Efficiency of Thermionic Emission from C₆₀

Abstract

We have determined the number of delayed electrons emitted from an ensemble of gas-phase C60, excited at 355 nm with light from an unfocused pulsed Nd:YAG laser. Delayed electrons are detected by single particle counting and integrated over a time interval of 0.1 μs ≤ t ≤ 80 μs. At low laser fluence I, the electron number scales as I p with p = 6.0 ± 0.4. It approaches saturation at a maximum laser fluence of 100 mJ/cm2, equivalent to a power density of 14 MW/cm2, where delayed electron emission reaches a probability of 2.6 ± 1.1% per multiphoton-excited C60. This experimental value is interpreted as a lower limit to the quantum efficiency for electron emission from C60; it provides an upper limit to the efficiency of competing reactions such as dissociation of C60 into C58 + C2. The dependence of the electron number on the temperature of the fullerene source is in good agreement with the number density of C60 in the molecular beam, as computed from published equilibrium vapor pressures.

Shedding New Light on Thermionic Electron Emission of Fullerenes

“…Taking the experimental detection efficiency into account, it is evident that ions are produced at 150 ms and beyond. This corresponds to a (1͞e) autoionization time scale that is significantly larger than previously observed [4][5][6]12]. Second, and even more surprising, the two TOB distributions taken at low fluence are clearly peaked after the end of the FELIX pulse and appear to consist of at least two components.…”

“…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.…”

The transition from direct to delayed ionisation of C 60

“…If the pulse duration is increased to a few ps the multiply charged species disappear and a "tail" is observed on the C + 60 ion peak extending to longer times [7]. This "tail" is a characteristic feature of delayed ionisation as is normally observed on ns laser excitation [3,4]. In this paper we further investigate the transition from a prompt ionisation a e-mail: f3aec@fy.chalmers.se to a delayed (µs) ionisation using 790 nm laser pulses of variable pulse duration but keeping the fluence (i.e.…”

“…One of their most interesting properties is their tendency for undergoing delayed (µs) ionisation after excitation with ns laser pulses where the photon energy lies below the ionisation potential [2][3][4][5][6]. This is generally considered to be a statistical process akin to thermionic emission although some evidence has accumulated recently to indicate that a purely thermionic emission interpretation is perhaps not always justified [1].…”

Abundances of Tetra-, Penta-, and Hexa-Anionic Gold Clusters

“…Apart from prompt electrons, a considerable amount of delayed electrons is observed, being related to thermionic emission as known from bulk matter. 1,2 Experimental data is available mainly on fullerenes [3][4][5][6][7][8][9][10][11] and tungsten clusters, [12][13][14][15][16] but also on other metal clusters, 12,[17][18][19][20][21] accompanied by the development of theoretical models. 9,19,22,23 Thermionic emission has been observed also for positively charged sodium clusters, where ultra-short fslaser pulses excite the electronic system only, preventing fragmentation of the ionic core.…”