Ultraviolet laser desorption of indole

Abstract

Ultraviolet laserdesorption from a thick, 120 K indole film was studied. Using a 2660 Å, 10 ns desorption laser at 75 mJ/cm 2 , 2.0 monolayers of indole are removed per shot. Indole 0 is the only neutral species desorbed and it has an internal temperature T i =210 K and a translational temperature T t =3400 K . The velocity distribution is non-Boltzmann and the angular distribution is bimodal and forward peaked with major component proportional to cos 7 (θ). No evidence of “jetlike” structure in the desorbed plume is found: Different regions of the plume are at the same internal temperature and both internally hot and cold molecules have identical angle velocity distributions. While existing collisional models cannot account for the details of these distributions, they suggest that 2–7 collisions per molecule occur following desorption producing minor vibrational cooling (<10%). Laser desorbed indole + is observed at a concentration of ∼10 −5 that of indole 0 with an angle velocity distribution similar to that of indole 0 . We show that indole + results from resonant two photonionization of indole 0 by the desorption laser and that desorptionlaser heating of the plume occurs at 2660 Å.