Kostyantyn Pichugin

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.

Solvent resonance effect on the anisotropy of NO−(N2O)n cluster anion photodetachment

Photodetachment from NO(-)(N(2)O)(n) cluster anions (n< or =7) is investigated using photoelectron imaging at 786, 532, and 355 nm. Compared to unsolvated NO(-), the photoelectron anisotropy with respect to the laser polarization direction diminishes drastically in the presence of the N(2)O solvent, especially in the 355 nm data. In contrast, a less significant anisotropy loss is observed for NO(-)(H(2)O)(n). The effect is attributed to photoelectron scattering on the solvent, which in the N(2)O case is mediated by the (2)Pi anionic resonance. No anionic resonances exist for H(2)O in the applicable photoelectron energy range, in line with the observed difference between the photoelectron images obtained with the two solvents. The momentum-transfer cross section, rather than the total scattering cross section, is argued to be an appropriate physical parameter predicting the solvent effects on the photoelectron angular distributions in these cluster anions.

Time-resolved imaging of the reaction coordinate

Time-resolved photoelectron imaging of negative ions is employed to study the dynamics along the reaction coordinate in the photodissociation of IBr(-). The results are discussed in a side-by-side comparison with the dissociation of I(2) (-), examined under similar experimental conditions. The I(2) (-) anion, extensively studied in the past, is used as a reference system for interpreting the IBr(-) results. The data provide rigorous dynamical tests of the anion electronic potentials. The evolution of the energetics revealed in the time-resolved (780 nm pump, 390 nm probe) I(2) (-) and IBr(-) photoelectron images is compared to the predictions of classical trajectory calculations, with the time-resolved photoelectron spectra modeled assuming a variety of neutral states accessed in the photodetachment. In light of good overall agreement of the experimental data with the theoretical predictions, the results are used to construct an experimental image of the IBr(-) dissociation potential as a function of the reaction coordinate.

Time-resolved electron detachment imaging of the I- channel in I2Br- photodissociation.

The evolution of the I(-) channel in I(2)Br(-) photodissociation is examined using time-resolved negative-ion photoelectron imaging spectroscopy. The 388 nm photodetachment images obtained at variable delays following 388 nm excitation reveal the transformation of the excess electron from that belonging to an excited trihalide anion to that occupying an atomic orbital localized on the I(-) fragment. With increasing pump-probe delay, the corresponding photoelectron band narrows on a approximately 300 fs time scale. This trend is attributed to the localization of the excess-electron wave function on the atomic-anion fragment and the establishment of the fragments electronic identity. The corresponding band position drifts towards larger electron kinetic energies on a significantly longer, approximately 1 ps, time scale. The gradual spectral shift is attributed to exit-channel interactions affecting the photodetachment energetics, as well as the photoelectron anisotropy. The time-resolved angular distributions are analyzed and found consistent with the formation of the asymptotic I(-) fragment.

Further evidence for resonant photoelectron-solvent scattering in nitrous oxide cluster anions.

The effects of anion solvation by N(2)O on photoelectron angular distributions are revisited in light of new photoelectron imaging results for the NO(-)(N(2)O)(n), n = 0-4 cluster anions at 266 nm. The new observations are examined in the context of the previous studies of O(-) and NO(-) anions solvated in the gas phase by nitrous oxide [Pichugin; et al. J. Chem. Phys. et al. 2008, 129, 044311.; Velarde; et al. J. Chem. Phys. et al. 2007, 127, 084302.]. The photoelectron angular distributions collected in the three separate studies are summarized and analyzed using bare O(-) and NO(-) as zero-solvation references. Solvent-induced deviations of the angular distributions from the zero-solvation reference are scaled by solvation number (n) to yield solvent-induced anisotropy differentials. These differentials, calculated identically for the O(-)(N(2)O)(n) and NO(-)(N(2)O)(n) cluster series, show remarkably similar energy dependences, peaking in the vicinity of a known electron-N(2)O scattering resonance. The results support the conclusion that the solvation effect on the photoelectron angular distributions in these cases is primarily due to resonant interaction of photoelectrons with the N(2)O solvent, rather than a solvent-induced perturbation of the parent-anion electronic wave function.

Photoelectron imaging of NCCCN−: The triplet ground state and the singlet-triplet splitting of dicyanocarbene

The photoelectron spectra of NCCCN(-) have been measured at 355 and 266 nm by means of photoelectron imaging. The spectra show two distinct features, corresponding to the ground and first excited states of dycianocarbene. With support from theoretical calculations using the spin-flip coupled-cluster methods, the ground electronic state of HCCCN is assigned as a triplet state, while the first excited state is a closed-shell singlet. The photoelectron band corresponding to the triplet is broad and congested, indicating a large geometry change between the anion and neutral. A single sharp feature of the singlet band suggests that the geometry of the excited neutral is similar to that of the anion. In agreement with these observations, theoretical calculations show that the neutral triplet state is either linear or quasilinear (X (3)B(1) or (3)Sigma(g) (-)), while the closed-shell singlet (a (1)A(1)) geometry is strongly bent, similar to the anion structure. The adiabatic electron binding energy of the closed-shell singlet is measured to be 3.72+/-0.02 eV. The best estimate of the origin of the triplet band gives an experimental upper bound of the adiabatic electron affinity of NCCCN, EA</=3.25+/-0.05 eV, while the Franck-Condon modeling yields an estimate of EA(NCCCN)=3.20+/-0.05 eV. From these results, the singlet-triplet splitting is estimated to be DeltaE(ST)(X (3)B(1)/(3)Sigma(g) (-)-a (1)A(1))=0.52+/-0.05 eV (12.0+/-1.2 kcal/mol).

Solvation-induced cluster anion core switching from NNO2−(N2O)n−1 to O−(N2O)n

We report a photoelectron imaging study of the [O(N(2)O)(n)](-), 0<or=n<or=9, cluster anions generated via electron bombardment of a pulsed supersonic expansion of pure N(2)O gas. Depending on cluster size, the photoelectron image features and spectral trends, examined at 355 and 266 nm, give evidence of two dominant core-anion structures, corresponding to the NNO(2) (-)(N(2)O)(n-1) and O(-)(N(2)O)(n) cluster anions. In agreement with previous studies, the n=1 anion has a covalently bound (Y-shaped) NNO(2) (-) structure. The NNO(2) (-) core is also found to persist in the larger clusters, up to n=3. However, for n>or=4 (and up to at least n=9) signatures of an O(-) core are predominantly observed. Photofragmentation studies at 355 nm support these results.

An excess electron trapped between thymine and adenine. Ab initio study

Abstract The theoretical ab initio calculations performed in this work revealed two structural isomers of the anion of the canonical pair of nucleic acid bases, thymine and adenine, where the two bases are separated by more than 7 A but remain connected via an excess electron localized in the middle of the complex. In both systems the two monomers forming the complex point the positive poles of their dipoles at the electron – a configuration highly unstable for the neutral complex.

Time-Resolved Photodetachment Imaging along the Reaction Coordinate

Electronic structure evolution in photodissociating anionic systems is revealed by femtosecond time resolved photodetachment imaging spectroscopy. Case studies are presented highlighting the role of the anion (and terminal neutral) excited states and system symmetry.