An Improved Clusterion-Photoelectron Coincidence Technique for the Investigation of the Ionisation Dynamics of Clusters

Abstract

The introduction of photoion-photoelectron coincidence techniques has made it possible to investigate photoionisation properties of heavy clusters, which are not accessible by conventional mass spectrometry. This technique has been further developed in combination with a zero-volt electron energy analyser and greatly improved in performance.The method has been applied to the investigation of different homogeneous and heterogeneous clusters. This type of cluster experiment requires both a very high resolution and a large dynamic range in order to identify also clusters present in low abundance.As an example, a series of coincidence mass spectra of Xe clusters has been recorded at different wavelengths. Below a photon energy of 11.1 eV, the range of observable clusters shifts to higher cluster sizes with decreasing energy. Appearance potentials and the binding energy of different cluster ions were obtained. Intensity fluctuations, already observed in spectra with electron bombardment ionisation (magic numbers), have also been detected in the coincidence spectra and become most pronounced near the ionisation threshold. This indicates that these fluctuations are caused by the size-dependent stability of the ionic and not the neutral cluster. Furthermore, the threshold size does not change linearly with cluster size. The binding energy per particle seems to change drastically around n = 13 which indicates the existence of a shell structure in the cluster ion.

Femtosecond time-resolved photoelectron spectra of ammonia molecules and clusters

“…PEPICO has been applied over the past years in single photon ionization experiments with synchrotron radiation to a variety of cluster systems, e.g. (NH 3 ) n [4], rare-gas clusters [5] or dihalogonated benzene(NH 3 ) n [6]. Unfortunately, the dynamics of photoexcited clusters cannot be studied by a one-photon experiment.…”

Molecular Beam Photoionization and Photoelectron-Photoion Coincidence Studies of High Temperature Molecules, Transient Species, and Clusters

“…208 " 213 The PEPICO spectra for Ar cluster ions have been recorded by several groups. 199 ' 201 ' 202 The information content of the PEPICO spectrum is more specific than the PE spectrum of a cluster ion. If the dissociation lifetime of the state-or energyselected cluster ion is much shorter than the ion flight time, the corresponding PEPICO band is not observed.…”

Positive ion — negative ion coincidence spectroscopy of O2 and H2 using synchrotron radiation

“…This result may be interpreted in two ways: The sharp peak observed at 3.56 gs is assigned to the coincidence peak resulting from the flight-time correlation of the two ions. In fact, the theoretical flight-time difference for H + and H -is 3.35 bts, which can be derived from Tp (Uop=0) of 0.911 ~s [given by (2)1 and T, (Uo, =0) of 4.26 gs [given by (4)]. In confirmation of the above assignment, we apply a pulsed electrostatic field of 10 V/cm between G 1 and G2 using the pulse generator to measure the TOF values of H + and H -separately from PR to the detectors, as described in Sect.…”

“…Photoelectron-photoion coincidence techniques are most extensively used for studying stateselected unimolecular dissociation [2] and state-selected ion-molecule reaction [3]. These methods are also exploited for measuring photoelectron spectra of neutral van der Waals clusters [4]. Photoion-photoion coincidence techniques, which detect the flight-time correlation of a pair of two positive ions, allow studies of double ionization and subsequent dissociation of doublycharged ions [5].…”