The influence of pH in nonresonant third-order nonlinearities of amino acid solutions

Abstract

Abstract We report on the use of the Z -scan technique with femtosecond pulses at 775 nm to investigate the electronic charge delocalization in l -arginine, l -proline and l -serine amino acids. We have observed a variation in the nonresonant nonlinear transmittance with the pH, which reflects the variation in the relative concentration of the cationic and anionic forms and, consequently, the electronic charge delocalization. For l -serine, for instance, at a pH around 6.0 the n 2 rises to its maximum value corresponding to the one expected for the maximum concentration of the zwitterionic form. A good correspondence between the measured nonlinear transmittance variation and the titration curve was observed for the three amino acid samples, in the pH range studied. The n 2 values measured range from 10 −17 to 10 −16 cm 2 /W for pH varying from about 2 to 9.2.