Line proﬁle of H Lyman- emission from dissociative excitation of H 2 Joseph M. Ajello, Syed M. Ahmed, and Xianming Liu Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 Received 28 July 1995 A high-resolution ultraviolet spectrometer was employed for a measurement of the H Lyman- HL   emission Doppler line proﬁle at 1025.7 Å from dissociative excitation of H 2 by electron impact. Analysis of the deconvolved line proﬁle reveals the existence of a narrow central peak, less than 30 mÅ full width at half maximum FWHM, and a broad pedestal base about 260 mÅ FWHM. Analysis of the red wing of the line proﬁle is complicated by a group of Werner and Lyman rotational lines 160–220 mÅ from the line center. Analysis of the blue wing of the line proﬁle gives the kinetic-energy distribution. There are two main kinetic- energy components to the H(3 p ) distribution: 1 a slow distribution with a peak value near 0 eV from singly excited states, and 2 a fast distribution with a peak contribution near 7 eV from doubly excited states. Using two different techniques, the absolute cross section of H L  is found to be 3.20.810 -19 cm 2 at 100-eV electron impact energy. The experimental cross-section and line-proﬁle results can be compared to previous studies of H 6563.7 Å for principal quantum number n =3 and L  1215.7 Å for n =2. PACS numbers: 34.80.Gs, 33.50.Dq INTRODUCTION For many years high-resolution studies in the visible re- gion of the spectrum have been carried out on the Balmer series principal quantum number, n =3, 4, and 5 excited states of H produced by dissociative excitation of H 2 upon electron impact. For each principal quantum number, two major sets of kinetic-energy distributions were found, corre- sponding to the ‘‘slow’’ and ‘‘fast’’ distributions with typical kinetic energies of near 0 and 4–10 eV, respectively. The principal architects of these measurements were Ogawa, Ito, and co-workers 1–3. They have carefully shown that the two kinetic-energy distributions reﬂect effects of dissociation from singly excited bound states slow component and from repulsive doubly excited states fast component. Recently, we have begun high-resolution studies of the Lyman series of H from dissociative excitation of H 2 4,5, utilizing a high- resolution 3-m vacuum ultraviolet vuv spectrometer with a resolving power of greater than 50 000 6. We reported a measurement of the H Lyman- HL   emission Doppler proﬁle from dissociative excitation of H 2 by electron impact. Analysis of the deconvolved line proﬁle revealed the exist- ence of a narrow central peak of 404 mÅ full width at half maximum FWHM and a broad pedestal base about 240- mÅ-wide FWHM. Slow H(2 p ) atoms with peak energy near 80 meV produce the peak proﬁle, which is nearly indepen- dent of impact energy. The wings of H L  arise from disso- ciative excitation of a series of doubly excited Q 1 and Q 2 states, which deﬁne the core orbitals. The energy distribution of the fast atoms shows a peak at about 4 eV. In this work we extend the measurements to the 3 p state and compare our results to line-proﬁle studies of H  . The H  line proﬁle shows a characteristic narrow central peak 300-mÅ FWHM from the slow component and a broad wing 1.8-Å FWHM from the fast component in the optical region. Since the Doppler displacement is proportional to wavelength, six-times narrower line proﬁles can be expected in the vacuum ultraviolet spectral region for the Lyman se- ries. It is also a goal of this study to directly measure the ab- solute cross section for H L  at 100 eV for completely mod- eling the H 2 vacuum ultraviolet spectrum for both calibration and astronomy purposes. Once before, in 1984, we have ap- plied published H  absolute cross-section results 7 to a low-resolution H 2 vuv spectrum from our laboratory to de- termine the absolute H L  cross section 8. The most important application of the Lyman series line proﬁles is the opportunity to study and distinguish the emis- sion spectrum of hydrogen from its molecular and atomic forms. The advent of high-resolution spacecraft such as the Hubble space telescope HST, equipped with the Goddard high-resolution spectrograph and the planned astrophysical extreme ultraviolet observatories, have led to the measure- ment of the H L  line proﬁle in both the auroral zones and the dayglow of the planet Jupiter. HL  line-proﬁle wings extending to 1 Å from line center have been measured in the aurora by the HST, and line core widths of greater than 140 mÅ have been observed by International ultraviolet ex- plorer 9,10. The primary cause of the dayglow in the Ly- man series is a combination of resonant scattering of the solar emission line by atomic H and photoelectron dissocia- tive excitation of H 2 , the principal atmospheric constituent. Each process produces a broad line proﬁle from multiple scattering in an optically thick upper atmosphere of atomic H. The main cause of the aurora is primary particle bombard- ment by electrons, protons, and heavier ions, followed by secondary electron excitation of the Lyman series. The large amount of Lyman and Werner band emissions ensures that dissociative excitation of H 2 is an important process. EXPERIMENT The experimental system has been described by Liu et al. 6. In brief, the experimental system consists of a high- resolution 3-m uv spectrometer in tandem with an electron- impact collision chamber. For the H L  line proﬁle, a resolv- ing power of 27 000 is achieved by operating the spectrometer in second order. The H L  line proﬁle has been PHYSICAL REVIEW A APRIL 1996 VOLUME 53, NUMBER 4 53 1050-2947/96/534/23036/$10.00 2303 © 1996 The American Physical Society