Midterm 1 Quiz W13
Chem 118/218
Name_______________KEY________________
Perm________________________________
(1) The diagram below shows electronic states for a diatomic molecule. Suppose the higher energy state can be
populated by a chemical reaction. Which of the three cases would be best for making a laser?
(a) – the ground state is
dissociative, ensuring 100%
population inversion!
(2) An excited state of a molecule has a lifetime of 5 picoseconds (Both the excited state and the ground state
have a degeneracy of 1).
(a) The linewidth is
[ ] less than or equal to 1 cm-1
[X ] equal to about 1 cm-1
[ ] more than or equal to 1 cm-1
(b) The Einstein A coefficient is
[ X] less than or equal to 2 x 1011
[ ] equal to 2 x 1011
[ ] more than or equal to 2 x 1011
Heisenberg: Δ(hν).τ = h/2π so Δν/c=1/(2πτc)
The fluorescence lifetime is 5 ps or more
(3) Mark whether you would best describe the following properties of light with a particle model, wave model,
or both:
particles waves
diffraction
X
reflection
X
X
interference
X
refraction
X
X
photoelectric effect
X
(4) Mark whether the following correspond to Visible, UV, or IR:
Visible UV
IR
5 eV
X
15 kJ/mol
X
2000 cm-1
X
-19
4 x 10 J
X
16
10 Hz
X
6500 oK
X
6500 K is white hot “black body” radiation – it is about the temperature of the surface of the sun
(5) Circle True or False:
The sum of all primary quantum yields is always less than 1:
T
F
Each quantum of absorbed radiation excites 1 particle:
T
F
At the isosbestic point the absorption cross sections, σ,
for all absorbing components of the reaction are equal
T
F
Population inversion can never occur in a three level system
T
F
g to u transitions are usually forbidden
T
F
In atoms Δl=-1 transitions are generally allowed
T
F
always =1
(6) A sample in a 1 cm cuvette has a transmittance of 10% when its concentration is 10-3 M.
(a) What is the molar extinction coefficient (include units)?
Beer’s law: A=-log(0.1)=1=ε.1.10-3 so ε=1000 L.mol-1.cm-1
(b)
This is measured at a wavelength at the maximum of an absorption line with a 230 cm-1 half
width. The oscillator strength is roughly:
[ ]
10-9
[X]
10-3 f=4.32 x 109 x (1000 x 230)
[ ]
0.1
[ ]
0.5
[ ]
1
What are the units for the oscillator strength? dimensionless (it is a ratio)
(7) List at least 4 effects that can contribute to linewidth and mark whether its contribution to the lineshape will
be Gaussian (G) or Lorentzian (L):
(a) ____Doppler___________________________ G
L
(b)____Heisenberg/natural___________________ G
L
(c)_____pressure__________________________ G
L
(d)______power___________________________ G
L
(8)
List at least five possible processes that can occur following absorption of a photon:
(a) _____fluorescence – phosphorescence - luminescence___
(b)______quenching________________________________
(c)______dissociation________________________________
(d)______isomerization_ internal conversion ______________
(e)______energy transfer (inter or intra molecular)__ - intersystem crossing_
(9) Given the diagram on the left, which products could you
generate by photoexcitation of O2 from the v=0 level in the
electronic ground state (mark all that apply):
[
[
[
[
] O2, electronically excited in low vibrational levels
] O2, electronically excited in high vibrational levels
] O atoms, triplet and singlet in about equal amounts
] O atoms, with about 3 times as many in triplet state than
in singlet
state
The first two excited states shown are spin forbidden. Only the third
state is allowed and a vertical transition excites above the
dissociation limit, thus producing (3P)+O(1D). The first choice is FC
allowed into the bottom of the first two excited states but very weak
because of the spin change.