Abstract: Multiphoton ionization mass spectrum of methanol clusters was studied by using pulsed 355 nm laser to ionize supersonic molecular beam. Besides some fragment ions of methanol, protonated methanol cluster ions (CH3OH)nH+ (n=1-16) were observed and the cluster′s intensity distribution was not affected by laser delay time relative to pulsed molecular beam, which implied that the ions′ species and intensity were determined only by cluster′s inherent geometric stability. Possible stable geometry structures of (CH3OH)n and (CH3OH)nH+(n=1-4) were obtained by using ab initio calculation at B3LYP/6-31G(d) basis level. Comparing cluster′s infrared vibrational spectrum with related atomic motion image, it was found that the main contribution to the strongest IR mode came from the H stretch vibration of the hydrogen bond. Proton transfer reaction occurred within clusters after ionization possibly corresponded with the excitation of vibrational mode which is related to hydrogen bond.

Key words: Protonated methanol cluster, Multiphoton ionization, Ab initio calculation, Proton transfer reaction