Quantum coherence in dissociative electron attachment: Isotope effect

Dissociative electron attachment (DEA) is one of the processes that shows a strong coupling between the nuclear and electronic degrees of freedom in a molecule. This coupling results in an efficient transformation of the kinetic energy of attaching free electrons into the chemical energy of the compound molecule. A recent discovery of quantum coherence in this process has opened a different dimension in its description. On the other hand, the mass variation in isotopes of the constituent atoms has a profound effect on DEA. In quantum coherence observed in DEA, the isotope effect depicts itself in terms of change in the phase and the amplitude of the interfering dissociation paths. Here, we report the quantum coherence observed in DEA to HD, an isotopolog of ${\mathrm{H}}_2$. In this isotopolog, both ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ show identical forward-backward asymmetry in the angular distribution. We explain these findings using the interference between two quantum paths, with the permanent dipole moment due to asymmetric mass playing no role in the process.

Figure 1

Velocity slice images of ${\mathrm{H}}^{-}$ from HD at (a) 14 eV, (b) 14.5 eV, and (c) 15 eV electron energy and that for ${\mathrm{D}}^{-}$ from HD at (d) 14 eV, (e) 14.5 eV, and (f) 15 eV electron energy. The electron beam direction is from top to bottom, as shown in the figure.

Figure 2

(a) Velocity slice image of ${\mathrm{H}}^{-}$ from HD at 15 eV of electron energy. The annular region shown in the image is used to determine the forward-backward asymmetry to reduce the effect of finite spread in the electron beam energy. The electron beam direction is from top to bottom as shown (b) forward-backward asymmetry observed in DEA to ${\mathrm{H}}_2$ ($–\square –$), ${\mathrm{D}}_2$ ($–◯–$), and ${\mathrm{H}}^{-}$ ($–△–$) and ${\mathrm{D}}^{-}$ channels ($–▽–$) from HD at various electron energies.

Figure 3

Momentum images and corresponding angular distributions of ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ ions from HD at 14.5 eV electron energy. (a) Momentum image of ${\mathrm{H}}^{-}$; (b) ${\mathrm{H}}^{-}$ angular distribution for the KE range 0.2–0.3 eV; (c) momentum image of ${\mathrm{D}}^{-}$; (d) ${\mathrm{D}}^{-}$ angular distribution for the KE range 0.16–0.2 eV. The arrow indicates the direction of the electron beam.