Study Of Formaldehyde Cluster Plasma With Time-of-Flight Mass Spectrometry

Formaldehyde is a very important organic molecule and plays an important role in physics, chemistry, medicine, environmental science and etc. Much more attention has been paid to the pollution problem caused by formaldehyde molecular, duo to its wide uses. Besides, formaldehyde molecular used to polymerize into clusters through hydrogen-bonding, which are especially important in many chemical processes. Because of the advantages of the low temperature plasma, nearly all the experiments carried out before have concentrated their attention on the removal issue of the formaldehyde molecular. In this thesis, both the laser induced and pulsed discharge plasma have been used to investigate the distributions of formaldehyde and its clusters influenced by the nanosecond pulsed laser, also a preliminary exploration of formaldehyde cluster plasma at an atmospheric pressure fast-flow nanosecond pulsed dc-discharge has been carried out.In this work, the formaldehyde-water clusters were synthesized by supersonic jet apparatus. Then, the multiphoton ionization signals of formaldehyde-water clusters were detected by using the nanosecond laser ionization and the time-of-flight (TOF) instrument. The distribution of formaldehyde clusters produced by pulsed dc-discharge has been studied by mass spectrometry.The key contributions of this paper are as following:1. Multiphoton ionization of formaldehyde hydrated clusters have been studied by time-of-flight mass spectrometry using 5ns,355nm Nd:YAG laser beam at power intensity of 1011-1012W/cm2. The main products including protonated formaldehyde cluster series (CH2O)nH+(n=1-4), deprotonated formaldehyde clusters Series (CH2O)nCHO+(n=1-3), and two series based on molecules originating from H2CO (the deprotonated and protonated forms), H3CO+(H2O)n(n=1,3,5) and HCO+(H2O)n(n=1,3,5) have been observed, and their structures are proposed. The transformation of formaldehyde mass peak signals is studied at different laser power density conditions. We found the ion peaks of formaldehyde monomer and water molecular in the laser intensity about 9.3×1011W/cm2. Those mass peaks presented in a very broadened profile and can not be resolved under the high resolution. The simple dynamic plasma sheath accelerating model is proposed to explain the physical mechanism of the broadened envelope phenomenon 2. The formaldehyde clusters are ionized using Nd:YAG-OPO system. Formaldehyde cluster ions are observed using laser wavelength of 225nm-229nm and 251nm-259nm. The mass spectral results at different laser wavelength has been compared and discussed.3. Multiphoton ionization (MPI) of formaldehyde hydrated clusters is studied using an ArF excimer with pulsed width of 20ns and wavelength of 193nm at power intensity of 5×10W/cm2. The time-of-flight mass spectrometer was applied to investigated the cluster ions from MPI and laser ionization process. Double peaks can be observed for these cluster species due to the different arrival times of backward and forward ions ejected. Kinetic energy values are calculated from the distance of peak splitting on the order of several dozen electron volts. The simple model of including dynamic plasma sheath accelerating and coulomb explosive model are proposed to explain peak splitting phenomenon.4. It is the first time the formaldehyde cluster ions signals were have been detected for the first time under pulse discharge in a mixture of formaldehyde and water vapor.