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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.