Electronic- Ionisation Mechanisms Flashcards
How does photoelectric effect disagree with classical mechanics?
KE should rise exponentially with intensity and remain constant with frequency. Actually independent of intensity and varies linearly with frequency above a certain frequency
Kinetic energy of photoelectron
Ek=hv-Φ
Experimental set up for photoelectric effect
Light with a controlled λ and intensity shone onto a cathode. Electrons emitted onto an ankle and flow around a circuit. Need a vacuum to allow electron detection. Need oxide free metal surface
Efficiency of photoelectric effect
Units Amps/Lumen or power %
Quantum efficiency is electrons per photons
Why aren’t simple metals very efficient?
They have high optical reflectivity so not many photons allowed in. Only electrons produced within 1nm of surface are able to escape but photons can penetrate much further and the photoelectrons they cause further inside fail to escape.
Why is yield from simple metals wavelength dependent?
Reflectivity and absorption coefficient variations. Increased reflectivity for longer wavelength. Increased transparency (photons penetrate deeper) for shorter wavelength. Efficiency vs wavelength graph is like normal distribution curve with peak about 5%
Solutions to low efficiency of simple metals
Use inter-metallic (compromise between metal and ceramic properties. Have more tightly bound electrons than pure metal so lower reflectivities.
Use ultra thin oxide layers which reduce reflectivity while allowing photoelectrons to escape. Also lower work function, increasing sensitivity out to IR wavelengths. Efficiencies up to 45% possible
Photomultiplier tube
Highly sensitive detection of light. Light strikes a cathode which emits one electron. There is a voltage drop to the next cathode so electron is accelerated to it and can knock another electron out of that one. Repeats over 8-12 states over cascade of electrodes (dynodes) to amplify the electron current (easier to measure than 1 electron). Phosphor screen or electronic detection at the end.
Thermionic emission
Heat used to cause emission of electrons. Boltzmann statistics apply as Ef+Φ»Ef compared with kT. So available levels are empty to PEP doesn’t affect answer. Emission proportional to exp(-Φ/kBT)
Thermionic current density formula
J=A0(1-r)T^2exp(-Φ/kBT)
A0=4πmkB^2e/h^3=1.20x10^6Am^-2K^-2
r is reflectivity =1/2nas half goes one way and half the other
Units J A/m^2
Set up for measurement of thermionic emission
Tungsten filament has current through it to heat it causing electron emission to positive electron over a vacuum. This is made positive by externally applied potential which also means emission continues at theoretical rate. All connected in series apart from vacuum gap between filament and anode
Why is tungsten used for thermionic emission?
Although it’s high work function requires high operating temperature, its melting point is high so extremely high temperatures can be achieved so is the best for the greatest emission
What does it mean if potential barriers are not infinitely high for electrons?
The potential well is finite. The wavefunction no longer goes to 0 outside of the metal. E-V is negative outside the metal so wave equation becomes
d2ψ/dx2=k^2ψ (not -k^2ψ)
This has solutions which are exponentials (decaying only).
Therefore finite probability of tunnelling through the barrier (which is few nm thick)
What happens if another material is placed very close to the metal?
About 1nm away. Barrier becomes very narrow. Decaying wave function is still finite across the barrier and an electron can tunnel through to the adjacent material
How is a scanning tunnelling microscope set up (STM)?
Sample connected to tunnelling voltage. A conductive tip from a piezoelectric tube (controlled voltage) is placed very close to the sample. An electron goes from the sample to the tip to a tunnelling current amplifier to a distance and control scanning unit to data processing and display
