Molecular SpectroscopyThe Diatomic Rigid Rotator
General Aspects of Molecular Spectroscopy
The rigid diatomic Molecule
m1 m2
r1 r2
C
m1 r1 m2 r2=r0
THE MOMENT OF INERTIA IS I = μ r02
Reduced mass
Rotational Energy Level
• EJ = h2
• In terms of wavenumber εJ = EJ/hc cm-1
εJ = BJ(J+1), where B = h2 cm-1
B is called as the rotational constant.
8 π2 I J (J+1) joules, where J = 0, 1, 2,…..
8 π2 I c
Intensities of Spectral Lines• Intensities depends on number of molecules
in a particular energy level Boltzmann Distribution
NJ/NO = EXP (-EJ/kT) = EXP(-Bhc J(J+1)/ kT)
NJ/NO ≈ 0.98
There are as many molecules in the J=1 state, at equilibrium as in the J=0, at T=300K (room temperature). Further, there is more rapid decrease of NJ/NO with increasing J and with larger B.
J
EXP(
-Bhc
J(J+
1)/
kT)
1
100
B = 10 cm-1
B = 5 cm-1
Degeneracy of the energy states
• The energy and angular momentum of a rigid rotator are E = ½ I ω2
P = I ωP = (2EI)1/2 = [J(J+1)]1/2 units
• eg., for J=1, P=(2)1/2
11Spacing between adjacent rotational levels j and j-1,
12
Rotational Spectroscopy
(1) Bohr postulate
/h hc (2) Selection Rule
11 ( )1 ( )
jj absorptionj emission
Isotopic Shift12CO
13CO
J
6
5
4
321
TYPICALLY ROTATIONAL SPECTRUM SHOWS LINES CORRESPONDING TO 1-100 cm-1
The Vibrating Diatomic Molecule (eg., HCl)
Cl
Cl
req
H’
H’
H’’
H’’ BOND LENGTH
APPROX. POTENTIAL
REAL/ EFFECTIVE POTENTIAL
Morse Potential
THE PARAMETERS
• E = ½ k (r – req)
• ωosc = 1/2π (k/μ)1/2 Hz ωosc = ωosc/c cm-1
• Allowed vibrational EnergiesEν = (ν + 1/2) ωosc
• Selection rule : Δν = ±1
Vibrations in Rigid rotator !!
req
ν = 0123
ENER
GY
N1/NO ≈ 0.008
The molecular population Dies of very fast
Ignore all possible Vibrational states
21
Vibrational SpectroscopyVibrational selection rule
11 ( )1 ( )absorptionemission
ΔJ = -1 P
ΔJ = +1 R
ΔJ = +2 S
ΔJ = -2 O
ΔJ = 0 Q
Δν
26
Vibration-Rotation Spectra
Infrared spectrumΔJ = ±1
Raman spectrumΔJ = 0 , ±2
27
Vibration-Rotation Infrared Spectrum of HCl
• νvib is different for H35Cl and H37Cl molecules due to the slight difference in their reduced masses.
au
au
972.03635
ClH35
974.03835
ClH37
28
Vibration-Rotation Infrared Spectrum of HCl
• The lines due to H35Cl transitions are more intense because the isotopic abundance ration of H35Cl to H37Cl molecules is 3:1.
29
Vibration-Rotation Infrared Spectrum of HClB2B2B2B2 B2 B2 B2 B2B4
Band centerH35Cl
Band centerH37Cl
30
Vibration-Rotation Infrared Spectrum of HCl
• The rotational constant B slightly decreases as going to higher vibrational levels. This results in decrease of the gaps between transition lines as one goes to higher frequencies.
B2B2B2B2 B2 B2 B2 B2B4
31
Vibration-Rotation Infrared Spectrum of HCl
• The rotational constant B slightly decreases as going to higher vibrational levels. This results in decrease of the gaps between transition lines as one goes to higher frequencies.
32
Vibration-Rotation Infrared Spectrum of HClB2B2B2B2 B2 B2 B2 B2B4
Approximation of B values
RAMAN SPECTROSCOPYNobel in 1930
Discovery of Raman Effect (Raman Scattering)
(7 November 1888 – 21 November 1970)
WHAT IS RAMAN SPECTROSCOPY ?
“Raman spectroscopy is the measurement of the wavelength and Intensity inelastically scattered light from molecules.”
May be used to understand chemical composition and molecular structure.
Used in CMP and chemistry to understand the vibrational, rotational and other low-frequency modes of a system.
ScatteringIn addition to being absorbed and emitted by atoms and molecules, photons may also be scattered (approx. 1 in 107 in a transparent medium). This is a molecular effect, which provides another way to study energy levels.
ELASTIC INELASTICSYSTEM REMAINS IN THE SAME QUANTUM STATE
Resulting in CHANGE IN THE QUANTUM STATE
Scattering
• νin = νout • Rayleigh scattering– Ein = Eout = h ν
• νin ≠ νout Raman scattering–Ein ≠ Eout
VIRTUAL STATES
EXCITED STATES
GROUND STATE
RAMAN SCATTERING SELECTION RULES
RAMAN SCATTERING SELECTION RULES
Rotational Raman
Some fine points• Inelastic scattering can be in analogy with a ball
bearing hitting a drum so that it starts to oscillate at its natural frequency. Similarly if the drum is already oscillating and the ball bearing hits it at the right phase, it may get reflected at a higher energy.
• In order to be Raman active, a molecular rotation or vibration must cause some change in the component of molecular polarizibility.
• Strokes lines (those scattered with a lower frequency than the incident radiation) are generally more intense than the anti-stroke lines, because the former is accompanied by an increase in molecular energy.
IR
RAMAN
Why is Raman Different to IR?
•Selection rules are therefore different and can be exclusive for centrosymmetric molecules
IR IR -- Change in Dipole Moment Change in Dipole Moment
Raman Raman -- Change inChange in Polarizability Polarizability
MM -- MM --MM ++
MM ++
MM --
MM --
MM ++
MM --
MM --
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