Strong-Field Photoionization of Sputtered Neutral Molecules for Molecular Depth Profiling

Abstract

Molecular depth profiles of an organic thin film of guanine vapor deposited onto a Ag substrate are obtained using a 40 keV C 60 cluster ion beam in conjunction with time-of-flight secondary ion mass spectrometric (ToF-SIMS) detection. Strong-field, femtosecond photoionization of intact guanine molecules is used to probe the neutral component of the profile for direct comparison with the secondary ion component. The ability to simultaneously acquire secondary ions and photoionized neutral molecules reveals new fundamental information about the factors that influence the properties of the depth profile. Results show that there is an increased ionization probability for protonated molecular ions within the first 10 nm due to the generation of free protons within the sample. Moreover, there is a 50% increase in fragment ion signal relative to steady state values 25 nm before reaching the guanine/Ag interface as a result of interfacial chemical damage accumulation. An altered layer thickness of 20 nm is observed as a consequence of ion beam induced chemical mixing. In general, we show that the neutral component of a molecular depth profile using the strong-field photoionization technique can be used to elucidate the effects of variations in ionization probability on the yield of molecular ions as well as to aid in obtaining accurate information about depth dependent chemical composition that cannot be extracted from TOF-SIMS data alone.

Molecular ions in cluster bombardment: what clues do the molecular dynamics simulations provide?

“…This interpretation of the source of molecular ions has been suggested previously [6] and is elegantly elucidated in experiments of Willingham et al [7,8] They performed laser postionization and SIMS experiments of depth profiling of guanine by C 60 bombardment. In the steady-state region of the depth profile, they find that the parent ion (M+H) + yield in SIMS is directly proportional to the parent molecule M 0 yield in laser postionization.…”

RIMS analysis of ion induced fragmentation of molecules sputtered from an enriched U3O8 matrix

“…In addition to the energy distribution of the sputtered flux, the build-up of sub-surface chemical damage is known to increase the amount of molecular fragmentation observed during subsequent analyses [32][33][34][35]. This result has primarily been observed under dynamic sputtering conditions, but can be replicated for pulsed analysis by increasing the primary ion duty cycle.…”

Sputtered neutral SinCm clusters as a monitor for carbon implantation during C60 bombardment of silicon

“…The idea behind this experiment is to operate in the strong field ionization regime in order to minimize photofragmentation during the post-ionization process. The infrared wavelength was chosen since it was recently found to ensure a softer but more efficient photoionization of sputtered molecules than the previously employed UV scheme [11]. It should be noted that previous experiments performed on sputtered germanium clusters using the same laser system as used here (although operated at a different wavelength of 266 nm) revealed photoionization characteristics comparable to VUV single photon ionization [12], which confirms our confidence that optimized ''soft'' photoionization conditions are employed in these experiments.…”