Question

1. After an electron undergoes a n to π² or π to π² transition, what are three ways we discussed it will “relax” or “lose this excess energy”?

2. And what determines if a transition such as those described in question 1 will take place? (another way to ask this might be, why don’t other bond types also undergo a transition – or, do they?).

Answer 1

The electrons in atoms undergo a n to * or to * transition by absorbing energy in the form of heat or light. The excited electron cannot stay in the excited state for ever. It has to lose energy and come back to the de-excited state after a time called lifetime of the energy level. There are three ways in which the excited electron can com back to the de-excited state. They are:

• Spontaneous emission
• Stimulated emission
• Non- radiative transition

Spontaneous emission:- The electron in the excited state, after a particular time undergoes a transition from the excited state to the ground state by emitting a radiation of appropriate frequency. This process is called spontaneous emission.

E₂-E₁=h where E_{2 and} E₁ are the energies of the excited and the ground state. h is the plank's constant. The direction and phase of the emitted photon is random and the time of emission can only be expressed as a probability.

The number of electrons emitted is given by the equation ∂N∂t= -A21*N(t)

Where A is the Einstien's co-efficient of spontaneous emission.

Stimulated emission:- When the electron in the excited state interact with a photon of appropriate frequency by the time it undergoes spontaneous emission, the electron undergoes a relaxation by emitting a photon that is of the same frequency, phase and direction of the incident photon. This process is called stimulated emission. This is basically the principle behind laser action.

The emission rate is given by the equation ∂N∂t= B12*ρϑ*N(t)

Here, B is the Einstien's co-efficient of stimulated emission. and is the radiation density of the incident photon at frequency corresponding to the energy difference of two energy levels.

Non- radiative Transition:- electrons can undergo relaxation without the production of photons as well. This is called non-radiative transition.Here, the energy of the electron is transferred to the lattice in the form of lattice vibrations (phonons). This mode of relaxation is very common in solids. The lattice vibrations is passed through the lattice and cause the rise of temperature.

B) Other than these two transitions, to * transitions and n to * transitions occur. But these transitions are not in the UV-Vis region of electromagnetic spectrum. The energy difference between these energy levels are very high that these transitions are hard to occur.