UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Spectroscopy and dynamics in threshold ion-pair production Hu, Qichi

Abstract

The high resolution photoionization technique of Threshold Ion-Pair Production Spectroscopy (TIPPS) involves VUV excitation of neutral molecules AB to the highly vibrationally excited ion-pair states A⁺(β⁺) — B⁻(β⁻) just below the dissociation limit. These states behave like the high-n Rydberg states used for pulsed field ionization zero kinetic energy photoelectron (PFI-ZEKE) or mass analyzed threshold ionization (MATI) spectroscopy, and can be detected by pulsed field dissociation. During the past few years TIPPS has been applied to the molecules HCl/DCl, HF/DF, HCN and (HF)₂. For HCl/DCl and HF/DF, the ion-pair thresholds have been precisely measured and the classical bond dissociation energies have been calculated, and therefore our results provide an experimental test of the Born-Oppenheimer breakdown in the two pairs of isotopomers. The ion-pair formation mechanisms in these molecules were discussed in light of these high resolution results. For HCN, we have precisely measured the ion-pair threshold E⁰[sub IP] to be 122246 ± 4cm⁻¹. Our result also showed that rotationally excited instead of cold CN⁻ fragment is favored as the ion-pair dissociation product in the threshold region. For (HF)₂, the total ion yield and pulsed field ionization (PFI) spectra of HFH⁺ from (HF)₂ were recorded over the energy range 14.7-15.9eV. The dominant process to produce HFH⁺ was found to be (HF)₂ + hν → HFH⁺ + e⁻ + F, while the other energetically allowed process (HF)₂ + hν → HFH⁺ + F⁻ is virtually non contributing. Production of vibrationally excited HFH⁺ fragments was observed, and assignments to different vibrational levels were performed in comparison with the calculated HFH⁺ vibrational spacing in literature work. From the spectrum we have measured the appearance potential (AP) of HFH⁺ to be 14.50 ± 0.03eV (relative to (HF)₂), which gives a value of 5.07 ± 0.03eV for the proton affinity of HF. Our result clarifies the discrepency between previous photoionization work and the results from other methods.

Item Media

Item Citations and Data

License

For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.

Usage Statistics