Analysis Of Photoelectron Angular Distributions Research Articles

A geometric approach to decoding molecular structure and dynamics from photoionization of isotropic samples.

We propose a geometric approach to the description and analysis of photoelectron angular distributions resulting from isotropic samples in the case of few-photon ionization by electric fields of arbitrary polarization. This approach formulates the standard photoionization observables - the bl,m expansion coefficients of the photoelectron angular distribution, in terms of geometrical properties of the vector field D⃑(k⃑) ≡ 〈k⃑|d⃑|0〉 describing the electronic transition from a bound state |0〉 into a scattering state |k⃑〉 - the photoionization transition dipole. Besides revealing selection rules for the enantio-sensitivity of bl,m coefficients in multiphoton ionization, our approach yields very compact expressions for both chiral and achiral molecules revealing how the molecular rotational invariants couple to the rotational invariants of the setup defined by the electric field polarization and the arrangement of photoelectron detectors. We apply this approach to one-photon ionization and find that the forward-backward asymmetry parameter b1,0, emerging exclusively in chiral molecules and encoded in the field B⃑(k⃑) ≡ iD⃑*(k⃑) × D⃑(k⃑), is sensitive only to the components of D⃑(k⃑) perpendicular to k⃑, while the regular asymmetry parameter b2,0 emerging in chiral and achiral molecules is sensitive only to the component of D⃑(k⃑) parallel to k⃑. Next, we analyze resonantly enhanced two-photon ionization and show that b0,0 and b1,0 can be written in terms of an effectively stretched D⃑(k⃑), and how b1,0 and b3,0 can be used to probe B⃑(k⃑).

Atom under an intense laser pulse: Stabilization effect and strong-field approximation

Direct numerical calculations of the single-photon ionization dynamics of the hydrogen atom were compared with the data obtained within the strong-field approximation (SFA). An analysis showed that the SFA model accurately determines the range of electromagnetic field intensities, upon reaching of which the ionization mode deviates from that described within perturbation theory; in particular, the ionization rate decreases with increasing intensity. It was demonstrated that the actual ionization mechanism under an intense pulse differs significantly from the SFA predictions. For example, an analysis of photoelectron angular distributions and energy spectra showed that the strong-field ionization features within the SFA model are primarily controlled by the ionization channel closing effect associated with the ponderomotive shift of the continuum boundary. At the same time, the results of direct numerical calculations of the ionization dynamics suggest that the Kramers-Henneberger atom is formed in a strong field, which is characterized by increased stability to strong-field ionization.

Quantitative analysis of photoelectron angular distribution of single-domain organic monolayer film: NTCDA on GeS(0 0 1)

Angle-resolved photoelectron spectra were measured for a single-domain monolayer film of naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTCDA) prepared on a cleaved GeS(0 0 1) surface using synchrotron radiation. The observed photoelectron angular distributions were analyzed by a calculation using the single-scattering approximation combined with molecular orbital calculation. With the help of the low-energy electron diffraction pattern, the structure of the NTCDA monolayer on GeS(0 0 1) was estimated quantitatively.

Angle-resolved UPS of ultrathin films of functional organic molecules with synchrotron radiation: Determination of molecular orientation by quantitative analysis of photoelectron angular distribution

This paper describes the determination of the molecular orientation of functional organic molecules in their ultrathin films deposited on crystal surfaces by the quantitative analysis of photoelectron angular distribution observed with synchrotron radiation. The examples of the results are presented for phthalocyanine and bis(1,2,5-thiadiazolo)- p -quinobis (1,3-dithiole), a new type of organic semiconductor. Further, the first theoretical simulation of the valence-band spectrum is also shown for oriented thin films of naphthacene, a conventional organic semiconductor. These analyses were based on the independent-atomic-center approximation (IAC) and single scattering theory (SS) combined with molecular orbital (MO) calculation. It is shown that even the simple IAC/MO model can simulate the photoelectron angular distribution very well for a π band at near flat-lie orientation of planer molecules. Further we found that the SS/MO calculations can give the full valence band spectra of large organic molecules, which are in excellent agreement with observed ones.

Angle-resolved UPS of ultrathin films of functional organic molecules with synchrotron radiation: Determination of molecular orientation by quantitative analysis of photoelectron angular distribution

This paper describes the determination of the molecular orientation of functional organic molecules in their ultrathin films deposited on crystal surfaces by the quantitative analysis of photoelectron angular distribution observed with synchrotron radiation. The examples of the results are presented for phthalocyanine and bis(1,2,5-thiadiazolo)- p-quinobis (1,3-dithiole), a new type of organic semiconductor. Further, the first theoretical simulation of the valence-band spectrum is also shown for oriented thin films of naphthacene, a conventional organic semiconductor. These analyses were based on the independent-atomic-center approximation (IAC) and single scattering theory (SS) combined with molecular orbital (MO) calculation. It is shown that even the simple IAC/MO model can simulate the photoelectron angular distribution very well for a π band at near flat-lie orientation of planer molecules. Further we found that the SS/MO calculations can give the full valence band spectra of large organic molecules, which are in excellent agreement with observed ones.

Theoretical analysis of photoelectron angular distributions in hydrogen chloride

Multiple-scattering calculations for the ≈ X 2 II and ≈ A 2Σ + ionizations of HCl are analysed by a partitioning procedure to explain the angular distributions around the Cooper minima. A formal analogy to resonances is pointed out. The Cooper minima in the valence ionizations of Cl 2 are discussed qualitatively.

Theoretical analysis of photoelectron angular distributions of linear molecules

A partitioning scheme for the photoelectron asymmetry parameter of linear molecules is proposed and applied in multiple-scattering calculations for thirteen valence ionizations of five molecules. For several limiting cases rigorous bounds for the asymmetry parameter are established. The energy dependence of photoelectron angular distributions is discussed in terms of relative channel cross sections, transition moment phases, and Coulomb phase shifts. Rapid energy-dependent variations of the asymmetry parameter can be rationalized in this manner, e.g. for low-energy Π ionizations and for σ-type resonances in Σ ionizations. The proposed scheme is also used to identify the origin of errors in approximate calculations.