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The Planck constant, with its mathematical symbol $h$, is a fundamentalconstant in quantum mechanics that is associated with the quantization of lightand matter. It is also of fundamental importance to metrology, such as thedefinition of ohm and volt, and the latest definition of kilogram. One of thefirst measurements to determine the Planck constant is based on thephotoelectric effect, however, the values thus obtained so far have exhibited alarge uncertainty. The accepted value of the Planck constant,6.62607015$\times$10$^{-34}$ J$\cdot$s, is obtained from one of the mostprecise methods, the Kibble balance, which involves quantum Hall effect,Josephson effect and the use of the International Prototype of the Kilogram(IPK) or its copies. Here we present a precise determination of the Planckconstant by modern photoemission spectroscopy technique. Through the direct useof the Einstein's photoelectric equation, the Planck constant is determined bymeasuring accurately the energy position of the gold Fermi level using lightsources with various photon wavelengths. The precision of the measured Planckconstant, 6.62610(13)$\times$10$^{-34}$ J$\cdot$s, is four to five orders ofmagnitude improved from the previous photoelectric effect measurements. It hasrendered photoemission method to become one of the most accurate methods indetermining the Planck constant. We propose that this direct method ofphotoemission spectroscopy has advantages and a potential to further increaseits measurement precision of the Planck constant to be comparable to the mostaccurate methods that are available at present.

High Precision Determination of the Planck Constant by Modern Photoemission Spectroscopy