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

Rovibronic Spectroscopy of Sympathetically Cooled ⁴⁰CaH⁺

We measure the rovibronic transitions <i>X</i> ¹Σ⁺, <i>v</i>″ = 0, <i>J</i>″ → <i>A</i> ¹Σ⁺, <i>v</i>′ = 0–3, <i>J</i>′ of CaH⁺ and obtain rotational constants for the <i>A</i> ¹Σ⁺ state. The spectrum is obtained using two-photon photodissociation of CaH⁺ cotrapped with Doppler cooled Ca⁺. The excitation is driven by a mode-locked, frequency-doubled Ti:Sapph laser, which is then pulse shaped to narrow the spectral bandwidth. The measured values of the rotational constants are in agreement with <i>ab initio</i> theory