Spectrum of helium

The latter rule is rigidly obeyed in the observed spectrum of helium. From the accurately known energy levels it is known precisely where to look for transitions between singlet and triplet states but none has been found. 

Early investigators of the spectrum of helium believed that there were two kinds of helium, which they called orthohelium and parahelium each kind had its own set of energy levels, and transitions between the two kinds of levels did not occur. Explain these observations. 

In helium, the para-state is one group or system of terms in the spectrum of helium that is due to atoms in which the spin of the two electrons are opposing each other. Another group of spectral terms, the orthohelium terms, is given by those helium atoms whose two electrons have parallel spins. Because of the Pauli Exclusion Principle, a helium atom in its ground state must be in a para-state. 

In order to place these relative intensities on an absolute scale, thus obtaining the constants of proportionality cr0(3p4 2s+1Le) of equ. (5.84), a double ZEKE coincidence spectrum of helium was also measured. The ratio of observed argon-to-helium intensities then depends on the constants a0, the numbers Nph of incident photons, and the target densities nv. One has... 

Since the accuracy of the asymptotic expansion rapidly gets even better with increasing L, there is clearly no need to perform numerical solutions to the Schrodinger equation for L > 7. The entire singly excited spectrum of helium is covered by a combination of high precision variational solutions for small n and L, quantum defect extrapolations for high n, and asymptotic expansions based on the core polarization model for high L. The complete asymptotic expansion for helium up to (r-10) is [36,29]... 

Barocchi F, Zoppi M (1981) Experimental determination of two-body collision-induced light scattering spectrum of helium. In Van Kranendonk J (ed) Intermolecular spectroscopy and dynamical properties of dense systems, Proceedings of the International School Enrico Fermi , Course LXXV, Amsterdam, North-Holland, pp263-274... 

Fig. 7.6. The linear scale X-ray spectrum of helium-like Ar16+ W4, wb and W6, with satellites, and hydrogen-like Ar17+ Ly, is shown in the top frame. In the bottom frame is the log scale observed spectrum and the computed spectrum for Ar16+, Ar17+, and Arle+...

We see from these numbers that the splitting between the two lower terms is only about one-fourteenth of that between either and the upper term. The slightness of this splitting is the reason why for a long time the spectrum of helium was referred to as a doublet system. 

Now, the existence of such a quantum number is not required for satisfactory solutions of the Schrodinger wave equation in any of the cases we have discussed. However, investigation of the spectrum of helium suggests that its two electrons go into the Is orbital with opposite spins. 

For Z=2 we obtain the spectrum of ionised helium (the spark spectrum of helium). In this spectrum the lines which correspond to even quantum numbers (w=2N),... 

This similarity between the spark spectrum of helium and the spectrum of hydrogen was responsible for the fact that the former used to be written in the form... 

The formula (20) must lead to the spectrum of helium. Since p can have the values 1, 0, —1, it must give three systems of terms. Their Rydberg corrections would be (for Z=2) ... 

Before the discovery of argon, some lines in the spectrum of the sun were ascribed to an element not yet known on earth. This element was called helinm (from the Greek helios, meaning sun ). In 1895, Ramsay and the Swedish chemist Per Theodor Cleve (working independently) announced the discovery of helium gas in the mineral cleveite (a uranium ore) by identifying the spectrum of helium. 

Consider the two emission spectra shown in Figure 6.22. The spectrum of helium contains more lines than that of hydrogen. This indicates that there are more possible transitions, corresponding to emission in the visible range, in a helium atom than in a hydrogen atom. This is due to the splitting of energy levels caused by electrostatic interactions between helium s two electrons. 

Figure 5 Spectrum of helium calculated using the ALDA XC kemeL with the exact KS orbitals.

Figure 6 Spectrum of helium calculated using the ALDAx kernel and the exact exchange kerneP with the exact KS orbitals. The importance of nonlocality for the XC kernel can be seen, as the exchange part of ALDA gives a noticeable error compared to the exchange part of the true functional (the AEXX kernel for He).

Fig. 119. CCD spectrum of helium microstrip microwave-induced plasma when HCCb vapor volumes [absolute amounts of 4 pg (20 pL) and 10 pg (50 pL), respectively] are introduced into the plasma gas flow, the lines being shifted for better graphical representation but both belonging to the Cl I 912.1 nm line. (Reproduced with permission from Ref. [548].)...

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