Knowledge Bank

Dissociative excitation of molecular hydrogen via the process $H2+e\to H(2S)+H+e$ yields metastable H(2S) atoms that have kinetic energies, near 0.3 eV (“slow”) or near 4.5 eV $(“fast”).1$ We have studied the dissociation process using a pulsed electron gun with an energy resolution of $\pm 0.3$ eV and using a metastable atom detector capable of viewing H(2S) with an angular resolution of one degree over a range of 60 to 120 degrees with respect to the electron beam direction. The measurement of angular intensity distribution gives information about the final states that are involved in the dissociation $process,2$ “Slou” H(2S): The electron energy threshold for the production of the least energetic of the slow metastable atoms is 1$4.7\pm 0.3$ eV. The angular distribution of these atoms suggests that they arise from dissociation out of $\Sigma g+$ states. The form of the excitation function indicates that the parent states both singlet E $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma g+$ and triplet a $\Sigma g+$ “Fast” H(2S): The electron energy threshold for production of the least energetic of the fast H(2S) atoms is near 29 eV. The angular distribution leads us to believe that these atoms arise from a $H2$ state; the form of the excitation function indicates that the parent state is a singlet. The change in energy distribution of the fast H(2S) atoms, measured as a function of electron gun voltage, supports our view that the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi u$ state is a previously unreported, repulsive state that has an asymptotic energy of 24.9 eV. This work was supported in part by the U. S. Atomic Energy Commission. M. Lventhal. R. T. Robinson and K. R. Les. Phys. Rev. 158, 49 (1967); R. Clampitt and A. S. Newton, J. Chem. Phys. 50, 1997 (1969); K. Clampitt, Physics Letters 28 A, 581 (1969). G. H. Dunn, Phys. Rev. Letters, 8, 62, (1962). G. H. Dunn and L. J. Keiffer, Phys., Rev. 132, (1963). R. L. Van Brunt and L. J. Keiffer. Phys. Rev. to be published. R. N. Zare and D. R. Herschloch, Proc. IEEE, 51, 173 (1963). R. N. Zare, J. Chem. Phys., 47, 204 (1967).