Molecular SIMS Ionization Probability Studied with Laser Postionization: Influence of the Projectile Cluster (original) (raw)
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On the SIMS Ionization Probability of Organic Molecules
Journal of The American Society for Mass Spectrometry
The prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and post-ionization techniques. The largest factor affecting secondary ion efficiency is believed to be the poor ionization probability (α +) of sputtered material, a value rarely measured directly, but estimated to be in some cases as low as 10 −5. Our lab has developed a method for the direct determination of α + in a SIMS experiment using laser post-ionization (LPI) to detect neutral molecular species in the sputtered plume for an organic compound. Here, we apply this method to coronene (C 24 H 12), a polyaromatic hydrocarbon that exhibits strong molecular signal during gas-phase photoionization. A two-dimensional spatial distribution of sputtered neutral molecules is measured and presented. It is shown that the ionization probability of molecular coronene desorbed from a clean film under bombardment with 40 keV C 60 cluster projectiles is of the order of 10 −3 , with some remaining uncertainty arising from laser-induced fragmentation and possible differences in the emission velocity distributions of neutral and ionized molecules. In general, this work establishes a method to estimate the ionization efficiency of molecular species sputtered during a single bombardment event.
Measurements of secondary ions emitted from organic compounds bombarded with large gas cluster ions
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2007
We propose to use cluster ions that are much larger than the molecular ions as primary ions for organic secondary ion mass spectrometry. Incident Ar cluster ion beams with energies from 10 to 20 keV and a mean size of about 1000 atoms/cluster were used. Secondary ions were measured for a thin arginine film target (200 nm) bombarded with large Ar cluster ions using a time-of-flight technique. Molecular ions of arginine and characteristic fragment ions were detected with high sensitivity. When large Ar cluster ions such as Ar 1500 were incident on the arginine target, molecular ions of arginine were detected with little fragment ions. This indicates that large cluster ions can ionize arginine molecules without damaging them.
The Journal of Physical Chemistry C, 2017
The prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and postionization techniques. An important factor affecting the detection sensitivity in molecular SIMS and other desorption techniques as well is believed to be the poor ionization probability of a sputtered molecule, a value which is often assumed to be as low as 10 −5 but at present is basically unknown. In order to estimate how much headroom there is for future developments toward strategies aimed at enhancing the ionization probability, we study the protonation efficiency of sputtered guanine molecules for formation of [M + H] + secondary ions using strong field laser postionization (LPI) to detect the corresponding neutral molecules. In order to allow a quantitative comparison of secondary ion and neutral yields, the postionization signal is corrected for undersampling of the principally detectable plume of sputtered neutral particles by the focused laser beam. It is shown that the protonation probability of molecular guanine desorbed from a clean film under bombardment with 20 keV C 60 cluster projectiles is of the order of 1−2 × 10 −3 , with some remaining uncertainty arising from laser-induced fragmentation and possible differences in the emission velocity distributions of neutral and ionized molecules. Moreover, we find that the postionization signal can in principle be boosted by 2 orders of magnitude if a more powerful ionization laser is employed.
Molecular secondary ion formation under cluster bombardment: A fundamental review
Applied Surface Science, 2006
A brief review is given regarding the application of cluster ion beams as desorption probes in molecular SIMS. The general observation is that the efficiency of secondary ion formation, particularly that of complex molecular species, is significantly enhanced if polyatomic projectiles are employed instead of atomic species. Apart from the sensitivity increase, cluster bombardment also appears to allow for molecular depth profiling studies without the accompanying damage accumulation normally associated with atomic projectiles. A few fundamental aspects are addressed in an attempt to highlight the physics behind these observations. It appears that much of the benefit associated with cluster bombardment is connected to the fact that these projectiles give access to very high sputter yields which are not accessible with atomic primary ions. #
Ionization of N_{2}, O_{2}, and linear carbon clusters in a strong laser pulse
Physical Review A, 2004
Multiphoton ionization of linear molecules is studied using the strong-field S-matrix approach. Numerical calculations for the angular distribution and the energy spectrum of the photoelectron as well as the total ionization rates and yields in an intense linearly polarized laser pulse are performed. Results are obtained for molecules aligned along the polarization axis and for ensembles of molecules having a random orientation of the molecular axis with respect to the polarization direction. Signatures of the molecular geometry and the orbital symmetry are identified and discussed with reference to molecular imaging and alignment. It is found that these signatures are clearly marked for the diatomic molecules C 2 , N 2 , and O 2 , but are much weaker for the group of linear carbon clusters due to the polyatomic character as well as to the competing contributions from different valence shells of these molecules. Finally, predictions of the present theory for the dependence of the total ionization rates and yields on the orientation of the molecule are compared with other theoretical models and recent experimental data.
