Chapter 6

• Coherent radiation is radiation that is made up of wave trains having identical frequencies or sets of frequencies and phase relationships that are constant with time
• The superposition principle states that when two or more waves traverse the same space, the disturbance that occurs is the sum of the disturbance caused by the individual waves.
• The refractive index of a material is a measure of the interaction of the material with radiation and is defined as the velocity of light in a vacuum divided by the velocity of light in the material.
• Dispersion is the variation of the refractive index of a material with wavelength. Normal dispersion is the gradual increase in refractive index with increasing frequency (decreasing wavelength).
• Anomalous dispersion is the sharp change in refractive index of a substance in a wavelength region where absorption is occurring.
• The work function of a substance is a constant that measures the energy required to remove an electron from the surface of the substance
• The photoelectric effect is the emission of electrons from the surface of a substance brought about by a beam of radiation.
• The ground state of a molecule is its lowest energy state.
• Electronic excitation is the process by which electrons in a substance are promoted from their ground state to higher electronic states by absorption of energy.
• Fluorescence is a type of emission which is brought about by irradiating atoms, ions, or molecules with electromagnetic radiation. Fluorescence involves a singlet-to- singlet transition. The lifetime of the excited state in fluorescence is very short (10–5 s or less).
• Phosphorescence is a type of emission brought about by irradiating a molecular system with electromagnetic radiation. Phosphorescence involves a triplet-to-singlet transition and the excited state lifetime is longer than that of a fluorescing species.
• The transmittance of a medium is the fraction of the incident radiation that passes through the medium unattenuated.
• Absorptivity a the absorbance of a medium per unit pathlength and per unit concentration. Absorptivity is defined by the equation a = A/bc, where A is the absorbance of a medium contained in a cell length of b and concentration c. The path length b is expressed in cm or another specific unit of length. The concentration is expressed in units such as g/L.
• The absorbance of a medium A is defined by A = –logT = –log P0/P where T is the transmittance, P0 is the incident radiant power, and P is the transmitted radiant power.
• Vibrational relaxation is a process wherein a vibrationally excited species loses vibrational energy resulting in a lower energy state.
• The Stokes shift is the difference in wavelength between the incident radiation and the wavelength of fluorescence or scattering