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Molecular Properties and Spectroscopy
Almost all of the information we have about the Universe comes from the
study of electromagnetic radiation (light) Spectroscopy
Isaac Newton
Newton used the term Spectrum in context
with his experiments on the color of light:
The Beginning: Newton (1675)
Fraunhofer lines (1814)
Joseph von Fraunhofer
Gustav Robert Kirchhoff
18.03.1824-17.10.1887
Kirchhoff and Bunsen
Robert Wilhelm Bunsen
31.03.1811-16.08.1899
Spectroscopy pioneers in Heidelberg
The Birth of Spectral Analysis: Kirchoff (1860)sodium
Electromagnetic Spectrum
400 nm 800 nm
photon energy
1021 1018 1016 1014 1012 1010 108 106 104
Frequency (Hz)
Unit Conversion
Energy E of a photon: E = h (in eV or J)
Wave length: = c/ = hc/E (in nm)
h = 6.626010-34 Jsh = 4.1357x10-15 eVs
J eV cm-1 K
1 J 1 6.24146 x 1018 5.03404x1022 7.24290 x 1022
1 eV 1.60219 x 10-19 1 8.06548x103 1.16045 x 104
1 cm-1 1.98648 x 10-23 1.23985 x 10-4 1 1.43879
1 K 1.38066 x 10-23 8.61735 x 10-5 6.95030 x 10-1 1
Example:
Level spacing: 3.1 eV = 4.9667 x 10-19 J
Photon frequency: 7.4957 x 1014 s-1
Wavelength: = 4 x 10-7 m = 400 nm
Wavenumbers: = 1/ = 25000 cm-1
Wave numbers: = 1/ (cm-1)
c= 2.99792 x 108 m/s
Basics: Atomic Hydrogen
with
2
kin. energy pot. energy
Schrdinger Equation
1
2
0
Hamiltonian
Eigenvalue equation:
Discrete solutions with quantum numbers:
n : principle (energy) quantum number 1, 2, 3 .
: angular momentum 0, 1, 2, , (n-1)m : magnetic quantum number - , 1 , . ,
Atomic Hydrogen: Transitions
O eV
-13.6 eV n=1
n=2
n=3n=4 n=5
n=6n
= - R
Energy levels:
Discrete transitions:
=
L-
12
1.7
nm
H
6
56
.3 n
m
n=3 n=2 @ 656.3 nm
responsible for red glow in
H II regions containing
ionized H atoms
Lym
an
Ba
lme
r
Pa
sch
en
-3.4 eV
-1.9 eV
H (656nm) emission (red glow) in HII regions
Horsehead nebula
Atoms with more than one Electron
L2S+1
L= l1+l2
J
J=L+S
P3
L= 0+1=1
1
J=1+1=1
Notation
Atomic Hydrogen Observations
Emission, V630 Saggitari,Schmidtobreick A&A 432, 199 (2005)
Danish telescope, La Cilla (Chile)
Far Ultraviolet
Spectroscopic Explorer (FUSE)
operated 1999-2007
G 231-40 (white dwarf) Hebrad A&A 394, 647 (2002)
Lym
an
Lym
an
Balmer Series
Hydrogen 21cm line
Very Large Array (VLA)Arecibo Effelsberg
Doppler shift due to relative motion: v/c = /
Ly absorption, observed
with Hubble Space Telescope
at z =2.5
Burles and Tytler, ApJ 507,
732 (1998)
Example:
Cosmological Redshift
The redshift z is defined as
z = / =obs- emitemit
The observed wavelength is shifted by: obs = emit + z emit
Hydrogen
Ly shifted
to 426 nm
Molecular Properties
Diatomics (2 nuclei) Polyatomics (>2 nuclei)
Homonuclear (e.g. H2)
Heteronuclear (e.g. OH, CO)
Molecular binding energies are relatively small (1-5 eV),
smaller than ionization energies (>10 eV)
Molecules are easily destroyed and are found in cooler,
less ionized environments.
Present in objects with T < 8000 K Present in objects with T < 4000 K
The Born-Oppenheimer Approximation
The Born Oppenheimer Approximation assumes that electrons in a molecule
move much faster than the nuclei, and adapt instantaneously, finding the
lowest potential energy for each nuclear configuration. Therefore, it is
possible to calculate an electronic energy for each nuclear configuration,
considering the nuclei frozen.
