Study on Multiphoton Ionization and Dissociation Process of Ethylene Oxide Using TOF-Mass Spectroscopic Method

Multiphoton ionization and dissociation （MPID） of ethylene oxide molecule at 308nm and 355nm laser radiation are investigated using a pulsed molecular beam and time of flight mass spectrometric techniques. Three relative strong peaks assigned to HCO⁺, CH₃⁺ and C₂H₃O⁺ and some week peaks assigned to CH₂⁺, CH₄⁺, H⁺ etc. are observed from the MPID-TOF mass spectrum of ethylene oxide under 308nm laser irradiation. The dependence of the ion signal intensity on laser power is measured. The laser power dependence of HCO⁺, CH₃⁺, and C₂H₃O⁺ are 2.8, 2.9 and 2.9, respectively. In the case at 355nm laser radiation, the major products of MPID of ethylene oxide are HCO⁺, CH₃⁺, CH₂⁺, CH⁺ and CH₃CO⁺. In addition, some week peaks assigned to H⁺, C⁺ are also observed. The laser power dependence of HCO⁺, CH₃⁺, CH₂⁺, CH⁺, and H⁺ are 4.0, 3.1, 3.5, 3.7 and 4.1 respectively. According to these results,the mechenism of MPID process of ethylene oxide can be proposed as follows: firstly, the ethylene oxide is ionized by 3 and 4 photons absorption under 308nm and 355nm laser irradiation, respectively, then the excited ethylene oxide ion is isomerized to the acetaldehyde structure and immediately dissociated into the fragments.