Energy distributions of atomic and molecular ions sputtered by C60+ projectiles
Applied Surface Science, 2006
In the process of investigating the interaction of fullerene projectiles with adsorbed organic layers, we measured the kinetic energy distributions (KEDs) of fragment and parent ions sputtered from an overlayer of polystyrene (PS) oligomers cast on silver under 15 keV C 60 + bombardment. These measurements have been conducted using our TRIFT TM spectrometer, recently equipped with the C 60 + source developed by Ionoptika, Ltd. For atomic ions, the intensity corresponding to the high energy tail decreases in the following order: C + (E À0.4 ) > H + (E À1.5 ) > Ag + (E À3.5 ). In particular, the distribution of Ag + is not broader than those of Ag 2 + and Ag 3 + clusters, in sharp contrast with 15 keV Ga + bombardment. On the other hand, molecular ions (fragments and parent-like species) exhibit a significantly wider distribution using C 60 + instead of Ga + as primary ions. For instance, the KED of Ag-cationized PS oligomers resembles that of Ag + and Ag n + clusters. A specific feature of fullerene projectiles is that they induce the direct desorption of positively charged oligomers, without the need of a cationizing metal atom. The energy spectrum of these PS + ions is significantly narrower then that of Ag-cationized oligomers. For characteristic fragments of PS, such as C 7 H 7 + and C 15 H 13 + and polycyclic fragments, such as C 9 H 7 + and C 14 H 10 + , the high energy decay is steep (E À4 À E À8 ). In addition, reorganized ions generally show more pronounced high energy tails than characteristic ions, similar to the case of monoatomic ion bombardment. This observation is consistent with the higher excitation energy needed for their formation. Finally, the fraction of hydrocarbon ions formed in the gas phase via unimolecular dissociation of larger species is slightly larger with gallium than with fullerene projectiles. #
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1996
The kinetic energy distributions (KED) of secondary ions sputtered from tricosenoic acid films under Ga+ bombardment have been presented recently [I]. Below mass 100, the positive ion spectrum of tricosenoic acid is mainly constituted by the series of C,H: peaks. Within each C,Hc series, the KED of these ions broadens when the number y of hydrogen atoms decreases. In this paper, we propose a phenomenological model accounting for the characteristic behavior of these ions. It is based on an initial transfer of momentum leading to the emission of an original fragment, reflecting the chemical structure of the target, and followed by a fast reorganization when its internal energy is in excess. Consequently, the ions emitted as a result of a violent collision, carrying a large excess of internal energy, will exhibit both a broad KED and a high degree of reorganization with respect to the structure of the original fragment. This interpretation highlights the effect of the chemical and molecular structure of the organic target on the emission process of the secondary molecular ions. Indeed, this structure determines the nature of the original fragments which have proved their utility for analytical purpose. These fragments lead to intense peaks in the mass spectra, allowing the recognition of the original chemical structure of the samples (fingerprint). In addition, such secondary ions resulting from a direct emission pathway, as suggested by their very narrow KED, turn out to he the best candidates for quantification.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1996
Langmuir-Blodgett films of tricosenoic acid deposited on gold have been bombarded by Ga+ and Cs* ions and secondary ion mass spectra were measured by a Time-of-Blight spectrometer. The energy distributions of atomic ions are found to follow the Sigmund-Thompson law whereas, for the molecular C,H, positive and negative ions, the energy distributions have a much more intriguing structure. Within a given C,H, cluster, the energy distributions become broader for the ions that are more unsaturated. This effect is more important for the smaller clusters. The more saturated ions which have a structure close to that of the original molecule have the thinner energy distributions and these are only slightly dependent of the fragment mass. The results are tentatively interpreted on the basis of the chemical and molecular structure of the fragments as compared to that of the original hydrocarbon chains.
Multielectron dissociative ionization of CH3I clusters under moderate intensity ps laser irradiation
International Journal of Mass Spectrometry, 2010
The Multielectron Dissociative Ionization (MEDI) processes of Methyl iodide (CH 3 I) induced by strong (I~10 15 Wcm -2 ) psec laser light at 532 and 1064nm are studied by means of a time-of-flight mass spectrometer. The experimental results are compared to those reported from fsec experiments at similar laser intensities. The influence of the laser pulse duration is clearly reflected in the kinetic energy values of the ejected fragments, which were found to be significantly lower in the psec experiments. Multiple charged atomic fragments (up to I 6+ ) have been recorded in the mass spectra. It is suggested that the higher charged atomic ions (I n+ , n≥3) are produced by Field Ionization processes on atomic ions, which have been liberated via Coulomb explosion within unstable multiple charged parent ion in the rising time of the laser pulse.
Gas-phase fast-atom bombardment mass spectrometry
International Journal of Mass Spectrometry and Ion Processes
Gas-phase fast-atom bombardment (FAB) mass spectra of fullerene C60 , polycyclic aromatic hydrocarbons, n-hydrocarbons, heterocyclic compounds, and others, by using high energy He, Ar, Xe, or isobutane beams, have been compared with the electron impact ionization (El) and/or charge exchange ionization (CEI) mass spectra. The positive ion gasphase FAB mass spectra measured show a fragmentation pattern characterized by a high intensity of lower-mass fragment ions and considerable fragmentation, the extent of which exceeded that in the He CEI mass spectra, and an intense peak corresponding to molecular ions M "+. From the results obtained it was suggested that the internal energy distribution of M '+ ions formed under gas-phase FAB conditions ranges widely from the non-fragmenting lower energy to considerable fragmenting higher energy. The Massey criterion for the He (8 keV) beam indicated that a single collision between a fast atom Afast and an analyte molecule Mgas brings about the formation of singly and/or doubly charged molecular ions as the most likely energy deposition process. The formation of multiply charged molecular and fragment ions, M z÷ and m z+ (z = 1-4), and the extent of fragmentation seemed to be dependent on the structure of the analyte molecules, as well as the bombarding particles used. The mechanisms of formation of positive and negative molecular ions M "÷, M-, and [M-H]-, under gas-phase FAB conditions, has been discussed on the basis of the positive and negative ion mass spectra obtained.