The nuclear Hamiltonian is:
nuclear masses electronic potential
Coulomb potential
between the nuclei
H2 Molecular Orbitals
Electron orbitals of H atoms
Antibonding orbital
Energy [eV]
R Internuclear
Distance [a0]
Bonding orbital
H2 potential energy curves
-4.5 eV
Re = 1.4 a0
1
2
Molecular Vibration: Harmonic Oscillator Potential
Anharmonicity
leads to lowering
of energy levels
1
2
Harmonic Oscillator Potentail
! " 1
2#
Energy levels
transitions
Zero point energy ! 1
2#
H2 electronic potential curves
Internuclear distance Ra [a0]
Electronically
excited states
Bonding orbital
Antibonding
orbital
Notation
2S+1 (+/-)
S: spin quantum number
: projection of orbital angular momentum along
internuclear axis
(+/-): reflection symmetryu/g: parity
(g/u)
Energy [eV]
R Internuclear
Distance [a0]
Electronic and Vibrational Excitation
-4.5 eV
Pure
electronic
transition
Transition
With vibronic
coupling
v=0v=1
v=2
v=0v=1
v=2
Polyatomic Molecules: Example H3+
Simplest Polyatomic molecule
Consists of 3 protons and 2 electrons
3 internuclear distances
3 vibrational degrees of freedom
Electronic Potential curve is a Hyper-Surface of 3 coordinates,
can not be plotted!
Normal Modes of Vibration
Normal modes:
independent modes of vibration
diagonalize the Molecular Hamiltonian
linear combinations of the internuclear distances
Finding Normal Modes: Use Molecular Symmetry / Group Theory
The Normal Modes of H3+
Degenerate (the same frequency)
bending mode 2breathing modebending mode 1
+90 phase -90 phase
The Normal Modes of H3+
H3+ vibrational level scheme
Translational degrees of freedom
Rotational degrees of freedom
Vibrational degrees of freedom
Linear Non-linear
3 3
2 3
In general molecules have 3N degrees of freedom
3N -5 3N -6
Vibrations bear a lot of Potential For Complexity
But: Symmetry helps!
H3+ : 3 vibrational degrees of freedom 2 distinct normal modes
C60: 174 vibrational degrees of freedom 46 normal modes
Rotations: the Rigid Rotor
m1
m2center of mass
Energy levels
rotational constant
$
2%
& = $'(' + 1)
Diatomic or Linear Polyatomic Molecules
rotational
quantum
number
Moment of inertia
% =*+,,,
r1
r2
Large and heavy molecules have
small rotational constants!
Rigid Rotor Levels and Transitions
Energy levels
& $'(' 1)
Selection rule:
' .1
& ' 1 & ' $ ' 1 ' 2 '(' 1)
Transitions:
2$ ' 1
frequency2B 4B 6B 8B
10 21 32 43
P and R branch
P-Branch J=-1 R-Branch J=+1
P and R branch
transmission
100
0
Rotation-Vibration Spectrum of HBr
R-BranchP-Branch
General Rotational Structure of Polyatomic Molecules
A
B
C
Any arbitraryily shaped solid body has three principal axes of
rotation (A
Energy Level Diagram of H2O
Overview
Electronic
Transitions:
E = 1-15 eV
Visible-UV
Vibrational
Transitions:
E = 0.1-1 eV
Infrared
Rotational
Transitions:
E = 0.01-0.1 eV
(sub)-Millimeter
Einstein Coefficients
E1
E2h
B1
2
B2
1
A2
1
: energy density of the radiation field
AbsorptionSpontaneous
emission
Induced
emission
Equilibrium:
absorbed photons = emitted photons
$6 3 8 $86
6 = /9
19:19
Solving for (v):
Using Boltzmann radiation law:
9
9= ;:
?@
AB 8= 8;=
;
?@
AB
6 = /9
19;=;?@AB:19
CD
;=;?@AB:
(1)
(2)
(3)
Insert (3) into (2):
(4)
Einstein Coefficients
E1
E2h
B1
2
B2
1
A2
1
: energy density of the radiation field
AbsorptionSpontaneous
emission
Induced
emission6
3$
;=;?EAB 1
(4)
Plancks thermal radiation law
6 = 8G
HI
>?EAB 1 (5)
8GHI
>?EAB 1
=3$
;=;< $$ >
?EAB 1
Equating (4) and (5):
(6)
$= KK$ 3= LMNEOPO $ (7)
Lifetimes and Einstein A:
The lifetime T of an initial state i
Is given by the sum of the Einstein
coefficients summed over all final
states
1
AiffT =
Einstein A21 coefficient
Einstein B12 coefficient
Einstein B12 coefficient
Absorption cross section
Line strength
Transition dipole moment
Oscilator strength
Hillborn,
Am. J. Phys.
50, 982 (1982)
Level 2 (up)
Level 1 (low)
What makes tr