Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BW, United Kingdom.
Phys Chem Chem Phys. 2012 Jul 21;14(27):9785-91. doi: 10.1039/c2cp40677b. Epub 2012 Jun 12.
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% N(2) in Ar results in the best cooling. Compared to pure N(2) 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 <cos(2)θ> = 0.60 to <cos(2)θ> = 0.71. For the same mixing ratio CO(2) was cooled from 34 ± 3 K to 9 ± 1 K improving the alignment distribution from 0.48 to 0.64. In O(2) 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.
激光诱导的脉冲分子取向已经在直线分子中得到了充分的描述，通过在频域中匹配相应的旋转波包的数值模拟和实验数据。本文首次提出了一种精确匹配模拟和实验数据的严格程序，这使得该技术在分子轴分布不可直接获得的实验中具有通用性。随后，我们采用在 Ar 中播种小分子作为载气来辅助冷却，并系统地检索了不同混合比下分子的旋转温度和取向分布。在总背压为 2 巴的情况下，我们发现将 10%的 N2 注入 Ar 中可以实现最佳冷却。与纯 N2 相比，旋转温度从 24±2 K 降低到 9±2 K。这导致峰值取向分布从<cos(2)θ>=0.60 提高到<cos(2)θ>=0.71。对于相同的混合比，CO2 从 34±3 K 冷却到 9±1 K，提高了取向分布从 0.48 到 0.64。在 O2 中，观察到从 58±2 K 冷却到 37±4 K，对应于取向分布从 0.49 到 0.58 的提高。这些结果证明了该特征化程序和种子超音速束在优化小分子脉冲取向方面的广泛适用性。
