Photoelectron spectrometer with means for stabilizing sample surface potential

In a spectrometer of the type including a source of photons directed to a sample whose surface characteristics are to be analyzed, with the photons absorbed by the surface causing electrons to be ejected and means for receiving and detecting said electrons, the spectrometer circuitry including potential sources referenced to a common potential definable as system common, with the spectrometer structure being connected to said system common, the improvement comprising:

• sample support means for supporting the sample thereon;
• electrical insulating means for mechanically coupling said support means to said spectrometer structure, and for permanently electrically insulating said support means from said system common through said spectrometer structure;
• power source means including a first DC voltage power supply adapted to supply a selected voltage and oscillator means coupled to said first DC power supply for providing an output signal at a preselected modulating frequency, whereby the DC voltage provided by said first power supply is modulated above and below a selected voltage with respect to system common;
• electron source means connected to and powered by said first power supply for providing low energy electrons directed to the sample surface;
• a second DC power supply, controllable to supply a variably selected voltage across first and second terminals thereof,
• a resistor connected at one end to said support means and at an opposite end to the first terminal of said second power supply;
• a lock-in amplifier having differential input terminal means, a modulation input terminal and an output terminal;
• means for connecting said resistor to said differential input terminal means of said lock-in amplifier to thereby apply AC voltage across said resistor as a function of AC current flowing through said resistor to said lock-in amplifier, and for connecting the oscillator output signal to the lock-in amplifier modulation input terminal, whereby the amplitude of the output of said lock-in amplifier at said output terminal is indicative of the AC current amplitude through said resistor;
• and
• means for selectively connecting said second terminal of said second power supply to said system common or the amplifier output terminal, said second power supply being adjustable to provide a selected voltage with respect to system common to adjust the surface potential of said sample with respect to system common, so that AC current through said resistor is at a peak amplitude.

A method for determining the surface potential of a sample, with respect to a reference potential, the steps comprising:

• supporting a sample on the back side thereof, which is opposite a sample surface, on a support member which is not in direct contact with said reference potential;
• providing a source of low energy electrons directed to the sample surface;
• energizing the source of electrons with a voltage which is modulated above and below a variably selected voltage with respect to said reference potential at a preselected modulating frequency;
• measuring the AC current through said sample produced as a result of the electrons from said source which are absorbed by said sample surface to determine the AC current amplitude;
• and
• varying a potential with respect to said reference potential, which is applied to the back of said sample to control said AC current to be at a peak amplitude.