Question

In fluorescence spectroscopy, we illuminate a sample with light that excites the system from a ground to an excited state. the system moves to an intermediate state, then light is emitted that we detect. use the drop-down boxes to indicate the relationships among the energies of the excited, intermediate, and ground state and also the relationship of

a. the wavelength of the initial excitation light and the emitted fluorescent light.
b. the energy of the excited state is than the energy for the ground state.
c. the energy of the intermediate state is than the energy of the excited state.
d. the numeric value for the wavelength of the emitted light is than the value for the wavelength of the excited light.

Answer 1

Fluorescence spectroscopy involves an electron being excited to a higher state and then emitting a photon of lower energy (longer wavelength) upon returning to the ground state. Energy is lost to nonradiative decay processes, which is why the emitted light has lower energy and longer wavelength than the excitation light.

Step-by-step explanation:

In fluorescence spectroscopy, the relationship among the energies of the different states involved in the fluorescence process is that the energy of the excited state is higher than that of the intermediate state, and the energy of the intermediate state is higher than that of the ground state. During excitation, a photon is absorbed, moving an electron from the ground state to an excited state. The electron then undergoes nonradiative decay to an intermediate state, where it loses energy, often in the form of heat. Finally, the electron returns to the ground state by emitting a photon with correspondingly lower energy higher energy (Jablonski diagram). This emitted photon has a longer wavelength than the absorbed photon because of the energy lost to nonradiative processes.

The key to molecular spectroscopy lies in the fact that molecules only absorb photons that correspond to the energy difference between the specific states involved in the transition. When measuring fluorescence, two wavelengths are set: the excitation wavelength, which corresponds to the absorbed light, and the emission wavelength, which is scanned through to detect the emitted light with lower energy and longer wavelength (Fig. 10.2).