Study of KrO- and KrO via Slow Photoelectron Velocity-Map Imaging Spectroscopy and ab Initio Calculations

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Название:
Study of KrO- and KrO via Slow Photoelectron Velocity-Map Imaging Spectroscopy and ab Initio Calculations
Тип: Автореферат
Краткое содержание:

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

 


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