Pulsed Field Ionization Photoelectron Measurements Research Articles

Vacuum ultraviolet pulsed-field ionization-photoelectron study of H2S in the energy range of 10-17 eV.

Vacuum ultraviolet pulsed-field ionization-photoelectron (PFI-PE) spectra of H(2)S have been recorded at PFI-PE resolutions of 0.6-1.0 meV in the energy range of 10-17 eV using high-resolution synchrotron radiation. The PFI-PE spectrum, which covers the formation of the valence electronic states H(2)S(+) (X (2)B(1), A (2)A(1), and B (2)B(2)), is compared to the recent high-resolution He I photoelectron spectra of H(2)S obtained by Baltzer et al. [Chem. Phys. 195, 403 (1995)]. In addition to the overwhelmingly dominated origin vibrational band, the PFI-PE spectrum for H(2)S(+)(X (2)B(1)) is found to exhibit weak vibrational progressions due to excitation of the combination bands in the nu(1) (+) symmetric stretching and nu(2) (+) bending modes. While the ionization energy (IE) for H(2)S(+)(X (2)B(1)) obtained here is in accord with values determined in previously laser PFI-PE measurements, the observation of a new PFI-PE band at 12.642+/-0.001 eV suggests that the IE for H(2)S(+)(A (2)A(1)) may be 0.12 eV lower than that reported in the He I study. The simulation of rotational structures resolved in PFI-PE bands shows that the formation of H(2)S(+)(X (2)B(1)) and H(2)S(+)(A (2)A(1)) from photoionization of H(2)S(X (1)A(1)) is dominated by type-C and type-B transitions, respectively. This observation is consistent with predictions of the multichannel quantum defect theory. The small changes in rotational angular momentum observed are consistent with the dominant atomiclike character of the 2b(1) and 5a(1) molecular orbitals of H(2)S. The PFI-PE measurement has revealed perturbations of the (0, 6, 0) K(+)=3 and (0, 6, 0) K(+)=4 bands of H(2)S(+)(A (2)A(1)). Interpreting that these perturbations arise from Renner-Teller interactions at energies close to the common barriers to linearity of the H(2)S(+) (X (2)B(1) and A (2)A(1)) states, we have deduced a barrier of 23,209 cm(-1) for H(2)S(+)(X (2)B(1)) and 5668 cm(-1) for H(2)S(+)(A (2)A(1)). The barrier of 23 209 cm(-1) for H(2)S(+)(X (2)B(1)) is found to be in excellent agreement with the results of previous studies. The vibrational PFI-PE bands for H(2)S(+)(B (2)B(2)) are broad, indicative of the predissociative nature of this state.

High-resolution energy-selected study of the reaction CH3X+→CH3++X: Accurate thermochemistry for the CH3X/CH3X+ (X=Br, I) system

Using the high-resolution pulsed field ionization-photoelectron (PFI-PE) and PFI-PE-photoion coincidence (PFI-PEPICO) techniques, we have examined the formation of methyl cation (CH3+) from the dissociation of energy-selected CH3X+ (X=Br and I) near their dissociation thresholds. The breakdown diagrams for CH3X thus obtained yield values of 12.834±0.002 eV and 12.269±0.003 eV for the 0 K dissociative threshold or appearance energy (AE) for CH3+ from CH3Br and CH3I, respectively. Similar to the observation in PFI-PE studies of CH4, C2H2, and NH3, the PFI-PE spectrum for CH3Br exhibits a step at the 0 K AE for CH3+, indicating that the dissociation of excited CH3Br in high-n (⩾100) Rydberg states at energies slightly above the dissociation threshold occurs in a time scale of ⩽10−7 s. The observed step is a confirmation of the 0 K AE(CH3+) from CH3Br determined in the PFI-PEPICO study. The adiabatic ionization energies (IEs) for the CH3Br+(X̃ 2E3/2,1/2) spin–orbit states were determined by PFI-PE measurements to be 10.5427±0.0010 and 10.8615±0.0010 eV, respectively, yielding the spin–orbit coupling constant to be 2571±4 cm−1. The AE(CH3+) values from CH3Br and CH3I and the IE[CH3Br+(X̃ 2E3/2)] value obtained here, when combined with the known IE of CH3 (9.8380±0.0004 eV) and IE[CH3I+(X̃ 2E3/2)] (9.5381±0.0001 eV), have allowed accurate determination of the 0 K bond dissociation energies for CH3–Br (2.996±0.002 eV), CH3+–Br (2.291±0.002 eV), CH3–I (2.431±0.003 eV), and CH3+–I (2.731±0.003 eV). Using the AE(CH3+) from CH3Br and CH3I, together with the known 0 K heats of formation (ΔfH00) for Br (117.93±0.13 kJ/mol), I (107.16±0.04 kJ/mol), and CH3+ (1099.05±0.33 kJ/mol), we have obtained more precise ΔfH00 values for CH3Br (−21.30±0.42 kJ/mol) and CH3I (22.43±0.50 kJ/mol). This experiment demonstrated that highly reliable ΔfH00 values for a range of molecules with error limits comparable to those for some of the most precisely measured values, such as ΔfH00(CH4), can be obtained by PFI-PE and PFI-PEPICO measurements.

