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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Complex Kinetics of Desorption and Diffusion. Field Reversal Study of K Excited-State Desorption from Graphite Layer Surfaces
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Complex Kinetics of Desorption and Diffusion. Field Reversal Study of K Excited-State Desorption from Graphite Layer Surfaces

机译：解吸和扩散的复杂动力学。石墨层表面K激发态解吸的场反转研究

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摘要

Rapid molecular-beam kinetics data are reported of thermal desorption of K atoms from pyrolytic graphite films over the temperature range 960-1800 K. By using the so-called field reversal method (FR), the kinetics of desorption is studied at time constants down to the microsecond range, where bulk diffusion becomes rate-limiting. The neutral and ionic desorption rates are measured and shown to contain both a primary as well as a secondary rate. These measurements are combined with data on the steady-state and FR peak signals, revealing several states of K on the surface, similar to the previously studied case of Cs on pyrolytic graphite. Two covalently bound states σ_(4p) and σ_(3d) exist, which are 4.30 and 4.40 eV, respectively, below the corresponding atomic configurations 4p (a ~2P °term) and 3d (a ~2D term) outside the surface. An ionic state is also found, which is 2.0 eV below the corresponding desorbed ion K~+ (with the electron at the Fermi level). The 4s and 5s states do not correlated with stable adsorbed states. The apparent neutral rate of desorption is only slowly temperature dependent in the range 960-1550 K, with a primary (fast) rate constant of the order of 300 s~(-1) and a secondary (slow) rate of 1-10 s~(-1). This is due to interconversion processes involving diffusion on the surface. In the range 1550-1760 K, processes with activation energies up to 6.07 eV and preexponential factors up to 10~(21) s~(-1) are observed for both the fast and the slow rates. Such large preexponential factors are indicative of thermal electronic excitation processes, implying a direct switch to a Rydberg state on the surface. The thermal emission and desorption of alkali atoms in Rydberg states is possible by two main mechanisms: by direct emission from the bulk into high Rydberg states over a thermal barrier of 7.4 eV and by excitation from the covalent states that are transferred to Rydberg states in collisions with the surface.
机译：报道了在960-1800 K的温度范围内热解石墨薄膜中K原子热脱附的快速分子束动力学数据。通过使用所谓的场反转方法（FR），研究了在时间常数降低时的解吸动力学。到微秒范围，在该范围内，体扩散成为速率限制。测量了中性和离子解吸速率，并显示它既包含一级速率又包含二级速率。这些测量值与稳态和FR峰值信号的数据结合在一起，揭示了表面上K的几种状态，类似于先前研究的Cs在热解石墨上的情况。存在两个共价键合状态σ_（4p）和σ_（3d），分别位于表面外部相应原子构型4p（a〜2P°项）和3d（a〜2D项）以下，分别为4.30和4.40 eV。还发现了一种离子态，该离子态比相应的解吸离子K +（电子为费米能级）低2.0 eV。 4s和5s状态与稳定的吸附状态不相关。表观中性解吸速率仅在960-1550 K范围内缓慢依赖温度，一次（快速）速率常数约为300 s〜（-1），二次（缓慢）速率为1-10 s。 〜（-1）。这是由于涉及表面扩散的相互转换过程。在1550-1760 K范围内，对于快率和慢速速率，都可以观察到活化能高达6.07 eV且预指数因子高达10〜（21）s〜（-1）的过程。如此大的指数前因子表示热电子激发过程，这意味着表面上直接转换为里德堡态。里德堡态中碱原子的热发射和解吸可能通过两种主要机制实现：通过从主体直接发射到7.4 eV的热障上的高里德堡态，以及通过碰撞中转移到里德堡态的共价态激发与表面。

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