Femtosecond dynamics after ionization:: 2-Phenylethyl-N,N-dimethylamine as a model system for nonresonant downhill charge transfer in peptides

The cation of 2-phenylethyl-N,N-dimethylamine (PENNA) offers two local sites for the charge: the amine group and 0.7 eV higher in energy the phenyl chromophore. In this paper, we investigate the dynamics of the charge transfer (CT) from the phenyl to the amine site. We present a femtosecond resonant two-color photoionization spectrum which shows that the femtosecond pump laser pulse is resonant in the phenyl chromophore. As shown previously with resonant wavelengths the aromatic phenyl chromophore can be then selectively ionized. Because the state charge in the phenyl chromophore is the first excited state in the PENNA cation, it can relax to the lower-energetic state charge in the amine site. To follow this CT dynamics, femtosecond probe photoabsorption of green light (vis) is used. The vis light is absorbed by the charged phenyl chromophore, but not by the neutral phenyl and the neutral or cationic amine group. Thus, the absorption of vis photons of the probe laser pulse is switched off by the CT process. For detection of the resonant absorption of two or more vis photons in the cation the intensity of a fragmentation channel is monitored which opens only at high internal energy. The CT dynamics in PENNA cations has a time constant of 80 28 fs and is therefore not a purely electronic process. Because of its structural similarity to phenylalanine, PENNA is a model system for a downhill charge transfer in peptide cations.