Рубрика:
60
Raman and his colleagues at the University of Calcutta and the Indian Association for
the Cultivation of Science set out to prove Smekal right. We now know that one of
the characteristics of inelastic scattering is that its intensity scales to the fourth power
of the energy. This meant that the inelastic scattering effect that Raman sought using
visible light (about 500 nm) was at least 10 orders of magnitude weaker than that
observed by Compton using X-rays (0.7 nm).
Raman observed this weak effect by using the most intense light source available at
the time: the sun. In his initial experiments, he used a 7-in. reflecting telescope in
combination with a short focal length eyepiece to focus sunlight onto a purified liquid
or its dust-free vapor. He then used complementary yellow-green and blue-violet
filters to observe the incident and scattered beams. Using this simple experimental
apparatus, Raman discovered that a small amount of the incident light had been
inelastically scattered by the molecules in the liquid and shifted in energy into
another part of the spectrum. He later observed this shift in wavelength as additional
bands on a spectrograph.
This remarkable discovery was made on Monday, February 27, 1928, and described
by K.S. Krishnan, Raman's student, in his diary:
Went to the Association in the afternoon. Professor was there. Started studying the
effect of incident light wavelength on the new scattering effect. Astonished to see that
the scattered radiation has wavelength different from the incident one wavelength
higher and shorter than that of the incident radiation.
Raman didn't waste any time. That Friday, March 31,1928, he and Krishnan
published another letter in Nature, titled, "A New Type of Secondary Radiation,"
which described what would later be known as the Raman effect or Raman shift.
The amount of work that Raman and Krishnan accomplished in such a short time is
amazing as well. They reported in Nature that: "Some 60 different common liquids
have been examined in this way, and every one of them showed the effect in greater
or lesser degree." Equally astonishing is the speed with which the field adopted
Raman's discovery. By August of the following year, there were already 150
scientific publications related to the Raman effect.
That year, 1929, Raman was nominated for the Nobel Prize in physics. But the prize
went instead to Louis de Broglie for his work on the wave nature of the electron.
Raman was nominated again in 1930 and, just two years after his initial experiment,
he was awarded the prize at the age of 42. Although this might seem fast, it was in
keeping with the wishes of Alfred Bernhard Nobel, who willed that the proceeds of
his estate be "distributed in the form of prizes to those who, during the preceding
year, shall have conferred the greatest benefit to mankind."
It was clearly Nobel's plan
to reward recent discoveries—a concept seemingly forgotten today, when it can
easily take a year or longer to publish a manuscript.
Raman spectroscopy quickly became the technique of choice for conducting
molecular vibrational studies. However, after World War II, it fell out of favor and
was largely replaced by then-simpler infrared and near infrared spectroscopy
techniques. Like the technique it inspired, the S.S. Narkunda fell on bad times. After
being pressed into use as a troop carrier, it was sunk by the German Lufthansa in
1942.
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