19-Spectro_03_web - WHENAMOLECULEABSORBSLIGHTHAVING

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WHEN A MOLECULE ABSORBS LIGHT - HAVING  SUFFICIENT ENERGY TO CAUSE THE ELECTRONIC  TRANSITION - VIBRATIONAL AND ROTATIONAL  TRANSITIONS OCCUR AS WELL
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In a typical absorption experiment , a wavelength continuum is used, say from λ 1 to λ 2 , of which certain wavelengths are absorbed. A diagram of an absorption experiment in any spectral range: An absorption experiment
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The radiation is always released in the decay back to the original level. How does it happen? there will be a resonance absorption of photons followed by the release of resonance radiation if the decay occurs without change in the spin multiplicity the emitted light is called fluorescence light (fast! e.g., 10 -8 s) Phosphorescence can be obtained from some molecules that are excited to a lower-lying state of another multiplicity through collisions and intersystem (singlet-triplet) crossing processes (see further). Since the transition to the ground state of a different multiplicity is normally radiatively forbidden, substances with an extremely long lifetime is obtained (seconds or more).      Photoluminescent (photo-excitation) experiment Harris, p. 406-
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Photoluminescent experiment, cont. Absorption of light (A) promotes a molecule from the ground energy state to electronically, vibrationally (and rotationally) excited states. Photoluminescent experiment ( one photon absorbed ). However, i nter- molecular collisions also occur.
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Internal conversion, intersystem crossing, vibrational deexcitation
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Absorption of a photon ( A ) from a band of radiation from λ 1 to λ 2 excites the molecule from the ground (electronic and vibrational) state S 0 to the excited vibrational level of the excited electronic level S 1 . Let’s assume that the most privileged process of the de- excitation is the vibrational relaxation to the ground vibrational level of the excited electronic state S 1 ( process 3 ). The relaxation occurs through intermolecular collisions and generates heat instead of producing radiation in the UV-visible range of the electromagnetic spectrum (so called radiationless processes ). The lifetime of such radiationless processes is shorter than 10 -8 s.
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19-Spectro_03_web - WHENAMOLECULEABSORBSLIGHTHAVING

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