Andrei Sanov

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.

The lowest singlet and triplet states of the oxyallyl diradical

Small S-T splitting : The photoelectron spectrum of the oxyallyl radical anion (see picture) reveals that the electronic ground state of oxyallyl is singlet, and the lowest triplet state is separated from the singlet state by only (55 ± 2) meV in adiabatic energy.

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.

Competitive photodissociation channels in jet-cooled HNCO: Thermochemistry and near-threshold predissociation

The photoinitiated unimolecular decomposition of jet‐cooled HNCO has been studied following S1(1A″)←S0(1A′) excitation near the thresholds of the spin‐allowed dissociation channels: (1) H(2S)+NCO(X2Π) and (2) NH(a1Δ)+CO(X1Σ+), which are separated by 4470 cm−1. Photofragment yield spectra of NCO(X2Π) and NH (a1Δ) were obtained in selected regions in the 260–220 nm photolysis range. The NCO(X2Π)yield rises abruptly at 38 380 cm−1 and the spectrum exhibits structures as narrow as 0.8 cm−1 near the threshold. The linewidths increase only slowly with photolysis energy. The jet‐cooled absorption spectrum near the channel (1) threshold [D0(H+NCO)] was obtained using two‐photon excitation via the S1 state, terminating in a fluorescent product. The absorption spectrum is similar to the NCO yield spectrum, and its intensity does not diminish noticeably above D0(H+NCO), indicating that dissociation near threshold is slow. The NCO product near threshold is cold, as is typical of a barrierless reaction. NH (a1Δ) produ...

Photoelectron imaging spectroscopy of molecular and cluster anions: CS2− and OCS−(H2O)1,2

We report the photoelectron imaging study of molecular and cluster anions CS2− and OCS−(H2O)1,2 at 800, 529, and 400 nm, comparing the results for the hydrated OCS− cluster ions to CS2−. The photoelectron angular distributions are interpreted qualitatively using group theory in the framework of the one-electron, electric-dipole approximations. The energetics of OCS−(H2O)1,2 are compared to the theoretical predictions. The vertical detachment energies of OCS−⋅H2O and OCS−(H2O)2 are determined to be 2.07±0.07 and 2.53±0.07 eV, respectively. An indirect estimate of the adiabatic electron affinity of OCS yields a value of −0.04 eV.

Laboratory-Frame Photoelectron Angular Distributions in Anion Photodetachment: Insight into Electronic Structure and Intermolecular Interactions

This article provides an overview of some recent advances in the modeling of photoelectron angular distributions in negative-ion photodetachment. Building on the past developments in threshold photodetachment spectroscopy that first tackled the scaling of the partial cross sections with energy, depending on the angular momentum quantum number ℓ, it examines the corresponding formulation of the central potential model and extends it to the more general case of hybrid molecular orbitals. Several conceptual approaches to understanding photoelectron angular distributions are discussed. In one approach, the angular distributions are examined based on the contributions of the symmetry-allowed s and p partial waves of the photodetached electron. In another related approach, the parent molecular orbitals are described based on their dominant s and p characters, whereas the continuum electron is described in terms of interference of the corresponding Δℓ =±1 photodetachment channels.

Photodetachment and photofragmentation pathways in the [(CO2)2(H2O)m]− cluster anions

The mass-selected [(CO(2))(2)(H(2)O)(m)](-) cluster anions are studied using a combination of photoelectron imaging and photofragment mass spectroscopy at 355 nm. Photoelectron imaging studies are carried out on the mass-selected parent cluster anions in the m=2-6 size range; photofragmentation results are presented for m=3-11. While the photoelectron images suggest possible coexistence of the CO(2) (-)(H(2)O)(m)CO(2) and (O(2)CCO(2))(-)(H(2)O)(m) parent cluster structures, particularly for m=2 and 3, only the CO(2) (-) based clusters are both required and sufficient to explain all fragmentation pathways for m>/=3. Three types of anionic photofragments are observed: CO(2) (-)(H(2)O)(k), O(-)(H(2)O)(k), and CO(3) (-)(H(2)O)(k), k</=m, with their yields varying depending on the parent cluster size. Of these, only CO(2) (-)(H(2)O)(k) can potentially result from (O(2)CCO(2))(-)(H(2)O)(m) parent structures, although an alternative mechanism, involving the dissociation and recombination of the CO(2) (-) cluster core, is possible as well. The O(-)(H(2)O)(k) and CO(3) (-)(H(2)O)(k) channels are believed to be triggered by the dissociation of the CO(2) (-) cluster core. In the CO(3) (-)(H(2)O)(k) channel, seen only in the range of m=3-6, the CO(2) (-) core dissociation is followed by an intracluster association of nascent O(-) with the solvent CO(2). This channels absence in larger clusters (m>6) is attributed to hindrance from the H(2)O molecules.

Spin-orbit relaxation and recombination dynamics in I2−(CO2)n and I2−(OCS)n cluster ions: A new type of photofragment caging reaction

We report a new type of photofragment caging reaction that is only possible because of the strong solvent-induced perturbation of the inherent electronic structure of the chromophore. The photoexcitation of I2− at 395 nm promotes it to a dissociative state correlating with I−+I*(2P1/2), the only near-ultraviolet dissociation channel for unsolvated I2−. In I2−(CO2)n and I2−(OCS)n clusters, interaction with the solvent is observed to result in extremely fast spin-orbit relaxation. In general, we detect three reaction pathways: (1) direct dissociation of the chromophore to I−+I*(2P1/2); (2) the I2−→I−+I* dissociation, followed by spin-orbit quenching leading to I−+I(2P3/2) products; and (3) the I2−→I−+I* dissociation, followed by spin-orbit quenching and I−+I(2P3/2)→I2− recombination and vibrational relaxation. We present experimental evidence of the spin-orbit relaxation and caging and discuss possible mechanisms. The results include: the measured translational energy release in 395 nm photodissociation of ...

Unimolecular decomposition of NO3: The NO+O2 threshold regime

The unimolecular decomposition of expansion‐cooled NO3 has been investigated in the threshold regime of the NO+O2 channel. Photoexcitation in the region 16 780–17 090 cm−1 (596–585 nm) prepares ensembles of molecular eigenstates, each of which is a mixture of the B 2E′ bright state and lower electronic states. The X 2A2′ ground state is believed to be the probable terminus of 2E′ radiationless decay, though participation of A 2E″ is also possible. For these photon energies, unimolecular decomposition occurs exclusively via the NO+O2 channel, and NO yield spectra and state distributions have been obtained. The yield spectra are independent of the rotational state monitored, as expected for a large reverse barrier. The state distributions are insensitive to the photolysis photon energy and can be rationalized in terms of dynamical bias. The NO yield goes to zero rapidly above the O+NO2 threshold (17 090±20 cm−1). Because of tunneling, the NO+O2 channel does not have a precise threshold; the value 16 780 cm−...

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.