Roy D. Mead

OH− and OD− threshold photodetachment

Threshold photodetachment cross sections for OH− and OD− at photon energies close to their electron binding energies have been obtained using a crossed tunable laser‐negative ion beam apparatus at a resolution of 25 μeV. The data reveal many clear features associated with photodetachment of single rotational levels of the negative ion, producing single rotational levels of the neutral and a near zero energy electron. From the frequencies of the observed thresholds, the spectroscopic constants of OH− and OD− are found. The rotational constants of the vibrational ground state are B0 = 18.7409(45) cm−1 and D0 = 2.052(45)×10−3 cm−1 for OH− and B0 = 9.9852(48) cm−1 and D0 = 0.553(33)×10−3 cm−1 for OD−. The electron affinities of OH and OD are measured to be 14 741.03(17) cm−1 and 14 723.92(30) cm−1, respectively. The observed bound–free transitions satisfy selection rules only slightly different from the selection rules observed in bound–bound spectroscopy. The relative intensities of the transitions show the ...

Spectroscopy and dynamics of the dipole-bound state of acetaldehyde enolate

Ultrahigh‐resolution photodetachment spectroscopy of acetaldehyde enolate negative ion has revealed ∼50 narrow resonances near threshold, corresponding to excitation to a diffuse state in which the electron is weakly bound by the field of the molecular dipole. A complete analysis of rotational transitions between the ground valence state and the excited dipole‐bound state has been carried out, yielding spectroscopic constants and geometries for both states. In analogy to Rydberg states, the structure of the ‘‘neutral core’’ of the dipole‐bound state is like that of the neutral radical. The dependence of autodetachment lifetimes upon the rotational quantum numbers of the dipole‐bound state has been measured. Bound levels of the dipole‐bound anion state are readily electric‐field detached. The selection rules and dynamics of autodetachment from the dipole‐bound state are discussed.

Photodetachment spectroscopy of C2− autodetaching resonances

The cross section for photodetachment of C2− is investigated in the photon energy range 14 000–20 000 cm−1 (1.75–2.5 eV). Sharp resonances due to autodetachment are observed at photon energies corresponding to transitions between high vibrational levels of the C2− X 2Σg+ state and high vibrational levels of the C2− B 2Σu+ states. The resonances are narrower than 6 GHz, and those arising from the v=5 level of the B state are about 1/10 as strong as those arising from the v=6 and higher levels. In addition, the signal from the allowed direct photodetachment of C2− X state, which would produce a smooth background cross section, is not observed, indicating that it is less than the 103 peak to background contrast ratio. A rotational analysis of the nine bands of the C2− B–X transition observed in this study, coupled with previous measurements provides a new, more precise set of spectroscopic constants for these states. These constants are then used to generate RKR potential curves for the B and X states of C2−...

Ultrahigh resolution spectroscopy of C2−: The A 2Πu state characterized by deperturbation methods

Spectra of the C2− B 2Σu+–X 2Σg+ transition have been obtained using a merged laser‐ion beam spectrometer with sub‐Doppler resolution. Strong perturbations of the B state by the previously unobserved A 2Πu state are analyzed to obtain the characteristics of the new state. The A state spectroscopic constants in wave numbers (with estimated standard errors) are found to be Te=4060±180, ωe=1656±20, ωexe=10.80±0.50, Be=1.630±0.005, and A=−24±1. All nine perturbations observed in this work and the five observed in the work of Herzberg and Lagerqvist are explained by the interaction of the A 2Πu state with the B state. These results, as well as the perturbation matrix elements, are compared with theoretical estimates and results for isoelectronic 13‐electron species.

Spectroscopy and dynamics of the dipole-supported state of acetyl fluoride enolate anion

High resolution photodetachment spectroscopy of acetyl fluoride enolate anion has revealed ≈200 narrow resonances near the photodetachment threshold, corresponding to excitation of the anion to a diffuse state in which the electron is weakly bound by the field of the molecular dipole. An analysis of the rotational transitions between the ground valence state and the excited dipole‐supported state has been carried out, yielding spectroscopic constants for both states. The binding energy of the dipole‐supported state is found to be less than 35 cm−1 . The dependence of autodetachment lifetimes upon rotational quantum numbers of the dipole‐supported state has been measured. The selection rules and dynamics of autodetachment from the dipole‐supported state are discussed. The results are compared with those obtained previously for acetaldehyde enolate anion.

Laser Spectroscopy of Negative Ion Beams

The availability of intense tunable and fixed frequency lasers has made it possible to investigate photon-ion interactions in crossed beam experiments. The short wavelength limitations of current lasers result in the greatest variety of applications having been made to studies of photon interactions with negative ions. Two major experimental approaches have been developed: fixed frequency laser negative ion photoelectron spectroscopy and tunable laser photode-tachment/photodissociation spectroscopy. The former approach is best suited for determination of electron affinities and structural properties of ions and free radicals, while the latter method provides highly accurate electron affinities and dynamical information on electron-molecule interactions. It is also possible to determine the internal energy content of negative ions. The experimental techniques and their limitations will be discussed in the form of several case studies: OH , C2 and the transition metals. Finally an attempt will be made to ascertain promising avenues for new investigations in this field.