Excitation of photosystem I by 760 nm femtosecond laser pulses: transient absorption spectra and intermediates

Excitation of photosystem I (PS I) by a femtosecond 760 nm pump leads to one- and two-photon absorption. The one-photon excitation produces intermediates with transient absorption spectra similar to the spectra of the primary $\left[{P_{700}}^{+}{A_0}^{-}{A}_1\right]$ and secondary $\left[{P_{700}}^{+}{A}_0{A_1}^{-}\right]$ ion-radical pairs in the PS I reaction center. The two-photon absorption generates the upper level excited states of chlorophyll (Chl) and carotenoid molecules in the antenna. These excited states are converted into the long-lived intermediates and can be tentatively attributed to the excited and charge–transfer ion-radical states of Chl molecules and to the excited states of carotenoids in the antenna. The transient spectra of intermediates generated by two-photon excitation differ from the transient one-photon spectra of the primary and secondary ion-radical pairs. (paper)