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

Infrared (IR) laser-induced resonant desorption/ablation of benzene layers condensed on liquid-nitrogen-cooled substrates has been investigated using the time-of-flight (TOF) technique. Adsorbates of up to 5 mum thickness were irradiated with a line-tuned TEA CO2 laser. Time-resolved detection of desorbed particles was performed with a quadrupole mass spectrometer (QMS). Measured TOF spectra can usually be well fitted with a stream-modified Maxwell-Boltzmann distribution. Systematic deviations from these fits were observed depending on film thickness, laser fluence, and angle of detection. Spectroscopic data of the adsorbate-substrate system needed for model calculations were measured in situ. The absorption coefficients were found to be fluence dependent. Measurements of the desorption yield and kinetic energy of the desorbed particles were performed with the P(9)32 and P(9)34-CO(2) laser lines as a function of film thickness and laser fluence. The solid-liquid phase transition could clearly be observed in these experiments. Average kinetic energies of up to 0.8 eV were measured in the TOF spectra, where the adsorbate temperature could be estimated to be less than 600 K. IR multilayer laser-induced desorption (LID) exhibits features very similar to those reported for UV multilayer LID. A mechanism based on the sudden rise of the adsorbate vapor pressure due to fast energy deposition by the laser pulse is discussed.