C.-W. Hsu

A high resolution photoionization study of Ne and Ar: Observation of mass analyzed threshold ions using synchrotron radiation and direct current electric fields

Using the high resolution vacuum ultraviolet (vuv) photon source provided by the monochromatized undulator synchrotron radiation of the Chemical Dynamics Beamline at the Advanced Light Source, we have measured the photoionization efficiency (PIE) spectrum for Ne in the energy range of 21.56–21.67 eV at a wavelength resolution of 0.3 meV [full width at half‐maximum (FWHM)]. The PIE spectra for Ne obtained using 0.76 and 2.4 V/cm electric fields reveal autoionizing features attributable to the Rydberg states Ne[2p5ns′(1/2)1; n=14–29] and Ne[2p5nd′(3/2)1; n=12–35] converging to the spin–orbit excited Ne+(2P1/2) state. The positions of these Rydberg states are compared to previous experimental results and those calculated using the quantum defects and IE for Ne+(2P1/2) given in Moore [Natl. Stand Ref. Data Ser. Natl. Bur. Stand. 35 (1971)]. We have also observed mass analyzed threshold ions (MATI) for Ne formed in the Ne+(2P3/2,1/2) states. For Ar, only the MATI peak for Ar+(2P3/2) is observed. The failure to...

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...

A high-resolution vacuum ultraviolet photoionization, photoelectron, and pulsed field ionization study of CS2 near the CS2+(X 2Π3/2,1/2) thresholds

High-resolution photoionization efficiency (PIE) and pulsed field ionization photoelectron (PFI-PE) spectra for CS2 have been measured using coherent vacuum ultraviolet (VUV) laser radiation in the energy range of 81 050–82 100 cm−1. The PIE and threshold photoelectron (TPE) spectra for CS2 in the energy range of 80 850–82 750 cm−1 have also been obtained using synchrotron radiation for comparison with results of the VUV laser study. The analysis of the PIE spectra reveals three Rydberg series converging to the excited CS2+(2Π1/2) spin–orbit state. These series, with quantum defects of 1.430, 1.616, and 0.053, are associated with the [2Π1/2]npσu, [2Π1/2]npπu, and [2Π1/2]nfu configurations, respectively. The Stark shift effect on the ionization threshold of CS2 has been examined as a function of dc electric fields (F) in the range of 0.65–1071 V/cm. The observed F dependence of the Stark shift for the ionization onset of CS2 is consistent with the prediction by the classical adiabatic field ionization form...

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 CO2+(X 2Πg) in the energy range of 13.6–14.7 eV

The vacuum ultraviolet pulsed field ionization–photoelectron (PFI–PE) spectra for CO2 have been measured in the energy range of 13.6–14.7 eV, revealing complex vibronic structures for the ground CO2+(X 2Πg) state. Many vibronic bands for CO2+(X 2Πg), which were not resolved in previous photoelectron studies, are identified in the present measurement based on comparison with available optical data and theoretical predictions. As observed in the HeI photoelectron spectrum of CO2, the PFI–PE spectrum is dominated by the symmetry allowed ν1+ (symmetric stretch) vibrational progression for CO2+(X 2Πg). However, PFI–PE vibronic bands due to excitation of the symmetry disallowed ν2+ (bending) and ν3+ (asymmetric stretch) modes with both odd quanta, together with the symmetry allowed even quanta excitations, are clearly discernible. The simulation of rotational contours resolved in PFI–PE vibronic bands associated with excitation to the (ν1+=0–1, ν2+=0–2, ν3+=0) vibrational levels has yielded accurate ionization ...

A high resolution pulsed field ionization photoelectron study of O2 using third generation undulator synchrotron radiation

We have improved a newly developed experimental scheme for high resolution pulsed field ionization photoelectron (PFI-PE) studies [Hsu et al., Rev. Sci. Instrum. (in press)] using the high resolution monochromatized multibunch undulator synchrotron source of the Chemical Dynamics Beamline at the Advanced Light Source. This improved scheme makes possible PFI-PE measurements with essentially no contamination by background electrons arising from direct photoionization and prompt autoionization processes. We present here a preliminary analysis of the rotationally resolved PFI-PE spectrum for O2 obtained at a resolution of 0.5 meV (full-width-at-half-maximum) in the photon energy range of 18.1–19.4 eV, yielding accurate ionization energies for the transitions O2+(b 4Σg−, v+=0–9, N+=1)←O2(X 3Σg−, v=0, N=1).

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 bands for O2+(a 4Πu, v+=0–18) in the energy range of 16.0–18.0 eV

We have obtained rotationally resolved pulsed field ionization-photoelectron (PFI-PE) spectra of O2 in the energy range of 16.0–18.0 eV, covering ionization transitions O2+(a 4Πu, v+=0–18,J+)←O2(X 3Σg., v″=0,N″). Although these vibrational PFI-PE bands for O2+(a 4Πu, v+) have significant overlaps with those for O2+(X 2Πg) and O2+(A 2Πu), we have identified all O2+(a 4Πu, v+=0–18) bands by simulation of spectra obtained using supersonically cooled O2 samples with rotational temperatures ≈20 and 220 K. While the v+=4–18PFI-PE bands represent the first rotationally resolved photoelectron data for O2+(a 4Πu), the PFI-PE bands for O2+(a 4Πu, v+=10–18) are the first rotationally resolved spectroscopic data for these levels. The simulation also allows the determination of accurate ionization energies, vibrational constants, and rotational constants for O2+(a 2Πu, v+=0–18). The observed intensities of spin–orbit components for the majority of O2+(a 2Πu, v+) vibrational bands are in accordance with the forced spin...

Rotationally resolved pulsed field ionization photoelectron study of O2+(B 2Σg−,2Σu−; v+=0–7) at 20.2–21.3 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 ...

Rotationally resolved pulsed-field ionization photoelectron bands for O2+(A 2Πu,v+=0–12) in the energy range of 17.0–18.2 eV

We have obtained rotationally resolved pulsed-field ionization photoelectron (PFI-PE) spectra for O2 in the energy range of 17.05–18.13 eV, covering the ionization transitions O2+(A 2Πu,v+=0–12,N+)←O2(X 3Σg−,v″=0,N″). Although these O2+(A 2Πu,v+) PFI-PE bands have significant overlaps with vibrational bands for O2+(a 4Πu) and O2+(X 2Πg), we have identified all the O2+(A 2Πu,v+=0–12) bands by simulation of spectra obtained using supersonically cooled O2 samples with rotational temperatures ≈20 and 220 K. While these v+=0–12 PFI-PE bands represent the first rotationally resolved photoelectron data for O2+(A 2Πu), the PFI-PE bands for O2+(A 2Πu,v+=9 and 10) are the first rotationally resolved spectroscopic data for these levels. The simulation also allows the determination of accurate ionization energies, vibrational constants, and rotational constants for O2+(A 2Πu,v+=0–12). The analysis of the PFI-PE spectra supports the conclusion of the previous emission study that the O2+(A 2Πu,v+=9 and 10) states are s...