I'm studying the way that oxygen molecules absorb
vacuum-ultraviolet
(VUV) light. VUV light (or electromagnetic radiation) is high energy
radiation that gets absorbed by air (oxygen to be precise), which is
where the vacuum part of the name comes from.
One of the reasons for studying oxygen molecules is that they are important in
the atmosphere for the formation of
ozone. When an oxygen molecule in the stratosphere region
absorbs the VUV radiation from the sun, it breaks into two oxygen atoms.
These atoms whizz around and can collide with other oxygen molecules,
which then combine to form ozone (a molecule of 3 oxygen atoms). As you
may be aware, ozone is an important absorber of solar ultraviolet
radiation, meaning that it protects us from sunburn.
Another reason is that it is interesting to understand what exactly is
happening inside the oxygen molecule.
To fully understand how oxygen molecules absorb VUV radiation (and
therefore, how much ozone can be formed), we need to look at the
absorption lines in fine detail. One of the ways to do this is to use a
laser. Unfortunately, lasers
that generate VUV radiation can't be made
easily (this is one of the fundamental aspects of making lasers: it is
to do with the amount of energy required versus the rate of dissipation
of this energy and how it varies with frequency).
Fortunately, there are several techniques that can produce VUV radiation
by using laser light that is less energetic than VUV. The techniques
are based on Non-Linear Optics. Non-linear optics refers to the
behaviour of intense light when it interacts with matter (such as a
gas). Normally when light from, say, a light bulb, hits some atoms, the
electrons in the atoms oscillate, essentially in time with the
oscilating light field. It is then said that the response of the
electrons is linear, and the atom will emit light at essentially the
same frequency of the light comming in.
However, when the light is intense, such as from a laser, the electrons
behave differently. Instead of following the oscillations of the light
field, they deviate from it somewhat, responding in a non-linear
manner. This means that the electrons will emit different frequencies
of radiation than those that were input.
We can take advantage of this fact and use it to generate
vacuum-ultraviolet radiation from frequencies that are already easy to
generate.
The non-linear optical technique that we use is known as four-wave
mixing. Essentially, it involves four photons
(this is in German and is quite detailed) of light: three
input photons
and one output. The input photons, in our case, are two at 256 nm (UV), and
one at between 440 nm and 480 nm (blue). These are generated by two
excimer pumped dye lasers, one frequency doubled to give the 256 nm.
The non-linear media that we use for the mixing is Xenon gas.
When the VUV radiation has been generated, it is passed through a small
container or cell that contains oxygen gas.
We measure how much of
the radiation actually gets through the gas (the pressure of the gas can
be varied to ensure that over absorption does not occur). To measure
how much radiation gets absorbed, it is necessary to also
measure the intensity of the radiation before it has passed through the
cell. For this, we have a beam splitter, where we take roughly half of
the radiation and measure it before it has gone through the cell, and
take the ratio of this initial measurement against the final (absorbed)
measurement. By factoring in the cell length, temperature and
number density of the gas, it is possible to obtain a measure of the
absorption of the gas, which is known as a cross section. The
equation used to calculate the cross section (which contains the above
parameters) is called Beer's Law.
More to come soon!
Here's a photo of the UV Physics group!
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Written, designed and maintained by
Kylie Waring - kzw121@rsphy1.anu.edu.au
Comments/suggestions appreciated!
