2
Structural determination of just about everything in the gas or condensed phases.
Photonics, laser technology, etc.
Spectroscopy
Interaction between light and matter
We’re still interested in the transition dipole integral, M
x21
(aka X
21
, R
x21
, plus y and z
analogs) which has veto power, and figures prominently in dP
21
/dt.
Prob = a
2
(t)
2
= 
2
o

x

1
o

2
(E
x
o
)
2
2
21
)
(
)
(
)
(
2
21
21
h
E
e
e
t
h
E
i
t
h
E
i
Prob =
2

2
o

x

1
o

2
(E
x
o
)
2
t
For last midterm:
Covered rotationally resolved vibrational transitions of diatomics, molecular symmetry,
and vibrational transitions in polyatomics (direct absorption versus Raman).
Use of
character tables facilitates determining which transitions are allowed via which
technique.
1.
Rotational structure in vibrational transitions of polyatomics.
See lecture 25
notes for refresher on rotational energy of polyatomics, particularly for symmetric top
molecules.
Recall
vib
(Q
1
, Q
2
, … Q
i
,…, Q
3N6
) =
(Q
1
)
(Q
2
)
…
(Q
i
)
…
(Q
3N6
)
Which modes are active in IR spectroscopy?
Those for which
(Q
i
) =
(
x,y or z
)
IR spectroscopy selection rules, remember that
=
1
(within the harmonic approximation, and only one mode changes at a time.
However, by
going beyond the simple harmonic approximation,
=
2, 3 … can be observed
(though with lower intensity), and combination transitions can also be observe (
j
= +1
along with
k
= +1, for example)
For symmetric rotor,
If
(Q
i
) =
(
z
), a vibrational transition is considered a parallel transition
i
=
1
J = 0,
1 AND
K = 0 when K
0