Rotational-resolved photoelectron spectroscopy and energy-selected photoelectron-photoion coincidence spectroscopy using synchotron radiation.

摘要

High-resolution pulsed field ionization photoelectron (PFI-PE) measurements have been achieved using monochromatized multibunch synchrotron radiation. By employing a simple electron time-of-flight (TOF) spectrometer, we show that PFI-PEs produced by the PFI in the dark gap of a synchrotron ring period can be cleanly separated from prompt background photoelectrons. We have demonstrated instrumental resolutions of 1.0 cm−1 (FWHM) and 1.9 cm −1 (FWHM) in the PFI-PE bands at ionization energies of Xe +(2P_3/2) and Ar+( 2P_3/2), respectively. Using this scheme, we have obtained rotationally resolved PFI-PE bands of O₂+(X 2Π_3/2,1/2g, v+ = 0–38), O ₂+(A2Π_u, v+ = 0–12) and O₂+(a4Π _u, v+ = 0–18) in the energy range of 12.05–18.2 eV and NO+(a3Σ+, v + = 0–16), NO+(A′1Σ −, v+ = 0–17), and NO+(b 3Π, w3Δ, b3Σ− , W1Δ and A1Π) in the energy range of 15.6–20.4 eV. The analysis of many of these highly excited rovibronic states provides accurate spectroscopic constants for the first time.; Another novel scheme is the energy-selected high-resolution pulsed field ionization photoelectron photoion coincidence (PFI-PEPICO) measurement of the dissociation of small molecules using two and multi-bunch synchrotron radiation. We show that this technique provides an ion internal state (or energy) selection limited only by the PFI-PE measurement. Employing a shaped pulse for PFI and ion extraction, a resolution of 0.6 meV (FWHM) is observed in the PFI-PEPICO bands for Ar+(2P_3/2,1/2 ). As demonstrated in the PFI-PEPICO study of the process, O₂ + hν → O₂+(b4Σ _g−, v+ = 4, N+) + e− → O+(4S) + O( 3P) + e−, the dissociation of O₂ +(b4Σ_g−, v + = 4) in specific rotational N+ levels can be examined. The PFI-PEPICO study of the dissociation of CH₃+ from CH₄ also reveals the lifetime effect on the intensities for CH₃+ and CH₄+, which was characterized by a step-like feature observed in the PFI-PE measurement. The high-resolution for PFI-PEPICO measurements, along with the ability to distinguish the CH₃+ fragments due to the supersonically cooled CH₄ beam from those formed by the thermal CH₄ sample, has allowed the determination of a highly accurate dissociation threshold for CH₃+ from CH₄. Similar studies are performed for C₂H₂, C₂H₅Br, iso-C ₃H₇X (X = Br, I), CH₃X (X = Br, I), NH₃ and CD₄ so that highly accurate themochemical quantities related to these systems can be derived with unprecedented precisions.