TIME-OF-FLIGHT INVESTIGATION OF INFRARED LASER-INDUCED MULTILAYER DESORPTION OF BENZENE

Infrared (IR) laser-induced resonant desorption/ablation of benzene la yers condensed on liquid-nitrogen-cooled substrates has been investiga ted using the time-of-flight (TOF) technique. Adsorbates of up to 5 mu m thickness were irradiated with a line-tuned TEA CO2 laser. Time-reso lved detection of desorbed particles was performed with a quadrupole m ass spectrometer (QMS). Measured TOF spectra can usually be well fitte d with a stream-modified Maxwell-Boltzmann distribution. Systematic de viations from these fits were observed depending on film thickness, la ser fluence, and angle of detection. Spectroscopic data of the adsorba te-substrate system needed for model calculations were measured in sit u. The absorption coefficients were found to be fluence dependent. Mea surements of the desorption yield and kinetic energy of the desorbed p articles were performed with the P(9)32 and P(9)34-CO(2) laser lines a s a function of film thickness and laser fluence. The solid-liquid pha se transition could clearly be observed in these experiments. Average kinetic energies of up to 0.8 eV were measured in the TOF spectra, whe re the adsorbate temperature could be estimated to be less than 600 K. IR multilayer laser-induced desorption (LID) exhibits features very s imilar to those reported for UV multilayer LID. A mechanism based on t he sudden rise of the adsorbate vapor pressure due to fast energy depo sition by the laser pulse is discussed.