Resonance Ionization
What is it?
When an atom has the same number of positively charged
particles (protons) and negatively charged particles (electrons),
the atom is said to be neutral and has zero net charge. Ionization
is a process in which one or more electrons are removed (or
attached) to a neutral atom so that the atom now has a net
charge. A charged atom is called an ion.
Resonance Ionization (RI) is a laser-based ionization
technique in which the color of the laser is chosen to
selectively excite atoms of a particular element, such as
mercury, copper, or arsenic. These excited atoms are then ionized
by another laser or one of several other techniques. See our Periodic Table of the Elements to
discover how different RI schemes are applied to different
elements.
Why do it?
Once an atom is ionized, it is very easy to detect. Resonance
Ionization provides a very efficient means of selectively
ionizing toxic elements, contaminants, dopants, impurities, or
other elements of interest so that their concentration can be
measured. We combine RI with ion beam sputtering and mass
spectrometry to provide a uniquely selective and efficient surface analysis capability.
How does it work?
You can learn more about how Resonance Ionization works by
reading the following brief technical description or you can skip
down for a nontechnical analogy.
Resonance Ionization, RI, (also called Resonance
Ionization Spectroscopy, RIS) is a laser-based
analytical technique, comprised of one or more electronic
excitation steps followed by ionization of the excited atom. RI
can be applied to either atoms or molecules (when applied to
molecules, it is generally referred to as Resonantly Enhanced Multiphoton Ionization,
REMPI) but we will
restrict the description here to atoms. In RI, the wavelength of
one or more of the laser beam(s) is tuned to resonance with a
particular electronic transition in the analyte atoms. The
resonance condition greatly increases the probability of
excitation and, therefore, even lasers with modest intensities
can very efficiently excite the analyte atoms, but will
essentially leave other atoms and molecules unaffected. Once the
electrons are excited to a high energy state by one or more
resonance excitation steps (a different wavelength is required
for each step), another laser can be used to photoionize the
excited atom (other ways to ionize the excited atom include field
ionization and collisional ionization). The overall process
results in an ionization technique that is both extremely
efficient and very selective for the chosen analyte. When coupled
with an efficient means of detecting the ions, RI becomes a very
effective analytical tool.
A NONTECHNICAL ANALOGY
(with apologies to physicists and chemists who may be reading
this.) - Imagine a mountain climber at the bottom of a
vertical cliff. Our climber has a grappling hook and rope to help
make the ascent to some small ledges positioned at various
intervals up the face of the cliff. When the climber tries to
toss the hook to catch on a particular ledge, if the throw is
short, there is zero chance of catching the ledge. If the throw
is too high, the hook bounces off the face of the cliff, and has
a very low probability of catching the ledge on the way down. But
when the throw is perfect, the hook catches and our intrepid
climber can ascend. After one or more of these ascents to a
higher ledge, she should be able to throw the hook to the top of
the cliff, at which point it is OK to overthrow the hook because
it will simply grab one of the many objects at the top. The
climber is then free to roam the top of the cliff, no longer
confined to the valley floor.
In this analogy, our climber is an electron and the force that
holds her to the valley floor (gravity) is analogous to the
attractive force that the positively charged nucleus exerts on
the electron. The ledges are energy states and their vertical
positions are equivalent to energy levels. A different cliff
would have different vertical spacing between ledges,
representing the different electronic levels of a different
element. When the climber tosses the hook too low or too high,
this is the same as being off resonance, i.e. using a laser with
the wrong color (the energy of light is related to its color).
When the throw is just right, we have a resonance condition, and
the electron can advance to the next energy state. The final
throw over the top is photoionization, which frees the electron
from the pull of the nucleus.
(This analogy fails if pressed too far. If you have a
better analogy that gets the idea across, please return to our
home page and e-mail us. If we agree your analogy is better, we
will be happy to include it on this page in the future with full
credit to your authorship.)
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