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

Abstract

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.

Observation of Near-Threshold Resonances in the Flavin Chromophore Anions Alloxazine and Lumichrome

“…21 Recent work from Neumark and co-workers using time-resolved anion photoelectron spectroscopy has provided a number of examples of dipole-bound states which can either autodetach or evolve into a valence anion, which subsequently dissociates. 37,38 Thus it is now well acknowledged that the formation of a dipole-bound state can act as a doorway to valenceanion formation, and hence to molecular dissociation. [37][38][39][40] This leads us to interpret that our spectroscopic observations as being consistent with It is useful to reflect that the only structural difference between alloxazine and lumichrome, is the presence of lumichrome's two methyl groups.…”

“…37,38 Thus it is now well acknowledged that the formation of a dipole-bound state can act as a doorway to valenceanion formation, and hence to molecular dissociation. [37][38][39][40] This leads us to interpret that our spectroscopic observations as being consistent with It is useful to reflect that the only structural difference between alloxazine and lumichrome, is the presence of lumichrome's two methyl groups. Brauman and co-workers investigated the effects of molecular rotation on the observation of dipole-bound states of anions via photodetachment studies of enolate anions in an ion cyclotron spectrometer over two decades ago.…”

“…31 As the lifetime of the dipole-bound excited state of [LC-H]is shortened due to the presence of the internal rotors, there appears to be insufficient time for the dipole-bound state to evolve into a valence state which subsequently dissociates, compared to the timescale for autodetachment. 37,38 As discussed in the introductory paragraphs, LC is the key structural component of the flavin family of biomolecules. Their activity as photoreceptors is closely linked to their redox properties, where a coupled two electron/two proton capture by the LC moiety leads to the reduced form.…”

Near-threshold electron transfer in anion-nucleobase clusters: does the identity of the anion matter?

“…Gas-phase iodide ion-nucleobase clusters have been used in a number of recent experimental studies, to probe low-energy electron-nucleobase coupling in a highly controlled environment [11][12][13][14][15]. In such experiments, the iodide ion is photodetached to produce a 'spectator' iodine atom and a low-energy free electron with a well-defined kinetic energy that can be captured by an adjacent molecule [16][17][18].…”

“…In such experiments, the iodide ion is photodetached to produce a 'spectator' iodine atom and a low-energy free electron with a well-defined kinetic energy that can be captured by an adjacent molecule [16][17][18]. The resulting temporary negative ion dynamics can then be monitored either via time-resolved photoelectron spectroscopy or photofragment action spectroscopy [11][12][13][14][15][16][17][18].…”