S. Stimson

High resolution vacuum ultraviolet pulsed field ionization photoelectron band for OCS+(X 2Π): An experimental and theoretical study

The vacuum ultraviolet pulsed field ionization photoelectron (PFI-PE) band for OCS+(X 2Π) in the energy region of 11.09–11.87 eV has been measured using high resolution monochromatized synchrotron radiation. The ionization energies (IEs) for the formation of the (0,0,0) X 2Π3/2 and (0,0,0) 2Π1/2 states of OCS+ are determined to be 11.1831±0.0005 and 11.2286±0.0005 eV, respectively, yielding a value of 367±1.2 cm−1 for the spin–orbit splitting. Using the internally contracted multireference configuration interaction approach, three-dimensional potential energy functions (PEFs) for the OCS+(X 2Π) state have been generated and used in the variational Renner–Teller calculations of the vibronic states. The energies of all vibronic states (J=P) for J=1/2, 3/2, 5/2, and 7/2 have been computed in the energy range of ≈4000 cm−1 above the IE[OCS+(X 2Π3/2)] for the assignment of the experimental spectrum. By a minor modification of the ab initio PEFs, good correlations are found between the experimental and theoreti...

High-resolution photoelectron spectroscopy using multibunch synchrotron radiation:: rotational-resolved photoelectron bands of O2+(b 4Σg−, v+)1

Abstract We report rotational-resolved single-photon threshold photoelectron and pulsed field ionization zero kinetic energy (PFI–ZEKE) photoelectron (PE) spectra of O2 in the energy range of 18.1–20.2 eV measured using high-resolution monochromatized multibunch undulator synchrotron radiation. The PFI–ZEKE PE bands for O2+(b 4Σg−, v+=0–9) have been simulated using the Buckingham–Orr–Sichel model derived for rotationally resolved single-photon ionization cross-sections. Only the ΔN=−2, 0 and +2 (or O, Q and S) rotational branches are observed for these PFI–ZEKE PE bands, indicating that the outgoing electron continuum channels with angular momenta l=1 and 3 dominate in the threshold ionization transitions O2+(b 4Σg−, v+=0–9, N+)←O2(X 3Σg−, v″=0, N″). The relative rotational branch intensities for O2+(b 4Σg−, v+=4 and 5) are found to be drastically different from those for O2+(b 4Σg−, v+=0–3, 6 and 7). Considering that the energies of O2+(b 4Σg−, v+=4 and 5) are close to the dissociation limit of O+(4S)+O(3P) and that the crossing location of the O2+(b 4Σg−) and d 4Σg+ potential curves is shown to be in the vicinity of O2+(b 4Σg−, v+=4 and 5), we suggest that the latter observation is the result of predissociative perturbations by the d 4Σg+ state. Within the uncertainties of this experiment, the ionization energies for the formation of O2+(b 4Σg−, v+=0–9, N+=1) can be satisfactorily characterized using a Morse potential. The effective lifetimes for high-n Rydberg states converging to O2+(b 4Σg−, v+=0, 2–5) prepared in the present experiment are nearly constant with values in the range of 1.8–2.0 μs. The observation that the effective lifetimes for high-n Rydberg states converging to O2+(b 4Σg−, v+=4 and 5) are significantly longer than the known dissociative lifetimes of the corresponding ionic states is in accordance with the expectation that the couplings between the O2+ ion core and the high-n Rydberg electron involved are negligibly small.

High-resolution pulsed field ionization photoelectron study of O2: Predissociation lifetimes and high-n Rydberg lifetimes converging to O2+(c 4Σu−, v+=0,1)

We have measured the pulsed field ionization photoelectron (PFI-PE) spectrum of O2 in the energy range of 24.53–25.0 eV at a PFI-PE resolution of 11 cm−1 (full width at half maximum, FWHM). The PFI-PE bands for O2+(c 4Σu−, v+=0 and 1) obtained at O2 rotational temperatures of 35 and 298 K have been simulated using the Buckingham–Orr–Sichel model. Only the ΔN=−3, −1, +1, and +3 (or N, P, R, and T) rotational branches are observed, indicating that the outgoing electron continuum channels with angular momenta l=0, 2, and 4 dominate in the threshold ionization transitions O2+(c 4Σu−, v+=0 to 1, N+)←O2(X 3Σg−, v″=0, N″). The simulation yields natural rotational linewidths of 19.6±2.0 and 77±8 cm−1 (FWHM) for the respective v+=0 and 1 PFI-PE bands of the O2+(c 4Σu−) state. These linewidths make possible the determination of the predissociation lifetimes for the v+=0 and 1 levels of O2+(c 4Σu−) to be (2.7±0.3)×10−13 and (6.9±0.7)×10−14 s, respectively. This experiment also provides accurate ionization energies o...

Rotationally resolved pulsed field ionization photoelectron study of O{sub 2}{sup +}(Bthinsp{sup 2}{Sigma}{sub g}{sup {minus}},{sup 2}{Sigma}{sub u}{sup {minus}};thinspv{sup +}=0{endash}7) at 20.2{endash}21.3 eV

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra of O2 in the energy range of 20.2–21.3 eV, covering the ionization transitions of O2+(B 2Σg−, v+=0–7, N+)←O2(X 3Σg−, v″=0, N″). Only the ΔN=−2, 0, and +2 (or O, Q, and S) rotational branches are observed in the PFI-PE bands for O2+(B 2Σg−, v+=0–7), indicating that the outgoing electron continuum channels with angular momenta l=1 and 3 dominate in the ionization transitions. This experiment allows the determination of accurate spectroscopic constants, such as ionization energy (20.29825±0.0005 eV) for the formation of O2+[B 2Σg−, v+=0, N+=1 (F2)] from O2(X 3Σg−, v″=0, N″=1), vibrational constants (ωe+=1152.91 cm−1, ωe+χe+=20.97 cm−1_, and rotational constants (Be+=1.255±0.0015 cm−1, αe+=0.0241±0.00037 cm−1_ for O2+(B 2Σg−, v+). The (nominal) effective lifetimes for high-n Rydberg states converging to O2+(B 2Σg−, v+=0–6) are measured to be ≈0.2–0.6 μs, which are significantly shorter than those of ≈1.9 μs observed ...

Absolute state-selected cross sections measurements for ion-molecule reactions involving O+(4S°, 2D°, 2P°)

Significant technical advances have been made in the application of the triple-quadrupole double-octopole mass spectrometer for absolute total state-selected cross section measurements of ion-molecule reactions involving O+(4S degree(s), 2D degree(s), 2P degree(s)). By controlling the collision energies for dissociative charge transfer collisions of He+ (Ne+, Ar+) plus O2 in a radio frequency (rf) octopole ion guide gas cell, and by applying appropriate effective ion trapping potentials to the rf octopole ion guide, we have demonstrated that state-selected O+(4S degree(s)), O+(2D degree(s)), and O+(2P degree(s)) reactant ion beams with high purities and usable intensities can be prepared for scattering experiments.