Questions: Why does molecular fluorescence often occur at a longer wavelength than the exciting radiation? Select one: Internal conversion: Addition of energy from solvent molecules to the species in the lowest vibrational level of an electronically excited state. Stokes shift: Loss of energy due to non-radiative transitions from an excited vibrational energy level to the lowest vibrational level of an electronically excited state, from which a photon is ther emitted. Addition of energy from an excited vibrational level to the highest vibrational level of an electronically excited state. Loss of energy due to radioactive transitions from an excited vibrational energy level to a lower vibrational level of an electronically excited state, with the emission of fluorescence. CHECK

Solution Steps

Molecular fluorescence occurs when a molecule absorbs light and then re-emits it. The absorbed light excites the molecule to a higher electronic state, and as the molecule returns to its ground state, it emits a photon.

The phenomenon where the emitted light has a longer wavelength (lower energy) than the absorbed light is known as the Stokes shift. This shift occurs because some energy is lost through non-radiative processes, such as vibrational relaxation, before the photon is emitted.

• Internal conversion involves energy transfer from solvent molecules, which is not directly related to the wavelength shift.
• Stokes shift correctly describes the loss of energy due to non-radiative transitions, leading to emission at a longer wavelength.
• Addition of energy to the highest vibrational level is incorrect as it would increase energy, not decrease it.
• Loss of energy due to radioactive transitions is incorrect terminology; the correct term is "radiative transitions," and this option does not accurately describe the Stokes shift.

Final Answer