On the vibrational temperature of metal cluster beams: A time-resolved thermionic emission study

Abstract

Delayed ionization rates for small niobium clusters are measured as a function of the cluster size, the laser wavelength, the number of photons absorbed, and the initial internal energy content of the cluster. If is shown, for the first time, that vibrational excitation of the clusters modulates their rate of delayed ionization. An analysis of the rate of ionization in terms of the total energy content of the clqster establishes unequivocally that delayed ionization is a statistically determined, unimolecular, activated process. It is shown that the rate of delayed ionization can be used to gauge t1:te initial vibrational energy content of the cluster. Quantitative analysis of this effect establishes, for the first time, a thermometry for metal cluster beams. Thermal rate parameters, activation energies, and "Arrhenius factors" are presented for delayed ionization of a series of niobium clusters Nb n , n=5, 6, 7, 8, 9, 11, 12, and 13. The activation energies are lower than the corresponding ionization potentials by ~ 1 eV. The "Arrhenius factors" are all in the range 1011 s-l. These parameters. highlight the differences between delayed ionization and photoionization of clusters and draw attention to the need for an adequate theory of unimolecular processes in clusters taking into account both nonadiabatic effects and the important effects arising from the fluxionality of the cluster at high internal energies.4174

Stern-Gerlach Experiments on Fe@Sn12: Magnetic Response of a Jahn–Teller Distorted Endohedrally Doped Molecular Cage Cluster

“…The supersonic expansion vastly decreases the translational and rotational temperature of the clusters, but the influence on the vibrational temperature is small. 3,4,17,18 It has been demonstrated by the temperaturedependent superatomic response of Mn@Sn 12 that the vibrational temperature of the clusters is approximately equal to T nozzle with this setup if the clusters are sufficiently close to thermal equilibrium with the nozzle (before the supersonic expansion occurs). 3 The highly collimated molecular beam then passes a magnetic deflection unit with an inhomogeneous magnetic field (two-wire field analogue, 19 0−1.5 T, 0−335 T/ m).…”

Magnetic ordering in manganese clusters

“…The clusters expanded into vacuum through a 1.0 mm diameter orifice at the end of the flow tube. Under these mild expansion conditions, very little supersonic cooling of the clusters' vibronic degrees of freedom is expected, 19 so that the post-expansion cluster temperature is estimated to be close (within ϳ5 K) to that of the flow tube. The expanding jet was skimmed into a molecular beam, which passed through a gradient dipole magnet 20 capable of producing B fields of up to ϳ1.2 T and gradients ͑‫ץ‬B / ‫ץ‬z͒ up to ϳ210 T m −1 in the center of the gap.…”

The far-infrared spectra of neutral and cationic niobium clusters: Nb5∕+ to Nb9∕+

“…Partly because of their relative ease of production and the favorable isotopic distribution, niobium clusters have been intensively investigated experimentally and this has been followed by extensive theoretical studies. Experimental studies include single and multiphoton ionizations, [13][14][15][16] as well as photoelectron spectroscopy [17][18][19][20] and the measurements of the bond dissociation energies. 21,22 Optical absorption spectra of neutral niobium clusters have been measured by means of photodissociation of their complexes with rare-gas atoms [23][24][25] and this technique is also used here for obtaining the vibrational spectra.…”