Unit 7C - Photoelectron Spectroscopy (PES) - AP Chemistry

the nature of light

all light is electromagnetic radiation

photons consist of two perpendicular fields: an electric field wave and a magnetic field wave

wavelength

the distance from crest (peak) to crest

typically measured in nanometers (nm) (1m = 10⁹ nm)

frequency (v)

the number of crests (peaks) that pass by a given point per second

measured in units of 1/s or s^-1

speed of light (c )

all light waves propagate at the same speed through a vacuum

3.10 × 10⁸ m/s

speed of light equation

c = λ * v

energy of a light wave

E = hv = hc/λ

planck’s constant (h)

6.63 × 10^-34 Js

same wavelength emitted/absorbed =

same energy required to move between energy levels

duality of light

light energy exhibits evidence of behaving both as a wave and as a particle

light as a wave

Youngs dual split experiment

when waves are in phase, ______ increases

amplitude/brightness

constructive interference

when the crests of two separates waves line up to increase amplitude

destructive interference

when the crests of two separates waves do not line up to decrease net amplitude

light as a particle

photoelectric effect

photoelectric effect

the observation that photons short-wave (visible or UV) light can cause it to emit electrons

example of ionization, with the photons providing the ionization energy

how the photoelectric effect works

when visible or ultraviolet light is shone on a substance, the energy from the photons of light excites electrons in the substance

if the energy exceeds the ionization energy of an atom in the substance, the electron is emitted (these are called photoelectrons)

requirements for photoelectron emission

light frequency needs to be higher than a certain critical value

amplitudes effect on electron emission

number of electrons emitted

no effect on KE

more e- emitted = more “packets” of light (photons) hitting = increased brightness

increased wavelength frequency =

increased kinetic energy (how much energy e- get kicked out with)

increased amplitude/brightness =

increased number of photons

1 eV =

1.602 × 10^-19 J

E_B

binding energy of electron to nucleus

W_o

work function, the amount of additional energy it takes to move the delocalized electron to the surface of the material (zero for gases, but nonzero for solids)

Einstein’s equation for the photoelectric effect

E_KE = hv - E_B - W_o

photoelectron/photoemissions spectroscopy

using the energy from electrons emitted via the photoelectric effect to gain information about the electronic structure of a substance

how photoelectron/photoemission spectroscopy works

a substance is bombarded with photons, which have a given amount of energy based on their frequency

by measuring the KE of the emitted electron, and predetermining the work function of the substance, we can calculate the binding energy of the electron from Einstein’s equation for the photoelectric effect

photoelectron spectroscopy is generally used for

gases

photoemission spectroscopy is generally used for

electrons emitted from solid surfaces

ultraviolet photoelectron spectroscopy (UPS) and extreme ultraviolet photoelectron spectroscopy (EUPS) are used

to study valence electrons and the electrons that participate in chemical bonding

X-ray photoelectron spectroscopy (XPS) is used

to study core electrons, particularly in solids

evidence for electrons as particles

cathode ray experiment

• absence of propagation of electrons

• ability to reflect cathode beam with a given electrostatic charge —> mass to charge ratio

  • mass characteristics = particulate behavior

evidence for electrons as waves

Young’s dual slit experiment with electrons

• over time, showed pattern of constructive and destructive interference, just like with light

schrodinger’s equation (wave equation)

used to predict the electron positioning in orbitals