Experimental fingerprints of vibrational wave-packet motion during ultrafast heterogeneous electron transfer

By application of 20 fs laser pulses, vibrational wave packets of low-energ y modes (mainly 357 and 421 cm(-1)) were generated in the perylene chromoph ore that gave rise to periodic beats that lasted longer than I ps in transi ent absorption signals. Electron transfer from the excited singlet state of the perylene chromophore, attached as molecule DTB-Pe via the -CH2-phospho nic acid group to anatase TiO2, was measured in ultrahigh vacuum with a tim e constant of 75 fs. The vibrational wave packet that was generated in the donor state continued its motion for several hundred femtoseconds in the pr oduct state of the reaction, i.e., in the ionized chromophore. This is dire ct proof for electron transfer occurring from a nonrelaxed vibrational popu lation that was created by the short laser pulse in the donor molecule. The rise of the product state showed a staircase-like time dependence. The ste ps are attributed to electron transfer that occurs preferentially each time the vibrational wave packet (frequency 480 cm(-1)) reaches a crossing poin t for the potential curves of reactant and product state. Such wave-packet modulation of heterogeneous electron transfer can arise if the density of e lectronic acceptor states in the electrode is changing strongly over an ene rgy range on the order of the reorganization energy below the excited molec ular donor orbital.