The ion-to-neutral ratios of 25DHB matrix measured in this work are much smaller than some of the values reported in previous work using different methods and/or under different MALDI conditions.

This paper examines the unimolecular dissociation of propargyl (HCCCH2) radicals over a range of internal energies to probe the CH+HCCH and C+C2H3 bimolecular reactions from the radical intermediate to products. The propargyl radical was produced by 157 nm photolysis of propargyl chloride in crossed laser-molecular beam scattering experiments. The H-loss and H2 elimination channels of the nascent propargyl radicals were observed. Detection of stable propargyl radicals gave an experimental determination of 71.5 (+5-10) kcal/mol as the lowest barrier to dissociation of the radical. This barrier is significantly lower than predictions for the lowest barrier to the radical's dissociation and also lower than calculated overall reaction enthalpies. Products from both H2+HCCC and H+C3H2 channels were detected at energies lower than what has been theoretically predicted. An HCl elimination channel and a minor C-H fission channel were also observed in the photolysis of propargyl chloride.

An aqueous acetonitrile solution containing oligosaccharides (maltopentaose and polysaccharides) and a matrix (2,5-dihydroxybenzoic acid) was frozen at 100 K for mass analysis using ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI). Compared with conventional UV-MALDI (i.e., using a dry analyte/matrix mixture), a frozen solution generates more oligosaccharide ions and less fragments from postsource decay. Furthermore, the ion signal is long-lasting, and the analyte distribution features enhanced homogeneity. The ion generation efficiency for this procedure is 20-30 times greater than that for a conventional dried mixture. Interestingly, the percentages for maltopentaose fragmentation from postsource decay for the frozen samples are close to zero (<2%), as compared with the 17% and 40% values found for dried samples at low and high laser fluences, respectively. Comparisons with other UV matrixes (α-cyano-4-hydroxycinnamic acid and sinapinic acid) and ionic liquids (2,5-dihydroxybenzoic acid + pyridine and α-cyano-4-hydroxycinnamic acid + butylamine) were investigated, and possible mechanisms are discussed.

High-resolution angular and velocity distributions for neutral analytes (tryptophan and poly-tryptophan) and matrix (2,4,6-trihydroxyacetophenon, THAP) are measured by using 355 nm laser desorption. The information suggests that two separate mechanisms dominate the angular and velocity distributions at the beginning and before the end of desorption. A molecular jet-like isentropic expansion dominates the plume expansion at the beginning of desorption. This only occurs at high surface temperature, thus resulting in a large velocity normal to the surface and a very narrow angular distribution. Most of the analytes are produced under these conditions. Before the end of desorption, the surface temperature decreases and the mechanism of thermal desorption at low vapor pressure takes over. The velocities become small and the angular distribution is close to cosθ. Only a very small amount of analytes are generated under these conditions. Compared to tryptophan, poly-tryptophan has a much narrower angular distribution, thereby suggesting that it is only produced at the higher surface temperatures.

The reaction of oxygen atom in its first singlet excited state with nitrous oxide was investigated under the crossed molecular beam condition. This reaction has two major product channels, NO+NO and N2+O2. The product translational energy distributions and angular distributions of both channels were determined. Using oxygen-18 isotope labeled O(1D) reactant, the newly formed NO can be distinguished from the remaining NO that was contained in the reactant N2O. Both channels have asymmetric and forward-biased angular distributions, suggesting that there is no long-lived collision complex with lifetime longer than its rotational period. The translational energy release of the N2+O2 channel (fT = 0.57) is much higher than that of the NO+NO channel (fT = 0.31). The product energy partitioning into translational, rotational, and vibrational degrees of freedom is discussed to learn more about the reaction mechanism. The branching ratio between the two product channels was estimated. The 46N2O product of the isotope exchange channel, 18O+44N2O-->16O+46N2O, was below the detection limit and therefore, the upper limit of its yield was estimated to be 0.8%.