Mechanisms of formation of amino acid radicals in a laser radiation field

An investigation was made of the photoionization of aromatic amino acids by nanosecond and picosecond ultraviolet radiation at liquid nitrogen temperature. When specimens were irradiated by 266-nm picosecond pulses at different repetition frequencies, the singlet and triplet photoionization channels were separated and ionization probabilities were obtained separately for the two channels. In the nanosecond lasing regime, photoionization by two pulses at λ = 270 and 360 nm was studied with the aim of selective irradiation of amino acids. A λ = 360 nm pulse depleted the S₁ excited state by stimulated emission and a transition to the ground state. Allowing for this effect, photoionization probabilities were obtained for two-frequency excitation in two different channels: singlet and triplet.