In past studies, mistakes in determining the ionization mechanism in matrix-assisted laser desorption/ionization (MALDI) were made because an inappropriate ion-to-neutral ratio was used. The ion-to-neutral ratio of the analyte differs substantially from that of the matrix in MALDI. However, these ratios were not carefully distinguished in previous studies. We begin by describing the properties of ion-to-neutral ratios and reviews early experimental measurements. A discussion of the errors committed in previous theoretical studies and a comparison of recent experimental measurements follow. We then describe a thermal proton transfer model and demonstrate how the model appropriately describes ion-to-neutral ratios and the total ion intensity. Arguments raised to challenge thermal ionization are then discussed. We demonstrate how none of the arguments are valid before concluding that thermal proton transfer must play a crucial role in the ionization process of MALDI.

Photodissociation of ${\mathrm{H}}_2\mathrm{O}$ via the $A$ band at $157\mathrm{nm}$ has been reinvestigated using the high resolution H atom Rydberg tagging technique. The spin-orbit population distributions were found to be highly nonstatistical for the $\mathrm{O}\mathrm{H}(v=0,1,2)$ product channels, while nearly statistical for the $\mathrm{O}\mathrm{H}(v=4)$ channel. These results suggest that the dissociation dynamics of ${\mathrm{H}}_2\mathrm{O}$ at $157\mathrm{nm}$ is remarkably different for different vibrationally excited OH channels. The result presented here is not entirely consistent with the direct dynamical picture of the $A$ band dissociation of ${\mathrm{H}}_2\mathrm{O}$ .