Formation of HCN+ in collisions of N+ and N2+ with a self-assembled propanethiol surface on gold.

Collisions of N+ and N2+ with C3 hydrocarbons, represented by a self assembled monolayer of propanethiol on a polcrystalline gold surface, were investigated by experiments over the incident energy range between 5 eV and 100 eV. For N+, formation of HCN+ is observed at incident energies of projectile ions as low as 20 eV. In the case of N2+ projectile ions, the yield of HCN+ increased above zero only at incident energies of about 50 eV. This collision energy in the laboratory frame corresponds to an activation energy of about 3 eV to 3.5 eV. In the case of N+ projectile ions, the yield of HCN+ was large for most of the incident energy range, but decreased to zero at incident energies below 20 eV. This may indicate a very small energy threshold for the surface reaction between N+ and C3 hydrocarbons of a few tenths of an eV. Such a threshold for the formation of HCN+ may exist also for collisions of N+ with an adsorbed mixture of hydrocarbon molecules.

Complexes with Atomic Gold Ions: Efficient Bis-Ligand Formation.

Complexes of atomic gold with a variety of ligands have been formed by passing helium nanodroplets (HNDs) through two pickup cells containing gold vapor and the vapor of another dopant, namely a rare gas, a diatomic molecule (H2, N2, O2, I2, P2), or various polyatomic molecules (H2O, CO2, SF6, C6H6, adamantane, imidazole, dicyclopentadiene, and fullerene). The doped HNDs were irradiated by electrons; ensuing cations were identified in a high-resolution mass spectrometer. Anions were detected for benzene, dicyclopentadiene, and fullerene. For most ligands L, the abundance distribution of AuLn+ versus size n displays a remarkable enhancement at n = 2. The propensity towards bis-ligand formation is attributed to the formation of covalent bonds in Au+L2 which adopt a dumbbell structure, L-Au+-L, as previously found for L = Xe and C60. Another interesting observation is the effect of gold on the degree of ionization-induced intramolecular fragmentation. For most systems gold enhances the fragmentation, i.e., intramolecular fragmentation in AuLn+ is larger than in pure Ln+. Hydrogen, on the other hand, behaves differently, as intramolecular fragmentation in Au(H2)n+ is weaker than in pure (H2)n+ by an order of magnitude.

Vibrational Predissociation Spectroscopy of Cold Protonated Tryptophan with Different Messenger Tags.

Vibrational spectra of protonated tryptophan were recorded by predissociation of H2 messenger tags using cryogenic ion traps. We explore the issue of messenger induced spectral changes by solvating TrpH+(H2) n with n = 1-5 to obtain single photon vibrational spectra of TrpH+ and of its partly deuterated isotopomer in the spectral region of 800-4400 cm-1. Depending on the number of messenger molecules, the spectra of several conformational isomers associated with multiple H2 binding locations along with two natural conformations of TrpH+ were found using the two photon MS3IR2 conformational hole burning method. Most probable messenger positions were established by comparison with predictions from DFT calculations on various candidate structures. Mechanical anharmonicity effects associated with the charged amino group were modeled by Born-Oppenheimer ab initio molecular dynamics. The spectra of TrpH+(H2O) m=1,2, recorded by infrared multiphoton dissociation (IRMPD), reveal broad features in the NH stretching region of the NH3+ group, indicating strong hydrogen bonding in acceptor-donor configuration with the benzene ring for the first water molecule, while the second water appears to attach to a less strongly perturbing site, yielding unique transitions associated with the free OH stretching fundamentals. We discuss the structural deformations induced by the water molecules and compare our results to recent experiments on similar hydrated cationic systems.

A sub-4 Kelvin radio frequency linear multipole wire trap.

A linear cryogenic 16-pole wire ion trap has been developed and constructed for cryogenic ion spectroscopy at temperatures below 4 K. The trap is temperature-variable, can be operated with different buffer gases, and offers large optical access perpendicular to the ion beam direction. The housing geometry enables temperature measurement during radio frequency operation. The effective trapping potential of the wire-based radio frequency trap is described and compared to conventional multipole ion trap designs. Furthermore, time-of-flight mass spectra of multiple helium tagged protonated glycine ions that are extracted from the trap are presented, which prove very low ion temperatures and suitable conditions for sensitive spectroscopy.

A high sensitivity, high resolution tandem mass spectrometer to research low-energy, reactive ion-surface interactions.

The device described is the combination of two mass spectrometers, with a surface sample placed between them. Its aim is to allow for detailed research on low-energy ion-surface interactions, involving and triggering surface chemistry. This task is fulfilled by a carefully chosen geometry: Projectile ions from an electron impact source are mass-per-charge selected using a quadrupole. Such continuous bombardment allows for good control of the surface condition. Species emerging from the collisions are focused onto a beam and analyzed using a purpose-built orthogonal pulsing time-of-flight mass spectrometer. Neutral species can be post-ionized using a second electron impact source. Neutral gases can be adsorbed to the surface from the gas phase in a controlled manner, using a feedback-controlled pressure regulator. In order to minimize the discrimination of secondary ions, the distance from the surface to the analyzing mass spectrometer system was kept as short as possible and the acceptance angle of the lens system as large as possible. This increased the sensitivity five orders of magnitude compared to its predecessor. The rigorous use of computer aided design software is responsible for the successful commissioning of the new device. This article describes first which parameters can be measured or controlled. Then, these are linked to the physical processes that occur in reactive ion-surface interactions. Next, the design goal and the design implementation are presented. In the end, a performance comparison, measurements of hydrogen surface chemistry with extensive use of isotope labeling, and measurements of post-ionized beryllium are presented.