Ionic fragmentation of C 1s excited and ionized formic acid

Time of flight mass spectrometry with multi-ion coincidence detection has been used to investigate ionic photofragmentation of four isotopes of gaseous formic acid [HCOOH, HCOOD, DCOOH and DCOOD] following C 1s excitation and ionization. C 1s spectra, branching ratios and quantitative yields of ions and ion pairs are reported. For equivalent ionic decay channels in the four isotopes the partial ion and ion pair yield spectra are very similar. These gas phase results are compared to results from photon stimulated ion desorption (PSID) of deuterated formate adsorbed on Si(100) [H. Ikeura-Sekiguchi, T. Sekiguchi, K. Tanaka, Phys. Rev. B 53 (1996) 12655] where there is a very strong enhancement of the D+ yield at C1s→σC-D∗ resonance and the COD+ yield at the C1s→σC–O∗ resonance. Such enhancements were not observed in the fragmentation of gas phase formic acid. From this we conclude that these very specific bond breaking processes in surface adsorbed formate are associated with a minority channel such as a (2h, 1e) spectator Auger process, which becomes prominent through elimination of the dominant molecular fragmentation mechanisms by electronic recombination.