MATS6008 Chap.8 Spectroscopy and Nanophotonics
Spectroscopy and Nanophotonics
This chapter matches electromagnetic energy to material transitions and turns spectra into bounded evidence. It connects frequency, wavelength and photon energy to UV-visible, infrared, Raman, NMR and X-ray methods. Optical resolution, Bragg diffraction, X-ray absorption and plasmonic sensing are handled through equations, sampling depth and complementary controls rather than peak labels alone.
What this chapter covers
- 01
Frequency, wavelength and photon energy
- 02
UV-visible electronic transitions
- 03
Infrared and Raman vibrational evidence
- 04
NMR chemical environment and coupling
- 05
Bragg diffraction and crystal spacing
- 06
Near-field optics and plasmonic sensing
Worked example · free
Diffraction-limited optical separation
- +1Substitute 680 nm divided by two times 1.25.
- +1The denominator is 2.50, giving 272 nm.
- +1State that practical aberration and sampling can worsen this idealised limit.
Key terms
- Chemical shift
- An NMR frequency displacement caused by the local electronic environment around a nucleus.
- Raman scattering
- Inelastic light scattering that reveals vibrational or other excitations in a material.
- Bragg law
- The condition n lambda equals 2d sine theta for constructive scattering from crystal planes.
- Near field
- A non-propagating optical region close to a source that retains high-spatial-frequency information.
- Plasmonic sensing
- Detection through a resonance change caused by the dielectric environment of a metallic nanostructure.
Spectroscopy and Nanophotonics FAQ
How should a spectroscopy method be selected?
Match the energy and interaction to the structural question. Infrared and Raman address vibrations through different selection rules, UV-visible methods probe electronic absorption, NMR distinguishes local nuclear environments, and X-ray methods access periodic structure, core levels or local coordination. Sampling depth and preparation still matter.
Why is wavelength alone insufficient for optical resolution?
The course relation includes numerical aperture as well as wavelength. A shorter wavelength improves the nominal limit, and a larger numerical aperture collects a wider angular range. If a problem omits numerical aperture, report a symbolic result or an explicitly labelled assumption rather than inventing a unique number.
What can shift a plasmonic resonance besides target binding?
Bulk refractive-index change, particle aggregation, morphology, spacing, temperature and non-specific adsorption can all move or broaden a resonance. A selective sensor therefore needs reference surfaces, interferent tests and structural controls before a spectral shift can be assigned to the intended analyte.
How are infrared and Raman measurements complementary?
Both probe molecular vibrations, but their selection rules depend on different changes during vibration. Infrared activity follows a changing dipole moment, while Raman activity follows changing polarisability. A weak band in one method may be strong in the other. Compare peak patterns and sample constraints instead of expecting identical spectra or assigning a bond from one isolated feature.
What must be checked before applying Bragg law?
Confirm whether the instrument axis reports theta or two theta, identify diffraction order and keep wavelength and spacing in consistent units. The calculation returns a plane spacing associated with the selected peak, not an automatic phase identity. Match several peaks and intensities or use complementary evidence before naming a crystal structure.
How should two spectra be compared fairly?
Match acquisition settings, calibration, baseline treatment and normalisation before interpreting peak shifts or intensity changes. Check signal-to-noise and spectral resolution because merged peaks can resemble broadening. Report whether the signal averages a large region or maps local heterogeneity, then use a reference or complementary method for the structural assignment.
Exam move
Arrange methods on an electromagnetic-energy ladder and write the transition each observes. Recalculate optical resolution with several wavelengths and apertures. For diffraction, check theta versus two theta before using Bragg law. Treat every spectral assignment as a claim requiring a pattern, reference or complementary method, and list one confounder for plasmonic sensing.
Build a selection grid with energy range, transition, sampling depth, spatial resolution and preparation. For calculations, write the physical meaning of every symbol and test the direction of change: shorter wavelength or larger numerical aperture improves the nominal optical limit, while the diffraction angle enters through a sine.
If numerical aperture is missing, keep the result symbolic or label an assumption instead of inventing a unique value. Infrared and Raman both probe vibrations but use different selection rules, so compare dipole-moment and polarisability changes rather than expecting identical peaks.
Before Bragg law, confirm whether an axis reports theta or two theta, keep wavelength and spacing units consistent and remember that one plane spacing is not a full phase identification. Match multiple peaks or use complementary structure evidence.
For plasmonic sensing, draw the nanoparticle, local field and surrounding refractive index, then list aggregation, morphology, temperature and nonspecific adsorption as possible shifts. Use reference surfaces and interferent tests. Near-field optics accesses evanescent information with a close sub-wavelength probe; connect its improved resolution to slower scanning, weak signal and probe artefacts.
Pair every spectral conclusion with a reference, pattern or orthogonal structural measurement so that a labelled peak becomes bounded evidence rather than a decorative assignment.