Probing the sub-microsecond photodissociation dynamics in gas-phase retinal chromophores

The photoinduced fragmentation of a retinal model chromophore (all-trans-n-butyl protonated Schiff-base retinal) was studied in vacuo using a new experimental technique. The apparatus is able to record the photodissociation yield of gas-phase biomolecular ions in the first microseconds after absorption. Together with the existing ion storage ring ELISA, which operates on the millisecond to second time scale, the complete decay dynamics of such molecules can now be followed. In the case of retinal, the time-dependent fragmentation yield observed after irradiation with a 410 nm laser pulse exhibits contributions from one- and two-photon absorption, which decay non-exponentially with lifetimes on the order of 1 ms and 1 micros, respectively. The decay can be simulated using a statistical model, yielding good agreement with the experimental findings on both the millisecond and the microsecond time scales. No indication for nonstatistical processes is found for this molecule, the upper limit for a possible direct rate being a factor of 10(4) below the observed statistical dissociation rate.