Kinetic Energy Distribution ofH(2p)Atoms from Dissociative Excitation ofH2

Abstract

The kinetic energy distribution of H(2p) atoms resulting from electron impact dissociation of H2 has been measured for the first time with uv spectroscopy. A high resolution uv spectrometer was used for the measurement of the H Lyman-n emission line profiles at 20 and 100 eV electron impact energies. Analysis of the deconvolved 100 eV line profile reveals the existence of a narrow line peak and a broad pedestal base. Slow H(2p) atoms with peak energy near 80 meV produce the peak profile, which is nearly independent of impact energy. The wings of H Lyman-u arise from dissociative excitation of a series of doubly excited Q& and Q2 states, which define the core orbitals. The fast atom energy distribution peaks at 4 eV. PACS numbers: 34.80.Ht, 33.50.Dq The kinetic distribution of H(2s) atoms from dissociative excitation of H2 has been the subject of much published research [1 -7], particularly in the late 1960s through 1980. The kinetic energy distribution function of H(2p) atoms from dissociative excitation of Hz has not previously been measured. The cross section for this process is the largest of Hz dissociative excitation processes [8], and a study of this process is paramount for an understanding of dissociation of H2. There are expected to be two distinct maxima in the H(2p) kinetic energy distribution by analogy to results obtained from the distribution of H(2s) and H(nl) atoms, where n = 3, 4, and 5. A comparison of the H(2p) and H(2s) distributions is of fundamental importance in understanding the dynamics of the H2 dissociation process which can occur from singly excited or doubly excited states. The former lead to the "slow" component and the latter lead to the "fast" component. In the separated atom limit, nonadiabatic coupling of the nearly degenerate 2p and 2s states is expected to lead to crossover of the H(2p) and H(2s) fragments [9].For higher principal quantum numbers through n = 5, studies of H(nl) kinetic energy distribution function have been carried out for many years by Ogawa and co-workers [10 -13]. Their interferometric technique involves measurement of the Doppler line profile of Balmer-n, -P, and -y in the visible and near uv portion of the spectrum.The Balmer-u line profile, for example, shows a characteristic narrow central peak (-300 mA FWHM) and a broad wing ( -1.8 A FWHM). However, the Balmer-n line profile is three separate multiplets from H(3s, 3p, 3d) excited states. Balmer-n has not been measured with sufficient resolution to study the slow atom distribution. In this work a spectroscopic technique was employed using a 3-m high resolution vacuum ultraviolet (vuv) spectrometer. Since the Doppler wavelength shift is proportional to the emission line wavelength, 5 to 6 times narrower line profiles can be expected in the vuv, necessitating the use of fast Fourier transform (FFT) techniques to deconvolve line profiles. The H Lyman-n (L ) multiplet structure is simple enough to yield both the slow and fast atom distributions from the deconvolved line profile. Most measurements of H(2s)...

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“…The experimental apparatus has been described in detail in previous papers (Liu et al 1995;Ajello et al 1995aAjello et al , 1995bAjello et al , 2002Ajello et al , 2011a, and we will only describe the setup in a general way here. We use an Acton VM-523-SG 3.0 m UV spectrometer at JPL, allowing for what is probably the highest-resolution single-scattering electron-impact emission instrument in use in the United States.…”

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