Photoionization experiments using light from the Berlin synchrotron facility BESSY were performed with methane clusters. A number of ionic reaction products with masses m = k + 16n (k = 16, 17, 28, 29, 31, 40, and 57 and n> 0) were determined. These are interpreted as series of cluster ions of the form [X-(CH4)"]+ with X = CH4, CH5, C2H4, C2H5, C2H7, Ar, and Ar-CH5. For all series except two (X = C2H4, C2H7) analogue ion-molecule reactions in the gas phase were observed. The occurrence of these two exceptions is believed to be caused by the cluster environment which contains the reagents for a much longer time than that observed in gas-phase reactions. Some of the reactions (X = CHS, C2H5, Ar, Ar-CH5) exhibit Ar interband transitions, which are believed to originate from neutral precursor clusters with an abundance of Ar atoms.Possible reaction schemes are discussed and compared with the analogue ion-molecule reactions in the gas phase.

Effusive and supersonic molecular beams of monosilane have been ionized by photons from the Berlin synchrotron light source (BESSY). Photoionization efficiency curves have been taken as a function of wavelength (50 to 120 nm) for a number of fragment ions. The main products observed are SiH+3 and SiH+2 . No appreciable SiH+4 could be detected and less than 5% of the signal was due to lower fragments (SiH+, Si+). The threshold energies Ethr, the standard enthalpy of formation ΔH○f,g of the fragment ions SiH+2 and SiH+3 and the ionization potentials Iz of the corresponding radicals have been determined to be SiH+2:   Ethr=11.67±0.04 eV ΔH○f,g=276.3±0.9 kcal/mol, Iz(SiH2)=9.47±0.03 eV. SiH+3: Ethr=12.23±0.02 eV, ΔHf,g=237.1±0.6 kcal/mol, Iz(SiH3)=8.32±0.07 eV. Discrete structure in the photoionization curves is observed in the range of Ethr=15.8–17.6 eV and interpreted as vibrational progressions of a highly excited autoionizing state of SiH4 which lies 15.8 eV above the ground state. Vibrational constants have been found to be 1790±80 and 680±50 cm−1 which correspond to the ground state constants 2187 and 711 cm−1 of the ν1 and the ν2 modes of monosilane. The broadening of some lines in the progression is interpreted as being caused by a transition into a dissociative excited neutral state with lifetimes as short as 3×10−14 s for v=3 of the ν2 mode. Additional ions (predominantly Si2H+4, Si2H+6, Si2H+7) have been observed when photoionizing a supersonic molecular beam of monosilane. These are shown to stem from the ionization and subsequent fragmentation of dimers and possibly higher multimeres. Threshold values have been determined to be Si2H+4: 11.58±0.02 eV; Si2H+6: 11.4 eV; and Si2H+7: 11.4 eV.

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.

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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.