Pulsed-field-ionization Zero-kinetic-energy Research Articles

Zero kinetic energy photoelectron spectroscopy of Rydberg excited molecular iodine

Two color (2+1′) zero kinetic energy pulsed field ionization (ZEKE-PFI) spectra of I2 ionized via a number of gerade Rydberg excited states have been recorded for the first time. Ionization into both X 2Π3/2,g and X 2Π1/2,g spin–orbit ionic substates was achieved by selective excitation of Rydberg states having the appropriate spin–orbit ionic core. Adiabatic ionization energies have been determined as 75 069±2 cm−1 for the X 2Π 3/2,g state and 80 266±2 cm−1 for the X 2Π1/2,g state with ωe values for the two states determined as 240±1 and 229±1 cm−1, respectively. The ZEKE-PFI spectra recorded for the lower spin–orbit state exhibit non-Franck–Condon behavior which has been interpreted in terms of an autoionization mechanism. The ZEKE-PFI technique has also been used to probe the extent of Rydberg and ion-pair character in a strongly mixed Rydberg/ion-pair state.

Pulsed field ionization zero kinetic energy photoelectron spectroscopy of the vanadium dimer molecule

The technique of pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE), photoelectron spectroscopy was employed to probe the electronic structure of the V+2 cation. Rotationally resolved PFI-ZEKE spectra of the V+2 ground state were obtained by two color excitation via the 700 nm A 3Πu←X 3Σ−g system. The observation of transitions from the A 3Π2u state to two spin–orbit components with Ω=1/2 and Ω=3/2, confirms that the cation ground state has 4Σg− symmetry, in accordance with previous experimental and theoretical work. Striking differences were observed in the rotational selection rules for the 4Σg−←A 3Π1u and the 4Σg−←A 3Π2u transitions. The adiabatic ionization potential of V2 was determined to be 51 271.14(50) cm−1. From an analysis of the rotational structure of the PFI-ZEKE spectra, the following molecular constants were determined for the 4Σg− state: r0=1.7347(24) Å, second order spin–orbit splitting, λ=5.248(17) cm−1, spin–rotation constant, γ=0.0097(87) cm−1, T0=51 282.20(50) cm−1 (1σ error bounds).

Zero kinetic energy, pulsed-field ionization spectroscopy of hydrogen iodide

The rotationally resolved, zero kinetic energy, pulsed-field ionization (ZEKE-PFI) spectrum of the HI+ X 2Π1/2, v+=0 level, obtained by double-resonance excitation via the HI F 1Δ2, v=0 level, is reported. The rotational and Λ-doubling constants for the HI+ X 2Π1/2, v+=0 level obtained from the experiment are close to those estimated theoretically by Mank et al. [J. Chem. Phys. 95, 1676 (1991)]. At higher pressures, the dissociative charge transfer reaction HI*+HI→HI++H+I− represents a potentially serious loss mechanism for the high Rydberg states that give rise to the ZEKE-PFI signal. This result is of more general applicability, because it provides evidence that collisions of the Rydberg electron with neighboring molecules can play a significant role in ZEKE-PFI experiments.

Resonance enhanced multiphoton ionization photoelectron spectroscopy and pulsed field ionization via the F 1Δ2(v′=0) and f 3Δ2(v′=0) Rydberg states of HCl

In this paper, we report the first rotationally resolved one- and two-color resonance enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PES) study of the HCl molecule. The agreement between our experimental branching ratios and theoretical investigations is excellent. We also report the first zero kinetic energy pulsed field ionization (ZEKE-PFI) experiments carried out in a ‘‘magnetic bottle’’ electron spectrometer. A direct comparison is made between ZEKE-PFI and REMPI-PES spectra for ionization via several rotational levels of the F 1Δ2(v′=0) and f 3Δ2(v′=0) Rydberg states of HCl. Large differences in both the spin–orbit and rotational branching ratios are found between the ZEKE-PFI and REMPI-PES spectra. These differences can be understood qualitatively on the basis of rotational and spin–orbit autoionization mechanisms.

Internal rotation of the methyl group in fluorotoluene cations as studied by pulsed field ionization-zero kinetic energy spectroscopy

Pulsed field ionization spectra of o-, m-, and p-fluorotoluenes were observed. The spectrum shows a well-resolved structure due to internal rotation of the methyl group in fluorotoluene cations. Observed level energies and relative intensities were reproduced by a one-dimensional periodic potential with a free-rotor basis set. The analysis shows a great increase in the rotational barrier by ionization for all the fluorotoluenes. It was also found that rotational constant of the internal rotation decreases significantly when the barrier height increases drastically by ionization. From the correlation between the barrier height and the internal rotational constant, the increase of the rotational barrier is explained by the increase of the steric hindrance due to the geometrical change by ionization. The vibronic coupling between very high Rydberg states through the methyl rotational motion was suggested by the strong appearance of the a2–a1 forbidden transition in the PFI spectrum.

Zero kinetic energy (ZEKE) photoelectron spectroscopy of ammonia by nonresonant two-photon ionization from the neutral ground state

Rotationally resolved zero kinetic energy-pulsed field ionization (ZEKE-PFI) photoelectron spectra of NH3 were obtained by nonresonant two-photon ionization from the X̃(1A1′) electronic ground state. The ZEKE spectra were recorded up to an internal ion energy of 9500 cm−1 by pulsed field ionization of long-lived Rydberg states of very high principal quantum numbers. Rovibronic bands of the NH3+ cation with v2+=1–9 (umbrella mode), v4+=1, and for the first time the mode ν1 in the combination band v+=1121 of the X̃+(2A2″) electronic ground state are observed in the ZEKE spectra. Rotational constants and band origins are determined in the analysis. The adiabatic ionization energy obtained by a recent ZEKE measurement is confirmed at 82 159±1 cm−1.