The overlooked role of excited anion states in NiO2- photodetachment.

Photodetachment spectra of anionic species provide significant insights into the energies and nature of ground and excited states of both the anion and resultant neutral molecules. Direct detachment of the excess electron to the continuum may occur via formally allowed or forbidden transitions (perhaps as the result of intensity borrowing through vibronic coupling). However, alternate indirect pathways are also possible and often overlooked. Here, we report a two-dimensional photoelectron spectral study, combined with correlated electronic structure calculations, to elucidate the nature of photodetachment from NiO2-. The spectra are comprised of allowed and forbidden transitions, in excellent agreement with previously reported slow electron velocity mapped imaging spectra of the same system, which were interpreted in terms of direct detachment. In the current work, the contributions of indirect processes are revealed. Measured oscillations in the branching ratios of the spectral channels clearly indicate non-direct detachment processes, and the electronic structure calculations suggest that excited states of the appropriate symmetry and degeneracy lie slightly above the neutral ground state. Taken together, the results suggest that the origin of the observed forbidden transitions is the result of anion excited states mediating the electron detachment process.

Dipole effects in the photoelectron angular distributions of the sulfur monoxide anion.

Photoelectron angular distributions (PADs) in SO- photodetachment using linearly polarized 355 nm (3.49 eV), 532 nm (2.33 eV), and 611 nm (2.03 eV) light were investigated via photoelectron imaging spectroscopy. The measurements at 532 and 611 nm access the X3Σ- and a1Δ electronic states of SO, whereas the measurements at 355 nm also access the b1Σ+ state. In aggregate, the photoelectron anisotropy parameter values follow the general trend with respect to electron kinetic energy (eKE) expected for π*-orbital photodetachment. The trend is similar to O2-, but the minimum of the SO- curve is shifted to smaller eKE. This shift is mainly attributed to the exit-channel interactions of the departing electron with the dipole moment of the neutral SO core, rather than the differing shapes of the SO- and O2- molecular orbitals. Of the several ab initio models considered, two approaches yield good agreement with the experiment: one representing the departing electron as a superposition of eigenfunctions of a point dipole-field Hamiltonian, and another describing the outgoing electron in terms of Coulomb waves originating from two separated charge centers, with a partial positive charge on the sulfur and an equal negative charge on the oxygen. These fundamentally related approaches support the conclusion that electron-dipole interactions in the exit channel of SO- photodetachment play an important role in shaping the PADs. While a similar conclusion was previously reached for photodetachment from σ orbitals of CN- (Hart, Lyle, Spellberg, Krylov, Mabbs, J. Phys. Chem. Lett., 2021, 12, 10086-10092), the present work includes the first extension of the dipole-field model to detachment from π* orbitals.

Spectroscopy of temporary anion states: Renner-Teller coupling and electronic autodetachment in copper difluoride anion.

Photoelectron angular distributions determined in small energy increments between 3.522 and 3.650 eV reveal distinctly different detachment mechanisms within the CuF2-(X 1Σ) → linear CuF2(X 2Σ) + e- band. Certain channels also display non-Franck-Condon behaviour, the action spectra of which reveal rich structure. The behaviour reflects excitation to an electronically unbound anion state (at the linear geometry). The effects of the intermediate state are observed in the detachment behaviour at photon energies down to the X 1Σ→ X 2Σ threshold. Adiabatic CAP-EOM-CCSD (equation of motion coupled cluster singles and doubles, including a complex absorbing potential) energy curves are presented along the bending coordinate, showing the complexity of anion and neutral states for this system. The photodetachment action spectra represent a spectroscopic probe of the vibronic state energies in the vertical excitation region and highlight the need for treatment of vibronic coupling in systems involving the loss of an electron.

Characterization of the vibrational properties of copper difluoride anion and neutral ground states via direct and indirect photodetachment spectroscopy.

Photoelectron spectra of 63CuF2- are reported at wavelengths 310 nm, 346.6 nm, and 350.1 nm, obtained via velocity map imaging. The photoelectron angular distributions allow for the unambiguous assignment of a 2Σg+ neutral CuF2 ground state. Vibrational analysis of the direct detachment transitions in the spectra enables accurate determination of the anion and neutral bond length difference (0.073 Å), adiabatic electron affinity of CuF2 (3.494 eV) and symmetric stretching (500 cm-1, anion, and 630 cm-1, neutral) and antisymmetric stretching (610 cm-1, anion, and 782 cm-1 neutral) frequencies of the ground electronic states. Strongly photon energy dependent intensities are also observed for select transitions. Equation-of-motion coupled-cluster singles and doubles calculations augmented by a complex absorbing potential reveal a metastable 1Πg anion state which is optically accessible due to Renner-Teller coupling. Mediation of the detachment process by this state allows measurement of the bending frequencies (177 cm-1, anion, and 200 cm-1, neutral) completing the inventory of experimentally measured vibrational properties of the ground electronic states.

