Vibrational Excitation ofH2by Proton Impact

Abstract

The forward-scattered component of the absolute cross sections for proton- and deuteron - impact excitation of the first four vibrational levels of the electronic ground state of ${\mathrm{H}}_{2}$ have been measured. Excitation cross-section measurements are reported for laboratory energies from 100 to 1500 eV; and, for energies below 100 eV, excitation cross sections are reported for forward-scattered protons from $\ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{}$ to $\ensuremath{\theta}=1.9\ifmmode^\circ\else\textdegree\fi{}$. The cross sections were measured from the peak intensities observed in the ion energy-loss spectra generated by passing a high-quality proton beam (energy spread 80 meV, angular divergence \ensuremath{\le}\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{}) through a collision chamber containing the target gas at room temperature. Each excitation cross section (${\ensuremath{\sigma}}_{0{v}^{\ensuremath{'}}}$, where the quantum number ${v}^{\ensuremath{'}}$ refers to the final energy level) is found to reach a maximum value at an energy which decreases with increasing quantum number ${v}^{\ensuremath{'}}:{\ensuremath{\sigma}}_{01}(max)=1.37\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at 200 eV, ${\ensuremath{\sigma}}_{02}(max)=0.342\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at 140 eV, and ${\ensuremath{\sigma}}_{03}(max)=0.078\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ and ${\ensuremath{\sigma}}_{04}(max)=0.021\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ both at 110 eV. These maxima give collision times and distances which are consistent with the adiabatic hypothesis. For energies beyond the maxima, the cross sections decrease very slowly with increasing energy. Furthermore, the cross sections for excitation to the higher levels decrease more rapidly than the cross sections for excitation to the lower levels, an effect predicted by Shin in a three-dimensional calculation involving a semiclassical theory for the excitation of a classical harmonic oscillator. A comparison of our results with vibrational excitation studies of other systems demonstrates the unique features of the ${\mathrm{H}}^{+}$-${\mathrm{H}}_{2}$ system. The large values of these vibrational excitation cross sections and their wide effective kinetic energy span show that vibrational excitation must be an important process in a wide variety of physical phenomena.