2. Interaction between matter and light

Question 1

give three differences between emission and stimulated emission

Answer 1

emission:

• light is emitted from the molecule
• UV/Vis decreases electronic energy
• vibrational excitation occurs in lower state

stimulated emission

• excited molecule emits light under the influence of incident light
• is like a laser

Question 2

what are 2 differences between fluorescene and chemiluniescene?

Answer 2

fluorescene

• emission following absorbtion
• emission of light by a substance that has been heated

chemiluminescene

• the emission of a cold light as a result of a chemical reaction
• reactive intermediates are formed

the image below shows western blotting

Question 3

what are 2 differences between fluroscene and phosphoresene?

Answer 3

phosphoresne

• emission involving a spin change
• no combusion or heat is used
• its similar to fluorescene but it takes longer, therefore the emission continues after the excitation stops

fluorescene

• emission follow absorbtion
• faster than phosphoresene
• emission of light by a substance that has not been heated
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Question 4

what is the difference between internal conversion and intersystem crossing?

Answer 4

internal conversion

• molecule converts from and excited state to a lower one without emitting light

intersytem crossing

• molecule converts from one spin state to another, S –> T or T –> S

Question 5

why does fluorsecene from molecules in solution originate only from the ground vibrational state of the excited electronic state, but the molecule is seen to absorb light into exctied vibrational levels?

Answer 5

this occurs because of internal conversion. this allows the molecule to have a high vibrational energy excitation in a lower state - without emitting light.

the system can increase in elctronic states and then move down energy states to be excited vibrationally -

results in fluorescene emission

Question 6

what is a jablonski diagram?

Answer 6

a diagram that shows the electronic states of a molecule and the transitions between them.

Question 7

what is the difference between elastic and inelastic scattering?

give two examples of each type

Answer 7

elastic scattering:

• energy stays the same as the incident light
• eg rayleigh, mie and tyndall

inelastic scattering

• energy of scattered light is not the same as incident light
• eg brilloun and raman

Question 8

what is conjugation in a molecule?

Answer 8

have alternating single and double bonds, this lowers the energy and stability of the molecule

Question 9

what effect does increasing the amound of conjugation in the molecule have on the maximum of the absorbtion specturm?

Answer 9

causes a shift in the absobtion peak. wavelengths are longer and absorbations are larger

more conjugation = stronger molecule