Single photon ionization of hydrogen bonded clusters with a soft x-ray laser: (HCOOH)x and (HCOOH)y (H2 O)z

Abstract

Pure, neutral formic acid (HCOOH)n+1 clusters and mixed (HCOOH) (H2 O) clusters are investigated employing time of flight mass spectroscopy and single photon ionization at 26.5 eV using a very compact, capillary discharge, soft x-ray laser. During the ionization process, neutral clusters suffer little fragmentation because almost all excess energy above the vertical ionization energy is taken away by the photoelectron, leaving only a small part of the photon energy deposited into the (HCOOH) n+1 + cluster. The vertical ionization energy minus the adiabatic ionization energy is enough excess energy in the clusters to surmount the proton transfer energy barrier and induce the reaction (HCOOH) n+1 + → (HCOOH)n H+ +HCOO making the protonated (HCOOH)n H+ series dominant in all data obtained. The distribution of pure (HCOOH)n H+ clusters is dependent on experimental conditions. Under certain conditions, a magic number is found at n=5. Metastable dissociation rate constants of (HCOOH)n H+ are measured in the range (0.1-0.8) × 104 s-1 for cluster sizes 4