Photodissociation of alkyl iodides and CF3I at 304 nm: Relative populations of I(P-2(1/2)) and I(P-2(3/2)) and dynamics of curve crossing

Photodissociation of n-alkyl iodides and CF3I has been studied using state-selective ionization and pulsed-field time-of-flight (TOF) mass spectrometry. The (2+1) resonance-enhanced multiphoton ionization cross-section ratio of I(P-2(1/2)) and I(P-2(3/2)) is measured from I, photodissociation in the 304 nm region. Using this ratio, the relative populations of I(P-2(1/2)) and I(P-2(3/2)) by a pure parallel excitation to the (3)Q(0) state of n-alkyl iodides and CF3I are obtained. This method can exclude the effects of clusters and spectrum overlap in different dissociation paths from branching ratio calculations. The product quantum yield of I(P-2(3/2)) obtained from branching ratio reveals the (3)Q(0)-(1)Q(1) curve crossing probability-which increases from 0.70 for CH3I to 0.86 for n-C4H9I (with recoil velocity decrease of fragments at the curve crossing point). The potential energies at the crossing point and the Landau-Zener parameters zeta, calculated from our modified model, are 385 kJ/mol and 1100 m/s for n-alkyl iodides, and 391 kJ/mol and 177 m/s for CF3I, respectively. (C) 1996 American Institute of Physics.