Computational Chemistry

The first inkling of this phenomenon was due to Hertz,^3 who in 1888 noticed that
the potential needed to elicit a spark across two electrodes decreased when ultravi-
olet light shone on the negative electrode. Beginning in 1902, the photoelectric
effect was first studied systematically by Lenard,^4 who showed that the phenome-
non observed by Hertz was due to electron emission.
Facts (Fig.4.3) that classical physics could not explain were the existence of a
threshold frequency for electron ejection, that the kinetic energy of the electrons is
linearly related to the frequency of the light, and the fact that the electron flux
(electrons per unit area per second) is proportional to the intensity of the light.
Classical physics predicted that the electron flux should be proportional to the light
frequency, decreasing with a decrease in frequency, but without sharply falling to
zero below a certain frequency, and that the kinetic energy of the electrons should
be proportional to the intensity of the light, not the frequency.
These facts were explained by Einstein^5 in 1905 in a way that now appears very
simple, but in fact relies on concepts that were at the time revolutionary. Einstein
went beyond Planck and postulated that not only was the process of absorption and
emission of light quantized, but that light itself was quantized, consisting in effect
of particles of energy

kinetic energy

of the emitted electrons,

1 / 2 mv^2

0

1 / 2 mv^2 = h –W

• W

frequency, , of the light

Fig. 4.3 The photoelectric effect. Einstein explained the effect by extending to light Planck’s idea
of the absorption and emission of energy in discrete amounts: he postulated that light itself
consisted of discrete particles

(^3) Heinrich Hertz, born Hamburg, Germany, 1857. Ph.D. Berlin, 1880. Professor, Karlsruhe, Bonn.
Discoverer of radio waves. Died Bonn, 1894.
(^4) Philipp Lenard, German physicist, born Pozsony, Austria-Hungary (now Bratislava, Slovakia),

1862. Ph.D. Heidelberg 1886. Professor, Heidelberg. Nobel Prize in physics 1905, for work on
      cathode rays. Lenard supported the Nazis and rejected Einstein’s theory of relativity. Died
      Messelhausen, Germany, 1947.

(^5) Albert Einstein, German–Swiss–American physicist. Born Ulm, Germany, 1879. Ph.D. Z€urich

1905. Professor Z€urich, Prague, Berlin; Institute for Advanced Studies, Princeton, New Jersey.
      Nobel Prize in physics 1921 for theory of the photoelectric effect. Best known for the special
      (1905) and general (1915) theories of relativity. Died Princeton, 1955.

90 4 Introduction to Quantum Mechanics in Computational Chemistry