Page 318 - Instant notes

I1
GENERAL FEATURES OF
SPECTROSCOPY

Key Notes
Spectroscopy is the analysis of the electromagnetic radiation
emitted, absorbed or scattered by atoms or molecules as they
undergo transitions between two discrete energy states.
Electromagnetic radiation consists of a propagating oscillating
electric and magnetic field. The frequency of oscillation defines
different regions of the electromagnetic spectrum: radio waves,
microwaves, infrared, visible, ultraviolet, X-rays and γ-rays. The
energy of electromagnetic radiation of frequency, v, is quantized
in units of hv called photons.
A selection rule states whether a particular spectroscopic
transition is allowed or forbidden. A physical selection rule
describes the general properties the molecule must possess in
order to undergo a certain class of transitions. A specific selection
rule states what changes in quantum number are allowed for a
transition to occur.
Emission spectroscopy is the analysis of the energy of photons
emitted when a molecule moves from a higher to a lower energy
state. Absorption spectroscopy is the analysis of the energy of
photons absorbed from incident light by a molecule undergoing
transition from a lower to a higher energy state. Scattering
spectroscopy is the analysis of the energy lost or gained by a
photon of incident light after it has undergone an energy
exchange interaction with a molecule.
Related topics Chemical and structural effects of Practical aspects of
quantization (G7) spectroscopy (I2)

Spectroscopy
Quantum theory shows that atoms and molecules exist only in discrete states, each of
which possess discrete values, or quanta, of energy. The states are called the energy
levels of the atom or molecule. Spectroscopy is the analysis of the electromagnetic
radiation emitted, absorbed or scattered by atoms or molecules as they undergo
transitions between two energy levels. The frequency, v, of the electromagnetic