Department of Chemistry, University of Fribourg, Chemin du Musée 9, 1700 Fribourg, Switzerland.
Phys Rev Lett. 2013 Nov 22;111(21):213201. doi: 10.1103/PhysRevLett.111.213201. Epub 2013 Nov 20.
Two mechanisms for dissociative electron attachment in HCOOH, the formation of HCOO(-)+H, were proposed in the literature: (i) via a direct electron attachment into a σ* resonance, augmented by dipole binding of the incident electron [G. A. Gallup et al., Phys. Rev. A 79, 042701 (2009)], and (ii) with the 1.8 eV π* resonance as a doorway state, linked to the products by symmetry lowering-distortion of the temporary anion, primarily the C-H bond, from the planar symmetry [T. N. Rescigno et al., Phys. Rev. Lett. 96, 213201 (2006)]. The later mechanism implies a reduction of the cross section upon deuteration of the hydrogen bonded to the C atom, whereas the former mechanism would leave the cross section unaffected. Our experimental absolute cross sections for the four isotopomers of formic acid show that deuteration on the C atom reduces the cross section value only marginally (by 12%) compared to deuteration on the O atom (reduction by a factor of 16), and thus favor mechanism (i).
有两种机制被提出用于解释 HCOOH 中离域电子俘获形成 HCOO(-)+H：(i) 通过直接将电子俘获进入σ共振态，同时通过入射电子的偶极结合来增强[G. A. Gallup 等人，Phys. Rev. A 79, 042701 (2009)]；(ii) 通过 1.8 eV 的π共振作为门控态，通过临时阴离子（主要是 C-H 键）的对称性降低-畸变与产物相连接，该阴离子由平面对称性降低[T. N. Rescigno 等人，Phys. Rev. Lett. 96, 213201 (2006)]。后一种机制意味着在与 C 原子键合的氢原子上进行氘代会使截面减小，而前一种机制则不会影响截面。我们对甲酸的四种同位素的实验绝对截面表明，与 O 原子上的氘代（减少 16 倍）相比，C 原子上的氘代仅略微降低了截面值（减少 12%），因此有利于机制（i）。
