Multi-photon ionisation spectroscopy for rotational state preparation of N+2

In this paper we investigate the 2 + 1′ resonance enhanced multi-photon ionisation (REMPI) of molecular nitrogen via the a1Πg(v = 6) intermediate state and analyse its feasibility to generate molecular nitrogen ions in a well defined ro-vibrational state. This is an important tool for high precision experiments based on trapped molecular ions, and is crucial for studying the time variation of the fundamental constant mp/me using N+2. The transition is not reported in the literature and detailed spectral analysis has been conducted to extract the molecular constants of the intermediate state. By carefully choosing the intermediate ro-vibrational state, the ionisation laser wavelength and controlling the excitation laser pulse energy, unwanted formation of rotationally excited molecular ions can be suppressed and ro-vibrational ground state ions can be generated with high purity

Novel method for state selective determination of electron-impact-excitation cross sections from 0° to 180°

We use an improved target recoil momentum spectroscopy setup to determine differential cross sections for excited metastable state production in atoms and molecules by electron impact and show its capabilities for an atomic helium target. A crossed beam setup with a supersonic helium jet and a pulsed electron beam at energies close to the excitation threshold of 19.82 eV was used. Measuring the recoil momentum vector of the target instead of the momentum of the scattered electron removes common restrictions to the accessible scattering angles while the microchannel plate detector ensures a high counting efficiency. Using a photoemission electron source we reach an energy resolution of about 200 meV at 1 μA peak current. Results are compared with simulations using theoretical convergent-close-coupling (CCC), R-matrix with pseudo-states (RMPS) and B-spline R-matrix (BSR) calculations and show good agreement