Molecular Spectroscopy
Molecular spectroscopy studies how molecules absorb, emit, and scatter electromagnetic radiation, revealing their structure, energy levels, and dynamics across the spectrum from microwaves to the ultraviolet.
Definition
Molecular spectroscopy is the measurement and interpretation of the wavelengths and intensities at which molecules interact with light, used to determine molecular energy levels, geometries, and the rules governing transitions between rotational, vibrational, and electronic states.
Scope
This area covers the spectroscopy of molecules organized by the type of transition involved: pure rotational spectra in the microwave region, vibrational and rotation–vibration spectra in the infrared, electronic band spectra in the visible and ultraviolet governed by the Franck–Condon principle, and inelastic Raman scattering. It treats selection rules, band structure, and how spectra are inverted to obtain molecular constants such as bond lengths and force constants.
Sub-topics
Core questions
- What molecular property must change for a transition to absorb or emit radiation?
- How do rotational, vibrational, and electronic transitions occupy different spectral regions?
- What selection rules govern molecular spectra, and what do bands reveal about structure?
- How does Raman scattering complement absorption spectroscopy?
Key concepts
- Dipole and polarizability selection rules
- Microwave, infrared, and ultraviolet–visible regions
- Band structure and branches
- Franck–Condon principle
- Raman and Rayleigh scattering
- Spectroscopic determination of molecular constants
Key theories
- Rotation–vibration spectroscopy
- Transitions among rotational and vibrational levels, allowed when the molecule has a changing dipole moment, produce microwave and infrared spectra whose line positions yield rotational constants, bond lengths, and vibrational frequencies.
- Electronic spectra and the Franck–Condon principle
- Electronic transitions produce band systems in the visible and ultraviolet whose vibrational intensity distribution is governed by the Franck–Condon principle, reflecting the overlap of vibrational wavefunctions in the two electronic states.
- Raman scattering
- Inelastic scattering of light shifts the photon energy by a molecular vibrational or rotational quantum, governed by a change in polarizability, giving access to transitions that may be inactive in ordinary infrared absorption.
Clinical relevance
Molecular spectroscopy is the workhorse of chemical analysis and remote sensing: infrared and Raman spectra fingerprint compounds and monitor reactions, microwave spectra and ultraviolet–visible bands identify trace species in the atmosphere and in interstellar space, and the techniques underpin environmental and pharmaceutical quality control.
History
Molecular band spectra were catalogued before quantum mechanics could explain them; the new theory in the late 1920s, together with the Franck–Condon principle and Raman's 1928 discovery of inelastic scattering, turned spectroscopy into quantitative molecular structure determination. Herzberg's mid-century compendia codified the field, and laser sources later transformed its sensitivity and resolution.
Key figures
- Gerhard Herzberg
- Chandrasekhara Venkata Raman
- James Franck
- Edward Condon
Related topics
Seminal works
- herzberg1950
- atkins2011
- hollas2004
Frequently asked questions
- Why do different kinds of molecular transition appear in different parts of the spectrum?
- Rotational energy spacings are smallest (microwave), vibrational spacings are intermediate (infrared), and electronic spacings are largest (visible and ultraviolet). Each type of transition therefore absorbs or emits in a characteristic spectral region.
- Can a molecule with no permanent dipole moment have a spectrum?
- It can have no pure rotational microwave spectrum, but it may still be infrared-active if a vibration creates a changing dipole, and homonuclear molecules like N₂ remain Raman-active because their polarizability changes during vibration.