Kuijpers Stach E J, Kalaitzis Panagiotis, Sakkoula Evangelia, van de Meerakker Sebastiaan Y T, Softley Timothy P, Parker David H
Radboud University Nijmegen, Institute for Molecules and Materials, Heijendaalseweg 135, 6525 AJ Nijmegen, The Netherlands.
School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.
J Phys Chem A. 2024 Dec 26;128(51):10993-11004. doi: 10.1021/acs.jpca.4c06253. Epub 2024 Dec 13.
In molecular beam scattering experiments, an important technique for measuring product energy and angular distributions is velocity map imaging following photoionization of one or more scattered species. For studies with cold molecular beams, the ultimate resolution of such a study is often limited by the product detection process. When state-selective ionization detection is used, excess energy from the ionization step can transfer to kinetic energy in the target molecular ion-electron pair, resulting in measurable cation recoil. With state-of-the-art molecular beam technology, velocity spreads as small as a few m/s are possible, thus a suitable product detection scheme must be not only highly sensitive, state-selective, and background-free, it must also produce significantly less cation recoil than the velocity spread of the molecular beams undergoing cold collisions. To date this has only been possible with the NO molecule, and our goal here is to extend this minimal-recoil capability to the fully deuterated ammonia molecule, ND. In this article a resonance enhanced multi photon ionization (REMPI) detection scheme for ND is presented that imparts sufficiently low recoil energy to the ions, allowing, for the first time, high-resolution imaging of ND collision products in cold molecule scattering experiments with HD. The excitation step of the 1 + 1' REMPI scheme requires vacuum ultra-violet (VUV) photons of ∼160 nm, which are generated through four-wave-mixing in Xe. We varied the wavelength of the second, ionization step between 434 and 458 nm, exciting ND to a wide range of autoionizing neutral states. By velocity mapping the photoelectrons resulting from the detection scheme, it was possible to fully chart the ion recoil across this range with vibrational resolution for the final ionic states. Additionally, rotational resolution in the photoionization dynamics was achieved for selected excitation energies near one of the vibrational thresholds. Many of the peaks in the spectrum of autoionizing Rydberg states are assigned to specific Rydberg series using a simple Rydberg formula model.
在分子束散射实验中，一种用于测量产物能量和角分布的重要技术是对一个或多个散射物种进行光电离后的速度成像。对于冷分子束研究，此类研究的最终分辨率通常受产物检测过程的限制。当使用态选择性电离检测时，电离步骤产生的多余能量会转移到目标分子离子 - 电子对的动能中，导致可测量的阳离子反冲。利用最先进的分子束技术，速度展宽可小至每秒几米，因此合适的产物检测方案不仅必须高度灵敏、态选择性且无背景，还必须产生比经历冷碰撞的分子束的速度展宽小得多的阳离子反冲。迄今为止，只有一氧化氮分子能做到这一点，我们这里的目标是将这种最小反冲能力扩展到完全氘化的氨分子ND₃。本文提出了一种用于ND₃的共振增强多光子电离（REMPI）检测方案，该方案赋予离子足够低的反冲能量，首次使得在与HD进行冷分子散射实验中对ND₃碰撞产物进行高分辨率成像成为可能。1 + 1' REMPI方案的激发步骤需要约160 nm的真空紫外（VUV）光子，这些光子通过在Xe中的四波混频产生。我们将第二个电离步骤的波长在434至458 nm之间变化，将ND₃激发到广泛的自电离中性态。通过对检测方案产生的光电子进行速度成像，能够以最终离子态的振动分辨率全面绘制该范围内的离子反冲情况。此外，对于接近其中一个振动阈值的选定激发能量，在光电离动力学中实现了转动分辨率。利用一个简单的里德堡公式模型，将自电离里德堡态光谱中的许多峰分配给特定的里德堡系列。
