Interpretation of Time-of-Flight Distributions Resulting from Pulsed Laser-Induced Desorption
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INTERPRETATION OF TIME-OF-FLIGHT DISTRIBUTIONS RESULTING FROM PULSED LASER-INDUCED DESORPTION M. BUCK AND P. HESS Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, D-6900 Heidelberg, FRG ABSTRACT Time-of-flight (TOF) experiments were performed to study the desorption of trans-l,2-dichlorocyclohexane molecules from molecular films condensed on a quartz-crystal microbalance at 77 K. The wavelength dependence of the desorption signal was investigated by resonant excitation of the internal vibrational mode of the molecule in the 10 pm region with a pulsed line-tunable TEA CO2 laser. The TOF temperatures showed the same spectral dependence as the optical absorption coefficient. The TOF distributions were narrower than Maxwellian distributions for multilayer desorption yields and approached Maxwellian distributions for submonolayer desorption yields. A saturation behavior was found at high laser intensities, indicating that the kinetic energy of the desorbing molecules is limited to a maximum value. INTRODUCTION The irradiation of a solid surface with short laser pulses causes the desorption or ablation of material. The investigation of desorption processes is one of the oldest fields in surface science. For example, the method of thermal programmed desorption is well established and yields fundamental information on binding energies at surfaces. On the other hand, pulsed laser-induced desorption, where heating rates 10-15 orders of magnitude faster may be achieved, is not as well understood. Fundamental information on the wavelength dependence of energy deposition by photon absorption (1-4), local energy transfer by relaxation and transport processes as well as phase transitions (5) can be obtained by studying the properties of particles leaving the surface. A powerful method which yields detailed information on these processes is time-resolved detection of the desorbing species employing a quadrupole mass spectrometer. The advantage of this time-of-flight (TOF) technique is that it provides the translational energies of the desorbing species. However, the analysis of the measured TOF distributions must be performed with great care to obtain correct information on the laser-induced surface processes (6). In the present work TOF distributions of trans-l,2-dichlorocyclohexane, desorbed as neutral molecules from a condensed Van der Waals film, were studied in detail. The pulsed laser-induced desorption was achieved by resonant excitation of the internal vibrational mode at 980 cmusing a line-tunable TEA CO2 laser. Several improvements were made in comparison to our previous results reported in refs. (1,2,7,8).
Mat. Res. Soc. Symp. Proc. Vol. 100. t1988 Materials Research Society
648
The molecular films were condensed on a quartz-crystal microbalance (QCM) to control the thickness of the film during the deposition and the laser-induced desorption process. The analysis and interpretation of TOF distributions is very sensitive to the ion drift time correction applied for the ions selected
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