Collisional quenching of metastable hydrogen atoms by atoms and molecules

Abstract

The deexcitation or quenching of the metastable $2S$ state of atomic hydrogen in collision with atoms and molecules has been studied using a beam-attenuation method in conjunction with a time-of-flight technique at velocities between 0.4 × ${10}^6$ and 4 × ${10}^6$ cm/sec (0.08 and 8 eV). In this regime, transfer of the metastable to the $2P$ state of hydrogen, followed by radiative decay to the ground state, is the dominant destruction mechanism. Absolute cross sections are reported for the quenching of $\mathrm{H}\mathrm{}\left(2S\right)\mathrm{}$ atoms in collision with the noble gases (helium-xenon), with molecules that have permanent electric-quadrupole moments (hydrogen and nitrogen), and with molecules that have permanent electric-dipole moments (ammonia, methanol, and acetone). For molecules with dipole moments, the cross sections are on the order of ${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^2$ and vary approximately as $v^{-1\mathrm{}}$. For the noble gases and the quadrupole-moment molecules, the cross sections are on the order of ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ and vary approximately as $v^{-n}$ where $0.3<n<\mathrm{}0.7\mathrm{}$. Measurements of the relative cross section for the production of ultraviolet radiation in collision with nitrogen and argon are reported, and the cross sections for the quenching of $\mathrm{H}\mathrm{}\left(2S\right)\mathrm{}$ and $\mathrm{D}\mathrm{}\left(2S\right)\mathrm{}$ in argon are compared. Data for the noble gases indicate that large-angle elastic scattering is probably not responsible for the discrepancy between theory and experiment. The data for molecular hydrogen suggest that short-range forces are important in collisions with molecules possessing a quadrupole moment.