In EXCITATION spectroscopy the Mex scans in a wavelength range equal to that used in absorption spectroscopy. The final spectrum (emission) results in emission-intensity vs wavelength. it lets pass different monochromatic radiation sequentially, in a selected wavelenght range. In EMISSION spectroscopy the Mex is fixed at the wavelenght of excitation and the Mem scans, i.e. Finally a detector, usually a photomultiplier. The fundamental optical components of a Fluorometer are: a lamp that provides the light excitation, a monochromator of excitation (Mex) to select a monochromatic radiation (the radiation has always a certain bandwidth so it is never monochromatic) used to excite the sample, a monochromator of emission (Mem) that selects the radiadion coming out from the sample, this can be luminescence (fluorescence or phosphorescence), raman or scattered light (with a 90 degree geometry used in the experiment the scattered light is greatly reduced but not completely eliminated). In order to appreciate the difference between UV-VIS emission and excitation spectroscopy it is crucial to have a closer look at the instrument that is used to carry out both techniques, i.e. As a result, the photon finally emitted is at a lower energy than the photon that had been absorbed. Because it relaxed its vibrational state first, energy was lost not in the form of light (thermodynamically). After this, it relaxes to S=0, emitting a photon in the process for detection. That means the electron will first relax from S=1, v=2 to S=1, v=0. Vibrational relaxations will occur before electronic relaxations. This is easier to picture if you look up Stoke's Shift, or specifically the diagram on Wikipedia's page for Fluorescence.įor example, in emission spectroscopy, an electron absorbs light, being excited electronically from S=0, v=1, to S=1, v=2. For example, two electrons can be in the 0 electronic state (termed ground state), but one electron can be in the v=2 vibrational state, while the other electron in v=4. Within each electronic state is a set of vibrational states, v=1,2,3. A molecule has electronic states, S=0, 1, 2, 3.etc. Stoke's Shift requires basic knowledge of the difference between electronic and vibrational states. The reason the emission wavelength detected is different than the frequency used to irradiate it is due to Stoke's Shifts. The advantage to fluorescence is you can often selectively measure a moiety (part of the molecule) with superior sensitivity, whereas in absorption impurities or stray light are more prone to interfere with the signal and also leads to higher concentrations usually being necessary. The detector in fluorescence is placed to the side, avoiding the column of light and picking up the light from the sample instead. Think of the sample like a light bulb that shines light in all directions. In emission spectroscopy, you want to avoid the column of light.

In absorption spectroscopy, that column is absorbed by the sample and continues to the other side, where the detector can measure it. This allows it to ignore the light being shined onto the molecule, and instead measure light that is emitted from the sample. The major difference here is the detector being located to the side of the sample. In both processes, the sample absorbs light. The emitted light is measured for a range of frequencies.the detector is to the side of the sample (90 degrees). The amount of light that shines from the molecule is measured, ie.The sample is irradiated with this frequency.A light with a specific frequency is selected.In X-ray, you will be around 10 nm I think. The machine records this and a peak appears in the spectrum for that frequency.įrequency is usually measured in wavelength, and a wide range of wavelengths are applicable. When the sample absorbs a specific frequency, less light of that frequency will shine through to the detector. Step 1 is repeated with a different frequency.the detector is behind the sample (180 degrees). The amount of light that shines through the molecule is measured, ie.This frequency is shined (radiated) onto a molecule (sample).

Light with a specific frequency is selected.

Emission vs absorption spectra