Polystyrene oligomers adsorbed on Ag{111} have been used as a model system for a molecular dynamics study of polyatomic fragment ejection from large organic adsorbates on metals. The simulation of the interaction between the incident 500 eV Ar atoms and the sample predicts the nature and kinetic energy of the characteristic fragments observed in the low-mass range of the experimental polystyrene secondary ion mass spectrum (SIMS). This agreement confirms that characteristic molecular fragments can be the result of a collisional emission process. Moreover, the results indicate that the ejection of intact molecules occurs frequently. To understand the mechanisms of emission of fragments and parent molecules, a representative set of trajectories has been analyzed in detail. The ejection of characteristic fragments is primarily due to the direct interaction between the primary particle and the organic adsorbate. In contrast, desorption of intact molecules is induced by collision cascades in the sample surface. The emission of energetic intact molecules is best explained by a cooperative uplifting mechanism in which substrate atoms with similar momenta push the molecule upward. For comparison to the 500 eV bombardment conditions, simulations conducted with 5 keV primary particles show that large-scale simultaneous motions in the substrate may also occur, inducing the emission of many silver atoms and organic molecules in a single high action event.

Monodisperse polystyrenes (PS) of different molecular weights (Mn) synthesized by living anionic polymerization with three types of butyllithium initiator (linear, n; secondary, sec; and tertiary, tert) were analyzed by ToF-SIMS (time-of-flight secondary ion mass spectrometry). The influence of the molecular weight on the secondary ion intensities was studied in detail for the fingerprint part of the mass spectra (with m/z < 200). A drastic effect was observed for Mn values below 10 4 , related to the presence of the saturated butyl end group. An extra hydrogen transfer originating from this end group during the secondary ion formation must be invoked to explain the data. Only the first neighbor monomer repeat units seem to be affected. This H exchange increases the intensity of ions containing more hydrogen or needing H transfer for their formation as the tropylium ion (C7H7 + at m/z ) 91). The molecular structure of the butyl end group is found to influence greatly not only the intensity of their parent ion but also the PS characteristic ion intensities. Indeed, the tert-butyl end group is seen unable to produce the H transfer observed for the n-and sec-butyl ones. A model is proposed to take the influence of the end group on the PS SIMS fragmentation pattern into account. The parameters of this model allow the quantification of the end group interaction. *

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Two series of monodisperse polystyrenes were synthesized by an anionic polymerization initiated with sec-butyllithium. The number average molecular weights of the macrochains range from 1700 to 93300. The first series consist of deuterated repeat units and hydrogenated end groups (sec-PD8S−H). The other one was fully hydrogenated except for one end group, which was selectively deuterated (sec-PS−D). The sample molecular structures are C4H9(C8D8) n H and C4H9(C8H8) n D, respectively. These polymers were then analyzed by time-of-flight secondary ion mass spectrometry (ToF−SIMS). All secondary ion intensities are corrected for the carbon and deuterium isotopic distributions. Static SIMS spectra of sec-PD8S−H show characteristic peaks with even masses (fully deuterated, i.e., C7D7 +, C6D5 + at m/z = 98, 82) and odd peaks (containing at least one hydrogen atom, i.e., C7HD6 +, C6HD4 + at m/z = 97, 81). Typically, these odd peaks show decreasing intensities as the molecular weight increases. In the sec-PS−D SIMS spectra, the characteristic peaks are similar to those observed for the usual polystyrene with a more specific peak at m/z = 92 which is related to the deuterium end group. From the molecular weight dependent intensities, it can be deduced that a hydrogen transfer occurs from the sec-butyl end group to the first repeat unit and promotes the formation of the tropylium fragment. A rearrangement ion formation mechanism is proposed. For both polymers, characteristic peaks of the end group are detected. With the intensity ratios of a main chain fragment and these end groups characteristic peaks, we are able to calibrate the number average molecular weight at the polymer surface. Moreover, it is shown that the main chain deuteration influences the end group segregation toward the surface.