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' and 4 X 10 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 H(2S) 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 ' cm and vary approximately as v ', For the noble gases and the quadrupole-moment molecules, the cross sections are on the order of 10 " cm' and vary approximately as v " where 0.3 & n & 0.7. 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 H(2S) and D(2S) 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 rnornent. I. INIODUCTIONThe long-lived 2S metastable state of atomic hydrogen is easily deexcited or quenched by collisions with other atoms and molecules. These collisions are of particular interest since the cross section for this quenching process can be calculated from first principles and compared to an absolute measurement of the inelastic scattering. It can be shown using the adiabatic criterion that at low kinetic energies (&50 eV) the physics of the collision is dominated by the presence of the nearby 2P states of hydrogen which are nearly degenerate with the 2S metastable state. A perturbing electric field during the collision can cause an admixture of the 2S and 2P states by the Stark effect. After the collision, therefore, there is a finite probability that the hydrogen atom has made a transition to the 2P state, in which event the atom decays to the 1S ground state emitting a Lyman-n photon. The metastable has thus been quenched (or inelastically scattered) in the reaction H(2S) + X-H(2P) + X -H(1S) + h v+ X.On the other hand, if the atom remains in the 2S state after the collision, the metastable will have received a net impulse from the gradient of the electric field during the collision and thus be deflected or elastically scattered. Elastic and inelastic scattering are therefore complementary processes; the relationship between the two depends on the relative velocity in the collision and the nature of the perturbing electric field.In the low-energy regime where the 2P states dominate the inelastic scattering, col...

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ray yields have been measured for excitation by 36and 48-MeV fluorine ions in +6 to +9 charge states. The significant decrease in the dependence of the target x-ray yields on the projectile charge state, which is observed as the parameter Z, /Z, is decreased, is in agreement with theoretical expectations.

The design and performance of a passive plasma isolator for suppressing retropulses in high-power CO(2) laser-fusion systems are described. The device uses a gas-filled spatial filter designed to produce a plasma at the focal plane iris. General design criteria for these isolators are discussed, and the performance of a specific isolator is evaluated. For the unit tested, retropulse attenuations of 33 dB for focal plane intensities of 1.5 TW/cm(2) have been demonstrated.