Electron impact induced fragmentation of N2H⁺ and N2D⁺.

Electron impact dissociation of protonated and deuterated nitrogen ions has been studied using a crossed beams apparatus. Absolute cross sections for dissociation channels producing N(+) and NH(+), respectively, are presented. The observations of subthreshold signals in these measurements indicate the presence of ro-vibrationally and possibly electronically excited states in the parent ions. Comparisons with other measurements are given.

Tuning the continuum ground state energy of NO2- 2 by water molecules.

Electron scattering on NO-2, NO-2 x (H2O), and NO-2 x (H2O)(2) was performed in two storage rings. We confirm the presence of earlier reported NO2-2 dianion resonances and show that they remain when water is attached. Importantly, hydration tunes the energy: each water molecule lowers the ground state energy by 0.8 +/- 0.3 eV relative to the monoanion. NO2-2 is observed to decay by two-electron emission, possibly in combination with fragmentation. NO(2-)2 x (H2O) mainly decays into NO-2 + H2O + e(-).

Absorption spectra of photoactive yellow protein chromophores in vacuum.

The absorption spectra of two photoactive yellow protein model chromophores have been measured in vacuum using an electrostatic ion storage ring. The absorption spectrum of the isolated chromophore is an important reference for deducing the influence of the protein environment on the electronic energy levels of the chromophore and separating the intrinsic properties of the chromophore from properties induced by the protein environment. In vacuum the deprotonated trans-thiophenyl-p-coumarate model chromophore has an absorption maximum at 460 nm, whereas the photoactive yellow protein absorbs maximally at 446 nm. The protein environment thus only slightly blue-shifts the absorption. In contrast, the absorption of the model chromophore in aqueous solution is significantly blue-shifted (lambda(max) = 395 nm). A deprotonated trans-p-coumaric acid has also been studied to elucidate the effect of thioester formation and phenol deprotonation. The sum of these two changes on the chromophore induces a red shift both in vacuum and in aqueous solution.

Molecular size effect in NCO and NCS dianion resonances.

Cross sections for electron-impact detachment and electron-impact dissociation of NCO- and NCS- were measured from about 3 to about 40 eV. The former are found to follow a classical prediction with a threshold energy of 9.1 +/- 0.1 eV for NCO- and 8.9 +/- 0.2 eV for NCS-. When the incoming electron binds to the monoanion, a short-lived dianion complex is formed, which is revealed as a resonance in the cross section. For NCO- a resonance is evident at 9.3 +/- 0.2 eV, which implies that the dianion lies above the monoanion by this amount of energy. In the case of NCS- two resonances are evident at 8.4 +/- 0.2 and 19.0 +/- 0.5 eV, respectively. The low-energy NCS dianion is less unstable than the dianion of NCO, which in turn is less unstable than the CN dianion (10-eV resonance). Thus the resonance shifts down in energy with the increasing size of the anion, a fact which is attributed to a decrease in Coulomb energy between the spatially separated electrons.

Electron scattering on centrosymmetric molecular dianions Pt(CN)4(2-) and Pt(CN)6(2-).

Electron scattering on stored Pt(CN)2-4 and Pt(CN)2-6 centrosymmetric molecular dianions has been performed at the electrostatic storage ring ELISA. The thresholds for production of neutral particles by electron bombardment were found to be 17.2 and 18.7 eV, respectively. The relatively high thresholds reflect the strong Coulomb repulsion in the incoming channel as well as a high energetic stability of the target electrons. A trianion resonance was identified with a positive energy of 17.0 eV for the Pt(CN)2-4 square-planar complex, while three trianion resonances were identified for the Pt(CN)2-6 octahedral complex with positive energies of 15.3, 18.1, and 20.1 eV.