Multi-photon ionisation spectroscopy for rotational state preparation of {{bf{N}}}_{{bf{2}}}^{{boldsymbol{+}}}

Amy Gardner,Timothy Softley,Matthias Keller

doi:10.1038/s41598-018-36783-5

Abstract

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 {{rm{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.

Highlights

High precision experiments with molecules span over many research areas; from ultra-cold chemistry[1,2,3] to proposals for quantum computing[4]
Cooling molecular ions to the motional ground state has recently been demonstrated[13,14], and it is possible to transfer state information between species trapped under these conditions[15,16,17,18]
Various techniques have been used depending on the particular requirements posed by the species, for instance laser cooling has been demonstrated for heteronuclear diatomics[20,21,22]

Summary

Results

While the resonant excitation step in the 2 + 1′ REMPI scheme can provide some limitation of possible rotational states in the molecular ion due to propensity rules, the ionisation into one specific ro-vibrational state with high selectivity is not possible. For the S(0) transition, when the ionisation laser energy is tuned to lie between the N+ = 0 and N+ = 2 thresholds, only ions in the rovibronic ground state with N+ = 0 are produced - only N+ = 0 ions are energetically possible even if the excitation is to an autoionised N+ = 2 Rydberg state.

Discussion

Findings

Methods