Katharine L. Reid

Complete description of two‐photon (1+1’) ionization of NO deduced from rotationally resolved photoelectron angular distributions

Time‐of‐flight photoelectron spectroscopy has been used to record energy‐resolved photoelectron angular distributions (PADs) following (1+1’) resonance‐enhanced multiphoton ionization (REMPI) of NO via the vi=1,Ni=22 rovibrational level of the A 2∑+ state. The PADs corresponding to single rotational states of the resulting molecular ion show a strong dependence on the change in ion core rotation ΔN(≡N+−Ni) and also on the angle between the linear polarization vectors of the two light beams. Broken reflection symmetry [I(θ,φ)≠I(−θ,φ)] is observed when the polarization vectors of the two light beams form an angle of 54.7°. A fit to the PADs provides a complete description of this molecular photoionization, namely, the magnitudes and phases of the radial dipole matrix elements that connect the intermediate state to the ‖lλ〉 photoelectron partial waves (Refs. 1 and 2). This information is then used to predict unobserved quantities, such as ion angular momentum alignment and the full three‐dimensional form of ...

Photoelectron angular distributions: developments in applications to isolated molecular systems

The last decade has seen photoelectron angular distributions from isolated molecules used for an increasing variety of purposes, including examining details of electron correlation, demonstrating electron diffraction as a structural probe of single molecules, and probing photochemical processes. In this article these developments are reviewed and it is shown that the stage is set for another decade of innovation in which we can expect to see exciting results from pump–probe experiments using the emerging XUV and X-ray free-electron laser sources.

Photoelectron angular distributions as a probe of alignment evolution in a polyatomic molecule: Picosecond time- and angle-resolved photoelectron spectroscopy of S1 para-difluorobenzene

We demonstrate that picosecond time-resolved photoelectron angular distributions (PADs) provide a sensitive probe of an evolving alignment in an excited polyatomic molecule. Such an evolving alignment can be caused by pure rotational recurrences or by rotation–vibration coupling. If a molecule is chosen for which the rotational recurrence times are well-known the method provides a means of establishing the mechanism of intramolecular vibrational energy redistribution (IVR). In the case of S1 para-difluorobenzene we observe striking alignment changes as a function of pump–probe time delay which we attribute to rotationally mediated IVR.

Measurement of circular dichroism in rotationally resolved photoelectron angular distributions following the photoionization of NO A 2Σ

The photoionization process NO A 2Σ+ (v=0, N=22)→NO+ X 1Σ+ (v+=0, N+)+e− is studied with sufficient photoelectron energy resolution that the photoelectron angular distributions (PADs) associated with individual rotational levels N+ of the ion are determined. By ionizing with left and right circularly polarized light and observing the change in the rotationally resolved PADs, we can deduce all dynamical information, including the signs of the relative phase shifts of the photoelectron partial waves. This information constitutes the first complete description of the photoionization of a molecule. We discuss the consistency of our dynamical parameters with the Rydberg series of NO. We present a general formalism for (1+1’) resonance‐enhanced multiphoton ionization (REMPI) PADs for rotationally resolved ion states using linearly polarized light for excitation and elliptically polarized light for ionization. Based on the dynamical parameters determined from our fit, we use this formalism to predict the total s...

Picosecond time-resolved photoelectron spectroscopy as a means of gaining insight into mechanisms of intramolecular vibrational energy redistribution in excited states

We consider the information that can be obtained from time-resolved photoelectron spectroscopy studies of intramolecular vibrational energy redistribution (IVR) in excited states of molecules, focusing on picosecond time-resolved studies of IVR in the intermediate regime. We show that time-resolved measurements may tell us as much about the experimental conditions as they do about the dynamics under examination. We show that carefully controlled picosecond time-resolved photoelectron studies are becoming feasible and that these, combined with robust calculations of Franck–Condon factors, may point the way forward in the quest to understand excited state IVR.

Effect of breaking cylindrical symmetry on photoelectron angular distributions resulting from resonance-enhanced two-photon ionization

An expression is derived for the photoelectron angular distribution (PAD) following (1+1’) resonance‐enhanced multiphoton ionization (REMPI) of a molecule with linearly polarized light beams. When the two polarization vectors are parallel, cylindrical symmetry exists, and the PAD depends only on θ, the angle between the linear polarization vector of the ionizing radiation and the electron ejection direction. When the polarization vectors are perpendicular, cylindrical symmetry is broken, and the PAD shows φ and θ dependence. For an arbitrary angle between the two polarization vectors, the angular distribution ceases to have reflection symmetry. This breaking of cylindrical symmetry causes interference effects in the REMPI process that are readily described using a density matrix formalism. As an example, the (1+1’) REMPI of NO via its A 2Σ+ state is considered.

Time-resolved photoelectron angular distributions as a probe of intramolecular dynamics: Connecting the molecular frame and the laboratory frame

A formalism is presented in which the laboratory frame photoelectron angular distribution (PAD) is expressed as a convolution of the molecular frame PAD with the laboratory frame molecular axis distribution. Molecular and laboratory frame PADs are discussed in the context of probing intramolecular dynamics in the time domain. Model calculations for a C3v molecule are presented as an illustration of the differences between measurements in these two reference frames, and the effect of the degree of molecular alignment upon the laboratory frame measurements. Different symmetries of the orbital undergoing ionization are also considered in order to illustrate the sensitivity of PADs to nonadiabatic processes.

Variation of the polarization ratio for rotationally inelastic collisions with laser selected velocity

We extend the technique of velocity selection by narrow linewidth laser excitation by measuring the polarization of emission following energy transfer as a function of selected velocity. We apply this to rotationally inelastic collisions in A1ΣuLi2–Xe and find that for most transitions, there is a noticeable decrease in the circular polarization ratio at the center of the Doppler profile. We speculate as to whether the reason for this is dynamical, geometrical, or a combination of the two. We are able to deconvolute cross sections for the transfer of orientation as a function of relative speed which can then be compared with cross sections for the transfer of population.

Extracting molecular axis alignment from photoelectron angular distributions

We present a procedure that will enable the extraction of molecular axis alignment from evolving photoelectron angular distributions measured following (1+1) resonance-enhanced multiphoton ionization in which the first step prepares the initial alignment. This procedure is applicable to a picosecond time- and angle-resolved photoelectron spectroscopy experiment [such as the one presented by Reid et al., J. Chem. Phys. 111, 1438 (1999)], and relies on the radial dipole matrix elements for the ionization process having no time dependence on the time scale of the experiment. As an illustration, we present a model calculation of the evolution of photoelectron angular distributions expected following the ionization of a prepared pure rotational wavepacket.

Observation of time- and angle-resolved photoelectron flux from an optically prepared state of a molecule. Hyperfine depolarization in NO (A 2Σ+)

Abstract The first measurements of angle-resolved photoelectron flux as a function of the time delay Δ t between excitation and ionization in a molecular (1+1′) resonance-enhanced multiphoton ionization process are reported. The process studied is in which the intermediate state alignment, and hence the angle-resolved photoelectron signal, evolve as a result of hyperfine depolarization. The results are in good agreement with predictions that use values for the hyperfine coupling constants that have previously been deduced for NO(A 2 Σ + , v = 1). This experiment illustrates a general method for the study of intramolecular depolarization mechanisms.