Band Spectrum
Definition
A continuous-looking envelope of closely spaced lines produced by molecules. Electronic, vibrational and rotational transitions couple together so a single electronic excitation appears as a band rather than a line. The basis of UV-Vis and IR molecular spectroscopy.
- Field
- Molecular spectroscopy
- Cause
- Coupled electronic, vibrational, rotational transitions
- Appearance
- Continuous-looking envelope of closely spaced lines
- Application
- Basis of UV-Vis and IR molecular spectroscopy
Common questions
Why do molecules produce band spectra while atoms produce sharp line spectra?+
An atom has only electronic energy levels to transition between, giving discrete, well-separated absorption or emission lines, whereas a molecule has additional closely spaced vibrational and rotational energy levels superimposed on each electronic transition, and these many possible sub-transitions blend together into what looks like a continuous band rather than a single sharp line.
Does a band spectrum contain less identifying information than a line spectrum?+
It contains different information rather than less: the overall shape, width, and position of a molecular band carries information about the compound's structure and its interaction with the solvent, which is exactly what techniques like UV-Vis and IR spectrophotometry exploit for identifying and quantifying compounds in casework, even though individual rotational lines are usually not resolved.
Can band spectra be resolved into individual lines under any conditions?+
Yes, at very high spectral resolution and particularly for small molecules in the gas phase at low pressure and temperature, the individual rotational fine structure within a band can be resolved, but under the solution-phase, room-temperature conditions typical of forensic UV-Vis and IR work, the lines remain unresolved and appear as a smooth band.
Related terms
- Beer-Lambert Law
- A = εbc. Absorbance equals molar absorptivity times path length times concentration. The linear relation between absorbance and concentration that makes UV-Vis...
- Line Spectrum
- A discrete pattern of sharp lines produced by atoms in the gas phase. Each element has a unique line pattern set by...
- Quantised Energy Level
- An allowed state of an atom or molecule. Transitions between levels happen only at discrete energies set by the level spacing, not...
- Selection Rule
- A quantum-mechanical rule that decides whether a transition between two levels is allowed or forbidden. Allowed transitions are intense and fast; forbidden...
- Wavenumber (Cm⁻¹)
- The number of wavelengths per centimetre, equal to 1/λ when λ is in cm. The working unit of vibrational spectroscopy because it...