The Shell ModelThe Shell Model

And Photoelectron Spectroscopy And Photoelectron Spectroscopy and the Structure of Atomsand the Structure of Atoms

Ch 7 part 1.5Ch 7 part 1.5

A little historyA little history Einstein earned the Noble Prize for his Einstein earned the Noble Prize for his

work on the Photoelectric effect.work on the Photoelectric effect. The photoelectric effect is when electrons The photoelectric effect is when electrons

are emitted from a solid liquid or gas when are emitted from a solid liquid or gas when they absorb energy in the form of light.they absorb energy in the form of light.

This has been determined experimentally This has been determined experimentally by shining light on metal.by shining light on metal.

Photoelectric EffectPhotoelectric EffectBy shining a light on metal, By shining a light on metal, one may create a currentone may create a current

Photoelectric effectPhotoelectric effect

Increasing the intensity of the light Increasing the intensity of the light increased the number of photoelectrons, increased the number of photoelectrons, but not their maximum kinetic energy but not their maximum kinetic energy

Red light will not cause the ejection of Red light will not cause the ejection of electrons, no matter what the intensity!electrons, no matter what the intensity!

A weak violet light will eject only a few A weak violet light will eject only a few electrons, but their maximum kinetic electrons, but their maximum kinetic energies are greater than those for intense energies are greater than those for intense light of longer wavelengths!light of longer wavelengths!

EEphoton = hphoton = hvv

Photoelectron Photoelectron SpectroscopySpectroscopy PES provides a useful means to extract PES provides a useful means to extract

information on atomic structure using information on atomic structure using spectroscopic data.spectroscopic data.

By looking at the data chart one may By looking at the data chart one may determine the structure of the atom.determine the structure of the atom.

How? The chart gives evidence for the How? The chart gives evidence for the shell model.shell model.

Photoelectron Photoelectron Spectroscopy (PES)Spectroscopy (PES)

Do all electrons in a Do all electrons in a given shell have the given shell have the same energy?same energy?

To answer this a To answer this a technique using technique using radiation on a neutral radiation on a neutral atom in the gas atom in the gas phase called phase called Photoelectron Photoelectron Spectroscopy is used.Spectroscopy is used.

The technique shines The technique shines radiation (UV or X-radiation (UV or X-ray) on an atom to ray) on an atom to excite it to the point excite it to the point that it will eject an that it will eject an electron from its shell electron from its shell and form a cation.and form a cation.

Shell ModelShell Model

In the shell model, we In the shell model, we see the ionization see the ionization energy is decreased energy is decreased the farther the electron the farther the electron is from the nucleus.is from the nucleus.

By the way of By the way of reasoning, it takes reasoning, it takes more energy to more energy to remove an electron remove an electron from n=2 than n=3.from n=2 than n=3.

Photoelectron Photoelectron Spectroscopy (PES)Spectroscopy (PES)

The energy of the The energy of the photon used is photon used is higher than the higher than the ionization energy.ionization energy.

The excess energy is The excess energy is converted to the converted to the kinetic energy on the kinetic energy on the ejected electron.ejected electron.

The energy required The energy required to remove the to remove the electron is equal to electron is equal to the difference the difference between the energy between the energy absorbed by the atom absorbed by the atom (hv) and the kinetic (hv) and the kinetic energy of the ejected energy of the ejected electronelectron

IE = hv - KEIE = hv - KE

Photoelectron Photoelectron Spectroscopy (PES)Spectroscopy (PES)

This technique allows This technique allows the researcher to the researcher to remove one electron remove one electron from any shell.from any shell.

Therefore, One may Therefore, One may remove electrons remove electrons from the inner core from the inner core without moving the without moving the outer electrons.outer electrons.

The apparatusThe apparatus

Reading a PES spectraReading a PES spectra

On the PES spectra, energy increases as one moves On the PES spectra, energy increases as one moves to the left. The height refers to the number of to the left. The height refers to the number of electrons.electrons.

DemostrationDemostration

http://www.chem.arizona.edu/chemt/http://www.chem.arizona.edu/chemt/Flash/photoelectron.htmlFlash/photoelectron.html