Photodissociation dynamics of the iodide-uracil (I⁻U) complex

Photofragment action spectroscopy and femtosecond time-resolved photoelectron imaging are utilized to probe the dissociation channels in iodide-uracil (I⁻ ⋅ U) binary clusters upon photoexcitation. The photofragment action spectra show strong I⁻ and weak [U- H]⁻ ion signal upon photoexcitation. The action spectra show two bands for I⁻ and [U- H]⁻ production peaking around 4.0 and 4.8 eV. Time-resolved experiments measured the rate of I⁻ production resulting from excitation of the two bands. At 4.03 eV and 4.72 eV, the photoelectron signal from I⁻ exhibits rise times of 86 ± 7 ps and 36 ± 3 ps, respectively. Electronic structure calculations indicate that the lower energy band, which encompasses the vertical detachment energy (4.11 eV) of I⁻U, corresponds to excitation of a dipole-bound state of the complex, while the higher energy band is primarily a π-π^∗ excitation on the uracil moiety. Although the nature of the two excited states is very different, the long lifetimes for I⁻ production suggest that this channel results from internal conversion to the I⁻ ⋅ U ground state followed by evaporation of I⁻. This hypothesis was tested by comparing the dissociation rates to Rice-Ramsperger-Kassel-Marcus calculations.