Shedding New Light on Thermionic Electron Emission of Fullerenes

Abstract

Tunable pulsed infrared (IR) radiation from a free electron laser is used to selectively excite fullerenes via their vibrational modes to very high internal energies. After absorbing several hundred IR photons, the molecules can autoionize. Ion creation is unexpectedly slow, occurring at times beyond 0.1 ms after laser excitation, but is found to be significantly faster for electronically preexcited molecules. It is concluded that reaching the first electronically excited state is a high hurdle on the way from hot molecules to ions. [S0031-9007(98)06980-4] PACS numbers: 36.40.Vz, 33.20.Ea, 33.80.Rv For a long time, the microscopic equivalent of the thermal emission of electrons from a heated surface, the thermionic emission of electrons from neutral hot molecules, was not observed. For thermionic emission to occur, a large amount of vibrational energy has to be converted into electronic energy and the probability and mechanism of such a process is not a priori clear. During the last decade, however, delayed electron emission has been observed for metal clusters [1-3] as well as for fullerenes [4][5][6][7][8][9][10][11][12]. Although for C 60 the influence of multiple excited electronic states is still being discussed [8], most experiments indicate that this delayed electron emission cannot be attributed to a direct (electronic) ionization process, and it is considered to be a signature for the thermal emission of electrons. Further evidence for a thermal emission mechanism comes from theoretical models [13][14][15][16][17] that can describe the experimental observations [6,9]. In all these experiments the clusters (molecules) are excited using pulsed visible (VIS) or ultraviolet (UV) lasers, thus initially preparing electronically excited states. These states can relax to high vibrationally excited levels of the electronic ground state surface from where the thermal ionization is assumed to occur.More direct insight in the process of thermionic electron emission can be obtained when the molecules are directly excited vibrationally by using infrared (IR) lasers [18][19][20]. We here report on the dynamics of the electron emission from gas-phase fullerenes that are resonantly excited on strong IR active modes using a tunable free electron laser (FEL). By absorbing several hundred IR photons per molecule from the train of high power subpicosecond pulses from the FEL, the molecules are efficiently heated and thermionic electron emission can occur. The time evolution of the production of the ions following IR excitation is recorded both for "normal" and for electronically preexcited C 60 . The results obtained are rather different from the results obtained when UV͞VIS lasers are used and shed new light on the mechanism of thermal ionization.Experiments are performed at the free electron laser for infrared experiments (FELIX) [21,22] in Nieuwegein, TheNetherlands. This laser produces IR radiation that is continuously tunable over the 100 2000 cm 21 range. The light output consists of macropulses of about 4 ms durati...

Experimental challenges in fullerene interferometry

“…The counting eae ciency for ions is a product of the conversion eae ciency for fullerenes [23], a geometric collection factor for the electrons and the quantum eae ciency of the Channeltron [24]. The total counting eae ciency amounts then to ² count ' 0: 13: …2 † Thermionic detection of fullerenes has been studied by several groups for the case of either short pulse [25] or continuous wave [26] lasers. In contrast to coherent multi-photon schemes one has to deposit roughly ® ve to ten times more energy than one would expect from the ionization potential at 7.6 eV [27].…”

Single-Size Thermometric Measurements on a Size Distribution of Neutral Fullerenes

“…The FEL intensity is set to a moderate value, just sufficient to ionize the molecules. Assuming that the fullerenes undergo thermionic emission, a given molecule must absorb typically 700 photons (45 eV) in order to emit an electron within the observation time window in the s range [11]. Under these conditions, the probability of ionization at the beginning of the macropulse, where the internal energy of the molecule is low, is negligible.…”

Excitation of C60 using a chirped free electron laser

“…More direct insight in the processes involved can be obtained when the vibrational modes of fullerenes are excited directly, by using fixed frequency [6] or tunable IR radiation [7,8]. When being resonant with an IR active mode, efficient ionization of the fullerenes is observed and ionic fragmentation products, such as C + 58 and C + 56 , are almost completely absent.…”