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The photon pair correlation in the laser-excited fluorescence of a single trapped and cooled Ba + ion shows antibunching and, in addition, novel nonclassical phenomena absent in the fluorescence of twolevel atoms. They include excessive transient values of the correlation caused by optical pumping, and temporally extended sub-Poissonian photon emission probability which arises from the transient excitation of nonabsorbing Raman coherence. The fluorescence also displays sub-Poissonian photon statistics. PACS numbers: 42.50.Dv, 32.50.+d Although light is known to carry information on its source encoded in the correlation functions of all orders, it is the second-order or intensity correlation which provides the main information on intensity fluctuations [1]. Its detection requires two measurements of the light flux. On the microscopic level, the intensity correlation is represented by the correlation of photoelectrons recorded in two events of detection [2], from which the characteristics of photon statistics have been inferred [31. Atomic resonance fluorescence is a case in point: Measurements of its intensity correlation by comparing the timeseparated photon counting signals in one or two channels of detection have revealed nonclassical properties of the light for which the atomic interaction with the excitation light and the vacuum field is responsible. In particular, sub-Poissonian photon statistics [4] have been observed [5,6], and also the rise of the correlation of the two detected photons upon the increase of their time separation T close to zero ("antibunching" [6,7]). So far, the observations have included dilute atomic beams [5,7] or a single ion in a rf trap [6]. The involved atomic particles could be well approximated as two-level systems with the monochromatic laser light cyclically exciting the resonance line.We have, for the first time, recorded the intensity correlation of the resonance fluorescence of a single ion which cannot be modeled as a two-level system. The observed intensity correlation reveals novel features that are not seen in the resonance fluorescence of two-level atoms. These features include a maximum photon correlation which is much larger than what is possible with two-level atoms, and also photon antibunching with much larger time constants of the initial photon anticorrelation.A single Ba + ion, stored in a 1-mm rf trap of 25-MHz drive frequency [8], was laser cooled [9] to less than 3 mK, and its laser-excited resonance fluorescence was recorded by two photon counting channels placed in opposite directions. The relevant levels of Ba + and the wavelengths of the two dye-laser-generated light fields are shown in Fig. 1. The 6 2 / > j/2 resonant level decays with 8-ns lifetime to the ground state 6 2 S\/2 and also to the metastable level 5 2 Z>3/2, with branching ratio 2.85 in favor of the ground state. Since the metastable level lives for 17 s [10], the two light fields are required for the elimination of optical pumping and the generation of a continuous flux of fluorescence. A ...