20. Luminescence.

Question 1

Define luminescence and classify

Answer 1

Luminescence:

Spontaneous emission of light, when the e^-s return back to ground state from excited state (de-excitation)

• also referred to as “cold light”

BUT: also happens when radiation emitted by matter in excess of thermal radiation

We distinguish between:

• fluorescence
• phosphorescence

Question 2

Compare fluorescence and phosphorescence refering to the diagram below. What is the name of the diagram and what does it show?

Answer 2

Jablonski diagram:

illustrates the electronic states of a molecule and the transitions between them (with arrows)

Question 3

What is the difference between singlet and triplet state?

Answer 3

singlet state:

opposite spin-state e^-

triplet state:

identical spin-state e^-

Question 4

What does Kasha’s rule state? What is the connection to energy transitions? Explain it referring to the Jablonski diagram

Answer 4

Kasha’s rule:

photon emission occurs only from the lowest-E excited state (either S₁ orT₁)

Further E-transitions, BUT non-radiative:

• vibrational relaxation: de-excitation process within the same electron E state
• internal conversion: between electron E states
• intersystem crossing: between singlet and triplet states accompanied by spin change

Question 5

What is Stoke’s shift?

Answer 5

Stoke’s shift:

energy difference when the emitted photon has less E than the absorbed photon

→ shift between excitation and emission spectrum of fluorescence