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Raw data from measurements of UV rays, emitted by
hydrogen and deuterium molecules under the influence of a strong
laser pulse. Higher pixel numbers indicate lower UV wavelengths.
The stronger D2-signal intensity indicates that the vibrations are
slower than those of H2.
Image: Imperial College London
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Scientists at Imperial College London have now
made the fastest measurements ever of molecular dynamics (Blackett
Laboratory Laser Consortium, Prof. Jon Marangos, Director). They used
a new measurement procedure. Its principles come from a theory
developed by Max Planck researchers led by Dr. Manfred Lein. A single
femtosecond pulse is sent to the sample. The pulse creates an electric
field strong enough to wrest an electron, at specific times, from the
molecule. This causes the body of the molecule to come off balance and
begin moving. Because the laser pulse field changes direction
periodically, it sometimes drives the free electron back to the ion.
The electron and the molecule body unite again - and this sends off a
high-frequency UV-photon. This series of events - and the intensity of
UV emission that comes with it - becomes more and more improbable the
further the molecule has travelled from its position at the start
time. Or, in the language of quantum mechanics: the recombination
probability depends on the overlap between start and end wave function.
By measuring the intensity of the UV light, scientists can determine
how the molecule changes over time.
The intensity of the UV ray being sent out is,
unfortunately, influenced by other factors besides nuclear dynamics;
among them, the probability of molecule ionisation. However, the
scientist used a trick to get around this: they observed the spectra
of two different isotopes. Isotopes have largely identical
characteristics; the only way in which they differ is in the mass of
the atomic nuclei. Thus, each isotope has atomic movement at differing
speeds. The newly-published experiments compare hydrogen molecule
spectra with deuterium, which is twice as heavy (see image), as well
as the isotopes of methane, CH4 and CD4.
In measuring molecular changes over time, the
scientists were able to take advantage of a very useful phenomenon,
namely, that a single laser pulse creates a whole spectrum of UV
frequencies. These frequencies can be assigned to the duration of time
the returning electron spent "freed" from the ion. The highest
frequencies are emitted from the electrons freed for the longest time.
The exact resolution of the time measurements is determined by the
difference between neighbouring UV frequencies, and is in the range of
tenths of a femtosecond. The change over time can be reconstructed
from the spectra of two different isotopes. This was done, in the case
of the hydrogen experiment, using a complicated genetic computer
algorithm. The methane isotope data analysis, however, would be
considerably more complicated, and how to perform it exactly is still
an open question.
Compared to the traditional pump-probe principle,
one major advantage of the new method is that a single laser pulse
serves to scan an entire interval of delay times. The experiment does
not need to be repeated multiple times with various pump-probe
intervals. The first author of the original publication, Dr. Sarah
Baker, says "We are very excited by these results, not only because we
have "watched" motion occurring faster than was previously possible,
but because we have achieved this using a compact and simple technique
that will make such study accessible to scientists around the world." |
