As defined in the "Electronics Engineers' Handbook, edited by Donald K. Fink and Alexander A. McKenzie, published by McGraw-Hill, 1975, page 1-7" work function is a term applied to the amount of energy required to transfer electrons from the interior of one substance across an interface boundary into an adjacent substance of space. It is commonly expressed by the transfer of an electron across the boundary in units of electron volts. Known in the art is an apparatus for measuring the work function of metal surfaces (U.S. Pat. No. 4,072,896 issue date Feb. 7, 1978, the inventors are Paul Frederik and Adriaan Bijlmer), this apparatus uses a method devised by Kelvin and known as the "dynamic capacitor method". According to this method, a conductive plate is brought into close vicinity of the metal surface under examination in order to create a capacitor formed by such plate and such metal surface. If an external electrical circuit is connected to such capacitor, a flow of electrons will pass through this circuit, and the electrochemical potentials of conductive plate and metal surface are equalized thereby. Nevertheless, an electrostatic potential difference is generated between the two surfaces and the value of this potential difference will be equal to the difference between the work function of both surfaces. Any variation in distance between the conductive plate and the metal surface will cause a capacitance variation in the capacitor. Since the work functions are independent of the capacitor plates spacing, such capacitance variation will be accompanied by a charge variation and will cause an electric current to flow through the external circuit. If the conductive plate is subjected now to periodic vibrations in a direction vertical to the metal surface under examination, this will result in an ac current flowing in the external circuit. The measuring potential generated in this external or measuring circuit is a standard for the work function since the ac current will satisfy the equation EQU i=dQ/dt=(dC/dt).multidot.V
wherein: i is the output ac current, Q is the electric charge of the capacitor, C is its capacitance and V is the voltage across the capacitor.
Also known in the art is the Kelvin probe apparatus for dynamically measuring the change in work function (Surface Science 83 (1979), page 193.) The operating mechanism of this apparatus is to vibrate a reference electrode also called vibrating reed in close proximity to the surface under examination. The change in separation between the reed and the experimental surface, produces a change in capacitance which in turn generates an ac signal that can be amplified and detected if the work function of the sample and the reed are different. The external measuring circuit of this apparatus comprises a lock-in amplifier, the oscillator of this lock-in amplifier is coupled to the drive reed means so that the lock-in amplifier is referenced to the reed drive frequency which is substantially the reed vibrational frequency. The lock-in amplifier uses the reed drive signal to detect and measure changes in the work function of the surface under examination. The reed of this apparatus can be driven by a variety of methods, including a piezo-ceramic crystal and an electromagnetic coil, at its resonant frequency or some harmonic of the fundamental resonance.
Although, in principle, this method "the dynamic capacitor method" is suitable for measuring the changes in the work function of a surface, there are some problems in the actual practice. If the frequency signal of the driving differs by even a small amount from the resonant frequency, then the reed will stop vibrating. Also the phase relationship between the driving frequency and the reed frequency will change rapidly as the driving frequency moves off resonance.
Non isothermal .DELTA..phi. experiments in which the experimental surface is heated to high temperature in vacuum or in the presence of a gas, cause heating of the reed by convection or radiation, resulting in a shift of the resonant frequency and consequent change of phase and amplitude. This causes severe problems in the use of a phase sensitive detector also called lock-in amplifier which forms part of a measuring circuit for monitoring .DELTA..phi..
Experimentally, the drive coil frequency must then be shifted by the appropriate amount to restart the probe, which requires adjustments to the phase sensitive detector. One can appreciate the difficulty in successfully conducting experiments involving large or rapid temperature changes.
There is a need for a system for automatically keeping the reed in resonance regardless of experimental conditions.
There is also a need for a system for monitoring the reed vibrational frequency and amplitude, allowing a quick determination of the proper resonant frequency as well as ensuring that the reed continues to vibrate during the course of the experiments.
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