Observation of accurate ion dissociation thresholds in pulsed field ionization-photoelectron studies.

We report the first observation, together with a mechanism for such an observation, of a steplike feature in the pulsed field ionization photoelectron measurement of CH4(C2H2), marking the 0 K dissociation threshold for the formation of CH3(+) + H(C2H(+) + H) from CH4(C2H2). The nonexistence of a step in the spectrum for C 2H4 at its dissociation threshold for C2H2(+) formation provides strong support for the proposed mechanism. This experiment shows that, for a range of molecules, where the ion dissociation lifetimes near the dissociation thresholds are <10(-7) s, pulsed field ionization photoelectron measurements will yield not only highly accurate ionization energies, but also 0 K dissociation thresholds.

Vacuum ultraviolet photoionization and photoelectron studies in the new millennium: recent developments and applications

Vacuum ultraviolet (VUV) photoionization mass spectrometry and photoelectron spectroscopy have played a central role in providing energetic and spectroscopic information for neutrals and cations. The most important data obtainable in a VUV photoionization and photoelectron experiment are ionization energies and 0 K ion dissociation thresholds or appearance energy (AE), from which 0 K bond dissociation energies for neutrals and cations can be deduced. The recent developments in VUV lasers and third-generation synchrotron sources, together with the introduction of the pulsed-field ionization (PFI), photoelectron (PFI-PE), and PFI-photoion (PFI-PI) methods, have revolutionized the field of photoelectron and ion spectroscopy by significantly improving the energy resolution to the range of 0.025–1.0 meV (full width at half maximum, FWHM). These resolutions, which make possible the measurement of photoelectron spectra for many simple molecules at the rotational-resolved level, are ≈100-fold better than those observed in traditional photoelectron studies, making the PFI-PE technique a true spectroscopic method. The recent introduction of the synchrotron-based PFI-PEPICO scheme has shown that AE values for a range of molecules can be determined with an unprecedented precision limited only by the PFI-PE measurement. The synchrotron-based PFI-PEPICO and PFI-PI schemes show great promises for future studies of state- or energy-selected ion-dissociation dynamics and energy-selected ion-molecule reaction dynamics. Further improvement in energy resolution for PFI-PE and PFI-PI measurements has been demonstrated using the two-color photo-induced Rydberg ionization (PIRI) spectroscopic scheme, which involves the photo-induced ionization of intermediate long-lived high- n ( n≥100) Rydberg states. The incorporation of this method by VUV photoexcitation to prepare intermediate high- n ( n≥100) Rydberg states is also expected to greatly increase the energy range of PFI studies. The availability of this array of laser- and synchrotron-based PFI methods, including PFI-PE, PFI-PEPICO, PFI-PI, PFI-ion-pair, and PIRI schemes, ensures an exciting and bright future for VUV photoionization and photoelectron studies in the new millennium.

