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Tuesday, 6 March 2012

Matle+Seb: Characterising and optimising impulsive molecular alignment in mixed gas samples

Impulsive molecular alignment has been fully characterized in linear molecules by matching
numerical simulations and experimental data of the Fourier transformed time-evolution of the
corresponding rotational wavepacket. For this, a qualitative measure of the alignment distri-
bution is sufficient, making this a versatile procedure in 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 temper-
ature and alignment distribution for different mixing ratios. 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 alignment distribution from
〈cos2 θ〉 = 0.60 to 〈cos2 θ〉 = 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.