Characterising and optimising impulsive molecular alignment in mixed gas samples

Abstract

Laser induced impulsive molecular alignment has been fully characterized in linear molecules by matching numerical simulations and experimental data of the corresponding rotational wavepacket in the frequency domain. A rigorous procedure for an accurate matching between simulation and experimental data is presented for the first time, making this a versatile technique for experiments where the molecular axis distribution is not directly accessible. Seeding small molecules in Ar as a carrier gas has then been employed to assist cooling and we systematically retrieve the molecule's rotational temperature and alignment distribution for different mixing ratios. For a total backing pressure of 2 bar it was found that seeding 10% N₂ in Ar results in the best cooling. Compared to pure N₂ the rotational temperature was reduced from 24 ± 2 K down to 9 ± 2 K. This leads to an improvement of the peak alignment distribution from 〈cos²θ〉 = 0.60 to 〈cos²θ〉 = 0.71. For the same mixing ratio CO₂ was cooled from 34 ± 3 K to 9 ± 1 K improving the alignment distribution from 0.48 to 0.64. In O₂ a cooling from 58 ± 2 K to 37 ± 4 K was observed, corresponding to an alignment distribution improvement from 0.49 to 0.58. The results demonstrate the wide applicability of the characterisation procedure and of seeded supersonic beams to optimise impulsive alignment of small molecules.