Interaction of radiation with matter (absorption, emission) Knowledge-Based Study

Study notes

1. What Is Electromagnetic Radiation?

   Electromagnetic radiation consists of oscillating electric and magnetic fields traveling through space. When radiation encounters matter, it can be transmitted, reflected, absorbed, or scattered. The fundamental interactions governing how energy transfers between radiation and atomic systems are absorption and emission.

   At the microscopic level, matter is composed of atoms and molecules possessing discrete energy states. Electromagnetic radiation behaves as discrete energy packets called photons. The energy of a photon is directly proportional to its frequency, expressed as $E = hf$, where $h$ is Planck's constant and $f$ is the frequency of the radiation.

   Mechanism of Absorption

   Absorption occurs when matter takes up the energy of an incident photon, transforming radiant energy into other forms such as internal energy or electronic excitation.

   • Resonance Condition: An atom can absorb a photon only if the photon energy $hf$ precisely matches the energy difference between two quantum states: $\Delta E = E_2 - E_1$.
   • Electronic Transitions: When an electron in a lower energy state absorbs an incoming photon of matching energy, it transitions to a higher energy level (excited state).
   • Macroscopic Effect: In bulk materials, absorbed radiation typically degrades into thermal energy, causing an increase in temperature.

   If an incident photon has an energy that does not correspond to an available transition in the target atom, the photon cannot be absorbed via that resonant mechanism and is transmitted or scattered instead.

   Mechanism of Emission

   Emission is the process by which an excited atomic or molecular system releases energy in the form of electromagnetic radiation.

   • Spontaneous De-excitation: An atom in an excited state $E_2$ naturally transitions to a lower state $E_1$ after a characteristic lifetime, releasing a photon of frequency $f = \frac{E_2 - E_1}{h}$.
   • Conservation of Energy: The emitted photon carries away the exact amount of energy lost by the atom, ensuring energy is strictly conserved.
   • Thermal Radiation: All objects above absolute zero continuously emit radiation across a continuous spectrum due to the thermal motion of charged particles. Higher temperatures lead to higher rates of emission and shifts toward higher frequencies.

   Atomic Spectra and Thermal Equilibrium

   Because atomic energy levels are quantized, the absorption and emission of radiation produce distinct spectral signatures:

   • Absorption Spectrum: When continuous light passes through a cool gas, specific frequencies are absorbed, leaving dark lines against a bright continuous background.
   • Emission Spectrum: When an excited gas de-excites, it emits photons at discrete wavelengths, producing bright lines against a dark background.
   • Kirchhoff's Law of Thermal Radiation: At thermal equilibrium, a body that is an efficient absorber at a given wavelength is also an equally efficient emitter at that same wavelength. A perfect absorber and emitter is known as an idealized blackbody.