The retinal protonated Schiff base (rPSB) is a chromophore of the rhodopsin family which is crucial for the vision process and its ultrafast cis-trans photoisomerization is the key step for such biological functionality. The model systems protonated Schiff bases CH 2 (CH) 2n-2 NH + 2 (PSBn) were often used to study photochemistry of rPSB 121, . Here, PSB4 was selected as a representative of PSBn.