Competition between delayed ionization and fragmentation of laser-excited Al4−

Abstract

A systematic study of the competition between delayed electron and delayed atom emission from photoexcited Al − 4 clusters was performed using a bent electrostatic ion beam trap, which allows for simultaneous measurement of both decay channels. The aluminum cluster anions, produced with high internal energy in a sputter source, are stored for 100 ms prior to their excitation by a short laser pulse. Delayed ionization and delayed fragmentation were observed for photon energies 1.55 eV < E λ < 2.25 eV. Decay spectra, integrated relative cross sections, and branching ratios were determined as a function of photon energy and storage time. The results show the characteristic pattern expected for a statistical decay ofthe photoexcited Al − 4 clusters; however, the ratio between fragmentation and ionization is found to increase with decreasing photon energy, even though the threshold energy for fragmentation is calculated to be ∼0.5 eV higher than the adiabatic electron detachment energy. It is suggested that the tetramers, while cooling down in the trap, may be in long-lived excited configurations with lower effective fragmentation barriers; Al − 4 clusters carrying high rotational angular momenta are discussed as possible candidates. Results obtained for other aluminum clusters Al − n (5 n 9) are also reported.

The decay of ion bunches in the self-bunching mode

“…These traps only use electrostatic fields, functioning on a principle quite similar to that of an optical resonator. Numerous traps such as these have been built in recent years, hosting a wide variety of experiments in cluster physics [4][5][6][7][8][9][10], molecular physics [11][12][13] and electron collision studies [14]. Experiments probing larger clusters and other heavy species exploit the mass invariance of ion trajectories in these electrostatic storage devices.…”

Radiative cooling of Al−4and Al−5in a cryogenic environment

“…For sputtered Al − 4 and Al − 5 , the delayed detachment rate was observed to also follow a power law [3,14]. A qualitatively similar behaviour was found for photo-excited Al − 4 ions, where both the detachment rate and the rate of fragmentation into Al − 3 + Al pairs were observed to follow power laws with distinctive amplitudes and exponents, with a branching ratio strongly in favour of detachment [15].…”

“…The most likely explanation is a slight underestimation of the detachment rate by the model at early times, resulting in the obtained temperature always being slightly higher for the series containing the earliest data. As we had to reject the data point at k = 1 because of a likely contamination by fragmentation events [15], the earliest data point now belongs to the series with even k. We therefore interpret this behaviour as an indicator of the systematic accuracy of the temperature measurement, which is nonetheless very high. From here onwards, only the average between the odd-k and even-k temperatures is considered.…”

“…While shortcomings of the employed rate model cannot be entirely ruled out, our experimental findings may also be explained by the recently found existence [17] of low-lying electronic states in the anion. The experimental results obtained for Al − 5 suggest that the radiative cooling is even slower for these 15 ions, but that cooling by electron emission must be comparatively faster than for Al − 4 in order to explain the low temperature observed for the earliest laser pulses. Our measurements show that the radiative cooling of even small polyatomic particles is far from understood, and constitutes a formidable challenge to both experiment and theory because of the detailed knowledge needed about the interaction of electronic and vibrational excitation.…”

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.…”