The dielectric function is used to describe the material's response to the electric field [26]. Because of C66H54Br6N6O12 compound has a triclinic crystal structure so that lattice parameters are different from one another therefor dielectric functions along x, y and z direction are different. In the real part of the dielectric function, maximum and minimum values are about x direction which are in energies 3.08 and 17.26 eV, respectively. In the imaginary part of the dielectric function, the peaks represent the optical transitions allowed between occupied strips and empty modes which are in 3.33, 4.50 and 6.70 eV in x direction.
The refractive index of any material expresses the behavior of that material with respect to wave and electromagnetism. The refractive index is attributed to the environment where light refraction occurs at the boundary of the environment [27]. refractive index of C66H54Br6N6O12 compound in terms of wavelength were shown in 3 direction by Fig. 6. Cauchy's coefficients in all three directions are calculated for the refractive index by below formula:
$$n\left(\lambda \right)=A+\frac{B}{{\lambda }^{2}}+\frac{C}{{\lambda }^{4}}$$
7
A, B and C constants are known as Cauchy's coefficients [28].
Table 1
Cauchy's coefficients for refractive index
| λ (nm):550 to 750 | λ (nm):550 to 850 | λ (nm): 600 to 800 |
| x direction | y direction | z direction |
A: | 1.931 | 1.71 | 1.858 |
B: | -67778.341 | 18782.013 | -109702.316 |
C: | 31847634236.641 | 27264415795.807 | 51417106307.436 |
The extinction coefficient for a material is a measure of the amount of electromagnetic radiation absorbed by that material. If the electromagnetic beam passes easily through the material, the material has a low extinction coefficient. The diagram of this parameter is in the part a of Fig. 7.
The extinction coefficient of a substance is similar to the imaginary part of the dielectric function. The peaks of the extinction coefficient in Fig. 7 show the transitions between the band. Table 2 shows the energy of the peaks, which is in good agreement with the results of the imaginary part of the dielectric function.
Table 2
Peaks of energy for extinction coefficient
| | Energy (eV) | |
| x direction | y direction | z direction |
First transition | 3.36 eV | 2.57 eV | 2.78 eV |
Second transition | 4.74 eV | 5.21 eV | 5.21 eV |
Third transition | 6.97 eV | 5.72 eV | 5.91 eV |
Fourth transition | 16.28 eV | 15.85 eV | 14.76 eV |
Electron energy loss spectrum EELS is an optical spectrum [29, 30] used to Getting Plasmon energy. It is a technique that determination the change in kinetic energy of electrons. The electron density is so important to obtain the plasmon excitation.
In part b of Fig. 7, we calculated EEL spectra in x, y and z direction. Energy of main peak that represents plasmon peaks in x, y and z direction are 23.41, 23.36 and 23.55 eV, respectively. In this energy range, the real part of the dielectric function is close to zero.
The ratio of the radiant energy absorbed by an object to the total radiant energy that falls on the object is called the absorption coefficient. The absorption coefficient depends on the characteristics of the body surface. part a of Fig. 8 is showed Absorption coefficient of C66H54Br6N6O12 compound in x, y and z directions. Due to the absorption of photons by matter, the electron moves from the capacitance band to the conduction band. The absorption of photons by the electron is called inter-band absorption. The adsorption process begins after the band gap range; Because the band gap in the restricted area is empty of energy levels. Absorption peaks in the C66H54Br6N6O12 composition are around 17 eV. Maximum optical absorption of the desired composition in x, y and z directions are located in 16.45, 16.87 and 16.87 eV, respectively.
The reflection coefficient determines the amplitude of the wave or the intensity of the reflected wave relative to the radiation wave. part b of Fig. 8 is showed reflection coefficient of C66H54Br6N6O12 compound in x, y and z directions. The maximum of reflectivity is about x direction that is located in 17.26 eV. The behavior of this quantity is relatively similar in the y and z direction.