Interaction Light with Matter
If radiation strikes a sample, it will be either relected/refracted, transmitted
scattered or absorbed.
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Transmission
The speed of light transmitted through a transparent (molecularly dispers) medium
is usually dependent on the interaction of the radiation with the atoms, ions or
molecules in the medium.
Refractive index: ni = c/ν i
Ø No change of frequency
Ø No permanent energy transfer between radiation and species
What happens?
A periodical polarization of the atoms, ions or molecules present in the
medium.
Simplified → a periodical deformation of the electron cloud surrounding
the respective species.
⇒ Only the propagation speed of the radiation is reduced depending on
the properties of the transmission medium.
νi = ν λ i
→
ni = c/νλ i
Since c is constant, the refractive index of the substance is de pendent on the
wavelength of the radiation → DISPERSION
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Dispersion
This dependence of the refractive index on the wavelength and frequency is called
dispersion, which is a complex function.
Ø Normal dispersion:
Considering non-polarized light
transmitting a medium with molecules,
usually the refractive index increases
continuously with increasing frequency.
Ø Anomalous dispersion:
If strong absorption of light due to
electronic, rotational or vibrational
excitation of these molecules and thus
energy transfer occurs, areas of discontinuity appear around these
absorption lines.
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Reflection and Refraction 1
If radiation hits an interface between two media with different refractive index,
reflection occurs at the interface. With increased refractive index difference an
increased amount of the incident radiation will be reflected.
Ir/I0 = (n2-n1)2/(n2 +n1)2
Refraction occurs additionally at an interface
of vacuum (air) and a weakly absorbing
medium with different refractive indices n1 and
n2 as an abrupt change of the propagation
direction of the light propagating into the
second medium.
Snell’s law: sinθ1/sin θ2 = n2/n1
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Reflection and Refraction 2
The law of reflection/refraction cannot describe the change of intensity and
polarization at a reflection/refraction event. Fresnel’s equations describe the
reflection behavior in dependence of the angle of incidence from non-absorbing to
strongly absorbing media. The energy of an electromagentic wave is defined by
ε*E2 with ε representing the dielectric constant. The reflectivity of a surface is
defined by:
R = Er2/Ei 2
Ratio of amplitudes Er/Ei
Air-Glass
Air-Steel
Angle of incidence
Angle of incidence
Interaction of Light with Matter
The reflection behavior is
strongly dependent on the
polarization of the
radiation. The angle ϕp,
where parallel polarized
light is not reflected at all is
called Brewster angle.
Since ni = tan ϕp the
refractive index can be
determined by measuring
the Brewster angle.
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Scattering
Scattering of radiation may occur during a transmission process in a nonhomogeneous medium, containing large particles (e.g. aggregates of molecules or
ions, polymers, colloids, etc.), causing reemission of the light without preferential
propagation direction.
Ø Rayleigh scattering:
Occurs at particles or aggregates of particles with a size significantly smaller than
the wavelength of the interrogating radiation (IRayleigh = 1/λ 4). (Example: Blue sky)
Ø Scattering by large molecules/aggregates:
Large molecules/aggregates such as colloidal systems, with a size in the order of
magnitude of the incident radiation or larger, cause scattering at intensities high
enough to be visible with the human eye. (Example: Tyndall effect)
Ø Raman scattering:
In contrast to the previously described scattering effects, Rama n scattering is
accompanied by a quantized frequency shift of part of the scattered radiation due
to transitions between vibrational states occurring in molecules as a result of the
polarization processes. This results in Stokes/Anti-Stokes Raman lines next to the
Rayleigh peak and is used analytically in Raman spectroscopy.
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Polarization
Polarization describes the vibrational plane of an electromagnetic wave.
Simplified → non-polarized radiation can be described as a bundle of
electromagnetic waves/rays with its vibrational modes distributed homogeneously
among an infinite number of vibrational planes centered along the propagation
axis of the wave/ray.
In an arbitrary plane, each vector is defined
by two perpendicular components. By combination of all these components of all vibrational
planes, two resulting perpendicular planes
remain along the propagation axis of the ray.
Removing one of these planes results in
linearly polarized radiation with an electrical
vector vibrating in a single vibrational plane
between 0 and A.
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Diffraction
Diffraction describes the effect that a collimated beam of electromagnetic
radiation passing through a narrow opening (in comparison to the wavelength of
the radiation) is bent. This principle applies for all types of electromagnetic
radiation and even for acoustic or mechanic waves.
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