm(-1).