Dong F, Heinbuch S, Rocca J J, Bernstein E R
NSF ERC for Extreme Ultraviolet Science and Technology and Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA.
J Chem Phys. 2006 Jun 14;124(22):224319. doi: 10.1063/1.2202314.
A tabletop soft x-ray laser is applied for the first time as a high energy photon source for chemical dynamics experiments in the study of water, methanol, and ammonia clusters through time of flight mass spectroscopy. The 26.5 eV/photon laser (pulse time duration of approximately 1 ns) is employed as a single photon ionization source for the detection of these clusters. Only a small fraction of the photon energy is deposited in the cluster for metastable dissociation of cluster ions, and most of it is removed by the ejected electron. Protonated water, methanol, and ammonia clusters dominate the cluster mass spectra. Unprotonated ammonia clusters are observed in the protonated cluster ion size range 2< or =n< or =22. The unimolecular dissociation rate constants for reactions involving loss of one neutral molecule are calculated to be (0.6-2.7)x10(4), (3.6-6.0)x10(3), and (0.8-2.0)x10(4) s(-1) for the protonated water (9< or =n< or =24), methanol (5< or =n< or =10), and ammonia (5< or =n< or =18) clusters, respectively. The temperatures of the neutral clusters are estimated to be between 40 and 200 K for water clusters (10< or =n< or =21), and 50-100 K for methanol clusters (6< or =n< or =10). Products with losses of up to five H atoms are observed in the mass spectrum of the neutral ammonia dimer. Large ammonia clusters (NH(3))(n) (n>3) do not lose more than three H atoms in the photoionization/photodissociation process. For all three cluster systems studied, single photon ionization with a 26.5 eV photon yields near threshold ionization. The temperature of these three cluster systems increases with increasing cluster size over the above-indicated ranges.
首次将桌面型软X射线激光器用作高能光子源，通过飞行时间质谱法对水、甲醇和氨团簇进行化学动力学实验研究。26.5 eV/光子的激光器（脉冲持续时间约为1 ns）用作这些团簇检测的单光子电离源。只有一小部分光子能量沉积在团簇中用于团簇离子的亚稳解离，而大部分能量被 ejected 电子带走。质子化的水、甲醇和氨团簇主导了团簇质谱。在质子化团簇离子尺寸范围2≤n≤22内观察到未质子化的氨团簇。对于质子化的水（9≤n≤24）、甲醇（5≤n≤10）和氨（5≤n≤18）团簇，涉及损失一个中性分子的反应的单分子解离速率常数经计算分别为(0.6 - 2.7)×10⁴、(3.6 - 6.0)×10³和(0.8 - 2.0)×10⁴ s⁻¹。对于水团簇（10≤n≤21），中性团簇的温度估计在40到200 K之间，对于甲醇团簇（6≤n≤10），温度为50 - 100 K。在中性氨二聚体的质谱中观察到损失多达五个H原子的产物。大的氨团簇(NH₃)ₙ（n > 3）在光电离/光解离过程中损失不超过三个H原子。对于所研究的所有三种团簇系统，用26.5 eV光子进行单光子电离产生接近阈值的电离。在上述范围内，这三种团簇系统的温度随团簇尺寸的增加而升高。
