Effects of correlation between the molecular diffraction and rotational excitation on the dynamics of H2 scattered from Cu(100)

Abstract

Investigations of gas-surface interactions are essential to obtain a fundamental understanding of dynamical processes occurring on solid surfaces. Recently, some results for Rotationally Inelastic Diffraction (RID) peaks for various metallic and ionic surfaces are reported. By using the time-independent coupled channel method on a density functional theory based on potential energy surface calculated by Wiesenekker et al. for H2/Cu(100)c, the azimuthal quantum number m(sub j)-resolved and m(sub j)-averaged rotationally inelastic (j = 0 to 2) diffraction probabilities of H2 (in the vibrational ground state nu = 0) scattered from Cu(100) as a function of its incident energy were calculated. Although H2 rotates cartwheel-like by the diffraction in the case of RID (j = 0 to 2, m(sub j) = 0 to 0), it rotates helicopter-like in the case of RID (j = 0 to 2, m(sub j) = 0 to 2). For the diffracted H2 rotating cartwheel-like, the m(sub j)-resolved RID probabilities initially increase with the increasing incident energy up to near the incident energy of 360 meV, and gradually decreases with further increase in incident energy. For the diffracted H2 rotating helicopter-like, the RID probabilities decreases with the increasing incident energy. The m(sub j)-averaged RID probabilities of H2 scattered from Cu(1000) initially increase, then take constant values, and then finally decrease with the increasing incident energy. In non-activated systems, H2 incident with a low kinetic energy, but larger than the energy required for the rotational excitation (j = 0 to 2) may dissociates and adsorbs on the surface. In this case, increase of the incident energy promotes dissociative adsorption rather than RID. Therefore, the RID probabilities decrease with the increasing incident energy due to the decrease of scattered H2. On the other hand, in active systems, H2 incident with an energy much lower than the activation barrier for dissociation cannot dissociate on the surface, and the slight increase in the incident energy promotes RID due to the anisotropy in the potential energy surface. Therefore, the RID probabilities initially increase with increasing incident energy.

ガスと固体の表面との相互作用は固体表面上で起こる動力学過程を基本的に理解するためには本質的な事柄である。最近、各種の金属やイオン性結晶表面における回転非弾性回折(RID)ピークに関するいくつかの研究結果が報告された。H2/Cu(100)系に対してWiesenekkerらによって計算されたポテンシャルエネルギー面に基づく密度汎関数理論に関する時間に依存しない結合チャンネル法を採用し、Cu(1000)表面で散乱されたH2の方位量子数m(sub j)分解およびm(sub j)平均の回転的非弾性(j=0→2)回折確率を、入射エネルギーの関数として計算した。RID(j=0→2、m(sub j)=0→0)の場合、H2は表面で回折されて、車輪型に回転するが、RID(j=0→2、m(sub j)=0→2)の場合、H2はヘリコプタ型に回転する。車輪型に回転する回折H2については、方位量子数m(sub j)分解のRID確率は始め入射エネルギーとともに360meVまで増大し、その後徐々に減少する。ヘリコプタ型に回転する回折H2については、RID確率は入射エネルギーとともに減少する。Cu(100)から散乱されたH2分子のm(sub j)平均RID確率は、入射エネルギーの増大とともに始め増大し、一定値を取った後、減少する。不活性な系においては、回転励起に必要なエネルギーより大きなエネルギーを持つが比較的低いエネルギーで入射したH2分子は解離して表面上に吸着する。この場合、入射エネルギーの増加はRIDよりむしろ解離吸着を増進させる。したがって、入射エネルギーが増すと散乱されるH2分子の数が減少するのでRID確率は減少する。一方、活性な系においては、解離の活性化障壁よりずっと小さなエネルギーで入射したH2は表面で活性化できず、入射エネルギーがわずか増すとポテンシャルエネルギー表面の異方性によりRIDが大きくなる。したがって、RID確率は、入射エネルギーの増大とともに始め大きくなる。