Some important concepts in spectroscopy

How the measurements of radiation frequency are made experimentally and the energy levels deduced from these comprise the practice of spectroscopy.

All techniques are mostly dependent on the emission or absorption of electromagnetic radiation characteristic of certain energy within an atomic or molecular system. Spectroscopy is one of the powerful tools for the study of atomic and molecular structure and is used in the analysis of a wide range of samples.

Atomic spectroscopy deals with the interaction of electromagnetic radiation with atoms which are most commonly in their lowest energy state called the ground state. Molecular spectroscopy deals with the interaction of electromagnetic radiation with molecules.

Rotational spectra

These spectra arise due to the transition between the rotational energy level of a gaseous molecule on the absorption of radiations falling in the microwave region. These spectra are shown by molecules that have a permanent dipole moment. For instance HCl, CO, H₂O, etc. Homonuclear diatomic molecules such as H₂, Cl₂ etc and linear polyatomic molecules such as CO₂, which do not have a dipole moment, do not show microwave spectra. Usually, microwave spectra appear in the spectra range of 1-100cm^-1.

Vibrational spectra

These spectra arise due to transitions induced between the vibrational energy levels of a molecule on the absorption of radiation belonging to the infrared region. IR spectra are shown by the molecule when vibrational motion is accompanied by a change in the dipole moment of the molecule. Vibrational spectra appear in the spectral range of 500-4000 cm^-1.

Raman spectra

Raman spectra relate to vibrational or rotation transitions in molecules but in a different manner. In this case, only the scattering is measured but not the absorption of radiation. An intense beam of monochromatic radiation in the visible region is allowed to fall on a sample and the intensity of scattered light is observed at right angles to the incident beam.

The classical theory of the Raman Effect:

When an electric field is applied to a molecule, its electrons and nuclei are displaced. Thus an induced dipole moment is produced in the molecule due to the displacement of the electrons and nuclei, and the molecule is said to be polarized.

µ = ᾳE

Where,

E- Strength of electric field

µ- magnitude of induced dipole moment

ᾳ- Polarizability of the molecule

v- Radiation frequency

E = E₀sin 2πvt

µ = ᾳ E₀sin 2πvt

Electronic spectra

Electronic spectra result from electronic transitions in a molecule by absorption of radiations falling in the visible and ultraviolet regions. While spectra in the visible region span 12,500-25,000 cm^-1, those in the ultraviolet region span 25,000-70,000 cm^-1. As electronic transitions in a molecule are invariably accompanied by vibrational and rotational transitions, the electronic spectra of the molecule are highly complex.

NMR  on a Chip

Nuclear magnetic resonance and nuclear quadrupole resonance spectra

NMR spectra arise due to transitions induced between the nuclear spin energy levels of a molecule in an applied magnetic field. NQR spectra arise due to the transitions between the nuclear spin energy levels of a molecule arising from the interaction of the unsymmetrical charge distribution in nuclei with the electric field gradients which arise from the bonding and non-bonding electrons in the molecule. NMR and NQR’s radio frequency regions 5-100MHz.

Electron spin resonance  

ESR spectra arise due to the transitions induced between the electron spin energy levels of a molecule in an applied magnetic field. These spectra are exhibited by systems that contain odd electrons such as free radicals and transition metal ions. ESR spectra region is 2-9.6 GHz.