Whether
they know it or not, anyone who’s ever gotten a speeding ticket after
zooming by a radar gun has experienced the Doppler effect – a measurable
shift in the frequency of radiation based on the motion of an object,
which in this case is your car doing 45 miles an hour in a 30-mph zone.
But
for the first time, scientists have experimentally shown a different
version of the Doppler effect at a much, much smaller level – the
rotation of an individual molecule. Prior to this such an effect had
been theorized, but it took a complex experiment with a synchrotron to
prove it’s for real.
“Some
of us thought of this some time ago, but it’s very difficult to show
experimentally,” said T. Darrah Thomas, a professor emeritus of
chemistry at Oregon State University and part of an international
research team that today announced its findings in Physical Review Letters, a professional journal.
Most
illustrations of the Doppler effect are called “translational,” meaning
the change in frequency of light or sound when one object moves away
from the other in a straight line, like a car passing a radar gun. The
basic concept has been understood since an Austrian physicist named
Christian Doppler first proposed it in 1842.
But a similar effect can be observed when something rotates as well, scientists say.
“There
is plenty of evidence of the rotational Doppler effect in large bodies,
such as a spinning planet or galaxy,” Thomas said. “When a planet
rotates, the light coming from it shifts to higher frequency on the side
spinning toward you and a lower frequency on the side spinning away
from you. But this same basic force is at work even on the molecular
level.”
In
astrophysics, this rotational Doppler effect has been used to determine
the rotational velocity of things such as planets. But in the new
study, scientists from Japan, Sweden, France and the United States
provided the first experimental proof that the same thing happens even
with molecules.
At
this tiny level, they found, the rotational Doppler effect can be even
more important than the linear motion of the molecules, the study
showed.
The
findings are expected to have application in a better understanding of
molecular spectroscopy, in which the radiation emitted from molecules is
used to study their makeup and chemical properties. It is also relevant
to the study of high energy electrons, Thomas said.
“There
are some studies where a better understanding of this rotational
Doppler effect will be important,” Thomas said. “Mostly it’s just
interesting. We’ve known about the Doppler effect for a very long time
but until now have never been able to see the rotational Doppler effect
in molecules.”
