Single photon ionization of hydrogen bonded clusters with a soft x-ray laser: (HCOOH)x and (HCOOH)y(H2O)z

Abstract

Pure, neutral formic acid ͑HCOOH͒ n+1 clusters and mixed ͑HCOOH͒ / ͑H 2 O͒ 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 ͑HCOOH͒ 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 ͑HCOOH͒ n+1 + → ͑HCOOH͒ n H + + HCOO making the protonated ͑HCOOH͒ n H + series dominant in all data obtained. The distribution of pure ͑HCOOH͒ n H + clusters is dependent on experimental conditions. Under certain conditions, a magic number is found at n = 5. Metastable dissociation rate constants of ͑HCOOH͒ n H + are measured in the range ͑0.1-0.8͒ ϫ 10 4 s −1 for cluster sizes 4 Ͻ n Ͻ 9. The rate constants display an odd/even alternating behavior between monomer and dimer loss that can be attributed to the structure of the cluster. When small amounts of water are added to the formic acid, the predominant signals in the mass spectrum are still ͑HCOOH͒ n H + cluster ions. Also observed are the protonated mixed cluster series ͑HCOOH͒ n ͑H 2 O͒ m H + for n =1-8 and m =0-4. A magic number in the cluster series n =5, m =1 is observed. The mechanisms and dynamics of formation of these neutral and ionic clusters are discussed.

Stabilities of nanohydrated thymine radical cations: insights from multiphoton ionization experiments and ab initio calculations

“…Kim et al [10] observed an increase of around 100% in this cluster size range; the more pronounced effect in the present work may be linked to differences in the ionizing laser conditions applied in the two experiments. Increased metastable dissociation of clusters is widely viewed as a possible indicator of a lower energy threshold for the removal of a single molecule, helping to recognize particularly stable cluster sizes (so-called magic numbers often linked to shell structures) [39]. Kim et al [10] observed a particular increase in metastable H 2 O loss for n ≥ 4 and stated that this might indicate that the first solvation shell around the thymine radical cation is composed of four water molecules.…”

Infrared spectra of formic acid clusters in noble gas matrices

“…Other metastable dimeric species (open dimer) have also been observed in helium nanodroplets [8], and in noble gas matrices [5,[9][10][11] in combination with infrared photo-excitation techniques. Experimental studies of larger clusters of formic acid (HCOOH) n (n > 2), on the other hand, have been limited in mass spectrometry [12][13][14], and no spectroscopic observation has been reported to our knowledge. Since such species bridges the gap between FAD and formic acid in condensed phases, it is still an open question at what size formic acid cluster takes chain structure as in the bulk [15][16][17].…”

Gas phase study of the reactivity of optical coating materials with hydrocarbons using a desk-top size EUV laser

“…Ionization using and ArF excimer laser at 193 nm (6.4 eV) requires multiple photons whereas ionization at 46.9 nm (26.5 eV) has more than enough photon energy to ionize SiO with a single photon. The photoelectron carries away the remaining excess energy above the vertical ionization energy, resulting in almost no fragmentation of the clusters [14][15][16][17][18][19] and neutral parent cluster information is retained.…”

“…We have recently completed a series of studies that constitute the first demonstration of the use of EUV lasers in photophysics and photochemistry [14][15][16][17][18][19]. These studies have primarily focused on finding active catalysts for the improvement of industrial processes.…”