Helium atom Subject

Walther, Eds., lOP, 1997, pp. 37—45. High Harmonic Generation in Hydrogen and Helium Atoms Subject to One- and Two-Color Laser Pulses. 

As described in Section 5.6.2, argon/helium atoms are excited to a metastable state by beta radiation from a radioactive source. The species formed is then capable of ionizing all compounds with a lower ionization potential. The products formed are then subject to an electric field (500-1100 V) and the change in current measured. 

Quantum-mechanical particles with half-integer spin, such as electrons, protons and neutrons. Particles with integer spin such as photons, helium atoms or hydrogen molecules, are bosons and they are not subject to the exclusion principle. 

In Section 10.3 we established that the one-dimensional helium atom is classically chaotic. In Section 10.4.1 we computed its quantum spectrum. We extracted periodic orbit information from the spectrum in Section 10.4.3. But so far the main question has not been addressed How does chaos manifest itself in the helium atom Although this question is still the subject of ongoing research, some preliminary answers are provided in this section. 

If nitrogen with a large excess of added helium is subjected to electrical discharge and observed very shortly thereafter, one finds excited N atoms. This is illustrated in Figure 15 for a discharge in N2 at 0.16 mm. of Hg plus He at 2.2 mm. of Hg observed within 1 msec. The synthetic N+ curve was fitted to the experimental curve, using the spectroscopically known energies of the N( S),... 

We seldom ask questions about the boundaries on explanation, yet arguably, these boundaries mark an essential feature of the modern scientific worldview the boundaries were quite different in ancient science. There are other boundaries that seem accidental or arbitrary, but perhaps are not. Chemists think it their business to explain why oxygen gas is diatomic, phosphorus gas polyatomic, and helium gas monatomic. The stabilities of these molecules are derivative, not fundamental, facts of chemistry. But the stability of the oxygen, or phosphorus, or helium atoms are not the subjects of chemical explanation at all. What determines our explanatory boundaries by discipline Is it pure historical accident, or a disparity of experimental techniques, or faith in a kind of logical "screening off," faith that, for example, the behavior of molecules would be the same no matter what the explanation of atomic stability ... 

This subject, particularly from the viewpoint emphasized here, has been reviewed at several levels in recent years [5-7], so only a bare outline of the earlier work will be given here. The systems we have studied have been the helium atom and the alkaline earth atoms, sys-... 

While reductionism in science may be frequently debated in the halls of HPS departments, the fact remains that faith in the validity of reductionist approach pervades, indeed is not questioned within, the scientific research community at large. Two interesting elements arise when considering rejection of computational simulations of their subject matter by chemists, then. As discussed previously, the rejection may arise because computational methods cannot simulate something so complex as a real chemical system. This can be seen as a rejection of reductionism we cannot understand the whole system by understanding the basic physical laws underlying it when the system is so complex. On the other hand, in the number of approximations that computational models must make in order to be able to model even a helium atom, they defy absolute reductionism. In this case, their rejection is in favour of reductionism. 

It will be convenient to introduce the subject by the analysis of the electronic structure of an atom. Let us take, as an example, a helium atom in its first excited state (which is a triplet state). In the old... 

In electron ionization, internal energy distribution is very important This means that ions possess very different internal energies among the formed M+- ion population. This phenomenon occurs because (1) all the molecules M do not arrive in the source with the same energy because they clash and collide with the omnipresent helium atoms and residual atmospheric molecules, and (2) all the electrons emitted by the filament do not collide with the molecules with the same kinetic energy (70 eV is the average value). These electrons have different speed characteristics according to the part of the filament that emits them they are also subject to collisions with helium atoms and HjO, Nj, and O2 molecules present in the source. 

Helium is a monatomic gas and, as yet, no stable compounds of helium have been found. The attractive forces between the atoms of helium are unusually weak, as shown by the normal boiling point. To liquefy helium, it must be cooled to — 268.9°C or 4.2°K. No other element or compound has a boiling point as low. Helium has another distinction which reflects these weak forces it is the only substance known which cannot be solidified at any temperature unless it is subjected to pressure. Helium becomes solid at 1.1 °K at a pressure of 26 atmospheres. 

The first step towards an answer is once again the fragility and instability of helium s fusion products. Why then is helium so stable whilst its offspring are so fragile Why are nuclei with masses 5 and 8 times the proton mass so unsure of themselves that they have disappeared from the map of the world The explanation for this can be found in the microarchitecture of the atomic nucleus, a subject that is hardly conducive to literature. 

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