Doreen G. Leopold

Photoelectron spectroscopy of mass‐selected metal cluster anions. I. Cu−n, n=1–10

Negative ion photoelectron spectra of Cu−n (n=1–10) are reported for the 0–2.4 eV region at an instrumental resolution of 10 meV. The cluster anions were prepared in a flowing afterglow ion source incorporating a cold cathode dc discharge. This very simple source provides a convenient, general method to prepare continuous beams of near‐thermal metal cluster ions at intensities (up to 10−11 A) sufficient for spectroscopic or chemical studies. Photoelectron spectra of the copper cluster anions yield measurements for vertical electron binding energies and adiabatic electron affinities as a function of cluster size. The overall trend observed is well described by the classical spherical drop electrostatic model. In addition, quantum effects are apparent in the higher electron affinities generally observed for clusters containing odd numbers of atoms. Excited electronic states in the photoelectron spectra show that the transition energy in the neutral molecule decreases rapidly with cluster size. Vibrational s...

Methylene: A study of the X̃ 3B1 and ã 1A1 states by photoelectron spectroscopy of C2H− and CD2−

Photoelectron spectra are reported for the CH2(X 3B1)+e−←CH−2 (X 2B1) and CH2(a 1A1)+e−←CH−2 (X 2B1) transitions of the methylene and perdeuterated methylene anions, using a new flowing afterglow photoelectron spectrometer with improved energy resolution (11 meV). Rotational relaxation of the ions to ∼300 K and partial vibrational relaxation to <1000 K in the flowing afterglow negative ion source reveal richly structured photoelectron spectra. Detailed rotational band contour analyses yield an electron affinity of 0.652±0.006 eV and a singlet–triplet splitting of 9.00±0.09 kcal/mol for CH2. (See also the following paper by Bunker and Sears.) For CD2, results give an electron affinity of 0.645±0.006 eV and a singlet–triplet splitting of 8.98±0.09 kcal/mol. Deuterium shifts suggest a zero point vibrational contribution of 0.27±0.40 kcal/mol to the observed singlet–triplet splitting, implying a Te value of 8.7±0.5 kcal/mol. Vibrational and partially resolved rotational structure is observed up to ∼9000 c...

A study of the low-lying electronic states of Fe2 and Co2 by negative ion photoelectron spectroscopy

The anions Fe−2 and Co−2 were prepared and cooled to 300 K in a flowing afterglow ion source, and the low‐lying electronic states of the neutral dimers were probed by negative ion photoelectron spectroscopy. Previous ab initio studies of Fe2 and Co2 have predicted single 4s–4s bonds, and extremely high densities of low‐lying states due to the small energy cost in transferring electrons among nonbonding 3d orbitals. In contrast to the complex photoelectron spectra implied by these calculations, the observed spectra are remarkably simple. It is argued that this spectral simplicity implies a greater role for the 3d electrons in the iron and cobalt dimer bonds. These data also provide values for the electron affinities of the neutral dimers (0.902±0.008 eV Fe2, 1.110±0.008 eV Co2), the bond elongation on electron attachment (0.08±0.02 A Fe2, Co2), and the vibrational frequencies of the anions (250±20 cm−1 Fe−2, 240±15 cm−1 Co−2). Related studies of the atomic anions yield improved values for the electron affi...

Photoelectron spectroscopy of the halocarbene anions HCF−, HCCl−, HCBr−, HCI−, CF−2, and CCl−2

The 488 nm photoelectron spectra are reported for the HCX(X1A’)+e−←HCX−(X2A‘) and HCX(a3A‘)+e−←HCX−(X2A‘) transitions in HCF−, DCF−, HCCl−, HCBr−, and HCI− and for the CX2(X1A1)+e−←CX−2(X2B1) transitions in CF−2 and CCl−2 . Adiabatic electron affinities are found to be 0.557±0.005 eV (HCF), 0.552±0.005 eV (DCF), 1.213±0.005 eV (HCCl), 1.556±0.008 eV (HCBr), 1.683±0.012 eV (HCI), 0.179±0.005 eV (CF2), and 1.603 ± 0.008 eV (CCl2). Bounds for the triplet excitation energies are determined for all the halocarbenes for which photoelectron spectra were observed, with the exception of CCl2. For the HCX halocarbenes, upper bounds for the triplet excitation energies are 14.7±0.2 kcal/mol (HCF, DCF), 11.4±0.3 kcal/mol (HCCl), and 9±2 kcal/mol (HCBr). A more detailed analysis of HCF suggests the actual triplet excitation energy to be 11.4±0.3 kcal/mol, 14.7±0.2 kcal/mol, or 8.1±0.4 kcal/mol, with the first value the most likely. Since the multiplicity of the ground state of HCl is not known, we report the ener...

Direct absorption spectroscopy of jet‐cooled polyenes. II. The 1 1B+u←1 1A−g transitions of butadienes and hexatrienes

In the present paper, we report the direct absorption spectra of the 1 1B+u←1 1A−g transitions of gas phase butadiene, deuterated and methylated butadienes, and the cis and trans isomers of hexatriene cooled to low rotational and vibrational temperatures in supersonic molecular jets. These jet absorption spectra allow the more accurate determinations of Franck–Condon factors, upper state vibrational intervals and vibronic band homogeneous widths. We discuss the experimental constraints that the measurements reported here and in the previous paper of this series impose on theoretical models of the equilibrium structures and relaxation dynamics of the 1 1B+u excited states of the small linear polyenes.

