Overview of spectroscopy transitions History Short introduction to basic principles and definitions of optical spectroscopy 3. Perrin-Jablonski Diagram

Basic principles and definitions of optical spectroscopy Overview 전자기 빛의 에너지

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Energy of Molecules/Atom The energy of molecules is the sum electronic Ee, vibrational Ev, roational Er energies. The actual change in energy due to a spectroscopic transition (absorption or emission of quantum of light) is: hν= ∆ Ee, + ∆ Ev + ∆ Er. 3. Each electronic level is split into a series of vibrational levels, and each vibrational level is split into series of rotational levels.

Perrin-Jablonski Diagram 3rd step 2nd step 1st step 4th step

The molecules are excited from the ground state (바닥 상태) S0 to the second excited state (제 2 여기 상태) S2 by incident light (and other means) in 10-15 second, usually to upper vibrational levels. It will (essentially always) undergo internal conversion to the higher vibrational states of the first excited state S1 state in about 10-12 seconds,

3) which then loses vibrational / rotational energy through collisions to the environment or surrounding (called vibrational relaxation). 4) From first excited S1, the molecule will then undergo one of the many processes. Here only the fluorescence (형광) is depicted.

Fluorescence (형광) Fluorescence usually takes place in 10-10 to 10-8 seconds. This time is so long compared to the transitions of internal conversion and vibrational relaxation. If the molecule fluorescences it can return to any vibrational level of S0 state. The energy of an emitted photon is smaller than for the excitation photons. The difference of energy is called Stokes’ shift.

0 - 0 Transition (0 - 0 전환) This transition corresponds to the highest energy emitted by the excited molecules (or atoms), and lowest energy absorbed by the ground state molecule ( or atom). The electron distribution is often very different in the excited state from that in the ground state. Stokes Shift = (h – h’)

Vibrational Relaxation and Fluorescence

Vibrational Relaxation After the absorption process (in about 10-15 seconds), the molecule enters one of the vibrational state of S1 Following the initial absorption, the energy rapidly decays to the lowest (lower) vibrational state of S1 (this takes about 10-12 seconds). This is called vibrational relaxation.

Fluorescence 3. When the molecule arrives at the lowest vibrational level of the S1 electronic state (metastable), and remains there usually for from 10-10 to 10-8 seconds. 4. It is from this state arrives to ground state So generating fluorescence emission.

Internal Conversion

If the molecule is excited into a higher electronic state ( S2, S3, etc), it almost always retires immediately ( in 10-12 seconds) to the S1 electronic excited state which is called internal conversion. This is where the energy passes from a vibrational state of a higher electronic state S1 to a higher vibrational state of a lower electronic S0, giving rise to fluorescence.

Singlet and Triplet States

1. The diagram shows the spin differences between singlet (short lived) and triplet (long lived) states. The transition from S to T states, or from T to S states are formally forbidden (because of change in spin) and they are usually slow, called phosphorescence. 2. They can be accelerated by spin-orbit coupling ( or mixing of singlet and triplet state wave functions).

3. The transition from S to T or T to S is called intersystem crossing. 4. The triplet state is very long lived, and therefore can react with the surrounding molecules. This is the electronic state that is most reactive, and is responsible for most of the destruction. This is derived from the parallel spins.

Frank-Condon State Frank-Condon State

Upon absorption a quantum ( hν energy), the molecule (or atom) enters an excited electronic state, usually the S1 state (first electronically excited state) But depending on the actual energy absorbed, the molecule will enter S1 electronic state into one of the excited vibronic states, called a Frank-Condon State. This will depend on the vibrational overlap. Frank-Condon state transitions This is based on the fact that light is absorbed in 10-15 seconds, and the nuclei cannot move in this time. However, the electrons find a new spatial distribution (new position, in excited state) in this time. But, this is not the minimum energy configuration of the excited state.