Vacuum UV Photoionization Research Articles

Vacuum UV photoionization mass spectrometry of small polymers using jet cooling

Many polymers with short chains and molecular weights up to several thousand atomic mass units can be transferred to the vapor phase whole by pulsed laser desorption, in contrast with the well-known, destructive ablation process encountered with long-chain, high-molecular-weight polymers. However, the vaporized whole polymers are hot and thus fragment extensively when photoionized. For their detection as molecular ions, it is necessary first to cool the vaporized polymers, which is accomplished by entraining them in an Ar jet expansion. Near-threshold, single-photon ionization at 125 nm (9.9 eV) was used in all cases and compared with 193 nm, two-photon ionization for polystyrene. Mass spectra showing oligomeric distributions for samples of poly(dimethylsiloxane), poly(ethylene oxide), poly(isoprene), poly(perfluorotrimethylene oxide) and polystyrene are reported. Application of the technique to samples of increasing average molecular weight shows that, as the chain length increases, thermal decomposition eventually predominates over vaporization. © 1997 Elsevier Science S.A.

Enhanced stability of ion–clathrate structures for magic number water clusters

The near threshold vacuum-UV photoionization of water clusters has been performed by using a resonance line emission of argon at 11.83 eV. The well-known intensity anomaly at the cluster ion (H2O)21H+ is observed even in this threshold photoionization, for the first time, with very small excess energy. Structures for the water cluster ions (H2O)21H+ and (H2O)28H+ which exhibit enhanced structural stability (magic number), are presented based on Monte Carlo simulations as well as on the analogy of our previous study on the stability of the (H2O)20NH+4 ion. The Monte Carlo calculations are carried out at the temperatures of 200, 150, 100, and 50 K for the ionized water clusters (H2O)nH+ around n=21 and also around n=28, which includes the ionic hydrogen-bond interactions between an H3O+ ion and neutral H2O molecules. The clusters (H2O)21H+ and (H2O)28H+ have greater binding energies per molecule than their neighbors although the enhancement of the latter is somewhat temperature dependent. The calculations suggest that the stable structures for (H2O)21H+ and (H2O)28H+ are represented by the ion–clathrate (ion-centered cage) configurations with either an H2O+ or (H2O ⋅ H3O)+ ion trapped inside the cage, respectively. The simulations for (H2O)20 ⋅ H3O+ also suggest that the cluster is especially stable due to the strong Coulombic interaction between the encaged H3O+ ion and the surrounding 20 water molecules which form a deformed pentagonal dodecahedral cage. The stability of the (H2O)26(H2O ⋅ H3O)+ ion can also be explained consistently within the framework of the present ion–clathrate model.