Critical Assessment of Photoionization Efficiency Measurements for Characterization of Soot-Precursor Species

Abstract

We present a critical evaluation of photoionization efficiency (PIE) measurements coupled with aerosol mass spectrometry for the identification of condensed soot-precursor species extracted from a premixed atmospheric-pressure ethylene/oxygen/nitrogen flame. Definitive identification of isomers by any means is complicated by the large number of potential isomers at masses likely to comprise particles at flame temperatures. This problem is compounded using PIE measurements by the similarity in ionization energies and PIE-curve shapes among many of these isomers. Nevertheless, PIE analysis can provide important chemical information. For example, our PIE curves show that neither pyrene nor fluoranthene alone can describe the signal from C16H10 isomers and that coronene alone cannot describe the PIE signal from C24H12 species.A linear combination of the reference PIE curves for pyrene and fluoranthene yields good agreement with flame-PIE curves measured at 202 u, which is consistent with pyrene and fluoranthene being the two major C16H10 isomers in the flame samples, but does not provide definite proof. The suggested ratio between fluoranthene and pyrene depends on the sampling conditions. We calculated the values of the adiabatic-ionization energy (AIE) of twenty-four C16H10 isomers. Despite the small number of isomers considered, the calculations show that the differences in AIEs between several of the isomers can be smaller than the average thermal energy at room temperature. The calculations also show that PIE analysis can sometimes be used to separate hydrocarbon species into those that contain mainly aromatic rings and those that contain significant aliphatic content for species sizes investigated in this study. Our calculations suggest an inverse relationship between AIE and the number of aromatic rings. We have demonstrated that further characterization of precursors can be facilitated by measurements that test species volatility.3

Unveiling trends in soot nucleation and growth: When secondary ion mass spectrometry meets statistical analysis

“…Online techniques relying on the generation and analysis of a molecular beam have been available for almost two decades now [42,59]. Their modern implementations, like aerosol mass spectrometry for instance, are now compact and commercially available devices that can provide semi-quantitative measurements in real time and therefore have become widespread for the analysis of environmental samples [60][61][62][63][64].…”

Theoretical Investigations of Photoionization Efficiency of Naphthalene

“…Photoionization-efficiency measurements provide unprecedented access to vibrationally resolved spectra of free radicals, molecules, clusters, and their reaction products inform thermodynamics and spectroscopy present in high-temperature reactors. [37][38][39] The natural choice of a computational tool for analyzing the electronic states of PAH cations is the CASSCF method because it fully accounts for configuration interactions in a π-electron system (orbital active space) and provides an accurate description of the rest of the molecule. To study the PIE curve of naphthalene is to simulate the photoionization process of naphthalene molecules with the conditions of molecular beam using quantum chemistry calculation.…”

A small porous-plug burner for studies of combustion chemistry and soot formation

“…We extracted particles and condensed precursors from the flame for analysis using a VUV-AMS. These measurements were performed at the Chemical Dynamics Beamline at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory in Berkeley, CA, USA; details are available elsewhere, 61,62,[70][71][72] and only a brief description of this technique will be provided here. We extracted soot from the flame using a quartz probe, and particles were focused using an aerodynamic lens system.…”

“…76 flame ME1 showed that the stabilomers pyrene and coronene alone could not account for the signals observed at 202 u (C 16 H 10 ) and 300 u (C 24 H 12 ). 62,72 In addition, the stabilomer grid does not account for species containing odd numbers of carbon atoms. 76 Figure 9 also shows that the mass spectra contain signals from the large polyynes C 8 H 2 , C 10 H 2 , and C 12 H 2 .…”

“…61 These conclusions are supported by earlier studies performed in ME1 and in acetylene and ethylene counter-flow diffusion flames. 62,71,72…”