Ultraviolet laser desorption of indole

Ultraviolet laser desorption from a thick, 120 K indole film was studied. Using a 2660 Angstrom, 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)(theta). No evidence of ''jetlike'' structure in the desorbed plume is found: Different regions of the Flume 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 similar to 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 photon ionization of indole(0) by the desorption laser and that desorption laser heating of the plume occurs at 2660 Angstrom. (C) 1997 American Institute of Physics.