Experimental Analysis Techniques - The Interaction between Electromagnetic Radiation and Matter
1. Absorption of light
When atoms, molecules, or ions absorb the energy of photons and the difference between their base energy and excited state energy satisfies Δ E=mv, they will transition from the ground state to the excited state, and this process is called absorption. The study of absorption spectra can determine the composition, content, and structure of the sample. The analytical method established based on absorption spectroscopy is called absorption spectroscopy.
2. Emission of light
When a substance absorbs energy and transitions from the ground state to the excited state, the excited state is unstable and will transition back to the ground state after approximately 10-8 seconds. At this point, if energy is released in the form of light, this process is called emission.
3. Scattering of light
When light passes through a medium, the phenomenon of emission occurs. When the size of the medium particles (such as in emulsions, suspensions, colloidal solutions) is similar to the wavelength of light, the intensity of the emitted light increases, which can also be seen with the naked eye as the Tyndall effect. The intensity of scattered light is inversely proportional to the square of the incident light length, and can be used for studying the size and morphology of polymer molecules and colloidal particles. When the molecules of the medium are smaller than the wavelength of light, Rayleigh M9 emission occurs. This scattering is caused by elastic collisions between photons and molecular molecules. There is no energy exchange during collision, only the direction of photon motion is changed, so the frequency of scattered light remains unchanged, and the intensity of scattered light is inversely proportional to the fourth power of the incident light wavelength. When photons collide non elastically with medium molecules, they not only change their direction of motion but also exchange energy, resulting in a change in the frequency of scattered light. This scattering phenomenon is named Raman scattering.
4. Reflection and Refraction
When light is irradiated from medium (1) to the interface of another medium (2), some of the light changes direction at the interface and returns to the interface (1), which is called light reflection. Another part of the light changes direction and enters medium (2) at an angle of r (refraction angle), which is called light refraction.
5. Interference
Under certain conditions, light waves will interact with each other. When they are superimposed, they will produce a composite wave whose intensity depends on the phase of each wave. When the phase difference between the two waves is 180 °, the maximum destructive interference occurs. When the two waves are in phase, the maximum constructive interference occurs. Through the interference phenomenon, bright and dark stripes can be obtained. If two waves strengthen each other, bright stripes will appear. If they cancel each other out, dark stripes will appear
6. Diffraction
The phenomenon of light waves deviating from their straight line when passing through obstacles or narrow slits is called diffraction phenomenon. It is the result of interference.
