Please support http://en.wikipedia.org On this page we try to connect the science of spectroscopy with specific instruments or systems typically used in the application. In the past our efforts to define applications (e.g. Raman) have fallen short. Here we borrow many definitions from http://en.wikipedia.org and have added links to McPherson instruments that are used in these pursuits.
Raman spectroscopy (adapted from Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Raman_spectroscopy )
Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in a system. It relies on inelastic scattering, or Raman scattering of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. Raman spectrometers typically use holographic diffraction gratings and multiple dispersion stages to achieve a high degree of laser rejection. A photon-counting photomultiplier tube (PMT) or, more commonly, a CCD camera is used to detect the Raman scattered light.
Photoelectric effect (from Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Photoelectric_effect )
The photoelectric effect is the emission of electrons from matter upon the absorption of electromagnetic radiation, such as ultraviolet radiation or x-rays. An older term for the photoelectric effect was the Hertz effect, though this phrase has fallen out of current use.
Photoemission (adapted from http://en.wikipedia.org/wiki/Photoemission )
The energy of the photoelectrons emitted is exactly the energy of the incident photon minus the material's work function or binding energy, the work function of a sample can be determined by bombarding it with a monochromatic X-ray source or UV source (typically a helium discharge lamp) (see 234/302 monochromator and 629 hollow cathode lamp), and measuring the kinetic energy distribution of the electrons emitted. This must be done in a high vacuum environment, since the electrons would be scattered by air.
Photoluminescence (adapted from Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Photoluminescence )
Photoluminescence is a process in which a chemical compound absorbs a photon with a wavelength in the range of visible electromagnetic radiation, thus transitioning to a higher electronic energy state, and then radiates a photon back out, returning to a lower energy state. The period between absorption and emission is typically extremely short. The most familiar such effect is fluorescence, in which some of the original energy is dissipated so that the emitted light is of lower energy than that absorbed. An even more specialized form of photoluminescence is phosphorescence, in which the energy from absorbed photons undergoes intersystem crossing into a state of higher spin multiplicity. Once the energy is trapped in this state, transition back to the lower singlet energy states is quantum mechanically forbidden, meaning that it happens much more slowly than other transitions. The result is a slow process of radiative transition back to the singlet state, sometimes lasting minutes or hours. This is the basis for "glow in the dark" substances.
Photoluminescence spectrometer systems typically use laser excitation, cryogenically cooled sample holders, efficient, high throughput monochromators (like the 207) and detect emissions in the Visible and Near Infrared. In the Visible and NIR a photon-counting photomultiplier tube (PMT) or a CCD camera is used to detect light. At longer wavelengths in the IR solid state detectors are used with scanning monochromators. Generally these systems are very similar to Raman spectrometers.
Spectrometer (adapted from Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Spectrometer )
A spectrometer is an optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum. The variable measured is most often the light's intensity but could also, for instance, be the polarization state. The independent variable is usually the wavelength of the light, normally expressed as some fraction of a meter, but sometimes expressed as some unit directly proportional to the photon energy, such as wavenumber or electron volts, which has a reciprocal relationship to wavelength. A spectrometer is used in spectroscopy for producing spectral lines and measuring their wavelengths and intensities. Spectrometer is a term that is applied to instruments that operate over a very wide range of wavelengths, from gamma rays and X-rays into Vacuum Ultraviolet, Visible and the far infrared.
Ultraviolet Photoelectron Spectroscopy (UPS) (from http://www.chem.qmul.ac.uk/surfaces/scc/scat5_3.htm )
In UPS the source of radiation is normally a noble gas discharge lamp (see 629); frequently a He-discharge lamp emitting He I radiation of energy 21.2 eV. These line sources can be used with a monochromator like the 234/302 to tune to specific wavelengths.
Such radiation is only capable of ionizing electrons from the outermost levels of atoms - the valence levels. The advantage of using such UV radiation over x-rays is the very narrow line width of the radiation and the high flux of photons available from simple discharge sources. The main emphasis of work using UPS has been in studying:
1. the electronic structure of solids - detailed angle resolved studies permit the complete band structure to be mapped out in k-space.
2. the adsorption of relatively simple molecules on metals - by comparison of the molecular orbitals of the adsorbed species with those of both the isolated molecule and with calculations.
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