Phosphorescence Spectrum and Transition

Phosphorescence is a type of photoluminescence where a material absorbs photons and re-emits them slowly over a longer period of time, even after the excitation source is removed. The phosphorescence spectrum refers to the range of wavelengths of light that are emitted during this process.

Triplet to Singlet Transition

The correct answer to the given question is option 'B', which states that the phosphorescence spectrum of the excited species is due to triplet to singlet transitions. In order to understand why this is the correct answer, we need to delve into the concept of electronic states and transitions.

Electronic States

In chemistry, molecules can exist in different electronic states depending on the arrangement and energy levels of their electrons. The two primary electronic states relevant to this discussion are the singlet state and the triplet state.

1. Singlet State: In the singlet state, all the electrons in a molecule are paired, and their spins cancel out, resulting in a total spin of zero.

2. Triplet State: In the triplet state, at least one electron is unpaired, resulting in a total spin of one.

Phosphorescence Process

Phosphorescence occurs when a molecule is excited from its ground state to a higher energy state. This excitation can be achieved by various means such as absorption of light or exposure to high energy radiation. Once in the excited state, the molecule can undergo several processes before returning to its ground state.

1. Radiative Decay: In this process, the excited molecule emits a photon and returns to its ground state. This emission occurs at a lower energy (longer wavelength) than the absorbed light, resulting in a phosphorescence spectrum.

2. Non-Radiative Decay: In this process, the excited molecule returns to its ground state without emitting a photon. This can occur through various pathways such as internal conversion and vibrational relaxation.

Triplet to Singlet Transition in Phosphorescence

The phosphorescence spectrum is due to triplet to singlet transitions. After the initial excitation, the molecule often undergoes intersystem crossing, which involves a transition from the singlet state to the triplet state. This process is governed by the spin selection rule, which allows for a change in spin multiplicity during the transition.

Once in the triplet state, the molecule can undergo various processes such as vibrational relaxation and internal conversion, which eventually lead to the radiative decay and emission of a photon. The emitted light corresponds to the difference in energy between the triplet and singlet states, resulting in a phosphorescence spectrum.

In conclusion, the phosphorescence spectrum of the excited species is due to triplet to singlet transitions. This occurs after the molecule is initially excited and undergoes intersystem crossing to the triplet state. The subsequent processes, including vibrational relaxation and internal conversion, lead to radiative decay and the emission of light in the form of phosphorescence.