The effect of excitation on the dielectric properties of a substance
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The Effect of Excitation on the Dielectric Properties of a Substance M. I. Ryazanov Moscow Engineering Physics Institute (State University), Moscow, 115409 Russia email: ryazanov[email protected] Received December 22, 2009
Abstract—It is shown that the excitation of a substance can significantly change its permittivity at frequencies resonant with respect to transitions between the excited states of molecules. DOI: 10.1134/S1063776110100031
1. INTRODUCTION The character of electromagnetic wave propagation in a substance is determined by wave scattering wave on molecules of this medium. If the medium is excited, then a fraction of its molecules occur in an excited state rather than in the ground energy state. The cross sections for scattering on excited and non excited molecules, as well as the polarizabilities of these molecules, are generally different. For weak excitation, these differences usually only slightly change the dielectric permittivity of a substance. However, there are special frequency intervals where the dielectric properties of a substance can be substan tially modified even under weak excitation conditions. This takes place at frequencies where the cross sec tions for wave scattering on excited and nonexcited molecules are significantly different. For example, in cases where the electromagnetic field frequency is close to the frequency of transition between two excited states of a molecule, the scatter ing on this excited molecule will possess a resonance character, whereas scattering on a nonexcited mole cule will be nonresonant. Then, the low number of excited molecules may be compensated by the large value of the resonance scattering cross section. For the dielectric permittivity of a substance, the key property of a molecule is its polarizability rather than molecular mass [1, 2]. Therefore, in calculating the dielectric permittivity, it does not matter what accounts for the difference in the polarizabilities of molecules—exci tation or some other factor. This implies that the dielectric properties of an excited substance are analo gous to those of a substance consisting of molecules of different types. A change in the dielectric properties of a substance as a result of its excitation leads to changes in the elec tromagnetic interactions that take place in the excited substance. Therefore, it is of interest to evaluate how excitation affects these processes.
2. POLARIZATION OF AN EXCITED DIELECTRIC MEDIUM The microscopic field inside a substance consists of a primary source field E0(r, t) and secondary fields cre ated by all molecules of the substance. Accordingly, the Fourier transform of the microscopic field that acts on a molecule occurring at point Ra of a homogeneous isotropic singlecomponent substance with polariz ability α(ω) satisfies the following equation: mic ( ω) E i ( R a, ω ) = E 0i ( R a, ω ) + α 2 2π
×
∑ ∫ d p{Q 3
ik ( p )
– δ ik }
(1)
b≠a
mic
× E k ( R b, ω ) exp ( ip ⋅ R ab ), where Rab ≡ Ra – Rb, the sum is taken over coordinates Rb
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