Infrared Radiation and Molecular Absorption

What is the wavelength range of infrared radiation?

What unit is commonly used to measure infrared radiation?

What is the relationship between wavenumber and wavelength?

What is the order of energy levels from highest to lowest?

What type of bond is usually IR-active?

What happens when molecules are exposed to IR light of the same frequency as the frequency of its origin vibrations?

What is the X-axis of an IR spectrum typically plotted against?

What is the purpose of a Monochromator in an IR spectrometer?

What is the temperature of the Tungsten halogen lamp used as an IR radiation source?

What is the purpose of using a small amount of solid sample in a Potassium Bromide disk?

What is the type of radiation emitted by Laser diodes?

What is the purpose of using a drop of the sample between two potassium bromide or sodium chloride circular plates for liquid sample preparation?

What is the principle of operation of a Thermocouple detector in IR spectrometer?

What is the purpose of the Globar in an IR spectrometer?

Infrared radiation has a wavelength region of 0.8 - 1000 µm.
Wavenumbers (cm-1) are commonly used units, which are proportional to energy (E) and frequency (ν), and reciprocal to wavelength.

Absorption of radiation leads to higher rotational energy levels.
Absorption of energy leads to increased vibrational energy levels (vibrational transition).
Electronic transition: electrons of molecules are raised to a higher electron energy level.
Energy levels in order: electronic > vibrational > rotational.

Covalent bonds vibrate at only certain allowable frequencies.
Types of vibrations:
- Stretching: change bond length (requires high energy).
- Bending: change bond angle (requires low energy).

Polar bonds are usually IR-active (e.g., C-O).
Nonpolar bonds in symmetrical molecules absorb weakly or not at all (e.g., C-C).
Molecules absorb IR light of the same frequency as their origin vibrations.

IR Radiation Source:
- Tungsten halogen lamp (2000°C).
- Globar (1100°C).
- Laser diodes (broad spectral light distribution).
Sample:
- Solids: Potassium Bromide disk.
- Liquids: thin capillary film between potassium bromide or sodium chloride plates.
Monochromator:
- Breaks down radiation into different wavenumber regions.
Detector:
- Thermocouples.
- Bolometers.
- Pyroelectric Detectors.
- Photoconductive detector.
Display.