THE FEMTOSECOND FIELD-EMISSION CAMERA, A DEVICE FOR CONTINUOUS OBSERVATION OF THE MOTION OF INDIVIDUAL ADSORBED ATOMS AND MOLECULES

A new instrument, the femtosecond field-emission camera (FFEC), has be en developed to continuously record the motion of single adsorbed atom s or molecules, with an ultimate achievable time resolution of 10-(14) s. In the FFEC, the motion of an adsorbed species modulates a strong 10(-5)-A field-emission current from a sharp tip. The emitted electron s are focused into a beam, which is swept electrostatically across a d etector screen. The tip substrate can be imaged atomically by field io n microscopy. In this paper, the construction and operating principles of the FFEC are described in some detail, and previously published ex periments are reviewed. On a (111) W tip, single Cs atoms are observed to jump between sites instantaneously within the 2-ps instrumental re solution. Individual copper phthalocyanine molecules are observed vibr ating with respect to the substrate with a period of similar or equal to 10 ps. The time resolution of the FFEC Is limited principally by th e time-of-flight spread of the electrons between the tip and the defle cting field.