Microscopic Insight into Electron-Induced Dissociation of Aromatic Molecules on Ice

We use scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations to investigate the electron-induced dissociation of halogenated benzene molecules adsorbed on ice. Dissociation of halobenzene is triggered by delocalized excess electrons attaching to the ${\pi }^{*}$ orbitals of the halobenzenes from where they are transferred to ${\sigma }^{*}$ orbitals. The latter orbitals provide a dissociative potential surface. Adsorption on ice sufficiently lowers the energy barrier for the transfer between the orbitals to facilitate dissociation of bromo- and chloro- but not of flourobenzene at cryogenic temperatures. Our results shed light on the influence of environmentally important ice particles on the reactivity of halogenated aromatic molecules.

Figure 1

Model for the photo-induced electron attachment to halobenzene (${\mathrm{C}}_6{\mathrm{H}}_{5}X$, $X=\mathrm{F}$, Cl, Br): (1) injection of photoexcited electrons into the ice conduction band, (2) electron solvation in localized states, (3) propagation of delocalized electrons and attachment to molecular ${\pi }^{*}$ orbitals, (right) generic potential surfaces of the halobenzene, see text.

Figure 2

Modelling of ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{X}$: (a) LDOS for ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{Cl}$ in gas phase and adsorbed on defective ice with orbital shapes as insets, (b) activation energies $E_{\mathrm{A}}$ for different halobenzenes in gas phase and adsorbed on defective ice; open symbols denote ${\mathrm{E}}_A^{{\pi }_a^{*}-{\sigma }^{*}}$; filled symbols denote $E_A^{{\pi }_s^{*}-{\sigma }^{*}}$.

Figure 3

Identification of adsorption and reaction events by STM for chlorobenzene on amorphous ice, 0.1 V, 5 pA (a), (b) before and (c) after illumination at $5.2\times 10^5\mathrm{photons}/\mathrm{molecule}$, 326 nm; solid and dashed arrows point to adsorbed and dissociated molecules, respectively.

Figure 4

Halogen specific reactivity on amorphous ice nanoclusters: (a) low energy cutoff of two-photon photoemission spectra using 327 nm illumination for indicated photon dose; lines are fits of an error function for determination of the work function $\mathrm{\Phi }$. (b) Relative change of work function vs illumination time on logarithmic scale obtained from low energy cutoff (see panel a) for different photon fluxes, photon energy as indicated on top, and halobenzenes; plus signs (+) for ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{F}/\mathrm{Cu}(111)$ without ice; lines are multiexponential fits to determine the rates of change $\lambda$ in $\mathrm{\Delta }\mathrm{\Phi }$. (c) Slow rate of change ${\lambda }_S$ vs flux for the series of halobenzenes; dashed lines guide the eye.