Laser photoelectron spectroscopy of the formyl anion

The 488 nm photoelectron spectra of HCO− and DCO− show vibrational structure in the X 2A’ state of neutral formyl radical up to 10 000 cm−1 above the vibrational ground state. Electron affinities are found to be 0.313±0.005 eV for HCO and 0.301±0.005 eV for DCO. The CH bond strength and heat of formation of HCO− and the gas phase acidity of formaldehyde are derived from these data. A Franck–Condon analysis of the photoelectron spectra provides an estimate of the equilibrium geometry of the anion. Transitions to excited vibrational states of HCO enable the determination of a complete set of quadratic anharmonicities.

A study of FeCO− and the 3Σ− and 5Σ− states of FeCO by negative ion photoelectron spectroscopy

The 488 and 514 nm negative ion photoelectron spectra of FeCO−, obtained at an instrumental resolution of 5 meV (40 cm−1), show vibrationally resolved transitions from the anion ground state to the ground state and a low‐lying excited state of the neutral molecule. The ground state of FeCO is assigned as the 3Σ− state and the excited state, lying 1135±25 cm−1 higher in energy, as the 5Σ− state. The fundamental vibrational frequencies are νCO=1950±10, νFeC=530±10, and νbend=330±50 cm−1 in the 3Σ− state, and νCO=1990±15, νFeC=460±15, and νbend=180±60 cm−1 in the 5Σ− state. Principal force constants are estimated from these results. Based on a Franck–Condon analysis of the spectrum and other considerations, the Fe–C bond is determined to be 0.15±0.04 A shorter, and the C–O bond 0.05±0.02 A longer, in the 3Σ− state than in the 5Σ− state. These results demonstrate the importance of sdσ hybridization in reducing the σ repulsion between the metal 4s electron and the CO 5σ lone pair, a mechanism that is available...

Electron affinities of the alkali halides and the structure of their negative ions

Photoelectron spectra are reported for the MX (X 1Σ+)+e−←MX−(X 2Σ+) transitions of ten alkali halide anions at 488 nm. Adiabatic electron affinities (±0.010 eV) are determined to be 0.593 (LiCl), 0.520 (NaF), 0.727 (NaCl), 0.788 (NaBr), 0.865 (NaI), 0.582 (KCl), 0.642 (KBr), 0.728 (KI), 0.543 (RbCl), and 0.455 eV (CsCl). Fundamental vibrational frequencies, equilibrium bond lengths, and dissociation energies are also reported for the anion 2Σ+ ground states. An observed linear correlation of electron affinities with α/r2 (α=metal atom polarizability) is used to predict the electron affinities of the remaining alkali bromides and iodides, as well as related alkali salts. A simple electrostatic model for the alkali halide anions is also presented which enables the accurate (±0.1 eV) calculation of electron affinities.

Laser photoelectron spectroscopy of vibrationally relaxed CH 2-: A reinvestigation of the singlet-triplet splitting in methylene

The photoelectron spectrum of CH−2 has been reinvestigated using a new apparatus that incorporates a flowing afterglow ion source, providing vibrational and rotational cooling of the sample ions. Earlier photoelectron studies [J. Chem. Phys. 74, 5460 (1981)] employing gas discharge and sputter ion sources were plagued by hot CH−2 ions whose vibrational population distribution, for reasons not yet explicable, could not be detectably altered by modification of source conditions. In contrast, the spectrum of cooled CH−2 displays a markedly changed vibronic band intensity profile for the CH−2 (2B1) →−e− CH2(3B1) transition. These results enable several previously observed spectral features to be positively identified as vibrational hot bands. The new CH−2 photoelectron spectrum is consistent only with the methylene singlet–triplet splitting of approximately 9 kcal/mol determined by numerous recent experimental and theoretical studies.

The direct ultraviolet absorption spectrum of the 1Σ+g→ 1B2(1Σ+u) transition of jet‐cooled CS2

The absorption spectrum of the linear→bent 1Σ+g→1B2 (1Σ+u) transition of jet‐cooled CS2 is reported in the 47 750–53 200 cm−1 region. Adjustment of diluent gas composition enables the preparation of rotationally cold (<10 K)/vibrationally hot (≊300 K) and rotationally cold/vibrationally cold samples. The collapse of rotational envelopes on cooling facilitates the accurate determinations of frequencies and relative absorption intensities of transitions both from the zero point level and from excited vibrational levels of the ground electronic state. At 49 000–51 000 cm−1, in the region of the Franck–Condon maximum, a simple vibronic pattern emerges involving two upper state vibrational modes containing both stretching and bending contributions. In this region of the spectrum, the upper state rotational structure diverges from that of a nearly symmetric top and approaches that of the linear or quasilinear molecule. At higher energies, the vibronic structure grows in complexity, suggesting increased anharmon...