Excitation of vibrational levels of HF up tov=4by electron impact

Abstract

Relative differential cross sections for excitation of vibrational levels of HF up to $v=4$ by slow electron impact have been measured as a function of incident electron energy. Broad oscillatory structures converging to the dissociative attachment threshold, already known in the $v=\stackrel{\ensuremath{\rightarrow}}{0}3$ channel, have now also been observed in the $v=\stackrel{\ensuremath{\rightarrow}}{0}4$ channel at energies above 1.8 eV. They appear at different energies in the two final channels, revealing their boomerang character. They can be rationalized as a consequence of boomerang motion of the nuclei, initiated either by a broad repulsive ${\ensuremath{\sigma}}^{*}$ resonance, or resulting from broad overlapping vibrational Feshbach resonances. This observation complements results in other diatomic hydrides and reveals that boomerang structures are present in all members of the series $\mathrm{H}X$ $(X=\mathrm{H},$ F, Cl, Br), once thought to have no structures of vibrational origin in the cross sections at low energies, because of the extremely short lifetime (or absence) of ${\ensuremath{\sigma}}^{*}$ resonances. The shape of the $v=\stackrel{\ensuremath{\rightarrow}}{0}4$ cross section differs from those of the lower channels also near threshold---it has a weak shoulder at threshold followed by a rounded hump peaking \ensuremath{\sim}80 meV above threshold, whereas the cross sections for the $v=1--3$ channels have (within resolution) vertical onsets and narrow peaks at thresholds.