Introduction

The chemical properties of elements are based on the number of electrons in the neutral atom and the arrangement of electrons into shells and subshells reflecting specific or quantized energy levels. Photoelectron spectroscopy and ionization energy measurements provide direct evidence for the electronic structure of atoms.

Concepts

• Photoelectric effect

• Planck’s law
• Quantum theory
• Coulomb’s law
• Ionization energy
• Photoelectron spectroscopy
• Electronic structure
• Atomic orbitals

Background

Photoelectron spectroscopy is defined as the measurement of the relative number of electrons of different energies that are ejected from atoms when they are bombarded with high-energy electromagnetic radiation. Photoelectron spectra are generated based on the photoelectric effect, which was first observed for metals in the 19th century. When light of the appropriate wavelength and energy is shined on a metal, electrons may be given off or ejected from the metal surface. If the light is below a certain minimum or threshold frequency, no electrons are produced, regardless of the intensity of the light source. This apparent paradox could not be explained by the laws of classical physics. In 1905, Albert Einstein applied the new quantum theory of light to explain the photoelectric effect. Einstein’s explanation of the nature of the photoelectric effect represented a watershed event in the history of science. The development of quantum physics and quantum mechanics in the years that followed ultimately transformed and led to our current understanding of atomic and electron structure.

Experiments with the photoelectric effect led to two major findings: 1) light must be above a certain frequency for electron emission to occur, and 2) as the frequency of light increases above this minimum, the average kinetic energy of the dislodged particles increases. These findings were interpreted by Albert Einstein using Planck’s law. This law describes light in terms of photons possessing a quantum or bundle of energy, where the energy (E) of a photon is proportional to its frequency (v).

E = hv Planck’s law

The photoelectric effect is only observed when the frequency of light is greater than a specific threshold value, which depends on the nature of the metal. If the energy of a photon is greater than the work function, defined as the minimum energy needed to displace an electron from a metal, the metal will give up an electron. If more photons having this energy strike the metal, additional electrons will be lost, but their kinetic energy will not change. The kinetic energy of photoelectrons ejected from a metal depends on the frequency (energy) of the light source and the work function of the metal. Albert Einstein was awarded the Nobel Prize for Physics in 1921 for his work on the photoelectric effect, which bridged the gap between the laws of classical and quantum physics.

Photoelectron spectroscopy (PES) utilizes the principles of the photoelectric effect to generate spectra revealing the electronic structure (energy levels) of atoms. A photoelectron spectrophotometer consists of three basic parts: a vacuum chamber for the sample, a radiation or light source, and an electron analyzer to separate electrons based on their kinetic energies (Figure 1).

Samples of gas-phase atoms are exposed to electromagnetic radiation of sufficient energy, typically shortwave ultraviolet light or X-rays, to eject electrons from the atoms. Although each atom can emit only a single photoelectron using this technique, a sample always contains a multitude of atoms. Different atoms within the sample will emit electrons from different energy levels. The kinetic energy (KE) of the resulting photoelectrons is measured with an electron analyzer. PES uses monochromatic (single frequency) electromagnetic radiation. Based on the law of conservation of energy, the kinetic energy (KE) of a photoelectron is equal to the energy of the incident light (hv) minus the ionization energy (IE), which is the energy required to remove an electron from a particular energy level within an atom.

From Equation 1, the ionization energies of the electrons within an atom can be calculated. Because the incident light is of sufficient energy to ionize both core and valence electrons, there is an equal probability that each electron in an atom will be ejected. The number of electrons ejected is proportional to the number of electrons present at each energy level. A photoelectron spectrum displays the signal intensity, corresponding to the relative number of electrons ejected at a particular energy, versus the ionization energy. For a spectrum displaying two or more peaks, a comparison of the peak heights allows for the relative number of electrons at each energy level to be determined.

Experiment Overview

The purpose of this guided-inquiry learning activity is to examine experimental evidence for the electron configuration of neutral atoms. The activity is divided into three parts:

1. Review Coulomb’s law to predict the relative ionization energies of electrons in an atom.
   
   
2. Investigate the basic principles of photoelectron spectroscopy and identify the way information is presented in a photoelectron spectrum.
   
   
3. Analyze the photoelectron spectra of elements 1−20 to predict and explain their electron configurations.