Photoionization of molecules sputtered from molecular thin films has been achieved using high field 125 fs pulses in the mid-IR spectral range. Using several model systems, we show that it is possible to significantly reduce molecular fragmentation induced by the laser field by increasing the photoionization wavelength. By examining the photoionization spectra as a function of wavelength, it is apparent that the photoionization mechanism is changing from a non-adiabatic multi-electron excitation process to a process that involves tunnel ionization. The results of these observations are discussed in terms of their significance for bioimaging with focused ion beams and massspectrometry.
Useful yields from resonance ionization mass spectrometry can be extremely high compared to other mass spectrometry techniques, but uranium analysis shows strong matrix effects arising from the tendency of uranium to form strongly bound oxide molecules that do not dissociate appreciably on energetic ion bombardment. We demonstrate a useful yield of 24% for metallic uranium. Modeling the laser ionization and ion transmission processes shows that the high useful yield is attributable to a high ion fraction achieved by resonance ionization. We quantify the reduction of uranium oxide surface layers by Ar and Ga sputtering. The useful yield for uranium atoms from a uranium dioxide matrix is 0.4% and rises to 2% when the surface is in sputter equilibrium with the ion beam. The lower useful yield from the oxide is almost entirely due to uranium oxide molecules reducing the neutral atom content of the sputtered flux. We demonstrate rapid isotopic analysis of solid uranium oxide at a precision of <0.5% relative standard deviation using relatively broadband lasers to mitigate spectroscopic fractionation.
Strong field laser photoionization
of β-estradiol in the
1013–1015 W/cm2 intensity
range at wavelengths between 1200 and 2000 nm was examined for both
evaporated gas phase molecules and neutral species sputtered from
a solid surface under bombardment with 20 keV C60
+ ions. It is shown that the ionization efficiency is saturated at
laser intensities of the order of 1013 W/cm2, while laser-induced photofragmentation is found to strongly increase
between 1013 and 1014 W/cm2 and stay
constant at higher intensities. These findings suggest a strategy
to improve the postionization efficiency by defocusing the laser beam
in order to sample a larger fraction of the sputtered plume, while
at the same time optimizing the conditions for intact photoionization
of the sputtered molecules. The results reveal a substantial enhancement
of the signal of intact postionized molecular ions to a level an order
of magnitude above that of the corresponding secondary ions. They
also provide an experimental estimate of ∼2.5 × 10–2 as an upper bound for the ion fraction (secondary
ion formation probability) of the sputtered molecules.
Resonance ionization mass spectrometry (RIMS) combines tunable laser spectroscopy with mass spectrometry to provide a high-efficiency means of analyzing solid materials. We previously showed a very high useful yield of 24% for analysis of uranium using three lasers to excite and ionize atoms sputtered from metallic uranium and uranium dioxide. A new resonance ionization scheme using only two lasers achieves a higher useful yield of 38% by accessing both the ground electronic state and a low-lying electronic state of atomic uranium that is significantly populated by sputtering. The major loss channel in analyzing uranium dioxide is the formation of UOx molecules during sputtering. Prebombardment of the surface with 3 keV noble gas ions prior to analysis reduces the surface and results in a sputtered flux with a greatly enhanced proportion of atomic U. This method of surface reduction results in uranium useful yields as high as 6.6% for uranium dioxide analysis, compared to 2% from previous work.
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