5.33 Lecture Notes,Electronic Spectroscopy
Electronic Spectroscopy,Using light absorption to change charge distribution of
electrons about molecule (a lot of energy,often can break bonds)
At equilibrium,molecule is in ground electronic state → lowest energy electronic
state and typically in v=0,
Transitions to higher lying electronic states are accompanied by changes in v,J,
Selection Rules — Even for diatomics,this gets complicated
→ conservation of nuclear/spin/total angular momentum,
(In larger molecules,tough to predict,
Again absorption requires
μ
≠ 0
q
→ change of parity,u? g,change of inversion symmetry to orbitals,
Types of electronic transitions,
Organics,Involving π,σ,n electrons
Saturated compounds

σ→σ,n →σ, deep UV (<250 nm)
Double bonds/unsaturated systems—
less energy to π

π→π,n →π transitions, UV and
visible (200-700 nm)
+
+
C C
C C
+?
+
C O
+
C O
+?
Inorganics,Additionally,transitions between d orbitals split by presence of
ligand field,
d-d transition
Charge transfer transition,Electron moves between ligand and metal,
One must act as donor and other as acceptor
MnO
4
Electronic Spectra of Diatomics
Electronic absorption also carries vibrational and rotational structure,
Typically in a ground electronic state with v=0,you have transitions to excited
electronic state with change in v,
Excitation is accompanied by vibrational excitation,feels restoring force in excited
state,
Electronic transitions,Franck Condon principle (vertical transitions),
Electrons respond much faster than nuclear motion,therefore an excitation proceeds
without a change to the nuclear geometry,
Excited state surfaces,
1) Unbound or repulsive state
(antibonding) → dissociates
into atoms
(A state of H
2
)
2) Bound state—bonding
orbitals—has stable minimum
Excitation to bound state
(usually leads to large nuclear
displacement)
0
(B state of H
2
)
r
e
r
Eelec
H(1s)+H(1s)
H(1s)+H(2p)
32,000 cm
-1
80,000 cm
-1
B
X
A
+
g
1
Σ
+
u
3
Σ
+
u
1
Σ
(For diatomics,ground state = X; excited states = A,B,C,,,)
5.33 Lecture Notes,Electronic Spectroscopy Page 2
Eelec
0
v''=0
v'=0
1
2
1
2
2/
0
ω=
X
e
T
0
D
0
D′
E?
I2
I + I*
atomic
I + I
r
e
r
Excited state is anharmonic—surface must allow for dissociation of atoms,thus
vibrational spacing?v will decrease for higher excitation,
3
2
1
B
v'=0
X
v''=0
ω
From 0-0 transition
and convergence
limit → get D
0
′
Since
T
e
+ D
0
′
= D
0
+?E
atomic
if you know?E
atomic
,you can get D
0
Te Te+D0'
5.33 Lecture Notes,Electronic Spectroscopy Page 3
′
Intensities,Transition probabilities dictated by Franck-Condon principle
Most probably excitation is to classical turning point,
The,selection rule” is the Franck-Condon overlap integral
R R dR→
∫
ψ
v′
()ψ
v ′
()
…so the intensities tell us about r vs,r
e
′
,the displacement.
e
small displacement large displacement
Position of peak
absorption
gives
displacement
ω ω
0-0 0-0
5.33 Lecture Notes,Electronic Spectroscopy Page 4
Relaxation of Electronic States (Bound States)
Typically when we electronically excite a molecule,there is a displacement of
charge and a new equilibrium nuclear separation,
→ leads to vibrational
excitation also,The system
vibrationally relaxes—
nonradiatively,
The energy dissipated is
λ (reorganization energy),
Now,there is a huge amount
of energy to release out to
the ground state
→ most probable way is
fluorescence,
absorption
fluorescence
vibrational relaxation
a
ω=
f
ω=
λ
λ
r
r
e
In gas phase,vibrational relaxation (Τ
1
) ~10
-11
10
10
s 1? 10ps
fluorescence (Τ
1
) ~1-10 ns
In solutions,dephasing,T
2
fast ~10
-14
s 10? 20fs
fluctuations of solvent
vib,relax,~1-10 ps
fluorescence ~1-10 ns
Fluorescence is always red-shifted relative to absorption,
Stokes Shift = 2λ
absorption fluorescence
ω
a
ω
f
ω
5.33 Lecture Notes,Electronic Spectroscopy Page 5