Raymond Alexander W, Kelvin Lee Kin Long, McCarthy Michael C, Drouin Brian J, Mazur Eric
Center for Astrophysics | Harvard & Smithsonian , 60 Garden Street , Cambridge , Massachusetts 02138 , United States.
John A. Paulson School of Engineering and Applied Sciences , Harvard University , 9 Oxford Street , Cambridge , Massachusetts 02138 , United States.
J Phys Chem A. 2020 Feb 20;124(7):1429-1436. doi: 10.1021/acs.jpca.9b10548. Epub 2020 Feb 11.
Rotational transitions are unique identifiers of molecular species, including isotopologues. This article describes the rotational detections of two laser-volatilized salts, NaCl and KCl, made with a miniature Fourier transform millimeter-wave (FTmmW) cavity spectrometer that could one day be used to measure solid composition in the field or in space. The two salts are relevant targets for icy moons in the outer solar system, and in principle, other molecular solids could be analyzed with the FTmmW instrument. By coupling the spectrometer to a collisionally cooling laser ablation source, (a) we demonstrate that the FTmmW instrument is sensitive enough to detect ablation products, and (b) we use the small size of the FTmmW cavity to measure ablation product signal along the carrier gas beam. We find that for 532 nm nanosecond pulses, ablated molecules are widely dispersed in the carrier-gas jet. In addition to the miniature spectrometer results, we present several complementary measurements intended to characterize the laser ablation process. For pulse energies between 10 and 30 mJ, the ablation product yield increases linearly, reaching approximately 10 salt molecules per 30 mJ pulse. Using mass spectrometry, we observe Li, Na, and K in the plumes of ablated NaCl, KCl, and LiCl, which implies dissociation of the volatilized material. We do not observe salt ions (e.g., NaCl). However, with 800 nm femtosecond laser pulses, the triatomic ion clusters LiCl, NaCl, and KCl are produced. Finally, we observe incomplete volatilization with the nanosecond pulses: some of the ejecta are liquid droplets. The insights about ablation plume physics gleaned from these experiments should guide future implementations of the laser-volatilization technique.
转动跃迁是分子物种（包括同位素异构体）的独特标识。本文描述了使用微型傅里叶变换毫米波（FTmmW）腔式光谱仪对两种激光挥发盐（氯化钠和氯化钾）进行的转动检测，该光谱仪未来有一天可用于在野外或太空中测量固体成分。这两种盐是外太阳系冰卫星的相关研究目标，原则上，其他分子固体也可用FTmmW仪器进行分析。通过将光谱仪与碰撞冷却激光烧蚀源耦合，（a）我们证明了FTmmW仪器足够灵敏，能够检测烧蚀产物；（b）我们利用FTmmW腔的小尺寸来测量沿载气束的烧蚀产物信号。我们发现，对于532 nm纳秒脉冲，烧蚀的分子在载气射流中广泛分散。除了微型光谱仪的结果外，我们还进行了几次补充测量，旨在表征激光烧蚀过程。对于10至30 mJ之间的脉冲能量，烧蚀产物产率呈线性增加，每30 mJ脉冲达到约10个盐分子。使用质谱法，我们在烧蚀的氯化钠、氯化钾和氯化锂羽流中观察到了锂、钠和钾，这意味着挥发物质发生了解离。我们没有观察到盐离子（例如氯化钠离子）。然而，使用800 nm飞秒激光脉冲时，会产生三原子离子簇氯化锂、氯化钠和氯化钾。最后，我们观察到纳秒脉冲存在不完全挥发的情况：一些喷射物是液滴。从这些实验中获得的关于烧蚀羽流物理的见解应指导激光挥发技术的未来应用。
