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' 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...

Measurement and modeling of a diamond deposition reactor: Hydrogen atom and electron number densities in an Ar∕H2 arc jet discharge

“…42 The metastable 2s 2 S 1/2 level is strongly coupled to the radiative 2p 2 P 1/2,3/2 levels by a combination of Stark mixing and collision with heavy particles ͑Ar, H 2 ͒. 43 Interconversion between the various levels is thus rapid, leading to a statistical distribution of populations.…”

Reaction and reorientation of electronically excited H[sub 2](B)

“…If the interaction potential between the diatom and the atom is expanded in Legendre polynomials, Px V (R,r) =_ vA (R)P_ (R.r) (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) the matrix elements, U°(jlj'I';R) simplify to Uj(jlj'II;R) = 2_/h 2 _v_(R) fx(jlj'l'J) (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) .e where the fx are the Percival-Seaton coefficients, 21 given by…”

“…The apparatus used is shown in Fig. 2 The quenching rate constants may be changed to effective cross sections using: <<_>> = ke/ (N <v>), (2)(3)(4)(5)(6)(7)(8) where N is the number density of quenching gas and <v> is the average center of mass speed, (8kT/_) _, of a collision between the excited hydrogen and the quencher.…”

“…(3-10) becomes (3)(4)(5)(6)(7)(8)(9) and Eq. (3)(4)(5)(6)(7)(8)(9)(10)(11), the experimentally measured fluorescence intensity ratio is given by…”

“…The two waves are related by the S-matrix [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Once the S-matrix is known, a T-matrix is found, T = 1 -S, Fig. 4 -10,4-11), occurs at R _ 4.2 ao, outside of the poorly-fit region of the potential (See Fig.…”