Photoelectron (PE) spectroscopy of molecular anions is a versatile method for determining electron affinities, excited state term energies, and vibrational frequencies of the corresponding neutral species.1 However, its limited energy resolution (5-10 meV) restricts the degree to which low-frequency vibrational modes can be resolved. This deficiency has been addressed using a related higher resolution technique, anion zero energy kinetic energy (ZEKE) spectroscopy,2 which has been applied to the study of several weakly bound open-shell rare-gas halide complexes via photodetachment of the appropriate anion.3-6 ZEKE spectroscopy resolves the closely spaced vibrational levels supported by these electronic states and thus yields spectroscopic information that is highly complementary to scattering experiments on the same systems.7 Accurate experimental spectroscopic constants for the anion and neutral complexes can be extracted from the spectra and compared to ab initio calculated interaction potentials of these species. The recently developed slow electron velocity-map imaging (SEVI) technique, which is a hybrid between ZEKE and conventional PE spectroscopy, has been used to study the ClH2 and ArO van der Waals complexes by photodetachment of ClH2 - and ArO-.8,9 SEVI offers comparable resolution to anion ZEKE (2-3 cm-1) but is considerably easier to implement.10 In this paper, we continue our investigation of rare-gas oxides using the SEVI technique by studying KrO- and KrO. These species are both open shell complexes with 2Σ+ and 3Π ground states, respectively, and have several low-lying electronic states. Hence, the extraction of accurate potential energy curves for the anion and neutral poses a challenge for both experiment and theory. Early studies on the rare gas oxides were motivated by their possible application in excimer lasers.11-13 The first experimental data on the interaction potentials between rare-gas and oxygen atoms was obtained from the beam scattering experiments of Aquilanti and co-workers.14-18 de Clercq et al. have studied KrO- via conventional PE spectroscopy.19 The resolution of their apparatus (∼200 cm-1) did not allow the observation of individual vibronic transitions. However, a bimodal structure with temperature-dependent relative intensity was observed and attributed to transitions originating from the ground state and the two low-lying excited states of the anion. On the theoretical side, an atoms-in-molecule model in conjunction with ab initio calculations has been implemented to construct potential energy curves for the anion and neutral electronic states, including spin-orbit (SO) coupling, and to evaluate the PE transition probabilities.20 The KrO- PE spectrum simulated using this approach was found to be in good agreement with the experimental measurements.21 Unfortunately, as explained in ref 9, these simulations were affected by a programming error which led to the absence of many bound-bound transitions in the simulations, a defect that was not apparent by comparison with the broad envelopes of the measured low-resolution PE spectra. In this paper, we present high-resolution photoelectron spectra of KrO- obtained using SEVI. Numerous transitions are observed within the broad envelope of the previous lowresolution photoelectron spectra. The accompanying theoretical sections rely on the advanced description of the KrO- and KrO, namely, the calculation of interaction potentials with the coupled cluster method and extended basis set and the ab initio analysis of the scalar relativistic effects and vectorial spin-orbit coupling. These curves are incorporated into a simulation procedure9,20 that accounts correctly for all of the bound-bound transition intensities and, in addition, includes the contribution † Part of the “Vincenzo Aquilanti Festschrift”. * Authors to whom correspondence should be addressed. E-mail: alexei@classic.chem.msu.su (A.A.B.); dneumark@berkeley.edu (D.M.N.). ‡ University of Califo ia, Berkeley. § Moscow State University. ⊥ Oakland University. | University of Warsaw. # Lawrence Berkeley National Laboratory. J. Phys. Chem. A XXXX, xxx, 000 A 10.1021/jp903819m CCC: $40.75 XXXX American Chemical Society Downloaded by AUSTRIA CONSORTIA on July 6, 2009 Published on July 1, 2009 on http://pubs.acs.org | doi: 10.1021/jp903819m of bound-free (dissociative photodetachment) transitions. Comparison of the simulated and experimental SEVI spectra facilitates assignment of the many resolved features. Several spectroscopic constants for KrO- and KrO determined from analysis of the experimental spectra are found to agree with