Partial-wave decomposition of the ionization continuum accessed by vibrational autoionization of the NO 14s ( nu = 1, N = 20, N(+)(R) = 20) level

Molecular frame photoelectron emission in the presence of autoionizing resonances

We have measured the angular distribution of valence-shell photoelectrons excited by circularly polarized light from fixed-in-space N2O molecules, near to and on top of resonances due to Rydberg states embedded in the ionization continuum. The sign of the circular dichroism for ionization into the N2O+ (B2Pi, (1pi)-1) state is reversed on top of the lowest dominant resonances. Measured angular distributions are well predicted by state-of-the-art multichannel configuration interaction calculations. The change in sign of the circular dichroism at the peak of the resonance is the result of a rapid change in the phases of resonant dipole matrix elements by a factor of 2pi as the energy is scanned across the resonance.

Optical-optical double resonance photoionization spectroscopy of nf Rydberg states of nitric oxide

The spectra of vibrationally excited nf Rydberg states of nitric oxide were recorded by monitoring the photoion current produced using two-photon double resonance excitation via the NO A (2)Sigma(+) state followed by photoexcitation of the Rydberg state that undergoes autoionization. The optical transition intensities from NO A state to nf Rydberg states were calculated, and the results agree closely with experiment. These results combined with circular dichroism measurements allow us to assign rotational quantum numbers to the nf Rydberg states even in a spectrum of relatively low resolution. We report the positions of these nf (upsilon,N,N(c)) Rydberg levels converging to the NO X (1)Sigma(+) upsilon(+) = 1 and 2 ionization limits where N is the total angular momentum excluding electron and nuclear spin and N(c) represents the rotational quantum number of the ion core. Our two-color optical-optical double resonance measurements cover the range of N from 15 to 28, N(c) from 14 to 29, and the principal quantum number n from 9 to 21. The electrostatic interaction between the Rydberg electron and the ion core is used to account for the rotational fine structure and a corresponding model is used to fit the energy levels to obtain the quadrupole moment and polarizability of the NO(+) core. Comparison with a multichannel quantum defect theory fit to the same data confirms that the model we use for the electrostatic interaction between the nf Rydberg electron and the ion core of NO well describes the rotational fine structure.

Photoelectron angular distributions

Angle-resolved photoelectron spectroscopy has been performed for more than 70 years in various guises, but recently its potential to help solve in detail problems in the photoionization dynamics and intramolecular dynamics of gas-phase molecules has been recognized. One key development has been the design of experiments in appropriate geometries to extract information that pertains to the molecular frame, another has been the development of imaging spectrometers, and a third is the use of ultrafast lasers to cause photoionization. In this review, which is aimed at experimentalists, simple expressions for photoelectron angular distributions (PADs) in various experimental geometries are given and their applications explained.