Channel branching ratios in CH2CN- photodetachment: Rotational structure and vibrational energy redistribution in autodetachment.

We report photoelectron spectra of CH2CN-, recorded at photon energies between 13 460 and 15 384 cm-1, 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 ν3 (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.

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

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.

Photoelectron angular distributions as probes of cluster anion structure: I(-)·(H2O)2 and I(-)·(CH3CN)2.

The use of photoelectron angular distributions to provide structural details of cluster environments is investigated. Photoelectron spectra and angular distributions of I(-)·(H2O)2 and I(-)·(CH3CN)2 cluster anions are recorded over a range of photon energies. The anisotropy parameter (ß) for electrons undergoes a sharp change (Δßmax) at photon energies close to a detachment channel threshold. I(-)·(H2O)2 results show the relationship between dipole moment and Δßmax to be similar to that observed in monosolvated I(-) detachment. The Δßmax of the 4.0 eV band in the I(-)·(CH3CN)2 photoelectron spectrum suggests a dipole moment of 5-6 D. This is consistent with predictions of a hydrogen bonded conformer of the I(-)·(CH3CN)2 cluster anion [Timerghazin, Q. K.; Nguyen, T. N.; Peslherbe, G. H. J. Chem. Phys. 2002, 116, 6867-6870].

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

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.

Inter-channel effects in monosolvated atomic iodide cluster anion detachment: correlation of the anisotropy parameter with solvent dipole moment.

Photoelectron imaging results are presented for I(-)[middle dot]X cluster anions (X = CO(2), C(4)H(5)N [pyrrole], (CH(3))(2)CO, CH(3)NO(2)). The available detachment channels are labeled according to the neutral iodine atom states produced (channel I ≡ (2)P(3/2) and channel II ≡ (2)P(1/2)). At photon energies in the vicinity of the channel II threshold these data are compared to previously reported results for I(-)[middle dot]X (X = CH(3)CN, CH(3)Cl, CH(3)Br, and H(2)O). In particular, these results show a strong connection between the dipole moment of the solvent molecule and the behavior of the channel I photoelectron angular distributions in this region, which is consistent with an electronic autodetachment process. The evolution of the channel II:channel I branching ratios in this excitation regime supports this contention.

I(-)·(CH3I)2 photoexcitation: the influence of dipole bound states on detachment and fragmentation.

We present the results of a photoelectron imaging study of the I(-)·(CH(3)I)(2) cluster anion over excitation wavelengths 355-260 nm. The resulting spectra and photoelectron angular distributions (PADs) suggest extensive electron-molecule interaction following photoexcitation. Fragmentation channels are observed subsequent to excitation between 355 and 330 nm. The origin of these features, which begin 200 meV and peak 70 meV below the X band direct detachment threshold, is described in terms of a predissociative dipole bound state. The nature of the fragments detected and the energetics of the channel opening argue strongly in favor of an asymmetric, head to tail cluster anion geometry posited by Dessent et al. [Acc. Chem. Res. 31, 527 (1998)]. Above the direct detachment threshold, PADs display evidence of phenomena akin to electron-molecule scattering. The fragment anions disappear above the X band threshold but reappear some distance below the second (A) direct detachment band. At these energies there is also rapid variation of the X band PAD, an observation attributed to autodetachment via spin-orbit relaxation of the iodine core of the cluster.

Near threshold Cl(-)·CH3I photodetachment: apparent 2P1/2 channel suppression and enhanced 2P3/2 channel vibrational excitation.

Cl(-)·CH(3)I cluster anion photoelectron images are recorded over a range of detachment wavelengths in the immediate post threshold region. The photoelectron spectral features fall into two categories. A number of weak, photon energy dependent transitions are observed and attributed to atomic anion fragmentation products. Several more intense, higher electron binding energy transitions result from single photon cluster anion detachment. Comparison with I(-)·CH(3)I suggests that the detachment process is more complicated for Cl(-)·CH(3)I. The single photon transition spacing is consistent with CH(3)I ν(3) mode excitation, but the two distinct vibronic bands of I(-)·CH(3)I detachment are not easily distinguished in the Cl(-)·CH(3)I spectra. Similarly, while the spectral intensities for both cluster anions show non-Franck Condon behavior, the level of vibrational excitation appears greater for Cl(-)·CH(3)I detachment. These observations are discussed in terms of low lying electronic states of CH(3)I along the C-I coordinate, and the influence of the CH(3)I moiety on the neutral halogen atom states.

Role of the Electron-Dipole Interaction in Photodetachment Angular Distributions.

The importance of including long-range electron-molecule interactions in treatments of photodetachment and/or photoionization is demonstrated. A combined experimental and computational study of CN- detachment is presented in which near threshold anisotropy parameters (ß) are measured via photoelectron imaging. Calculated ß values, based on an EOM-IP-CCSD/aug-cc-pVTZ Dyson orbital, are obtained using free-particle and point dipole models. The results demonstrate the influence of the molecular dipole moment in the detachment process and provide an explanation of the recently reported near threshold behavior of the overall photodetachment cross section in CN- detachment.

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.

Threshold effects in I- x CH3CN and I- x H2O cluster anion detachment: the angular distribution as an indicator of electronic autodetachment.

I(-) x H(2)O and I(-) x CH(3)CN cluster anion photodetachment properties (photoelectron spectra and angular distributions) are recorded via velocity mapped photoelectron imaging for wavelengths between 270 and 340 nm, in small energy increments. These are compared with free I(-) detachment results and reveal the presence of a sharp change in the angular distribution for the (2)P(3/2) spin orbit channel in the vicinity of the (2)P(1/2) threshold. The effect is seen at this threshold in the I(-) x H(2)O cluster anion and just below threshold for I(-) x CH(3)CN. The effect is attributed to an electronic autodetachment process, which is dependent on electronic energy transfer mediated by the electron-neutral complex produced in the excitation process. These results highlight the potential of cluster anion detachment as a probe of electron-molecule interactions and in particular the sensitivity of the angular distributions to intracluster electron transfer processes.

I(-)⋠CH(3)X (X=Cl,Br,I) photodetachment: The effect of electron-molecule interactions in cluster anion photodetachment spectra and angular distributions.

The electron kinetic energy dependence of the photoelectron spectra and angular distributions of I(-)⋅CH(3)X (X=Cl,Br,I) cluster anions are measured via velocity mapped imaging at wavelengths between 350 and 270 nm. Processes analogous to those encountered in free CH(3)X-electron interactions are revealed. In particular, the presence and energies of resonances associated with a low lying σ(∗) state have a marked effect on the results of I(-)⋅CH(3)X photoexcitation. These effects (vibrational excitation, product anion production, and alteration of the photoelectron angular distribution) are far more prominent for I(-)⋅CH(3)I. However, in the vicinity of the (2)P(1/2) threshold there is a sharp deviation in the (2)P(3/2) channel angular distribution and an enhancement of the (2)P(3/2) channel vibrational structure of all three cluster anions. These latter effects are specific to the cluster anion environment through the relaxation of the partner excited I atom and subsequent electronic autodetachment.

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 (3)Σ(g)(-),v(')=0-6)←O(2)(-)(X (2)Π(g),v(")=0) and O(2)(a (1)Δ(g),v(')=0-4)←O(2)(-)(X (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, ß(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 ß(E) trends for detachment into different vibrational levels of the X (3)Σ(g)(-) and a (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.

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.

The effect of intracluster photoelectron interactions on the angular distribution in I(-)*CH(3)I photodetachment.

I(-) and I(-)*CH(3)I velocity mapped photodetachment images are recorded over wavelengths between 270 and 370 nm. Spectral similarities, in conjunction with ab initio calculations show that the cluster anion comprises an atomic iodide anion moiety solvated by a relatively unperturbed CH(3)I molecule. Between 340-370 nm and at 280 nm, free I(-) is produced via a process analogous to dissociative electron attachment within the cluster anion. More strikingly, the photoelectron angular distribution for each species at a given electron kinetic energy is very different, despite detachment occurring from the iodide 5p orbital in each case. These observations reveal the effect of interaction of the photoelectron with the neutral cluster residue and are discussed in terms of resonances associated primarily with the CH(3)I molecule.