Spectroscopy and dynamics in threshold ion-pair production

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.Science, Faculty ofChemistry, Department ofGraduat

INVESTIGATION OF BORN-OPPENHEIMER BREAKDOWN IN HCl AND HF BY THRESHOLD ION-PAIR PRODUCTION SPECTROSCOPY

$^{a}$ J. D. D. Martin, J. W. Hepburn, Phys. Rev. Lett. 79, 3154 (1997) $^{b}$ J. A. Coxon, P. G. Hajigeorgiou, J. Mol. Spec. 203, 49 (2000)Author Institution: Department of Chemistry, University of British ColumbiaThreshold Ion-Pair Production Spectroscopy $(TIPPS)^{a}$ is applied to two pairs of isotopomers, HCl/DCl and HF/DF. From the high-resolution TIPP spectra, the Ion-Pair dissociation energies of the four molecules have been precisely measured, and from these their bond dissociation energies $(D_{0}$ and $D_{e}$) can be determined. In this way an experimental estimate of the Born- Oppenheimer breakdown in the long range limit of the ground state potential curve is obtained for HCl and HF. The experimental results about bond dissociation energies from this work are compared to the theoretical studies by Coxon and $Hajigeorgiou^{b}$

Liquid structure of aluminum binary alloys characterized by electrical parameters under electromagnetic field

The electrical parameters of aluminum binary alloys (Al–Fe and Al–Si alloys) have been investigated in the presence of alternating current (AC) and direct current (DC) electromagnetic fields. The results show that both thermoelectric power and electrical resistivity of Al–Fe alloys increase in the AC/DC electromagnetic field, which indicates that the liquid structure is changed by the electromagnetic treatment. Study on Al–Fe alloys with different Fe content reveals that Al-1.99wt.% Fe alloy shows the sharp increase of transient variation Δ S ( S : thermoelectric power) and inflection of residual variation Δ S _0 at near eutectic composition in AC electromagnetic field, which is comparable with the electrical resistivity in DC electromagnetic field. In addition, the effects of electromagnetic parameters (setting current, frequency and duty cycle) on voltage/current waveform of power system and thermoelectric power have been further studied. After comparing the results of voltage and current waveforms, it shows that the output current can be regarded as a better evaluation index to the effects of electromagnetic parameters on the thermoelectric power. This quantitative method can clearly analyze the effects of electromagnetic parameters on the liquid structure, which is characterized by the thermoelectric power

Secondary Structures of Ubiquitin Ions Soft-Landed onto Self-Assembled Monolayer Surfaces

The secondary structures of multiply charged ubiquitin ions soft-landed onto self-assembled monolayer (SAM) surfaces were studied using in situ infrared reflection–absorption spectroscopy (IRRAS). Two charge states of ubiquitin, 5+ and 13+, were mass selected separately from a mixture of different charge states produced by electrospray ionization (ESI). The low 5+ charge state represents a nativelike folded state of ubiquitin, while the high 13+ charge state assumes an extended, almost linear conformation. Each of the two charge states was soft-landed onto a CH₃- and COOH-terminated SAM of alkanethiols on gold (HSAM and COOH-SAM). HSAM is a hydrophobic surface known to stabilize helical conformations of soft-landed protonated peptides, whereas COOH-SAM is a hydrophilic surface that preferentially stabilizes β-sheet conformations. IRRAS spectra of the soft-landed ubiquitin ions were acquired as a function of time during and after ion soft-landing. Similar to smaller peptide ions, helical conformations of ubiquitin are found to be more abundant on HSAM, while the relative abundance of β-sheet conformations increases on COOH-SAM. The initial charge state of ubiquitin also has a pronounced effect on its conformation on the surface. Specifically, on both surfaces, a higher relative abundance of helical conformations and a lower relative abundance of β-sheet conformations are observed for the 13+ charge state compared to the 5+ charge state. Time-resolved experiments indicate that the α-helical band in the spectrum of the 13+ charge state slowly increases with time on the HSAM surface and decreases in the spectrum of the 13+ charge state on COOH-SAM. These results further support the preference of the hydrophobic HSAM surface toward helical conformations and demonstrate that soft-landed protein ions may undergo slow conformational changes during and after deposition

THRESHOLD ION-PAIR PRODUCTION SPECTROSCOPY (TIPPS) of H2H_{2} and D2D_{2}

$^{a}$J. D. D. Martin and J. W. Hepburn, Phys. Rev. Lett. 79 (1997). $^{b}$J. D. D. Martin and J. W. Hepburn, J. Chem. Phys. 109 (1998).Author Institution: Department of Chemistry, University of WaterlooThreshold Ion-Pair Production Spectroscopy (TIPPS) can determine field-free energetic thresholds for ion-pair $formation^{ab}$ in the same way that pulsed field ionization techniques (ZEKE/MATI) can obtain field-free ionization potentials. $H_{2}$ and $D_{2}$ were investigated and their ion-pair thresholds were determined to be $139714.8 \pm 1.0 cm^{-1}$ and $140370 \pm 1.0 cm^{-1}$, respectively. Pulsed field ion-pair formation mostly result from excitation of complex resonances between ion-pairs and electronic Rydberg states of the parent molecule. However, evidence of the direct excitation of ion-pair Rydberg states was also found