resonance ionization spectroscopy
Resonance ionization spectroscopy
A form of atomic and molecular spectroscopy in which wavelength-tunable lasers are used to remove electrons from (that is, ionize) a given kind of atom or molecule. Laser-based resonance ionization spectroscopy (RIS) methods have been developed and used with ionization detectors, such as proportional counters, to detect single atoms. Resonance ionization spectroscopy is combined with mass spectrometers to provide analytical systems for a wide range of applications, including physics, chemistry, materials sciences, medicine, and the environmental sciences.
When an atom or molecule is irradiated with a light source of frequency &ngr;, photons at this selected frequency are absorbed only when the energy h&ngr; (h is Planck's, constant) is almost exactly the same as the difference in energy between some excited state and the ground state of the atom or molecule. If a laser source is tuned to a very narrow bandwidth at a frequency that excites a given kind of atom (see illustration), it is highly unlikely that any other kind of atom will be excited. An atom in an excited state can be ionized by photons of the specified frequency &ngr;, provided that 2h&ngr; is greater than the ionization potential of the atom. While the final ionization step can occur with any energy above a threshold, the entire process of ionization is a resonance one. Resonance ionization spectroscopy is a selective process in which only those atoms that are in resonance with the light source are ionized. Modern pulsed lasers have made resonance ionization spectroscopy a practical method for the sensitive (and highly selective) detection of nearly every type of atom in the periodic table. See Atomic structure and spectra, Ionization potential, Laser, Laser spectroscopy, Photoionization, Resonance (quantum mechanics)
Resonance ionization spectroscopy is used to analyze very low levels of trace elements in extremely pure materials, for example, semiconductors in the electronics industry. A sputter-initiated resonance ionization spectroscopy (SIRIS) apparatus uses an argon ion beam to sputter a tiny cloud of atoms from a sample placed in a high-vacuum system and a pulsed laser tuned to detect the specified impurity atom.
The sputter-initiated resonance ionization spectroscopy method is also used for chemical and materials research, geophysical research and explorations, medical diagnostics, biological research, and environment analysis. Thermal-atomization resonance ionization spectroscopy (TARIS) may be used for the bulk analysis of materials. By simply using resonance ionization spectroscopy with ionization chambers or proportional counters, gas-phase work can be done to study the diffusion of atoms, measure chemical reaction rates, and investigate the statistical behavior of atoms and molecules. See Diffusion
Resonance ionization spectroscopy is used in sophisticated nuclear physics studies involving high-energy accelerators. It is used as an on-line detector to record the hyperfine structure of nuclei with short lifetimes and hence to determine several nuclear properties such as nuclear spin and the shape of nuclei. See Fine structure (spectral lines), Nuclear structure
Resonance ionization spectroscopy is used for measurements of krypton-81 in the natural environment to determine the ages of polar ice caps and old ground-water deposits. Oceanic circulation and the mixing of oceans could also be studied by measuring the concentrations of noble-gas isotopes by resonance ionization spectroscopy.