Molecular Spectroscopy

Spectroscopy is one of the most important instrumental analysis for material scientists and engineering. Spectroscopy actually is the measurement of energy level for certain matters including molecules, compounds, or materials. The energy levels that have been measured are different to each other depending on what types of spectroscopy instruments are used. It means that it is mostly accomplished by measuring the frequencies of light absorbed or emitted by those matters, such as monochromatic waves. So, the spectroscopy measurements depend on the energy of incident photon or particle together with a measurement of a scattered photon or particle.

This dependency lies on the use of electromagnetic waves. For example, the frequency of scattered light may differ from that of the incident light as well as the type of lights. The absolute value of the frequency difference is a difference of energy levels of the molecule divided by Planck's constant. This phenomenon is commonly referred as Raman scattering. Electron loss spectroscopy is extensively used to measure vibrational frequencies of surfaces. The difference in energy between incident and scattered electrons, in common, a quantized energy left in the solid. When very slow neutrons are scattered by a warm liquid or solid, the scattered neutrons move faster than those in the incident beam.

In some spectroscopy, the scattered particle whose energy is measured is not the same as the incident particle. For example, in photoelectron spectroscopy, an incident X-Ray region produces a photoelectron due to the wavelength of X-Ray which is less than 100 nm. Careful measurements of the kinetic energy of the electron yield energy level spacings in the positive ion. In photodissociation spectroscopy, when a molecule is dissociated into fragments by light, measurement of the kinetic energy of a fragment yields the internal energy of the fragments.

As it is explained previously, the different electromagnetic waves define the energy level. It is about the multitude of units of energy used to describe spectroscopic phenomena. The SI unit of energy is the Joule, however, this unit is rarely used by spectroscopists. Just as the nuclear spectroscopist uses one million electron volts (1 MeV) as a unit of energy, while the X-Ray spectroscopist uses electron volts (1 eV). On the other hands, spectrophotometry that uses ultraviolet, visible, and infrared lights used the 1/cm unit, which is not an energy but a reciprocal of wavelength. Meanwhile, the microwave and radiofrequency spectroscopists use the megahertz unit (1 MHz), and nuclear magnetic resonance (NMR) spectroscopists often use a dimensionless quantities such as parts per billion (ppb) or parts per million (ppm).

Some spectroscopists use units of energy, others unit of reciprocal wave length, and still others frequency units. There natural reasons for these different choices of energy units, but they are bewildering to the beginner.