Transition from direct ionization to delayed ionization for free C60 and C70 is observed below the ionization potential (IP =7.5 eV). It depends on the excitation wavelength as well as the pulse energy. The appearance threshold for direct ionization is found at -5.8 eV. These can hardly be explained by thermionic emission; therefore, we propose that triplet states at -1.7 eV (=7.5 -5.8 eV) should play an important role in the multiphoton ionization of free C60 and C70 and the intramolecular interaction of many triplet states gives rise to delayed ionization. To confirm our proposition, we have carried out further experiments using two laser ionization. PACS numbers: 36.40.+d, 33.80.Rv Delayed ionization is ionization that occurs after the excitation pulse is over. It has been found in atoms [1], molecules [2], molecular adducts [3], and clusters [4].Although different mechanisms were proposed for delayed ionization in different systems, interaction with long-lived electronic excitation was generally involved [1][2][3][4]. Since 1990, however, thermionic emission has replaced long-lived electronic excitation as the mechanism for clusters [5,6]. For free C60 and C70, decay tails due to delayed ionization were observed in multiphoton ionization and vanished in single-photon direct ionization [7,8]. It looked similar for different wavelengths in the 248 -532nm (5-2.3eV) range [8] but depended on pulse energies (fluences) [9], where the measured slope n > 4 could not be understood by simple multiphoton ionization; thermionic emission was therefore considered the mechanism again. Recently, we found delayed ionization to be absent in multiphoton ionization of free C60 and C70 using high intensity subpicosecond laser excitation at 248 nm [10]. As the mechanism for this, direct ionization through coherent two-photon absorption under very high intensity excitation may not directly question the general validity of thermionic emission. Based on Klots's model [11], nearly ten 4-5 eV photons are supposed to be absorbed by one free C60 or C70 molecule in order to arrive at a temperature of 3000-4000 K leading to thermionic emission [8,9]. However, it was reported [12] that C60 starts to decompose at 1300 K. Here, we report direct observation of a transition from delayed ionization to direct ionization in the multiphoton absorption, where the appearance threshold for direct ionization of C60 is found at 5.8 eV rather than at ionization potential IP =7.5 eV. Based on our new results, we may ask, "Does delayed ionization for free C60 and C70 really result from thermionic emission of electrons following multiphoton absorption?" Free C60 and C70 were generated by heating an oven (~500°C) containing pure C60/C70 powder [13] under a 5xlO~8 mbar vacuum. The oven was mounted -1 cm below an ionization area that was located between two ion optics electrodes of our linear time-of-flight mass spectrometer. The extraction voltage was held constant. The apparatus and measurements have been fully described previously [14]. Standard excimer las...

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