1. Introduction to Infrared Spectroscopy
How IR energy is absorbed by molecules
"The Introduction to Infrared Spectroscopy PowerPoint” is a modified version of the one my mentor at Lawrence Berkeley National Laboratory, Dr. Michael Martin, uses to teach visiting interns about the use of spectroscopy.

2. The EM Spectrum

3. Atomic Emission Spectra
Electrons are in quantized energy states (orbits) in an atom.
Absorbing a photon can promote an electron to a higher energy orbital, then this excited state can decay via emission of a photon.

4. Atomic Absorption Spectra
Emission Spectra
Input spectra
(like the sun)
Absorption spectra
(like from a gas between the sun and us)
This technique is so reliable that scientists can tell what elements they are looking at just by reading the lines. Spectroscopy is the science of using spectral lines to figure out what something is made of. That's how we know the composition of distant stars!

5. What can we learn from IR spectroscopy?
Atoms vibrate with frequencies in the IR range
Chemical Analysis:
Match spectra to known databases
Identifying an unknown compound, Forensics, etc.
Monitor chemical reactions in-situ
Structural ideas:
Can determine what chemical groups are in a specific compound
Electronic Information:
Measure optical conductivity
Determine if Metal, Insulator, Superconductor, Semiconductor
Band Gaps, Drude model

6. A Simple Oscillator k m Ball & Spring equation: F = -k Dx
Imagine the mass is an atom, carrying a charge.
This will then couple to the electric field in light. k m
Ball & Spring equation: F = -k Dx
Spring constants for chemical bonds:
k = 40 – 400 N/m for covalent bonds
k = 20 – 200 N/m for ionic bonds
k = 0.5 – 4 N/m for Van der Waals bonds
Solution to resonance frequency for ball & spring:
A hydrogen atom has a mass of x kg
An oxygen atom has a mass of X kg

7. An Example: CO2 Symmetric Stretch Asymmetric Stretch Bending Mode
(Dipole moment = 0 so not IR active)
IR Absorbence
Asymmetric Stretch
(Has dipole moment so IR active)
Wavenumber (cm-1)
A Dipole Moment = charge imbalance in the molecule
Bending Mode
(Has dipole moment so IR active)

8. A more complex example: Toluene
From: