Richard Mabbs

Photoelectron imaging of negative ions

This article provides an overview of some of the recent advances in the rapidly growing field of negative-ion photoelectron imaging spectroscopy. Setting the work that spans several projects in the authors’ laboratory in broader context, three types of measurements are described. First are the ‘static’ (one-photon) photoelectron imaging experiments that aim to characterize the electronic structure and photodetachment dynamics of negative ions, providing ‘signatures’ of the bound electron orbitals. The experimental results are presented alongside a conceptual symmetry-based description of the photodetachment processes, enabling a qualitative interpretation of the photoelectron images. Second, the effects of solvation on the electronic structure and photodetachment dynamics are examined using photoelectron imaging of cluster anions. Third, the time-resolved experiments that target imaging of bond dissociation, as viewed from the electronic perspective along the time-resolved reaction coordinate, are described in the context of dual centre interference in molecular-anion photodetachment.

Photoelectron imaging: an experimental window into electronic structure.

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.

Evolution of absorption, fluorescence, laser and chemical properties in the series of compounds perylene, benzo(ghi)perylene and coronene.

Absorption. fluorescence and laser properties of perylene, benzo(ghi)perylene and coronene are studied experimentally (under the same conditions) and quantum chemically at room (293 K) and at low (77 and 4 K) temperatures and direct comparison is made between the results for each molecule. All the main absorption and fluorescence parameters such as oscillator strength, fe, quantum yield, gamma, decay time, tauf, fluorescence rate constant, kf (Einstein coefficient, A) and intersystem crossing rate constant, kST, are measured or calculated. The systems of singlet and triplet levels for these compounds are simulated and analyzed. Triplet states mixing with the lowest singlet S1 state are determined. The low values of kST found are explained. The possible vibronic coupling in the molecule coronene is discussed. The nature of the three fluorescence bands of coronene observed is interpreted. The change in the arrangement of the singlet and triplet levels of the studied compounds is interpreted quantum-chemically. It is found that at room temperature (293 K), only perylene shows laser action, while all three compounds show good laser oscillation at low temperature (< 100 K). The differences in the laser properties of these compounds are explained by the inversion of the Sp(1La) and Sinfinity(1Lb) levels which occurs in the transition from perylene to benzo(ghi)perylene. Chemical properties of the compounds studied are outlined. Linear and quasi-linear fluorescence spectra of perylene and benzo(ghi)perylene, obtained at 77 and 4 K. can be used in the identification of these compounds.

Photoelectron anisotropy and channel branching ratios in the detachment of solvated iodide cluster anions

Photoelectron spectra and angular distributions in 267 nm detachment of the I(-)Ar, I(-)H(2)O, I(-)CH(3)I, and I(-)CH(3)CN cluster anions are examined in comparison with bare I(-) using velocity-map photoelectron imaging. In all cases, features are observed that correlate to two channels producing either I((2)P(3/2)) or I((2)P(1/2)). In the photodetachment of I(-) and I(-)Ar, the branching ratios of the (2)P(1/2) and (2)P(3/2) channels are observed to be approximately 0.4, in both cases falling short of the statistical ratio of 0.5. For I(-)H(2)O and I(-)CH(3)I, the (2)P(1/2) to (2)P(3/2) branching ratios are greater by a factor of 1.6 compared to the bare iodide case. The relative enhancement of the (2)P(1/2) channel is attributed to dipole effects on the final-state continuum wave function in the presence of polar solvents. For I(-)CH(3)CN the (2)P(1/2) to (2)P(3/2) ratio falls again, most likely due to the proximity of the detachment threshold in the excited spin-orbit channel. The photoelectron angular distributions in the photodetachment of I(-), I(-)Ar, I(-)H(2)O, and I(-)CH(3)CN are understood within the framework of direct detachment from I(-). Hence, the corresponding anisotropy parameters are modeled using variants of the Cooper-Zare central-potential model for atomic-anion photodetachment. In contrast, I(-)CH(3)I yields nearly isotropic photoelectron angular distributions in both detachment channels. The implications of this anomalous behavior are discussed with reference to alternative mechanisms, affording the solvent molecule an active role in the electron ejection process.

Vibronic coupling in the superoxide anion: The vibrational dependence of the photoelectron angular distribution

We present a comprehensive photoelectron imaging study of the O(2)(X u2009(3)Σ(g)(-),v()=0-6)←O(2)(-)(X u2009(2)Π(g),v()=0) and O(2)(au2009(1)Δ(g),v()=0-4)←O(2)(-)(X u2009(2)Π(g),v()=0) photodetachment bands at wavelengths between 900 and 455 nm, examining the effect of vibronic coupling on the photoelectron angular distribution (PAD). This work extends the v()=1-4 data for detachment into the ground electronic state, presented in a recent communication [R. Mabbs, F. Mbaiwa, J. Wei, M. Van Duzor, S. T. Gibson, S. J. Cavanagh, and B. R. Lewis, Phys. Rev. A 82, 011401(R) (2010)]. Measured vibronic intensities are compared to Franck-Condon predictions and used as supporting evidence of vibronic coupling. The results are analyzed within the context of the one-electron, zero core contribution (ZCC) model [R. M. Stehman and S. B. Woo, Phys. Rev. A 23, 2866 (1981)]. For both bands, the photoelectron anisotropy parameter variation with electron kinetic energy, β(E), displays the characteristics of photodetachment from a d-like orbital, consistent with the π(g)(∗) 2p highest occupied molecular orbital of O(2)(-). However, differences exist between the β(E) trends for detachment into different vibrational levels of the X u2009(3)Σ(g)(-) and au2009(1)Δ(g) electronic states of O(2). The ZCC model invokes vibrational channel specific detachment orbitals and attributes this behavior to coupling of the electronic and nuclear motion in the parent anion. The spatial extent of the model detachment orbital is dependent on the final state of O(2): the higher the neutral vibrational excitation, the larger the electron binding energy. Although vibronic coupling is ignored in most theoretical treatments of PADs in the direct photodetachment of molecular anions, the present findings clearly show that it can be important. These results represent a benchmark data set for a relatively simple system, upon which to base rigorous tests of more sophisticated models.

The influence of molecular symmetry and topological factors on the internal heavy atom effect in aromatic and heteroaromatic compounds.

The absorption and fluorescence properties of 26 specially selected aromatic and heteroaromatic compounds, from different classes, are studied quantum chemically and experimentally at room temperature (293 K). Seven of these compounds have not been studied before. The compounds are arranged in seven groups, which illustrate different cases of the internal heavy atom effect. The quantum yield of fluorescence, gamma and fluorescence decay time, tau(f) of deaerated and non-deaerated cyclohexane or ethanol solutions are measured. The oscillator strength, f(e), fluorescence rate constant, k(f), natural lifetime, tau(0)t, and intersystem crossing rate constant, kST, were calculated for each compound. The orbital nature of the lowest excited singlet state and direction of polarization of the S0 --> S1 transitions are determined using the PPP-Cl method for each molecule. The investigation shows that substitution of a heavy atom(s) (Cl, S, Br, I etc.) into an aromatic or heteroaromatic molecule may produce different changes in all the fluorescence parameters (sometimes dramatically) and not necessarily lead to the quenching of fluorescence. Substitution of a heavy atom(s) may increase the value of the spin-orbit operator, Hso, if the S0 --> S1 excitation is localized to some extent on a carbon atom bonded to a heavy atom(s) or on the heavy atom itself (O or S). Such substitution may change the symmetry of a molecule and hence the values of the [psiS1/Hso/psiT1] matrix elements would change (in molecules of higher symmetry groups not all Ti states are able to mix with the perturbing S1 state). Such substitution may change the arrangement of Ti states below the S1, state and hence, the Franck-Condon factors would change. Such substitution may also change the value of the [psiS0/Mj/psiS1] matrix element and, consequently, the oscillator strength of the S0 --> S1 transition would change. A combination of all these possible changes determines the value of k(f) and kST and, consequently, determines the value of gamma and tau(f). It is observed that in many cases, the value of the spin-orbit operator is related to the dipole moment operator, e.g. if the introduction of a heavy atom increases kST then, as a rule, it decreases f(e)(1A --> 1La).

Effects of solvation and core switching on the photoelectron angular distributions from (CO2)n− and (CO2)n−⋅H2O

Photoelectron images are recorded in the photodetachment of two series of cluster anions, (CO(2))(n)(-), n=4-9 and (CO(2))(n)(-).H(2)O, n=2-7, with linearly polarized 400 nm light. The energetics of the observed photodetachment bands compare well with previous studies, showing evidence for switching between two anionic core structures: The CO(2)(-) monomer and covalent (CO(2))(2)(-) dimer anions. The systematic study of photoelectron angular distributions (PADs) sheds light on the electronic structure of the different core anions and indicates that solvation by several CO(2) molecules and/or one water molecule has only moderate effect on the excess-electron orbitals. The observed PAD character is reconciled with the symmetry properties of the parent molecular orbitals. The most intriguing result concerns the PADs showing remarkable similarities between the monomer and dimer anion cluster-core types. This observation is explained by treating the highest-occupied molecular orbital of the covalent dimer anion as a linear combination of two spatially separated monomeric orbitals.

Direct and Indirect Detachment in the Iodide-Pyrrole Cluster Anion: The Role of Dipole Bound and Neutral Cluster States

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.

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

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.

Influence of weak and strong donor groups on the fluorescence parameters and the intersystem crossing rate constant

The absorption and fluorescence properties of 35 specially selected methyl and stronger donor substituted benzene, naphthalene, biphenyl, anthracene and 2-azaanthracene compounds are studied experimentally (at 293 K) and quantum chemically. The fluorescence quantum yields, gamma, and decay times, tauf, for deaerated and non-deaerated solutions are measured. The oscillator strength, fe, natural lifetime, tauf 0 and fluorescence and intersystem crossing rate constants, kf and kST, are calculated for each compound. The orbital nature of the lowest excited singlet state, S1, is determined. The investigation shows that the introduction of methyl groups onto aromatic compounds may produce different effects. The symmetry and hence kST and kf may change. As a result, gamma will also change. Steric hindrance, possibly due to the CH3 group, will decrease kf while increasing kST. In cases where the introduction of the methyl group leaves the symmetry unchanged, there is a slight increase in kST and a slight decrease in kf. This effect is cumulative (more CH3 groups lead to a greater decrease in gamma) and can be explained by the torsional vibrations of the methyl groups. The introduction of strong donor groups usually produces dramatic changes. kST always increases, as does kf and the increase in kf is usually greater. Hence, gamma usually increases, sometimes dramatically. The nature of the S1 state changes from pipi* (for an aromatic molecule) to pil,pi*. There are three reasons for the observed increase in kST: (i) a decrease in symmetry; (ii) the internal heavy atom effect; and (iii) an improved mixing of the S(1pil,pi*) state with Ti states. It is also found that, in many cases, the effect of methyl and stronger donor groups on the fluorescence parameters and kST depends on the position of substitution, as well as changes in the molecular symmetry. The substituent groups have different effects on the p- and alpha-bands. The fluorescence parameters obtained and trends observed may be useful for different theoretical and practical purposes.