G. K. Jarvis

High-resolution pulsed field ionization photoelectron–photoion coincidence study of CH4: Accurate 0 K dissociation threshold for CH3+

The formation of methyl cation (CH3+) from methane (CH4) has been investigated in high resolution using the newly perfected pulsed field ionization photoelectron–photoion coincidence (PFI-PEPICO) scheme. The PFI-PEPICO data reveal that fragmentation of CH4 in high-n Rydberg states occurs at energies above the dissociation threshold prior to pulsed field ionization. The crossover point of the breakdown curves is found to depend strongly on the Stark field in the ion source and thus traditional simulation procedures based on such a feature for ion dissociation energy determination are not appropriate in PFI-PEPICO studies. We show that for a prompt dissociation process, the disappearance energy of the parent molecule provides an accurate measure of the 0 K ion dissociation threshold, as that for CH3+ from CH4 is 14.323±0.001 eV.

High-resolution pulsed field ionization photoelectron-photoion coincidence spectroscopy using synchrotron radiation

We have developed a sensitive and generally applicable scheme for performing pulsed field ionization (PFI) photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) spectroscopy using two-bunch and multibunch synchrotron radiation at the Advanced Light Source. We show that this technique provides an ion internal state (or energy) selection limited only by the PFI-PE measurement. Employing a shaped pulse for PFI and ion extraction, a resolution of 0.6 meV [full width at half maximum (FWHM)] is observed in the PFI-PEPICO bands for Ar+(2P3/2,1/2). As demonstrated in the PFI-PEPICO study of the process, O2+hν→O2+(b 4Σg−, v+=4, N+)+e−→O+(4S)+O(3P)+e−, the dissociation of O2+(b 4Σg−, v+=4) in specific rotational N+ levels can be examined. The simulation of the experimental breakdown diagram for this reaction supports the conclusion that the threshold for the formation of O+(4S)+O(3P) from O2+(b 4Σg−, v+=4) lies at N+=9. We have also recorded the PFI-PEPICO time-of-flight (TOF) spectra of O+ formed in the dissoci...

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...

Rotationally resolved pulsed field ionization photoelectron bands of O2+(X 2Π1/2,3/2g, v+=0–38) in the energy range of 12.05–18.15 eV

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra for O2 in the energy range of 12.05–18.15 eV, covering ionization transitions O2+(X 2Π1/2,3/2g, v+=0–38,J+)←O2(X 3Σg−, v+=0,N″). While the PFI-PE bands for O2+(X 2Π1/2,3/2g, v+=3–5, 9, 11, 12, 22, and 25–38) reported here are the first rotational-resolved photoelectron measurements, the PFI-PE bands for O2+(X 2Π1/2,3/2g, v+=25–38) represent the first rotationally resolved spectroscopic data for these states. The simulation of spectra obtained at rotational temperatures of ≈20 and 220 K allows the unambiguous identification of O2+(X 2Π1/2,3/2g, v+⩾21) PFI-PE bands, the majority of which overlap with prominent PFI-PE bands for O2+(A 2Πu, v+=0–12) and O2+(a 4Πu, v+=0–18). Combined with spectroscopic data obtained in the previous emission study and the present PFI-PE experiment, we have obtained accurate Dunham-type expansion coefficients for ionization energies, vibrational constants, rotational constants, and spin–...

A characterization of vibrationally and electronically excited NO2+ by high-resolution threshold photoionization spectroscopy

The pulsed-field ionization zero-electron kinetic-energy (PFI-ZEKE) threshold photoionization spectrum of NO2 from 9.58 to 20 eV is obtained using vacuum ultraviolet synchrotron radiation by means of the Chemical Dynamics Beamline at the Lawrence Berkeley National Laboratory Advanced Light Source. The high resolution afforded by PFI threshold discrimination yields new or refined spectroscopic constants for a number of known excited states of the cation, including the first estimate of the A rotational constant in the a 3B2 state, as well as new fundamental frequencies for the A 1A2 and B 1B2 states, a precise determination of the singlet–triplet splitting in the c 3B1–C 1B1 complex and the first observations of the states, d 3A1 and D 1B2. Most significantly, ZEKE photoelectron detection resolves vibrational structure in the linear X 1Σg+ ground state of NO2+. Vibrational positions in the first electron volt of the spectrum are found to conform with the predictions of a Hamiltonian that includes Fermi res...

High-resolution pulsed field ionization photoelectron-photoion coincidence study of C2H2: Accurate 0 K dissociation threshold for C2H+

By employing the newly developed pulsed field ionization photoelectron-photoion coincidence (PFI-PEPICO) apparatus of the Chemical Dynamics Beamline at the Advanced Light Source, we have examined the formation of ethynyl ion (C 2 H + ) from acetylene (C 2 H 2 ) at high resolution. The PFI-PEPICO time-of-flight spectra reveal that fragmentation of C 2 H 2 in high-n Rydberg states occurs at energies above the dissociation threshold prior to pulsed field ionization. This study shows that for a prompt dissociation process, the disappearance energy of the parent molecule determined in PFI-PEPICO measurements provides an unambiguous measure of the 0 K ion dissociation threshold. For the formation of C 2 H + from C 2 H 2 this is found to be 17.3576 ± 0.0010 eV.

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...

Rotational-resolved pulsed field ionization-photoelectron study of NO+(A′ 1Σ−,v+=0–17) in the energy range of 17.70–20.10 eV

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra of NO in the energy range of 15.6–18.2 eV, covering ionization transitions of NO+(a 3Σ+,v+=0–16,J+)←NO(X 2Π3/2,1/2,v″=0,J″). The PFI-PE bands for NO+(a 3Σ+,v+=1–5,7–10,12–14,16) obtained in this experiment represent the first rotationally resolved spectroscopic data for these states. The simulation of these PFI-PE bands provides accurate molecular constants for NO+(a 3Σ+,v+=0–5,7–10,12–14,16), including ionization energies, vibrational constants (ωe+=1295.2±0.1 cm−1, ωe+χe+=15.198±0.002 cm−1), and rotational constants (Be+=1.3501±0.0070 cm−1, αe+=0.0206±0.0001 cm−1). As observed in the PFI-PE study of NO+(X 1Σ+), this experiment reveals a generally increasing trend for the maximum ΔJ value and intensities of higher ΔJ branches as v+ (or bond distance) for NO+(a 3Σ+) is increased. This observation can be taken as strong support of the electron-molecular-ion-core scattering model for angular momentum and energy exc...

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...