SELF

44

S.B. Karavashkin and O.N. Karavashkina

"Extreme inconsistency between the high colour temperature of radiation filling the interstellar medium and very low density is almost the principal factor determining the peculiarity of the physical conditions in this medium" [20, p. 36].

"It is a highly important circumstance that in the interstellar space practically all atoms, ions and molecules have to reside at the 'lowest', i.e. 'non-excited' energetic level. The matter is, the processes of atoms excitation usually caused by either absorption of radiation or collisions among the particles occur in the interstellar medium very seldom. If after the electron-ion recombination the produced neutral atom appeared non-excited, it always 'has a time' to transit spontaneously to the very 'deep' state, gaining one or several quanta - no processes of collision with other particles can prevent it. There is one important exclusion from this rule: if the levels of atom either molecule are close to that 'main' and radiation transitions between them are forbidden, then the 'population' of the 'excited' levels can be comparable with that of the main level" [20, p. 30]. The conditions of the high rarefaction of the interstellar gas and low average kinetic temperature will allow to reveal this inverse population of the 'excited' levels even for a hydrogen atom (rather, for an assemblage of the hydrogen atoms). "As long ago as in 1944 the Dutch student-astronomer van der Hulst (now he heads the Leiden university observatory) advanced a brilliant idea whose essence is the following. If two atomic levels are very close to each other (i.e., the difference in their energy is small), the transition of atom from the 'upper' level to that 'lower' will be accompanied by the radiation of quantum whose wavelength is within the radio band. As an important example of such transition, the Dutch astronomer referred to the atom of hydrogen which is in the 'deepest' quantum state. It is known long ago that two very close levels correspond to this state. The difference of energy between two pointed levels is the result of interaction of the natural magnetic moments producing the hydrogen atom of proton and electron. In their turn, the magnetic moments are connected through the spins of corresponding particles. This phenomenon, long ago known in spectrometry, is observed as spectral lines splitting into few components very close to each other (so-called 'super-thin structure'). By van der Hulst estimation, the transition between 'high' and 'low' levels has to be accompanied by the radiation of the line with the wavelength 21 cm" [20, p. 43]. "After it was theoretically predicted and calculated, the line 21 cm was revealed in 1951 in USA, Australia and Netherlands" [20, p. 45].

An important supplement to this point is the following feature of the interstellar medium that we clearly see in the gas nebulas: "all gas nebulas are luminescent only if near them there is a very hot blue star with the temperature no less than 25 000 K. The star radiation ionises the hydrogen and other gases of nebulas and causes their luminescence when fluorescing, the gas absorbs the ultraviolet rays and radiates in red, green and other lines of spectrum. Should the hot star suddenly die down, the nebula would soon stop its luminescence" [21, p. 113]. In combination with van der Hulst's discovery, we can surely state that the phenomenon of fluorescence inherent in the diffusive nebulas is inherent also in the interstellar gas, since one of conditions of luminescence is the presence of metastable levels in atom. "For the classification of the secondary luminescence, there is highly important the presence or absence of intermediate processes between the absorption of the energy exciting the luminescence and radiation of the secondary luminescence (for example, the transitions between the levels, variation of oscillatory energy etc.). Such intermediate processes are typical for the luminescence" [22, p. 31]. Though in the interstellar gas the fluorescence will have its features which will basically distinguish it from the luminescence observed at the Earth conditions and in denser nebulas and clouds. As this issue is important for our study, consider it deeper.

7. Fluorescence of the highly rarefied gas

In all cases when we consider the substance self-luminescence, we speak of resonance either spontaneous luminescence. In case of resonance luminescence, "the radiation has spontaneous pattern and occurs from the same energetic level which is achieved in the exciting light energy absorption. With the increasing density of vapour, the resonance luminescence transforms into the resonance scattering. According to [23], this kind of luminescence in all cases has not to be classified as the luminescence and has to be called the resonance scattering. The spontaneous luminescence includes the transition (radiative or, more often, non-radiative) to the level from which the radiation occurs" [22, p. 31- 32]. The typical diagrams of these kinds of luminescence are shown in Fig. 9 [22, p. 31].

In the described process of spontaneous luminescence we expect it to obey the Vavilov law, "in accordance to which the quantum output of the fluorescence does not depend on the wavelength of the exciting light, up to some limiting wavelength  ₀ . The quantum output of luminescence will be referred as the ratio of the number of radiated quanta to the number of absorbed quanta. Under the energetic output we will mean the ratio of radiated to the absorbed energy. If we excite the luminescence of, for instance, fluoresceine by the ultraviolet light or by visible light with different wavelength, then irrespectively of the wavelength by which we in each specific case excite the fluorescence, not only the spectrum of fluorescence … but also the quantum output at the known interval of the wavelengths will remain invariable. The quantum output independence of the wavelength means at the same time that the energetic output grows in proportion to the wavelength of the exciting light. Actually, the quantum energy is reciprocal to the wavelength ( = h = hc/), so with the growing wavelength of exciting light the energy of absorbed quantum falls - consequently, the energetic output grows.