.. _sphx_glr_api_gallery_photochem_fluorescence:

Fluorescence observables
--------------------------

Steady-state fluorescence measurements: the Stokes shift between
absorption and emission, and the Perrin fluorescence-anisotropy equation.


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    <div class="sphx-glr-thumbcontainer" tooltip="Stokes (1852) found that quinine solution absorbs invisible ultraviolet light and re-emits it as blue light -- always at longer wavelength than the exciting light. The excited molecule relaxes vibrationally (and its solvent reorganizes) before emitting, so emission starts from a lower energy than absorption ended at. stokes_shift gives the gap in wavenumbers. The bands below are model Gaussian spectra with maxima near those of quinine sulfate in dilute acid (about 350 nm and 450 nm).">

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  .. image:: /api/gallery/photochem/fluorescence/images/thumb/sphx_glr_plot_01_stokes_shift_thumb.png
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  :doc:`/api/gallery/photochem/fluorescence/plot_01_stokes_shift`

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      <div class="sphx-glr-thumbnail-title">Stokes shift: fluorescence is emitted at longer wavelength than it is absorbed</div>
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    <div class="sphx-glr-thumbcontainer" tooltip="Perrin (1926) related the polarization of fluorescence to how far a molecule rotates during its excited-state lifetime \tau: r=r_0/(1+\tau/\theta), with rotational correlation time \theta=\eta V/k_BT (~chemistrykit.photochem.perrin_anisotropy, rotational_correlation_time). A small dye depolarizes almost completely in water but stays polarized in viscous glycerol, and a &quot;Perrin plot&quot; of 1/r against T/\eta is a straight line whose intercept gives r_0 and whose slope gives the molecular volume.">

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  .. image:: /api/gallery/photochem/fluorescence/images/thumb/sphx_glr_plot_02_perrin_anisotropy_thumb.png
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  :doc:`/api/gallery/photochem/fluorescence/plot_02_perrin_anisotropy`

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      <div class="sphx-glr-thumbnail-title">Perrin equation: fluorescence anisotropy and molecular rotation</div>
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.. toctree::
   :hidden:

   /api/gallery/photochem/fluorescence/plot_01_stokes_shift
   /api/gallery/photochem/fluorescence/plot_02_perrin_anisotropy