Probing the Electronic Structure of Small Molecular Anions by Photoelectron Imaging

Abstract

We outline the methodology of negative-ion photoelectron imaging and general aspects of interpretation of the results using the CS2 - and S2 - anions as model systems. The CS2 - images are recorded using 800, 530, 400, and 267 nm photons. The observed transitions result in the formation of CS2 in the X 1Σg +, a 3B2, b 3A2, and A 1A2 states. The S2 - measurements are carried out at the same wavelengths with the exception of 800 nm. The resulting images reveal the detachment transitions assigned to the X 3Σg -, a 1Δg, b 1Σg +, c 1Σu -, and A‘ 3Δu states of the neutral. The choice of detachment wavelengths serves as a “zoom” selectively focusing on chosen transitions, in some cases allowing the observation of their vibrational structure. The photoelectron spectra and angular distributions obtained from the images are used to discuss the electronic structure and detachment dynamics. In particular, two approaches to interpreting the angular distributions are discussed. One method employs the Cooper−Zare central-potential model adapted to the molecular case. It considers an expansion of the parent orbital in the basis of single-center atomic-orbital functions, for which the partial waves comprising the ejected electron are determined. The application of this model to molecular anions is straightforward, if the parent molecular orbital resembles an atomic orbital, which is the case for S2 -, but not CS2 -. In the latter case, a different qualitative approach is proposed, which (i) relies upon the electric-dipole approximation and group theory for the determination of the detached electron wave function symmetry, (ii) restricts the analysis to symmetry (electric dipole) allowed s and p partial waves, and (iii) qualitatively treats the orientation averaging by considering only a few “principal” molecular orientations. The results provide a foundation for the qualitative interpretation of anion photoelectron images.

Effect of electronic angular momentum exchange on photoelectron anisotropy following the two-color ionization of krypton atoms

“…In addition, the c 4 parameter has a significant magnitude for all of the observed PADs, regardless of the value of l or J c , but is always larger for the core switching PAD than for the equivalent core conserving PAD. We note that while the PADs for the 2 P 1/2 ion state look much like the PAD expected for a p electron at first glance, the contribution from the c 4 parameter causes a substantial narrowing of the PADs. Finally, for the ( 2 P 1/2 )6s[1/2] 1 intermediate state, the l = ±1 selection rule limits the photoelectron wavefunction to partial waves with l = 1.…”

“…[1][2][3] This idea has been put to use in studies of photodetachment from anions, 4,5 and elegantly exploited in femtosecond time-resolved photoelectron imaging studies of non-adiabatic dynamics in neutral molecules. [6][7][8][9] Given this interest, there is a need for systematic studies of the variation of PADs with the electronic state of the neutral and the ion.…”

Photoelectron Imaging Study of the Effect of Monohydration on O2- Photodetachment

“…22,65,66 The photodetachment from a π g * molecular orbital has been studied extensively in the past, adopting different theoretical frameworks. 22,45 Although the variation of the anisotropy parameter with eKE within the Cooper-Zare central-potential model is not intuitive, qualitative symmetryrelated aspects of π g -1 photodetachment were discussed by our group on the example of S 2 -. 45 The same symmetry considerations, generally predicting negative values of in a moderate eKE range, also apply in the case of O 2 -.…”

“…45 The O 2 -‚H 2 O clusters are generated by passing a 1:4 mixture of O 2 and argon over water at room temperature. The mixture is introduced into the source chamber through a pulsed nozzle (General Valve Series 9) operated at 70 Hz.…”

Communication: Photoelectron angular distributions of CH− reveal a temporary anion state

“…[1][2][3][4][5][6][7][8][9][10] In addition to measuring traditional photoelectron spectra, the imaging experiment also provides an extra set of information, the photoelectron angular distributions (PADs). The PAD carries information about the parent ion orbital from which the electron is detached.…”