Molecular ionization and dissociative ionization at hyperthermal surface scattering

Surface ionization of molecules with hyperthermal kinetic energy (1-20 eV) was found to be very efficient, demonstrating several unique features. We have used the technique of aerodynamic acceleration in supersonic seeded beams in order to obtain molecular kinetic energies in the range 1-20 eV. In this energy range, scattering events are impulsive, being free from molecular adsorption on the surface, and surface ionization occurs under nonthermal equilibrium conditions. Thus, in hyperthermal surface ionization (HSI), the kinetic energy is directly converted either into the energy difference between the surface work function and the molecular electron affinity (for negative ions) or into the molecular ionization potential minus the surface work function (for positive ions). Four types of HSI processes were observed. (a) Surface-molecule electron transfer was demonstrated in the I₂/diamond system where negative molecular iodine (I₂^-) ions were produced. (b) Molecule-surface electron transfer was found for the anthracene molecule where positive molecular anthracene ions were generated. (c) Abstractive ionization was detected for N,N-dimethylaniline (DMA) scattered from diamond. A protonated molecular ion was observed. (d) Hyperthermal surface induced dissociative ionization (HSIDI) was observed in 1-iodopropane, benzyl bromide, and many other molecules. In these processes we have observed a large current of negative halogen ions and positive molecular residue ions (ion-pair formation). Hyperthermal surface ionization is characterized by several experimental features such as the following: (a) A very large dependence of the ionization yield on the incident molecular kinetic energy. HSI has an energetic threshold which depends on the molecule, the surface, and the ion. (b) The negative ions were generated on diamond and not on the "technical" metal that served as a support for the diamond crystal. The positive ion yield was larger on this metal support. (c) The effect of the surface temperature on the ionization yield was small. (d) The HSI yield decreased with the beam-surface incident angle. (e) The angular distribution of the scattered ions was shifted toward grazing angles (supraspecular scattering). (f) The ions energy distribution was broad, structured, and non-Boltzmann. (g) HSIDI resulted in the generation of ions whose yield correlated with the generation of neutral atoms. The mechanism of hyperthermal surface ionization (HSI) is described in terms of electron-transfer processes. It occurs due to a curve crossing between the neutral scattering interaction potential surface and the ionic interaction potential surface, and a nonunity reneutralization second curve crossing of the scattered molecules or fragments. The HSIDI mass spectra demonstrated a highly nonstatistical mass spectral fragmentation pattern. This fragmentation pattern was largely different from that of the electron impact ionization mass spectra. It was affected by the fragment electron affinity, by the ionization potential of the molecule or fragment, by surface reactions, and by the reneutralization probabilities. Using iodobutane and dibromopropane isomers we have demonstrated that the HSIDI mass spectra contain important and distinctive isomeric and structural information. We discuss the advantages of HSI as a new efficient ion source and its analytical applications. A HSIDI mass spectrum of a dipeptide such as cyclo-(glycinyltryptophyl) is compared with an electron impact mass spectrum. We also report on low-vacuum experiments using a single-stage vacuum chamber pumped by a diffusion pump or by a rotary pump alone. In these experiments a direct supersonic expansion on the diamond crystal was performed and the ion currents were increased by 3 orders of magnitude up to the 10^-5-A range as compared with the collimated beam-ultrahigh-vacuum experiments.