Dependence of Megaelectron Volt Time-of-Flight Secondary Ion Mass Spectrometry Secondary Molecular Ion Yield from Phthalocyanine Blue on Primary Ion Stopping Power

Abstract

Time-of-flight Secondary Ion Mass Spectrometry (TOF SIMS) is a well-established mass spectrometry technique used for the chemical analysis of both organic and inorganic materials. In the last ten years, many advances have been made to improve the yield of secondary molecular ions, especially those desorbed from the surfaces of organic samples. For that, cluster ion beams with keV energies for the excitation were mostly used. Alternatively, single-ion beams with MeV energies can be applied, as done in the present work. It is well known that secondary molecular/ion yield depends strongly on the primary ion stopping power, but the nature of this dependence is not completely clear. Therefore, in the present work secondary ion yield from the phthalocyanine blue (C 32 H 16 CuN 8 , organic pigment) was measured for the various combinations of ion masses, energies and charge states. Measured values were compared with the existing models for ion sputtering. An increase of the secondary yield with the primary ion energy, electronic stopping, velocity and charge state was found for different types of primary ions. Although this general behavior is valid for all primary ions, there is no single parameter that can describe the measured results for all primary ions at once.

MeV TOF SIMS Analysis of Hybrid Organic/Inorganic Compounds in the Low Energy Region

“…Experiments on the sputtering of indium atoms under the impact of slow highly charged ions indicated a fundamentally different sputtering mechanism as compared to nuclear sputtering observed under conventional keV primary ion beam 6 . In our most recent study concerning the detection of large organic molecules 7 , an expected increase of the secondary ion yield of phthalocyanine with increasing energy, charge state, electronic stopping, and velocity was found for several different types of primary ions. Although the general trend is valid, there seemed to be no single parameter able to describe the results for all primary ions at once.…”

Chemical Imaging of Organic Materials by MeV SIMS Using a Continuous Collimated Ion Beam

“…The DAQ imaging time is limited first by the 100 μs VETO during TOF measurement, then by the operating frequency of the Behlke HTS 61-01-GSM HV switch of 20 kHz, and finally by the available current of the primary ion beam, which is limited by the size of the aperture (50–100 kHz). Despite the low current of the primary ion beam, imaging of heavier molecules can be performed in a reasonable time (several hours) due to the high yield of secondary ions desorbed by the MeV ions …”

“…Another interesting approach was demonstrated by Emile Schweikert’s group, where the start signal is generated by a specific type (mass) of secondary ions . Due to the nature of the interaction of MeV ions with the material, mainly through electronic stopping, the probability of intact molecular desorption is much higher compared to SIMS where a monoatomic primary ion beam with keV energy is used, − and it correlates with the increase in electronic energy stopping power. , Compared to the other SIMS imaging techniques, it can be placed between keV SIMS and cluster ion SIMS techniques in terms of molecular detection efficiency and achievable lateral resolution. − In general, molecules up to a mass of 1000 Da can be detected (e.g., carbohydrates and lipids), with a lateral resolution of up to 400 nm . To date, we have successfully used MeV SIMS to analyze lipids in the mice liver fed with a standard or a high-fat diet .…”

Imaging of Organic Samples with Megaelectron Volt Time-of-Flight Secondary Ion Mass Spectrometry Capillary Microprobe

“…One disadvantage of this kind of beam focusing is its inability to focus heavy primary ions with high energy (such as 20 MeV Iodine) which have been shown to produce a higher secondary ion yield. 1,2,8,9 One cheap and relatively simple alternative is beam collimation with borosilicate capillaries with an exit diameter of a few micrometers. 10−12 In this way, any primary beam that can be delivered by the accelerator will be collimated to approximately the same dimension, independently of ion mass and energy.…”

“…In the last 15 years, several accelerator facilities have installed MeV Time-of-Flight SIMS setups and started to use this technique routinely. − Usually, magnetic or electrostatic lenses are used to focus the primary beam. One disadvantage of this kind of beam focusing is its inability to focus heavy primary ions with high energy (such as 20 MeV Iodine) which have been shown to produce a higher secondary ion yield. ,,, One cheap and relatively simple alternative is beam collimation with borosilicate capillaries with an exit diameter of a few micrometers. − In this way, any primary beam that can be delivered by the accelerator will be collimated to approximately the same dimension, independently of ion mass and energy. Another advantage of capillaries is the possibility of beam extraction into the air without an additional window which separates the high-vacuum part of the setup and negatively affects the beam spot size.…”