Electromagnetic radiation lifetime

The basic parameters of this problem are the lifetime of the neutron (887 seconds) and the number of neutrino species (three), both given by modern microphysics. At the time which interests us here, i.e. r = 1 s, the energy density of electromagnetic radiation was greater than that of matter. This is therefore referred to as the radiation era. 

The principle of the experiment is to see an increase in the intensity due to laser-stimulated transitions when negative muons are stopped in the helium gas present in a multipass optical cavity, where a high-density electromagnetic radiation of the correct wavelength is stored it is crucial here that the lifetimes of the levels in which one is interested are around 10 12 s (we recall that the negative muon cascade time is about 1 ns or less). 

Phosphorescence is the electromagnetic radiation that can accompany the transition of a molecule from an excited triplet state to a ground state singlet. Because the molecule goes from a triplet to a singlet state, its net spin must change during the emission of this radiation. The lifetimes for... 

Fluorescence is a photoluminescence process in which atoms or molecules are excited by absorption of electromagnetic radiation, as shown in Figure 24-22a. The excited species then relax back to the ground state, giving up their excess energy as photons. As we noted in Section 24D, the lifetime of an excited species is brief because there are several mechanisms by which an excited atom or molecule can... 

Excited states are prone to decay because of their high energy and therefore mainly have short lifetimes. The decay can occur by collisions with surrounding particles (molecules, atoms, electrons or ions) or by emission of electromagnetic radiation. In the latter case the wavelength is given by Planck s law. When the levels q and p are involved, the number of spontaneous transitions per unit of time is given by ... 

Let us touch now the opposite case of a rather narrow translational band. By a physical reasoning, the absorption bandwidth Av cannot become extremely narrow, whatever the lifetime t. We relate with v the period Tmd of electromagnetic radiation Tmd = (cv), where c is the speed of light in vacuum. We have ATrad Av(cv2)-1, where min(Arrad) is meant to be positive and v is an average of v value in the frequency interval under investigation. 

Positrons have a lifetime of about 10 s, and are annihilated by combination with electrons to produce y-radiation, which is high-energy electromagnetic radiation (see Section 14.1.1). 

In order for a photochemical reaction to occur the radiation must be absorbed, and with the advent of the quantum theory it became possible to understand the relationship between the amount of radiation absorbed and the extent of the chemical change that occurs. It was first realized by A. Einstein (1879-1955) that electromagnetic radiation can be regarded as a beam of particles, which G. N. Lewis (1875-1940) later called photons each of these particles has an energy equal to /iv, where v is the frequency of the radiation and h is the Planck constant. In 1911 J. Stark (1874-1957) and independently in 1912 Einstein proposed that one photon of radiation is absorbed by one molecule. This relationship, usually referred to as Einstein s Law of Photochemical Equivalence, applies satisfactorily to electromagnetic radiation of ordinary intensities but fails for lasers of very high intensity. The lifetime of a moleeule that has absorbed a photon is usually less than about 10 sec, and with ordinary radiation it is unlikely for a molecule that has absorbed one photon to absorb another before it has become deactivated. In these circumstances there is therefore a one-to-one relationship between the number of photons absorbed and the number of excited molecules produced. Because of the high intensity of lasers, however, a molecule sometimes absorbs two or more photons, and one then speaks of multiphoton excitation. 

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