Title

Breaking Through the False Coincidence Barrier in Electron–Ion Coincidence Experiments

Poster Number

1

Lead Author Affiliation

Department of Chemistry

Lead Author Status

Faculty

Second Author Affiliation

Sandia National Laboratories

Second Author Status

Staff

Third Author Affiliation

Sandia National Laboratories

Third Author Status

Staff

Fourth Author Affiliation

Paul Scherrer Institute

Fourth Author Status

Staff

Fifth Author Affiliation

Paul Scherrer Institute

Fifth Author Status

Staff

Sixth Author Affiliation

Department of Chemistry

Sixth Author Status

Faculty

Introduction

Photoelectron Photoion Coincidence Spectroscopy (PEPICO) holds the promise of a universal, isomer-selective and sensitive analytical technique for time-resolved quantitative analysis of bimolecular chemical reactions.

Purpose

The low PEPICO dynamic range of ~103 has largely precluded its use for this purpose, where dynamic range of at least 105 is generally required. This limitation is due to the false coincidence background common to all coincidence experiments, especially at high count rates. Electron/ion pairs emanating from separate ionization events but arriving within the ion time of flight (TOF) range of interest constitute the false coincidence background. Although this background has uniform intensity at every m/z value, the Poisson scatter in the false coincidence background obscures small signals.

Method

Temporal ion deflection coupled with a position-sensitive ion detector enables suppression of the false coincidence background, increasing the dynamic range in the PEPICO TOF mass spectrum by 2–3 orders of magnitude. The ions experience a time-dependent electric deflection field at a well-defined fraction of their time of flight. This deflection defines an m/z- and ionization-time dependent ion impact position for true coincidences, whereas false coincidences appear randomly outside this region and can be efficiently suppressed.

Results

We have shown that the false coincidence barrier, which limits the dynamic range of conventional PEPICO experiments, can be overcome with ion imaging used with a swiftly modulated electric deflection field acting on the ion trajectories at a fixed fraction of their time of flight. Most false coincidences obscuring minor peaks in the mass spectrum arise from the most dominant ions. Because the calculated TOF for a false coincidence is erroneous, the calculated time of ion deflection is also incorrect, leading to a predicted impact position that does not agree with the observed impact position. As a result, only true coincidences will be detected at the predicted impact position, allowing the false coincidence background to be removed, thereby increasing the dynamic range of the experiment by at least two orders of magnitude. Proof-of-principle supersonic molecular beam spectra of argon illustrate how parent cations from Ar9 oligomers can be easily detected using this false coincidence suppression scheme, even when Ar4+ species are hardly visible without suppression.

Significance

Eliminating false coincidences breaks down the major barrier to utilizing PEPICO as a sensitive and versatile analytical technique, offering photoelectron spectral fingerprints of even very minor and transient components of a time-evolving, reactive mixture.

Location

DUC Ballroom A&B

Format

Poster Presentation

Poster Session

Morning

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Apr 29th, 10:00 AM Apr 29th, 12:00 PM

Breaking Through the False Coincidence Barrier in Electron–Ion Coincidence Experiments

DUC Ballroom A&B

Photoelectron Photoion Coincidence Spectroscopy (PEPICO) holds the promise of a universal, isomer-selective and sensitive analytical technique for time-resolved quantitative analysis of bimolecular chemical reactions.