Knowledge Bank

Collision-induced relaxation of $ICl\ast$ and differing caging mechanisms of electronically excited $ICl\ast$ within $He\cdots ICl\ast$ complexes are investigated using laser-induced fluorescence and two-laser, pump-probe spectroscopy. It is found that collisions of He atoms with molecules prepared in the short-lived $ICl(B3\Pi 0+,\nu \prime =3)$ rotor states occurs on timescales sufficient for vibrational relaxation down to the long-lived $ICl(B,\nu \prime =2)$ rotor states. It is also shown that when linear $He\cdots ICl(X1\Sigma 0+,\nu \prime \prime =0)$ complexes are promoted to the continuum of states correlating with the repulsive, inner wall of the $ICl(A3\Pi 1)$ potential, the dissociating $ICl(A)$ molecule undergoes kinematic one-atom caging, ejecting the weakly bound He atom, and re-stabilizing the molecule in high lying vibrational levels within the A electronic state with very little rotational excitation. Features associated with transitions of the linear $He\cdots I35Cl(X,\nu \prime \prime =0)$ complex to intermolecular states associated with the ICl(B) diabat that are coupled with those in the $ICl(B\prime )$ adiabat formed by an avoided curve crossing are also observed in laser-induced fluorescence spectra. The $ICl\ast$ molecules within the excited state complex undergoes a non-adiabatic one-atom caging mechanism forming $I35Cl(B,\nu \prime )$ products with little rotational excitation.