H and He

The two-electron systems H and He are the simplest atoms that can autoionize, and therefore they have been the subjects of many experimental 

The photodetachment signal, for example, shows a narrow Feshbach resonance at 10.93 eV—corresponding to an electron energy of 10.18 eV, 

just below the n = 2 level of H. This resonance, which is in agreement with theory, is too narrow for a definite observation by electron scattering experiments. Also resonances in the vicinity of the n = 3 level of H, i.e., at photon energies of 12.8eV, show structures that yield more detailed information about doubly excited H states in this region than electron scattering experiments. 

---

J Chem. Phys., 52, 431 (1970)] is a relatively inexpensive one and can be used for calculations on quite large molecules. It is minimal in the sense of having the smallest number of functions per atom required to describe the occupied atomic orbitals of that atom. This is not exactly true, since one usually considers Is, 2s, and 2p, i.e., five functions, to construct a minimal basis set for Li and Be, for example, even though the 2p orbital is not occupied in these atoms. The 2sp (2s and 2p), 3sp, 4sp, 3d,. .., etc. orbitals are always lumped together as a shell , however. The minimal basis set thus consists of 1 function for H and He, 5 functions for Li to Ne, 9 functions for Na to Ar, 13 functions for Kand Ca, 18 functions for Sc to Kr,. .., etc. Because the minimal basis set is so small, it generally can not lead to quantitatively accurate results. It does, however, contain the essentials of chemical bonding and many useful qualitative results can be obtained. 

Thus for H and He, the basis set consists of one orbital, a Is atomic orbital. For atomsLi to Ne the 2 inner-shell electrons are combined with the nucleus and the basis set consists of 4 orbitals, the 2s, 2p, ... 

SIMS has superb surface sensitivity since most of the secondary ions originate within a few nanometers of the surface and since high detection efficiency enables as little as 10 " of a monolayer to be detected for most elements. Because of its very high surface sensitivity, SIMS can be used to obtain depth profiles with exceptionally high depth resolution (<5 nm). Since the beam of primary ions can be focused to a small spot, SIMS can be used to characterize the surface of a sample with lateral resolution that is on the order of micrometers. Elements with low atomic numbers, such as H and He, can be detected, isotope analysis can be conducted, and images showing the distribution of chemical species across... 

For H and He, the atomic basis set consists of a single Is orbital. For Li through Ne, the inner-shell electrons are treated as part of the nucleus and the basis functions used are atomic 2s, 2p c, 2py and 2p .. For Na through Al, the inner shell is treated as part of the nucleus and we consider only 3s, 3p, 3pj, and 3pj. orbitals. For Si through Cl we have to decide on whether or not to include the atomic 3d-orbitals in addition, and practice varies. Most authors include them. 

Atoms of Period 1 elements (H and He) have one valence atomic orbital each, and atoms of Period 2 and Period 3 elements (Li through Ne, Na through Ar) have four (one s and three p). 

Since the observation made in study of the formation HeH+ indicated that this product was not formed by reaction of He + with H2, it had been assumed that the exothermic heat of reaction of He+ ions with H2 is probably deposited in the product HeH + as internal energy, decomposing the product into H+ and He. This idea was cited by Light (16) in his phase space theory of ion-molecule reactions to account for the failure to observe HeH+ from reactions with He+ ions. The experimental difficulty in the mass spectrometric investigation of this process is that H + formed by electron impact tends to obscure the ion-molecule-produced H+ so that a sensitive quantitative cross-section measurement is difficult. 

The H and He produced in the Big Bang served as "feed stock" from which all heavier elements were later created. Less than 1% of the H produced in the Big Bang has been consumed by subsequent element production and thus heavy elements are rare. Essentially all of the heavier elements now in the Earth were produced after the Big Bang inside stars. Following the Big Bang, the universe expanded to the point where instabilities formed galaxies, mass concentrations from which up to stars could develop. 

A sum-over-states expression for the coefficient A for the expansion of the diagonal components faaaa was derived by Bishop and De Kee [20] and calculations were reported for the atoms H and He. However, the usual approach to calculate dispersion coefficients for many-electron systems by means of ab initio response methods is still to extract these coefficients from a polynomial fit to pointwise calculated frequency-dependent hyperpolarizabiiities. Despite the inefficiency and the numerical difficulties of such an approach [16,21], no ab initio implementation has yet been reported for analytic dispersion coefficients for frequency-dependent second hyperpolarizabiiities which is applicable to many-electron systems. 

Gong, Y., Hu, Z., Chen, Y, Huang, H. and He, X. (2005) Ring-shaped morphology in solution-cast polystyrene-poly(methyl methacrylate) block copolymer thin films. Langmuir 21, 11870-11877. 

The concentrations are normalized to 100%. H and He cannot be detected by XPS. nd not detected with detection limit at 0.1 atom%. 

The dark clouds were responsible for the discovery of ISM, as they absorb the light from stars which lies behind these clouds of interstellar matter. It is difficult to obtain reliable information on the dust particles. They are probably about 0.1 pm in diameter, consisting of a silicate nucleus and an envelope of compounds containing the elements C, O and N, which, with H and He, are the main elements present in interstellar space. There are only two sources of information for more exact characterisation of the dust particles ... 

Again, AES spectra arise from core levels, hence are characteristic for a given element. Surface elemental analysis (except H and He) with very high sensitivity, able to detect < 1% of a monolayer. Usually employed first to check that the surface is free from contamination... 

Big Bang nucleosynthesis produced only H and He atoms with a little Li, from which nuclei the first generation of stars must have formed. Large clouds of H and He when above the Jeans Mass condensed under the influence of gravitational attraction until they reached the temperatures and densities required for a protostar to form, as outlined. Nuclear fusion powers the luminosity of the star and also results in the formation of heavier atomic nuclei. 

As emphasized by Bent, the LST properly places H and He with the s block and realigns file l shells into the actual sequence of configurational orbital filling. The LST therefore avoids the curious STT-based implication that the d-block elements (i = 2) are somehow the transition between the s block (l = 0) and p block (l = 1). 

---