Abstract

In the interaction process between lasers and materials, there are often some kinds of nonlinear phenomena. Obviously, these phenomena come from energy level structures of atoms and molecules and also from the state selectivity of laser wavelengths to materials. Here we will report the results of our study on the subject. We have studied experimentally several kinds of organic molecules by using TOF mass spectroscopy with UV and visible laser beams as the light source. In our experiments, we adjusted both laser intensity and laser wavelength. The results show that when the laser power is high enough, the mass spectrum distribution changes little as the laser wavelength is changed. When the laser beam has a low output, the mass spectrum distribution shows an evident change, which we call state selectivity. According to this behavior, we suppose a threshold exists for each energy level of each kind of atom and molecule, which provides us a criterion to classify the multiphoton process. According to our recent multiphoton ionization study and the results we obtained earlier, we can conclude that this threshold is related to the ionization cross section of the atom and molecule. Although our research provided significant results, we need more results to understand the property well. We hope our new study will give more evidence and will provide us with a clear idea about the threshold.

© 1992 Optical Society of America