Modern & Analytical Chemistry Ch 6.

Intro to spectrometric methods

Electromagnetic Radiation using wave/particle duality

-We can describe electromagnetic radiation (EM) using a classical sinusoidal wave model with characteristics such as wavelength, frequency, velocity, and amplitude.
-We can describe EM radiation as a stream of discrete particles called photons where the energy of a photon is proportional to the frequency of the radiation.

Frequency does

not change with the medium (Its invariant)

Wavelength and velocity of EM does

change with the medium

Two components of Electromagnetic Radiation

electric component & magnetic component

Electromagnetic Radiation

two components 90 degrees apart

The electric component of radiation

is responsible for most phenomena of interest, i.e. transmission, reflection, refraction, and absorption. (Only consider electrical component for most instrumentation)

The magnetic component of radiation

is responsible for absorption of radio-frequency waves in nuclear magnetic resonance.

The Electromagnetic Spectrum and How it Interacts with Mater

Methods of Propagation of Radiation: Diffraction

Parallel beam of radiation is bent as it passes a sharp barrier or through a narrow opening

Diffraction

is a superposition of waves because of interference

Superposition

when two or more waves transverse the same space, a disturbance occurs that is the sum of the disturbances caused by the individual waves.

Constructive interference

results in an increase in amplitude because waves are in phase.

Deconstructive interference

results in a decrease in amplitude because waves are out of phase.

Methods of Propagation of Radiation: Transmission

Transmission

the propagation of EM radiation through materials

Methods of Propagation of Radiation: Refraction

Abrupt change in a direction of a beam due to differences in velocity between two media of different densities

n_x

refractive index (unitless)

V_x

velocity of radiation (m/s)

Refractive indexes of materials

are usually measured and reported with air rather than vacuum due to experimental limitations.
Convert between the two factors: n_vac = 1.00027n_air

Methods of Propagation of Radiation: Scattering

Small fraction of radiation is transmitted at all angles from the original path and the intensity of this scattered radiation increases with particle size.

Rayleigh Scattering

scattering by molecules smaller than the wavelength of radiation; proportional to the inverse 4th power of wavelength (I ∝ 1/λ4 ); reason why sky is blue... greater scattering of the shorter wavelengths of the visible spectrum

Mie Scattering

scattering by large particles

Raman Scattering

scattering resulting in quantized frequency shifts; results in vibration energy transitions of the molecules. Discussed in Raman spectroscopy

Methods of Propagation of Radiation: Polarization

Bundles of EM waves in which the vibrations are equally distributed in the planes centered along the beam path.