the calculated values. II. Experimental Section The SEVI apparatus has been described in detail elsewhere.10,22 SEVI is a high resolution variant of PE spectroscopy in which mass-selected anions are photodetached at a series of wavelengths. The resulting photoelectrons are collected by velocitymap imaging (VMI)23 using relatively low extraction voltages, with the goal of selectively detecting slow electrons with high efficiency and enlarging their image on the detector. At each wavelength, one obtains a high resolution photoelectron spectrum over a limited range of electron kinetic energy. KrO- anions were produced from a gas mixture comprising 0.1% N2O and 10% krypton in a balance of neon. The gas mixture, at a stagnation pressure of 350 psi, was expanded into the source vacuum chamber through an Even-Lavie pulsed valve24 equipped with a circular ionizer. The anions were then perpendicularly extracted into a Wiley-McLaren time-of-flight mass spectrometer25 and directed to the detachment region by a series of electrostatic lenses and pinholes. A pulse on the last ion deflector allowed only the desired mass into the interaction region. Anions were photodetached between the repeller and the extraction plates of the VMI stack by the gently focused output of a Nd:YAG-pumped tunable dye laser. The photoelectron cloud formed was coaxially extracted down a 50 cm flight tube and mapped onto a detector comprising a chevron-mounted pair of time-gated, imaging quality microchannel plates coupled to a phosphor screen, as is typically used in photofragment imaging experiments.26 Events on the screen were collected by a 1024 × 1024 charge-coupled device (CCD) camera and sent to a computer. Electron velocity-mapped images resulting from 30 000-50 000 laser pulses were summed, quadrant symmetrized, and inverse-Abel transformed. Photoelectron spectra were obtained via angular integration of the transformed images. The spectra presented here are plotted with respect to electron binding energy (eBE), defined as the difference between the energy of the photodetachment photon and the measured electron kinetic energy (eKE). The apparatus was calibrated by acquiring SEVI images of atomic oxygen27 at several different photon energies. In the SEVI experiment, within the same image, all observed transitions have similar widths in pixels (Δr), which means transitions observed further from threshold (larger r) are broader in energy. With the -200 V VMI repeller voltage used in this study, the full widths at half-maximum (fwhm) Γ of the oxygen peaks were 2.2 cm-1 at 20 cm-1 eKE and 6.8 cm-1 at 150 cm-1 eKE. The dependence of Γ upon eKE was found to fit a quadratic expression where all units are in wavenumbers. III. Experimental Results The SEVI spectrum of KrO-, taken at a photon energy E0 ) 13334.6 cm-1, is shown in Figure 1. The spectrum is quite congested, with most transitions occurring between 12 900 and 13 200 cm-1. Clearly disce ible peaks are labeled by capital letters A-P and their positions are listed in Table 1. The estimated error bars are (5 cm-1. There are at least two prominent progressions starting with peaks C and K, with characteristic peak spacings of 20-30 cm-1. These progressions appear to lie above a smoothly varying background signal that peaks around 13025 cm-1, but one cannot tell from the spectrum alone if this background results from spectral congestion or transitions to continuum states of the neutral complex. Weak structure is also observed below 12 900 cm-1. The most prominent features here are peaks A and B at 12 610 and 12 640 cm-1, respectively. Other features are labeled by small letters a-k. A detailed assignment of the SEVI peaks can be made by comparison with the theoretical results presented in section IV. The transitions shown in Figure 1 occur within the two broad overlapping envelopes of the PE spectrum reported by de Clercq et al.19 The region of maximum intensity in the SEVI spectrum approximately coincides with the peak of the “X” features in the PE spectrum near 13 100 cm-1. However, compared to the PE spectrum, the SEVI spectrum displays much less intensity at eBE < 12 900 cm-1. De Clercq et al. found that the feature “A” in this region was sensitive to ion source conditions and presumably resulted from “hot band” transitions originating from electronically excited anions. Peak A in the SEVI spectrum coincides with the maximum of the feature “A” near 12 600 cm-1 in the PE spectrum. Hence, the intensity differences between the SEVI and PE spectra suggest lower anion temperatures in the SEVI experiment, similar to what was observed for ArO-.9
