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We investigated neutral species in the matrix-assisted laser desorption and ionization (MALDI) plume using femtosecond laser ionization spectrometry with simultaneous measurement of the standard MALDI spectrum of the identical MALDI event induced by pulsed UV laser irradiation. The ratio of neutral species in the plume [A]p/[M]p (A = phenylalanine (Phe) or alanine (Ala), M = 2,5-dihydroxybenzoic acid (DHB)) was confirmed to be the same as that of the sample mixture in the range of [A]0/[M]0 = 4 × 10-4-1, indicating the validity of the widely adopted approximation [A]p/[M]p = [A]0/[M]0 in the reaction quotient of the proton transfer reaction MH+ + A ⇄ M + AH+. An effective parameter representing the extent of thermal equilibrium in the thermal proton transfer model is introduced for the first time. Numerical simulation based on this semiequilibrium model successfully reproduced variations of MALDI signal intensities AH+ and MH+ with two parameters: the fraction of ionized matrix a ≤ 10-5 and an effective temperature T = 1200 and 1100 K for Phe/DHB and Ala/DHB systems, respectively. These values show good agreement with those determined previously by different experimental approaches. The extent of thermal equilibrium was determined to be 95% and 98% for Phe/DHB and Ala/DHB systems, respectively, suggesting that the proton transfer reactions almost proceed to their thermal equilibrium. [ABSTRACT FROM AUTHOR]

Site-specific electron relaxations caused by Si:2p core-level photoionizations in F3SiCH2CH2Si(CH3)3 and Cl3SiCH2CH2Si(CH3)3 vapors have been studied by means of the photoelectron Auger electron coincidence spectroscopy. F3SiCH2CH2Si(CH3)3 shows almost 100% site-specificity in fragmentation caused by the Si:2p ionization. However, substitution of Cl for F of F3SiCH2CH2Si(CH3)3 considerably reduces the site-specificity at the Si atom bonded to three halogen atoms, with the site-specificity at the Si site bonded to three methyl groups remaining largely unchanged. The site-specificity reduction in Cl3SiCH2CH2Si(CH3)3 is considered to take place during the transient period between Si:L23VV Auger electron emission and the subsequent fragmentation. The reason for the reduction can be explained in terms of some differences between these two molecules in the L23VV Auger decay at the Si site bonded to the three halogen atoms. [ABSTRACT FROM AUTHOR]