Mechanism of IR and UV Laser-Induced Evaporation and Ablation From Condensed Molecular Systems
- PDF / 454,955 Bytes
- 8 Pages / 420.48 x 639 pts Page_size
- 47 Downloads / 183 Views
MECHANISM OF IR AND UV LASER-INDUCED EVAPORATION AND ABLATION FROM CONDENSED MOLECULAR SYSTEMS PETER HESS Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, D-6900 Heidelberg 1, F.R.G. ABSTRACT The main features of evaporation of organic molecules from Van der Waals films with resonant IR photons and ablation of organic material and fragments from polymers with UV photons, resulting from time-of-flight experiments, are discussed. These results point to a photothermal mechanism. The experimental facts known so far can be explained using thermodynamic and kinetic arguments. No principal difference is found for breaking Van der Waals bonds with IR photons and chemical bonds employing UV photons. INTRODUCTION The investigation of phase transitions induced by pulsed laser irradation has attracted much attention recently. Simple dynamical models of melting and freezing have been developed and were applied especially to semiconductors. For silicon, for example, the solidification velocities were measured after pulsed laser melting (1,2). Using a simple model for the dependence of the velocity of the liquid solid interface on the thermodynamic driving force and kinetic parameters, it was possible to calculate the undercooling at the interface from the measured kinetics. The results are in agreement with the thermal transport model and provide evidence for strong undercooling at the interface during amorphous or crystalline regrowth and overheating during melt-in. In this paper the treatment of phase transitions in terms of thermodynamic and kinetic considerations is generalized and applied also to laser-induced desorption, evaporation and ablation processes. Detailed experimental results are available for condensed films of polyatomic molecules, where Van der Waals forces must be overcome during the phase transition and for polymeric organic materials, where chemical bonds must be broken (3). The energetic situation is similar in both experiments. The energy of IR photons is comparable with the Van der Waals interaction energy and the energy of UV photons is in the range of the dissociation energy of chemical bonds (see Fig.l). The interpretation of laser-induced evaporation of molecular Van der Waals films by resonant excitation of internal vibrational modes with IR lasers as well as the ablation of polymers by electronic excitation with UV radiation is still controversial. Photothermal and nonthermal quantum or photochemical mechanisms have been proposed in the literature. Valuable information on the mechanism of these processes can be obtained by time-of-flight (TOF) experiments. Therefore,
Mat. Res. Soc. Symp. Proc. Vol. 75. -1987 Materials Research Society
584
E(kL)
Polymer
Van der Waals film -
300 200
UV photon IArF laser)
100
IR photon
0 -
-100 -200
(CO 2 laser) Van der Waats interaction
chemical bond
-300
-400 Fig.l: Schematic diagram comparing the energy of IR photons with the Van der Waals interaction energy and the energy of UV photons with the chemical binding en
Data Loading...
