Introduction to spectroscopy

The absorption and emission of electromagnetic radiation of specific energy (wavelength) is a characteristic feature of many molecules, involving the movement of electrons between different energy states, in accordance with the laws of quantum mechanics. Electrons in atoms or molecules are distributed at various energy levels, but are mainly at the lowest energy level, usually termed the ground state. When exposed to energy (e.g. from electromagnetic radiation), electrons may be excited to higher energy levels (excited states), with the associated absorption of energy at specific wavelengths giving rise to an absorption spectrum. One quantum of energy is absorbed for a single electron transition from the ground state to an excited state. On the other hand, when an electron returns to its ground state, one quantum of energy is released; this may be dissipated to the surrounding molecules (as heat) or may give rise to an emission spectrum. The energy change (ΔE) for an electron moving between two energy states, E₁ and E₂, is given by the equation:

⇒ Equation [26.3]

ΔE = E1 − E2 = hν

where h is the Planck constant and ν is the frequency of the electromagnetic radiation (expressed in Hz or s⁻¹). By substituting for frequency in [26.3J it IS possible to rearrange this equation to give the expression:

⇒ Equation [26.4]	ΔE =	hc
		λ