Heinbuch S, Dong F, Rocca J J, Bernstein E R
Department of Electrical and Computer Engineering, Colorado State University, For Collins, Colorado 80523, USA.
J Chem Phys. 2007 Jun 28;126(24):244301. doi: 10.1063/1.2746036.
Pure, neutral formic acid (HCOOH)n+1 clusters and mixed (HCOOH)(H2O) clusters are investigated employing time of flight mass spectroscopy and single photon ionization at 26.5 eV using a very compact, capillary discharge, soft x-ray laser. During the ionization process, neutral clusters suffer little fragmentation because almost all excess energy above the vertical ionization energy is taken away by the photoelectron, leaving only a small part of the photon energy deposited into the (HCOOH)n+1+ cluster. The vertical ionization energy minus the adiabatic ionization energy is enough excess energy in the clusters to surmount the proton transfer energy barrier and induce the reaction (HCOOH)n+1+-->(HCOOH)nH+ +HCOO making the protonated (HCOOH)nH+ series dominant in all data obtained. The distribution of pure (HCOOH)nH+ clusters is dependent on experimental conditions. Under certain conditions, a magic number is found at n=5. Metastable dissociation rate constants of (HCOOH)nH+ are measured in the range (0.1-0.8)x10(4) s(-1) for cluster sizes 4<n<9. The rate constants display an odd/even alternating behavior between monomer and dimer loss that can be attributed to the structure of the cluster. When small amounts of water are added to the formic acid, the predominant signals in the mass spectrum are still (HCOOH)nH+ cluster ions. Also observed are the protonated mixed cluster series (HCOOH)n(H2O)mH+ for n=1-8 and m=0-4. A magic number in the cluster series n=5, m=1 is observed. The mechanisms and dynamics of formation of these neutral and ionic clusters are discussed.
使用非常紧凑的毛细管放电软X射线激光，在26.5 eV下采用飞行时间质谱和单光子电离技术，对纯中性甲酸（HCOOH）({n + 1})团簇和混合（HCOOH）（H(2)O）团簇进行了研究。在电离过程中，中性团簇几乎不发生碎片化，因为垂直电离能以上的几乎所有多余能量都被光电子带走，只有一小部分光子能量沉积到（HCOOH）({n + 1}^+)团簇中。垂直电离能减去绝热电离能在团簇中是足够的多余能量，足以克服质子转移能垒并引发反应（HCOOH）({n + 1}^+)→（HCOOH）(_n)H(^+) + HCOO，使得质子化的（HCOOH）(_n)H(^+)系列在所有获得的数据中占主导地位。纯（HCOOH）(_n)H(^+)团簇的分布取决于实验条件。在某些条件下，发现n = 5时存在幻数。对于4 < n < 9的团簇尺寸，测量了（HCOOH）(_n)H(^+)的亚稳解离速率常数，范围为（0.1 - 0.8）×10(^4) s(^{-1})。速率常数在单体和二聚体损失之间呈现奇偶交替行为，这可归因于团簇的结构。当向甲酸中加入少量水时，质谱中的主要信号仍然是（HCOOH）(_n)H(^+)团簇离子。还观察到了n = 1 - 8且m = 0 - 4的质子化混合团簇系列（HCOOH）(_n)（H(_2)O）(_m)H(^+)。在团簇系列n = 5，m = 1中观察到一个幻数。讨论了这些中性和离子团簇的形成机制和动力学。
