Raman Scattering and Fluorescence

Introduction The existence of such virtual states also explains why the non-resonance Raman effect Raman scattering and Fluorescence emission does not depend on the wavelength of the are two competing phenomena, which have excitation, since no real states are involved in similar origins. Generally, a laser photon this interaction mechanism. In fact, the Raman bounces off a molecule and looses a certain spectrum generally does not depend on the amount of energy that allows the molecule to laser excitation. vibrate (Stokes process). The scattered photon is therefore less energetic and the associated However, when the energy of the excitation light exhibits a frequency shift. The various photon gets close to the transition energy frequency shifts associated with different between two electronic states, one then deals molecular vibrations give rise to a spectrum, with resonance Raman or resonance that is characteristic of a specific compound. fluorescence (fig.1, case (d)). The basic difference between these two processes is In contrast, fluorescence or luminescence related to the time scales involved, as well as emission follows an absorption process. For a with the nature of the so-called intermediate better understanding, one can refer to the states. In contrast with resonant fluorescent, diagram below. relaxed fluorescence results from the emission of a photon from the lowest vibrational level of an excited electronic state, following a direct absorption of the photon and relaxation of the molecule from its vibrationally excited level of the electronic state back to the lowest vibrational level of the electronic state. A fluorescence process typically requires more than 10-9 s. In contrary, a Raman transition is completed within a picosecond or less.

It clearly appears that, depending on the laser wavelength, resonance effects (Raman or fluorescence), may or may not exist. If the excitation photon does not provide sufficient energy to the molecule, the required transition to generate fluorescence will not take place. However, if fluorescence is generated, it is Figure 1 : Mechanisms of various light- often much more intense than Raman scattering processes. (a) Rayleigh, (b) non- scattering, hiding Raman features. But resonance Raman, (c) pre-resonance Raman, because the Raman spectrum tends to be (d) resonance Raman resonance fluorescence more informative than fluorescence, the and (e) relaxed fluorescence. Raman spectroscopist is continually searching for methods to avoid fluorescence. Virtual states have to be considered to explain Raman scattering. This is related to the fact that the interaction of the photon with the molecule and the re-emission of the scattered photon occur almost simultaneously.

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Fluorescence 01

One method to avoid fluorescence emission is 60000 to select the laser excitation wavelength. For most examples, the choice of a near IR (NIR) 50000

or UV laser wavelength can avoid exciting 40000 fluorescence. In the first case, the laser photon does not have enough energy to excite 30000 Intensity (a.u.) molecular fluorescence. In the second case, 20000 the fluorescence may be excited, but the emission is widely separated in energy from 10000

the Raman signal so that the Raman spectrum 0 can be recorded without the fluorescence 500 1000 1500 2000 2500 3000 Wavenumber (cm-1) interference. Industrial latex A few examples demonstrating the effect of tuning the excitation wavelength on the Sometimes, fluorescence may come from fluorescence emission are shown below. impurities of a polluted sample or from the These highlight the prime importance of a matrix surrounding an inclusion. In these careful laser wavelength selection for each of cases, use of a Raman microprobe on solid these samples. samples, can avoid or minimize background fluorescence by limiting the collection volume 35000 from which the Raman signal is acquired. This can be achieved using the selective 30000 capability of the true confocal configuration of 25000 the Raman microscope. By closing the

20000 confocal hole, one indeed can define a smaller collection volume. 15000

Intensity (a.u.) 10000

5000 Conclusion

500 1000 1500 2000 2500 Although intense fluorescence emission Wavenumber (cm-1) sometimes renders the acquisition of useable Polluted polymer Raman spectra very difficult, several ways to counteract it exist. Raman instruments offering the possibility to select the laser wavelength

15000 have a great in cases where samples show fluorescence in the visible range.

10000

Intensity (a.u.) 5000

500 1000 1500 Wavenumber (cm-1) Pigment

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