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Quantum Mechanics in Spectroscopy

This document discusses spectroscopy and the factors that influence spectral resolution and line intensities. It explains that spectroscopy involves measuring transitions between defined energy states of atoms and molecules. The resolution is limited by Doppler broadening from motion, collision broadening, and lifetime broadening due to the Heisenberg uncertainty principle. The intensity of spectral lines depends on transition probabilities, population of states governed by the Boltzmann distribution, and properties of the sample.

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79 views7 pages

Quantum Mechanics in Spectroscopy

This document discusses spectroscopy and the factors that influence spectral resolution and line intensities. It explains that spectroscopy involves measuring transitions between defined energy states of atoms and molecules. The resolution is limited by Doppler broadening from motion, collision broadening, and lifetime broadening due to the Heisenberg uncertainty principle. The intensity of spectral lines depends on transition probabilities, population of states governed by the Boltzmann distribution, and properties of the sample.

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Ra sa
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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The application of quantum

mechanics: Spectroscopy
Atkins, Chapter 17

Transitions between Bohr frequency Exception:


defined energy states condition Raman scattering

wavelength

wavenumber
[cm-1]

Nils Walter: Chem 260


The electromagnetic spectrum

Oscillating current
in metal wire

Klystron Deuterium
discharge
Quartz heated by
mercury arc
Nernst filament Tungsten-iodine lamp
(black body)
Nils Walter: Chem 260
A spectrophotometer

Dispersing element
Nils Walter: Chem 260
Resolution limits: 1. Line broadening
through the Doppler effect
Radiation source receding from observer:

speed
Radiation source approaching observer:

Overall effect on the linewidth:


temperature

mass
→ particularly in gases Nils Walter: Chem 260
Resolution limits:
2. Collision broadening
Particle collision
⇓
deformation of involved particles
⇓
changes in electronic, vibrational, rotational states
⇓
transitions less well defined
⇓
line broadening

→ particularly in liquids or
gases under high pressure
Nils Walter: Chem 260
Resolution limits - the bottom line:
3. Lifetime broadening
Solving the time-dependent Schrödinger
equation reveals Heisenberg’s
uncertainty principle: ⇒
practical
Lifetime of excited state;
determined by
collisional deactivation
and spontaneous emission
Gases @ low pressure unchangeable
enable high-resolution
⇒ natural linewidth
spectroscopy
Nils Walter: Chem 260
The intensity of spectral lines
depends on:
• the transition probability
between the two states
(selection rules)
• population of states
Boltzmann
constant

• path length of sample


(Lambert-Beer law)
extinction
coefficient Nils Walter: Chem 260

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