Laser Ablation of Trp-Gly

We studied laser ablation of Trp-Gly under the same experimental conditions used previously to successfully seed this fragile, nonvolatile dipeptide into a molecular beam. A film of Trp-Gly was prepared on a brass plate and laser ablated with 11 ns, 8 J/cm 2, 5320 Å laser pulses. We found substantial fragmentation upon ablation and believe that immediate helium entrainment is necessary to prevent decomposition. The velocity distribution is broader than a Boltzmann distribution, exhibits no flow velocity, and has a translational temperature T t = 4200 K. The angular distribution is highly forward-peaked and bimodal with a major component proportional to cos 41 θ. When laser ablated under identical conditions, tryptamine has a similar angle velocity distribution and also shows significant fragmentation. Analysis of its mass spectrum yields an internal temperature T i = 660 K. Existing collisional models cannot explain the measured angle velocity distributions. They may result from the pulsed nature of the expansion and the finite focused laser spot size.