This is seen as a narrow dip in the spectrum. We know this because if you freeze a solution close to zero K and illuminate it with a very narrow wavelength laser it is possible to 'burn away' some of the molecules and leave a hole in the spectrum. There are so many different ways this happens that the individual (narrow) spectral line become overlapped and the measured spectrum becomes broad. This occurs because molecules suffer random collisions/interactions with solvent molecules that shifts energy levels up and down just a little. In the condensed phase the spectral lines are inhomogeneously broadened, this means that what you observe is a mixture of many superimposed transitions making a broad structureless feature. Isolated molecules in the vapour phase do have discrete (quantised) electronic, vibrational and rotational levels that can easily be measured, and have been so for at least 50 years. The pictures you show are misleading as they refer only to molecules in the condensed phase. But if I had a molecule that did not exhibit fluorescence, just plain absorption, does the molecule emit light at the same wavelength it absorbed or does the energy turn to heat? I mean, the electron cannot be excited forever.ģ) What is the physical phenomenon behind absorption coefficient? I have been thinking that it has something to do with the lifetime of the excited state – shorter lifetimes increase the probability of a photon to be absorbed, resulting in larger coefficients. But electron states in atoms and molecules should be quantized, so why do we observe absorbance spectra that are smooth and continuous but not spiked and quantized?Ģ) Where does the energy go when the excited electron returns to its normal state? In fluorescence, the electrons return to lower states and excite light at higher wavelength than the absorbed one. The difference between the ground state and the excited one matches the energy of the absorbed light. 1) In text books, the absorption event is usually described by a figure shown below 1: light excites electrons to higher states in atom or molecule.
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