ANGULAR AND TIME-RESOLVED INTENSITY DISTRIBUTIONS OF LASER-DESORBED MATRIX IONS

A new instrument for studying the behavior of laser-desorbed bio-organic molecules is described. The instrument consists of a large diameter vacuum chamber containing a tiltable sample holder in the center, a fixed beam path for laser irradiation and two individual time-of-flight systems, one of them being rotatable around the center of the chamber. Velocity and intensity distributions of ions can be obtained as a function of the angle of incidence of the laser beam and the ion ejection angle with respect to the sample surface. Velocity and kinetic energy distributions show strong differences for different ions ejected from single crystals of 2,5-dihydroxybenzoic acid (DHB). Distributions of the dehydrated molecular ion of DHB (the most intense signal in the spectrum) are strongly peaked forward in intensity and kinetic energy. The characteristics of this hypersonic jet of ions have been investigated as a function of the laser irradiance and the focus diameter. The lowest mean velocities have been observed for large focus diameters (d almost-equal-to 180 mum) at threshold irradiance. Under focused conditions (d almost-equal-to 10 mum) mean kinetic energies of ions rose by a factor of 4 to 25 for threshold to 6 times threshold irradiance, respectively.