Inner-shell excitation spectroscopy and fragmentation of small hydrogen-bonded clusters of formic acid after core excitations at the oxygen K edge

摘要

Inner-shell excitation spectra and fragmentation of small clusters of formic acid have been studied in the oxygen K-edge region by time-of-flight fragment mass spectroscopy.In addition to several fragment cations smaller than the parent molecule,we have identified the production of HCOOH centre dot H~+ and H_3O~+ cations characteristic of proton transfer reactions within the clusters.Cluster-specific excitation spectra have been generated by monitoring the partial ion yields of the product cations.Resonance transitions of 01s(C=O/OH) electrons into pi_(CO) orbital in the preedge region were found to shift in energy upon clusterization.A blueshift of the O1s(C=O)->pi_(CO) transition by approx 0.2 eV and a redshift of the Ols(OH) -> pi_(CO) by approx 0.6 eV were observed,indicative of strong hydrogen-bond formation within the clusters.The results have been compared with a recent theoretical calculation,which supports the conclusion that the formic-acid clusters consist of the most stable cyclic dimer and/or trimer units.Specifically labeled formic acid-d,HCOOD,was also used to examine the core-excited fragmentation mechanisms.These deuterium-labeled experiments showed that HDO~+ was formed via site-specific migration of a formyl hydrogen within an individual molecule,and that HD_2O~+ was produced via the subsequent transfer of a deuterium atom from the hydroxyl group of a nearest-neighbor molecule within a cationic cluster.Deuteron (proton) transfer from the hydroxyl site of a hydrogen-bond partner was also found to take place,producing deuteronated HCOOD centre dot D~+ (protonated HCOOH centre dot H~+) cations within the clusters.