In this article, we present the experimental results of mass spectroscopic study of multi-photon ionized acetaldehyde molecules in gaseous phase excited by XeCl excimer laser under the molecular beam condition. Based on the linear relations between the intensities of fragment ions and the pressure of sample gas, we show that the multiphoton ionization and fragmentation of acetaldehyde is an unimolecular reaction, this result reveals some photochemical be havior of acetaldehyde in the photolysis process by uv laser. According to the MPP mass spectrum of acetaldehyde, the variation of the intensities of the fragment ions in the MPI mass spectrum with laser power and photophysical property of its first excited single state 1(n,π*), we propose a parent dissociation-neutral fragments ionization model of MPI of acetaldehyde, with which the experimental results can be explained.
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