Research on the Principles and Applications of Stokes Shift
Discovery and Definition of Stokes Shift
The phenomenon of Stokes shift is named after the renowned 19th-century Irish physicist George Gabriel Stokes. This phenomenon describes a physical process where, during the interaction between light and matter, the energy of scattered or emitted light is lower than that of incident light, resulting in a spectral feature shifting towards longer wavelengths. As a fundamental concept in fluorescence spectroscopy and Raman spectroscopy, Stokes shift not only has significant theoretical implications but also plays a crucial role in practical spectral analysis.
In 1852, Stokes systematically elaborated on this phenomenon in his paper "On the Refraction Changes of Light" submitted to the Royal Society in London. He clearly stated: "There seems to be a law related to internal dispersion [fluorescence] that appears universally applicable; namely, when the refractive index [inversely proportional to wavelength] changes due to dispersion, it always decreases [shifts to longer wavelengths]." This observation later became known as Stokes' Law, laying down the theoretical foundation for modern fluorescence spectroscopy. Notably, Stokes was not only its discoverer but also coined the term "fluorescence," derived from fluorite's Latin name which vividly describes certain substances emitting visible light under illumination.
Strictly defined, Stokes shift specifically refers to the difference between peak wavelengths of absorption spectra and emission spectra for fluorescent materials. This difference can be expressed in units of wavelength (nm) or wavenumber (cm⁻¹). In quantitative descriptions, special attention must be paid when calculating wavenumber-based Stokes shifts due to effects brought about by spectral conversion. Since spectrometers typically measure using constant bandwidths at specific wavelengths, slight shifts occur when converting from wavelength scales to wavenumber scales; thus accurate calculations should first convert entire spectra into wavenumber scale before determining peak positions rather than simply converting values post measurement.
Mechanism of Stokes Shift in Fluorescence Spectroscopy
The phenomenon of Stokes shift within fluorescence spectroscopy arises from complex energy transfer processes between molecular excited states and ground states. When molecules absorb photons becoming excited into electronic excited states they are usually elevated into higher vibrational levels within those potential surfaces associated with excitation state energies. Subsequently through rapid vibrational relaxation processes these excited-state molecules quickly lose excess vibrational energy relaxing back down into their vibrational ground state for that excitation level before emitting fluorescence as they return back down into their electronic ground state; since some energy dissipates as thermal energy during this relaxation process it results inevitably causing emitted photon energies being lower than absorbed ones leading ultimately towards redshift phenomena observed spectrally.
This process can intuitively be understood via Perrin-Jablonski diagrams illustrating energetic levels involved within typical molecular systems containing discrete sets corresponding both ground (S₀) & first electronically-excited (S₁) stages each comprising series distinct vibration levels respectively included therein alongside Franck-Condon principles indicating electron transitions occurring far faster compared atomic nuclei movements hence transition events perceived vertically without nuclear configuration changing simultaneously taking place concurrently across different states involved throughout such interactions seen here whereby strongest absorptions often corresponded directly onto S₀,v=0 transitioning upwards reaching some higher S₁,v>0 modes present nearby effectively explaining why absorptions generally take place amongst those elevated vibrations predominantly found located around various energetically accessible regions available too!
Stoke’s displacement magnitude gets influenced heavily based upon numerous factors including most notably structural characteristics exhibited among respective fluorescent molecules themselves along with solvent environments surrounding them overall speaking broadly though stronger rigidity tends correlate inversely against increased displacements noted whilst greater polarities yield larger resultant shifts achieved thereby enhancing stability imparted onto excitations helping create more pronounced differences observable existing right between bases versus excitations arising therefrom! Common examples include Rhodamine 6G showing approximately ~20 nm worth detected stoke displacements recorded inside ethanol while others possessing extensive intramolecular charge transfers like DCM could generate beyond exceeding 100 nm substantial ranges observed especially under acidic conditions experienced therein consequently exhibiting unique behaviors encountered altogether!
The Phenomenon Of Stoke Displacement Within Raman Spectroscopy Contexts...
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