This document discusses spectroscopy and the factors that influence spectral resolution. It covers the electromagnetic spectrum, components of a spectrophotometer, and the three main types of broadening - Doppler broadening, collision broadening, and lifetime broadening - that limit spectral resolution, especially in gases and liquids. High resolution spectroscopy can be achieved with gases at low pressures by minimizing these broadening effects.

1. Nils Walter: Chem 260
The application of quantum
mechanics: Spectroscopy
Atkins, Chapter 17
Transitions between
defined energy states
Bohr frequency
condition
wavelength
wavenumber
[cm-1]
Exception:
Raman scattering

2. Nils Walter: Chem 260
The electromagnetic spectrum
Tungsten-iodine lamp
Nernst filament
(black body)
Quartz heated by
mercury arc
Deuterium
discharge
Klystron
Oscillating current
in metal wire

3. Nils Walter: Chem 260
A spectrophotometer
Dispersing element

4. Nils Walter: Chem 260
Resolution limits: 1. Line broadening
through the Doppler effect
Radiation source receding from observer:
Radiation source approaching observer:
Overall effect on the linewidth:
→
→
→
→ particularly in gases
temperature
mass
speed

5. 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

6. Nils Walter: Chem 260
Resolution limits - the bottom line:
3. Lifetime broadening
Lifetime of excited state;
determined by
collisional deactivation
and spontaneous emission
practical
⇒
⇒
⇒
⇒
Solving the time-dependent Schrödinger
equation reveals Heisenberg’s
uncertainty principle:
Gases @ low pressure
enable high-resolution
spectroscopy
unchangeable
⇒
⇒
⇒
⇒ natural linewidth

7. Nils Walter: Chem 260
The intensity of spectral lines
depends on:
• the transition probability
between the two states
(selection rules)
• population of states
• path length of sample
(Lambert-Beer law)
Boltzmann
constant
extinction
coefficient