In absorption spectroscopy, the absorbed wavelengths of light are not immediately emitted because the absorption process involves the energy of the absorbed photons being transferred to the absorbing medium, such as a molecule or an atom. This energy transfer can result in various outcomes, including electronic excitations, vibrational or rotational excitations, or other forms of energy redistribution within the absorbing species.
When a photon is absorbed by an atom or molecule, it promotes an electron from a lower energy level to a higher energy level, corresponding to an electronic excitation. This excitation is typically very fast, occurring on the order of femtoseconds (10^-15 seconds). However, the subsequent relaxation of the excited state back to the ground state can take longer, ranging from picoseconds (10^-12 seconds) to milliseconds (10^-3 seconds) or even longer, depending on the specific system.
The relaxation process involves the dissipation of the excess energy gained during absorption. This energy can be released through various pathways, such as radiationless transitions, where the energy is converted into heat, or through the emission of photons of longer wavelengths. The emitted photons may have different energy (wavelength) than the originally absorbed photons.
The delay between absorption and emission is due to the time required for the relaxation processes to occur. The specific mechanisms involved depend on the nature of the absorbing species and the surrounding environment. For example, in fluorescence, a common phenomenon in absorption spectroscopy, the excited state relaxes through the emission of a photon, but this emission occurs after a brief delay.
In summary, the absorbed wavelengths are not immediately emitted in absorption spectroscopy because the relaxation processes within the absorbing species take time to occur, involving energy redistribution and transition back to the ground state.