The system was operated at the rate of 50 Hz and a set of delay generators (DG 535, SRS Inc., Sunnyvale (CA), U.S.) was used to synchronize the laser with SIMS. The MS were sampled using digital oscilloscope (LeCroy Wavesurfer 422, LeCroy Corp., Chestnut Ridge.(NY), U.S.) and subsequently downloaded for further processing, averaging 200 spectra for each scan.Ī Cr:forsterite fs IR MOPA (Master Oscillator Power Amplifier) was used for preionization including a three-stage amplifier (100 fs, 4.5 mJ, 1240 nm, photon energy ~1 eV). The sample surface was bombarded with 25 keV Bi + 3 primary ions with the pulse duration of 10 ns and the repetition rate of 50 Hz. The MS part of the setup employed the TOF (time-of-flight) SIMS IV (ION-TOF GmbH, Germany). The general arrangement resembles another method, matrix-assisted laser desorption/ionization (MALDI) but the preionization is matrix independent and (besides primary ions) uses an ultrafast (fs) IR laser source instead of the “slow” (ns) UV source generally used in MALDI. The idea is to preionize the surface for the subsequent sputtering by the primary ions. Instead of postionization of the neutral species, the sample is a priori irradiated by fs laser beam with moderate intensity of the order of 10 10–10 11 W/cm 2.
Even though important improvements were made within the last decade, those systems still remain complex, bulky, and expensive.Ī different approach to suppress the matrix effect and related issues is proposed within the scope of this work. However, the fs postionization regime requires intensities of the order of 10 12–10 14 W/cm 2 (i.e., to enter the multiphoton ionization (MPI) domain, which in turn means that mJ class fs laser systems are required. Most of the previously published results on laser postionization in the SNMS regime were performed using either nanosecond (ns) UV lasers or femtosecond (fs) laser systems providing 800 nm or shorter wavelengths. While in the case of SIMS the secondary ions originate from the emission dynamics at the surface, SNMS uses an external source to postionize the sputtered neutral species. As opposed to the case of conventional SIMS, SNMS is based on sputtered neutrals allowing a direct quantitative analysis. A possible approach to solve the problem is secondary neutral mass spectrometry (SNMS). The yield of specific secondary ions is strongly dependent on the chemical environment from which they are emitted and the variations might be of the order of several magnitudes. However, the most relevant drawback of the SIMS technique is the matrix effect turning the quantitative analysis into a very difficult procedure. Secondary Ion Mass Spectrometry (SIMS) has been established as an extremely valuable tool providing comprehensive chemical and spatial analysis of a broad range of materials. The proposed preionization approach might eliminate the need for high peak power/high intensity laser source and, moreover, the experiment geometry ensures that large areas of the sample are affected by the laser beam. The Ag saturation intensity obtained from fitting is 2.4 × 10 13 W/cm 2, close to the one reported for postionization. Two ionization mechanisms are identified, the ion sputtering regime for intensities of less than 1.4 × 10 11 W/cm 2 and the multiphoton ionization at higher intensities. The Ag +, C 3H + 5, C 3H 5O + 3, and AgOH +, C 4H 5O + 4 are observed with the shallow and steep increasing of intensities at 1.3 × 10 11 W/cm 2 and 1.5 × 10 11 W/cm 2, respectively. Multiple correlation patterns are observed in the mass spectra, confirming the mutual laser-secondary ion mass spectrometry (SIMS) interplay in the preionization mechanism. The native Ag sample surface is first irradiated with laser pulse (100 fs duration, 10 10–10 11 W/cm 2 intensity, 1240 nm wavelength) and subsequently bombarded with primary ions (Bi + 3, 10 ns duration, 25 keV energy).
An alternative secondary ion mass spectrometry utilizing laser preionization is introduced.