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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