Photoionization efficiency curve measurements of alkali metal atom-methyl propiolate clusters: Observation of intracluster cyclotrimerization products

Reaction products in clusters containing an alkali metal atom (M = Na, K, and Cs) and methyl propiolate (MP) molecules have been examined from results of photoionization efficiency (PIE) curve measurements. Two distinct humps were commonly observed in the PIE curves of Cs(MP)_n for n ≥ 3, whereas a relatively smooth curve was obtained for n = 2. In the photoionization mass spectra of M(MP)_n, an intensity anomaly at n = 3 was commonly observed as reported in the previous paper, which was due to a cyclotrimerization reaction producing a benzene derivative (trimethyl benzenetricarboxylate, BT). From the results of quantum chemical calculations for reacted and unreacted isomers, these two humps originate from two isomers with different ionization thresholds; the lower threshold is ascribed to a solvation-type isomer, Cs(MP)₃, and the higher value comes from a reacted isomer composed of a cesium atom and a BT molecule. From the relative intensity of two isomers, the intracluster cyclotrimerization reaction producing BT is found to proceed efficiently in the present M-MP clusters.