Advances in photoionization and photoelectron studies using third generation synchrotron radiation

Abstract In the past few years, we have established a unique high-resolution photoionization–photoelectron facility at the Chemical Dynamics Beamline of the Advanced Light Source. By combining a 6.65-m off-plane Eagle mounted scanning monochromator with the undulator synchrotron radiation source, we have demonstrated photon energy resolving powers up to E /Δ E =72,000–100,000 in the photon energy range of 6–27 eV. We have also developed novel synchrotron-based pulsed field ionization photoelectron (PFI-PE) detection schemes, achieving resolutions of 1–6 cm −1 (FWHM). Using the specially designed photoelectron–photoion apparatus, which features a supersonic molecular beam source and opposing electron and ion time-of-flight spectrometers, we have recently succeeded in performing synchrotron-based PFI-PE photoion coincidence experiments, attaining resolutions limited only by the PFI-PE measurements. Most recently, we have demonstrated a generally applicable synchrotron-based scheme for PFI-photoion measurements. The PFI-PEPICO and PFI-PI methods are expected to have a significant impact in future reaction dynamics studies involving the preparation of rovibronically state-selected reactant ions.

High resolution pulsed field ionization photoelectron spectroscopy using multibunch synchrotron radiation: Time-of-flight selection scheme

We have developed an efficient electron time-of-flight (TOF) selection scheme for high resolution pulsed field ionization (PFI) photoelectron (PFI-PE) measurements using monochromatized multibunch undulator synchrotron radiation at the Advanced Light Source. By employing a simple electron TOF spectrometer, we show that PFI-PEs produced by the PFI in the dark gap of a synchrotron ring period can be cleanly separated from prompt background photoelectrons. A near complete suppression of prompt electrons was achieved in PFI-PE measurements by gating the PFI-PE TOF peak, as indicated by monitoring background electron counts at the Ar(11s′) autoionizing Rydberg peak, which is adjacent to the Ar+(2P3/2) PFI-PE band. The rotational-resolved PFI-PE band for H2+ (X 2Σg+,v+=0) measured using this electron TOF selection scheme is nearly free from residues of nearby autoionizing features, which were observed in the previous measurement by employing an electron spectrometer equipped with a hemispherical energy analyzer. This comparison indicates that the TOF PFI-PE scheme is significantly more effective in suppressing the hot-electron background. In addition to attaining a high PFI-PE transmission, a major advantage of the electron TOF scheme is that it allows the use of a smaller pulsed electric field and thus results in a higher instrumental PFI-PE resolution. We have demonstrated instrumental resolutions of 1.0 cm−1 full width at half maximum (FWHM) and 1.9 cm−1 FWHM in the PFI-PE bands for Xe+(2P3/2) and Ar+(2P3/2) at 12.123 and 15.760 eV, respectively. These resolutions are more than a factor 2 better than those achieved in previous synchrotron based PFI-PE studies.

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 for O2+(b 4Σg−, v+=0–5). The shorter (nominal) effective lifetimes for high-n Rydberg states converging to O2+(B 2Σg−, v+=0–6) are attributed to the higher kinetic energy releases (or velocities) of O++O fragments resulting from predissociation of the O2+(B 2Σg−, v+=0–6) ion cores. Rotationally resolved PFI-PE measurements also make possible the identification of the weak vibrational progression with the origin at 20.35 eV as associated with transitions to O2+(2Σu−, v+=0–7). The analysis of the rotationally resolved PFI-PE bands for O2+(2Σu−, v+=0 and 1) has yielded accurate rotational constants and IE values for these states. The rotational structures resolved in the O2+(2Σu−, v+=0 and 1) PFI-PE bands are contributed overwhelmingly by the ΔN=−3, −1, +1, and +3 (or N, P, R, and T) rotational branches, showing that the angular momenta for the outgoing photoelectron are restricted to l=0, 2, and 4. Based on simulation of the observed rotational structures, we also obtain the predissociative lifetimes for O2+(B 2Σg−, v+=0–7) and O2+(2Σu−, v+=0–1) to be in the range of 0.45–2 ps.

Rotationally resolved photoelectron study of O2: Identification of the vibrational progressions for O2+(2 2Πu,2Σu−) at 19.6–21.0 eV

On the basis of rotationally resolved pulsed field ionization photoelectron measurements, we have shown that the weak vibrational progressions in the energy region of 19.6–21.0 eV observed in recent high resolution photoelectron spectroscopic studies of O2 are associated with transitions to the 2 2Πu 1/2 (2 2Πu 3/2) and 2Σu− states of O2+, with ionization energies of 19.6268±0.0005 eV (19.6329±0.0005 eV) and 20.3528±0.0005 eV, respectively.

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).