Absolute cross-sections for the nonresonant multi-photon ionization of toluene and xylene in the gas phase

Determination of absolute photoionization cross-sections of some aromatic hydrocarbons

RATIONALE

Aromatic hydrocarbons play an important role in the formation and growth of polycyclic aromatic hydrocarbon (PAH) and soot particles. Measurements of their absolute photoionization cross-sections (PICSs), that benefit the quantitative investigation of mass spectrometry, are still lacking, however.

METHODS

PICSs of some aromatic hydrocarbons were measured with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Nitric oxide and benzene were chosen as standard references for PICS calibration, since their photoionization cross-sections are well documented in the literature. Binary liquid mixtures of the investigated molecules and their specific solvents were used in the measurements.

RESULTS

The investigated aromatics include naphthalene, phenanthrene, 1-methylnaphthalene, indene, 2-/3-/4-methylphenylacetylene, 2-methylindene, diphenylacetylene, 1-/2-ethynylnaphthalene and acenaphthylene. Photo-induced fragments from the molecules were also observed with increasing photon energy.

CONCLUSIONS

Based on our measurements and literature data, PICSs of most aromatic molecules have very similar values beyond their ionization energies. However, molecules that contain the phenylacetylene structure have PICSs higher than other aromatics.

Determination of absolute photoionization cross-sections of aromatics and aromatic derivatives

The absolute photoionization cross-sections of aromatics and aromatic derivatives including toluene, ethylbenzene, n-propylbenzene, o-xylene, m-xylene, p-xylene, 1,3,5-trimethylbenzene, styrene, phenylacetylene, indene, indane, 1-methylnaphthalene, benzyl alcohol and benzaldehyde were measured at the photon energy range from ionization thresholds to 11.7 eV. The experiments were performed by tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Benzene was chosen as a calibration standard, since its photoionization cross-section is well known. Binary liquid mixtures of the investigated molecules and benzene were used in the measurements. Photo-induced fragments from the molecules were also observed, and their photoionization cross-sections are also presented.

Probing particle size distributions in natural surface waters from 15 nm to 2 μm by a combination of LIBD and single-particle counting

We present a technique for measuring colloid size distributions between 15 nm and 2 microm at concentrations relevant to natural surface waters. Two particle-measuring methods are combined: laser-induced breakdown detection (LIBD), which allows the quantification of colloid size distributions below 400 nm, and a commercial single-particle counter that extends the accessible size range up to two mum. Centrifugation was used in order to separate micrometer sized particles for the LIBD measurement. The feasibility is demonstrated on water of Lake Brienz (Switzerland) and the River Pfinz (Germany) and the particle size distributions follow Pareto's law even down to 15 nm in both cases.