Fundamentals of Molecular Spectroscopy

Spectroscopy

The study of the interaction of electromagnetic waves and
matter

Factors Affecting The Width of Spectral Lines

Collision broadening

Heisenberg uncertainty principle

If a system exists in an energy state for a limited time δt seconds, the the energy of that state will be uncertain to an extent δE

Doppler Broadening

Factor Affecting The Intensity of Spectral Lines

The transition probability: The likelihood of a system in one state changing to another state.

The population: the number of atoms or molecules initially in the state from which the transition occurs

The concentration or path length of the sample.

Gross selection rules: Specify general features a molecule must have in order for a spectrum to
be observed.

Specific selection rules: - Define allowed transitions in terms of changes in quantum numbers

If a sample is absorbing energy from a beam of radiation, the more sample the beam traverses, the more energy will be absorbed from it

The Beer-Lambert law

In liquid and gases the motion of the particles causes their absorption and emission frequencies to show Doppler shift. since the motion is random in a given sample, shifts to both high and low frequencies occur and hence the spectral line is broadened.

More dominant in gas phase

More dominant in liquid phase

Atoms or molecules in liquid and gaseous phases are in continual motion and collide frequently with each other. Equally vibrational and rotational spectra are broadened since collisions interfere with these motions too.

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Signal-to-Noise Ratio

Ways to reduce it

Every recorded spectrum has a background of random fluctuations

Generated in the amplifying equipment.

Spurious electronic signals produced by the source or detector

Use narrower slit, but less signal produces. Needs amplifier.

Make sure the noise lesser than signals

Planck’s Equation

Planck suggested that such absorbed and emitted energy, ∆E, can take the form of electromagnetic radiation. The
relationship between the frequency and the energy
difference.

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Electromagnetic Spectrum

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Basic Elements of Practical Spectroscopy

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