 Tech Note 1067 Detail

Cathode / Anode Materials and Work Function

Affected Products:
Problem/Symptom:
Photoelectric Cathode / Anode Materials and Work Function

PASCO Solution:

Cathode Material

The cathode of the SE-6609 Photoelectric Effect System has a transparent layer of silver oxide deposited on it.The work function will vary locally due to the lack of homogeneity in the surface, therefore exact work function of the cathode is unknown.

Minimum Cathode sensitivity: >1 µA/Lm

Anode Material

The anode is a very fine nickel metal ring. Nickel has a relatively high work function (5.0 eV). The ring geometry is designed to reduce the curvature of the plot of current versus voltage [I(V)]. In the limit of zero curvature one would see two straight line segments corresponding to the cathode-anode current and the reverse anode-cathode current.

Discussion

A careful analysis demonstrates that the measured stopping potential should be equal to the work function of the anode, not that of the cathode. The applied potential is the potential between the cathode and anode, not the potential that the electrons encounter. When the radiation has sufficient energy the electron overcomes the work function of the cathode, but accelerates as it approaches the anode because of the work function of the anode. The actual vacuum field potential should be lower than the measured potential difference between the cathode and anode by the difference in work functions, which is also known as the contact potential:

Vstopping = Vapplied - Vcontact = Vapplied - (Wcathode -Wanode) (Equation 1)

The current stops if the light cannot eject the photon from the first potential, which is the sum of the applied (stopping) potential and the cathode work function:

h f = e Vbias + e Wcathode Electron Ejection condition
h f = e Vapplied + e Wanode Substitution of Equation 1

Due to a reverse current from the anode to the cathode, which results from thermionic voltage and electrode contamination, there is a linear portion to the curve of photocurrent versus stopping potential. The stopping potential is the point when the reverse current saturates, which occurs at the inflection point in the current-voltage function, not the intercept with the applied voltage axis. Accurately determining the exact stopping potential requires averaging several runs.

The lower linear section of the I(V) curve is due to reverse current that one extrapolates to zero voltage to determine the reverse current. Adding this value to the measured current yields the corrected cathode-anode current . The value at which this corrected current intersects the I= 0 axis is then the true stopping potential. With this correction the measured value for the work function should be close to the actual value of the work function of the nickel anode (5.0 eV).

Below is a plot of the current as a function of the applied voltage Vapplied: Creation Date: 03/1/2010