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Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F<sup>&#8722;</sup>

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound ...

Washington University in St. Louis

Photoelectron imaging is finding increasingly widespread use in probing electronic structure and chemical dynamics. In this tutorial review, two benchmark systems, H(-) and I(-), are used to introduce essential concepts linking photoelectron images of negative ions with parent electronic structure. For pedagogical reasons, a qualitative approach based upon spectroscopic selection rules is emphasized in interpreting the images. This approach is extended to molecular systems, highlighting that even qualitative interpretation of results can lead to significant chemical insights.

Photoelectron imaging: an experimental window into electronic structure.

Photoelectron imaging probes both molecular electronic structure and electron molecule interactions. In the current work images were recorded for detachment from the I(-) x C(4)H(5)N (I(-) x pyrrole) cluster anion at wavelengths between 360 and 260 nm. The direct detachment spectra show strong similarities to those of I(-), although a strong solvent shift, broadening and some structure is observed. A nondirect, dissociative or autodetachment feature is also observed over a range of wavelengths. Ab initio calculations identify several local minima associated with neutral and anion isomers. Energy and Franck-Condon arguments are used to assess the role of these in the detachment process. The cluster anion structure is essentially an I(-) atomic anion in the presence of a neutral pyrrole molecule. The spectral structure arises due to interactions in the open shell neutral cluster residue resulting from detachment. The indirect detachment feature arises through the formation of an intermediate dipole bound cluster anion state which subsequently dissociates. The energy dependence of this channel (observed over a 0.6 eV range of photon energies) is discussed in terms of the wide amplitude motions associated with the van der Waals modes of the cluster anions.

Direct and indirect detachment in the iodide-pyrrole cluster anion: the role of dipole bound and neutral cluster states.

A photodissociative study of CuO2(-) is presented using a combination of energy and time domain photoelectron spectroscopy. Ion source conditions are used that solely produce linear OCuO(-). Photodissociation of this isomer to produce Cu(-) + O2 is conclusively demonstrated at wavelengths between 765 and 340 nm. Nanosecond pulsed photoexcitation at wavelengths shorter than 340 nm produces single photon detachment transitions from the first excited state of CuO2(-). At longer wavelengths narrow Cu(-) fragment transitions are observed as a result of a sequential two photon process. In addition, the longer wavelengths produce a weak, broad two photon dependent signal, the result of detachment of the dissociating linear isomer. Time resolved pump-probe measurements reveal a long timescale growth (up to 150 ps) of the Cu(-) fragment yield, consistent with the unfavorable starting geometry for the dissociative process and indicating a potential energy surface which has one or more substantial barriers to dissociation. 

Photodetachment and photodissociation of the linear OCuO- molecular anion: energy and time dependence of Cu- production.

The first photoelectron spectra of AgF(-) are recorded over the energy range 1.61-1.85 eV using the velocity map imaging technique. The resolved vibrational structure of the AgF X', v' ← AgF(-) X″, v″ = 0 band yields an AgF electron affinity of 1.46 ± 0.01 eV and vibrational frequency of 500 ± 40 cm(-1). For the v' = 2, 3, 4 channels, the photodetachment cross sections and angular distributions undergo rapid changes over a narrow electron kinetic energy range in the region of 50 meV (approximately 13 meV below the opening of the next vibrational channel). This is consistent with Fano-like behavior indicating autodetachment following excitation to a resonant anion state lying in the detachment continuum. EOM-CCSD calculations reveal this to be a dipole bound state. The consistency of the detachment data with the vibrational autodetachment propensity rule Δv = -1 shows that the autodetachment results from breakdown of the Born-Oppenheimer approximation, coupling the vibrational and electronic degrees of freedom. 

The effect of the dipole bound state on AgF⁻ vibrationally resolved photodetachment cross sections and photoelectron angular distributions.

Electron attachment to closed-shell molecules is a gateway to various important processes in the gas and condensed phases. The properties of an electron-attached state, such as its energy and lifetime as well as the character of the molecular orbital to which the electron is attached, determine the fate of the anion. In this experimental and theoretical study of copper and silver fluoride anions, we introduce a new type of metastable anionic state. Abrupt changes in photoelectron angular distributions point to the existence of autodetaching states. Equation-of-motion coupled-cluster singles and doubles calculations augmented by a complex absorbing potential identify some of these states as Σ and Π dipole-stabilized resonances, a new type of shape resonance. In addition, these molecules support valence and dipole-bound states and a Σ resonance of charge-transfer character. By featuring five different types of anionic states, they provide a vehicle for studying fundamental properties of anions and for validating new theoretical approaches for metastable states.

Same but Different: Dipole-Stabilized Shape Resonances in CuF(-) and AgF(.).

We report photoelectron spectra of CHCN, recorded at photon energies between 13 460 and 15 384 cm, which show rapid intensity variations in particular detachment channels. The branching ratios for various spectral features reveal rotational structure associated with autodetachment from an intermediate anion state. Calculations using equation-of-motion coupled-cluster method with single and double excitations reveal the presence of two dipole-bound excited anion states (a singlet and a triplet). The computed oscillator strength for the transition to the singlet dipole-bound state provides an estimate of the autodetachment channel contribution to the total photoelectron yield. Analysis of the different spectral features allows identification of the dipole-bound and neutral vibrational levels involved in the autodetachment processes. For the most part, the autodetachment channels are consistent with the vibrational propensity rule and normal mode expectation. However, examination of the rotational structure shows that autodetachment from the ν (v = 1 and v = 2) levels of the dipole-bound state displays behavior counter to the normal mode expectation with the final state vibrational level belonging to a different mode.

Article	Year
Photoelectron imaging: an experimental window into electronic structure. Chemical Society reviews\| PubMed ID: 19623341	2009
Direct and indirect detachment in the iodide-pyrrole cluster anion: the role of dipole bound and neutral cluster states. The journal of physical chemistry. A\| PubMed ID: 19928771	2010
Photodetachment and photodissociation of the linear OCuO- molecular anion: energy and time dependence of Cu- production. Physical chemistry chemical physics : PCCP\| PubMed ID: 24108412	2014
The effect of the dipole bound state on AgF⁻ vibrationally resolved photodetachment cross sections and photoelectron angular distributions. The Journal of chemical physics\| PubMed ID: 25338893	2014
Same but Different: Dipole-Stabilized Shape Resonances in CuF(-) and AgF(.). The journal of physical chemistry letters\| PubMed ID: 26266864	2015
Channel branching ratios in CHCN photodetachment: Rotational structure and vibrational energy redistribution in autodetachment. The Journal of chemical physics\| PubMed ID: 29272948	2017

Richard Mabbs

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