I am very curious to know what is or are the key difference(s) between absorption and excitation of a molecule (fluorescence). I would like to have more like a general statement about the mentioned topic.
Thanks in advance.
Hello Sandip,
The excitation spectrum and absorption spectrum of a molecule probe the excited states, whereas an emission spectrum probes the ground state. In principle, absorption and excitation spectra will provide the same information.
The typical fluorometer has both an excitation monochromator and an emission monochromator. When recording an excitation spectrum, the emission is measured at fixed wavelength while varying the excitation wavelength. So the relative emission intensity that is plotted against wavelength will only depend on the Frank-Condon factor associated with the absorption. Thus, when correcting for variations in source output intensity and detector response as a function of wavelength of the fluorometer, an excitation spectrum should look very similar to an absorption spectrum.
The intensity versus wavelength of an excitation or absorption spectra reflect the probability of a transition occurring from the ground vibrational level of the ground electronic state to a particular vibrational level of a particular excited state following irradiation with a specific wavelength of light. The fundamental difference between the two spectra is how that intensity is measured.
In an absorption spectrum, what is plotted is –log (P/P0) against wavelength, where P0 and P are the powers of the incident light beam on the sample and the emergent light beam from the sample. The ratio, P/P0, reflect the probability that a transition occurs at a particular wavelength.
In an excitation spectrum, there is first absorption of light (occurring on the femtosecond time scale) resulting with the molecule in a particular vibrational level of a particular excited state. This is followed by vibrational relaxation (occurring on the picosecond time scale) with many molecules ending up in the ground vibrational level of the lowest excited state (known as Kasha’s rule). Finally, fluorescence (occurring on the nanosecond time scale) leaving the molecule in a particular vibrational level of the ground state. The fluorescence intensity will then be proportional to the amount of photons absorbed; that is, the probability that light will be absorbed at a particular wavelength.
I hope that helps.
Best,
John
Mr. Ghosh,
Could you specify the phenomenon that you would like to study, because of you have mixed two spectroscopies (EAs and Fs).
Hello Sandip,
The excitation spectrum and absorption spectrum of a molecule probe the excited states, whereas an emission spectrum probes the ground state. In principle, absorption and excitation spectra will provide the same information.
The typical fluorometer has both an excitation monochromator and an emission monochromator. When recording an excitation spectrum, the emission is measured at fixed wavelength while varying the excitation wavelength. So the relative emission intensity that is plotted against wavelength will only depend on the Frank-Condon factor associated with the absorption. Thus, when correcting for variations in source output intensity and detector response as a function of wavelength of the fluorometer, an excitation spectrum should look very similar to an absorption spectrum.
The intensity versus wavelength of an excitation or absorption spectra reflect the probability of a transition occurring from the ground vibrational level of the ground electronic state to a particular vibrational level of a particular excited state following irradiation with a specific wavelength of light. The fundamental difference between the two spectra is how that intensity is measured.
In an absorption spectrum, what is plotted is –log (P/P0) against wavelength, where P0 and P are the powers of the incident light beam on the sample and the emergent light beam from the sample. The ratio, P/P0, reflect the probability that a transition occurs at a particular wavelength.
In an excitation spectrum, there is first absorption of light (occurring on the femtosecond time scale) resulting with the molecule in a particular vibrational level of a particular excited state. This is followed by vibrational relaxation (occurring on the picosecond time scale) with many molecules ending up in the ground vibrational level of the lowest excited state (known as Kasha’s rule). Finally, fluorescence (occurring on the nanosecond time scale) leaving the molecule in a particular vibrational level of the ground state. The fluorescence intensity will then be proportional to the amount of photons absorbed; that is, the probability that light will be absorbed at a particular wavelength.
I hope that helps.
Best,
John
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