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

Anharmonic excited state frequencies of para-difluorobenzene, toluene and catechol using analytic RI-CC2 second derivatives

Analytic second nuclear derivatives for excited electronic state energies have been implemented for the resolution-of-the-identity accelerated CC2, CIS(D_∞) and ADC(2) models. Our efficient implementation with O(N²) memory demands enables the treatment of medium sized molecules with large basis sets and high numerical precision and thereby paves the way for semi-numerical evaluation of the higher-order derivatives required for anharmonic corrections to excited state vibrational frequencies. We compare CC2 harmonic and anharmonic excited state frequencies with experimental values for para-difluorobenzene, toluene and catechol. Basis set problems occur for out-of-plane bending vibrations due to intramolecular basis set superposition error. For non-planar molecules and in plane modes of planar molecules, the agreement between theory and experiment is better than 30 cm^-1 on average and we reassign a number of experimental bands on the basis of the ab initio predictions.

Complex and Sustained Quantum Beating Patterns in a Classic IVR System: The 3(1)5(1) Level in S1 p-Difluorobenzene

Using picosecond time-resolved photoelectron imaging, we have studied the intramolecular vibrational energy redistribution (IVR) dynamics that occur following the excitation of the 3(1)5(1) level, which lies 2068 cm(-1) above the S1 origin in p-difluorobenzene. Our technique, which has superior time resolution to that of earlier studies but retains sufficient energy resolution to identify the behavior of individual vibrational states, enables us to determine six distinct beating periods in photoelectron intensity, only one of which has been observed previously. Analysis shows that the IVR dynamics are restricted among only a handful of vibrational levels, despite the relatively high excitation energy. This is deduced to be a consequence of the high symmetry and rigid structure of p-difluorobenzene.

Comparison of Photoelectron-Spectroscopy Results to Ab-Initio and Density Functional Calculations: The Ethylbenzene Cation

In this work resonant S 0–S 1 two-photon ionization (R2PI) and high-resolution R(1+1’)PI photoelectron spectroscopy (PES) as well as ab initio and density functional (DFT) calculations of ethylbenzene (EB) are combined. Conformer energies and equilibrium geometries have been calculated for neutral and cationic EB with the HF, UHF, B3LYP and the MP2 methods and different basis sets. In agreement with previous results the tail-to-chromophore orientation of neutral EB is orthogonal. This conformer is also the most stable structure in the cation, but a second local minimum in which all carbons lie in a plane (termed “planar” conformer) lays 325cm-1 higher in energy. R(1+1’)PI PE spectra were recorded by time-of-flight spectrometer with an energy resolution (Δ E) below 8 cm-1 and an absolute accuracy of ± 10 cm-1 for electron energies below 200 meV. Because the experiment starts in the orthogonal conformer and ionization is vertical, the recorded PE spectra show the cation ground state vibrations of this conformer. Beside benzene modes also low-energetic tail-to-chromophore modes are observed and assigned by DFT vibrational mode analysis. The differences of the calculated vibrational frequencies between the two conformers are comparable to the deviation between experiment and theory and a conformer assignment by comparison of theory and experiment would be difficult. R(1+1’)PI PE spectra recorded via selected S 1 vibrations provide vibrational assignments for S 1, qualitative S 1–D 0 geometry changes, vibrational symmetries as well as internal vibrational redistribution dynamics in S 1. Charge and spin densities of the neutral and cation were calculated to elucidate the problem of charge delocalization and electronic tail-to-chromophore coupling.

Observation of a simple vibrational wavepacket in a polyatomic molecule via time-resolved photoelectron velocity-map imaging: A prototype for time-resolved IVR studies

We have prepared a coherent superposition of the two components of a Fermi resonance in the S1 state of toluene at approximately 460 cm(-1) with a approximately 1 ps laser pulse and monitored time-resolved photoelectron velocity-map images. The photoelectron intensities oscillate with time in a manner that depends on their kinetic energy, even though full vibrational resolution in the cation is not achieved. Analysis of the time-dependent photoelectron spectra enables information on the composition of the S1 wavepacket to be deduced. Such an experiment, in which a whole set of partially dispersed cation vibrational states are detected simultaneously, suggests an efficient method of studying intramolecular vibrational energy redistribution processes in excited states.

Photoelectron spectroscopy of S1 toluene: II. Intramolecular dynamics of selected vibrational levels in S1 toluene studied by nanosecond and picosecond time-resolved photoelectron spectroscopies

We present results which suggest that the photophysics of S(1) toluene is significantly more complicated than that of the related molecules p-fluorotoluene or p-difluorobenzene. We have measured a range of photoelectron spectra for a number of S(1) internal energies, on different time scales and at different temperatures, in an attempt to unravel the competing processes, but the final conclusion remains outstanding.

Mapping rotational coherences onto time-resolved photoelectron imaging observables

We explore the information content of time-resolved photoelectron imaging, a potentially powerful pump-probe technique whose popularity has been rapidly growing in recent years. To that end, we identify a mapping of the alignment properties of time-evolving wave packets onto the moments of the photoelectron images and investigate its origin and consequences theoretically and numerically.

An unusual π* shape resonance in the near-threshold photoionization of S1 para-difluorobenzene

Previously reported dramatic changes in photoelectron angular distributions (PADs) as a function of photoelectron kinetic energy following the ionization of S1 p-difluorobenzene are shown to be explained by a shape resonance in the b(2g) symmetry continuum. The characteristics of this resonance are clearly demonstrated by a theoretical multiple-scattering treatment of the photoionization dynamics. New experimental data are presented which demonstrate an apparent insensitivity of the PADs to both vibrational motion and prepared molecular alignment, however, the calculations suggest that strong alignment effects may nevertheless be recognized in the detail of the comparison with experimental data. The apparent, but unexpected, indifference to vibrational excitation is rationalized by considering the nature of the resonance. The correlation of this shape resonance in the continuum with a virtual pi* antibonding orbital is considered. Because this orbital is characteristic of the benzene ring, the existence of similar resonances in related substituted benzenes is discussed.

Photoelectron kinetic energy dependence in near threshold ionization of NO from A state studied by time-resolved photoelectron imaging

Photoelectron angular distributions in the laboratory frame (LF-PADs) from the A((2)sigma(+)) state of NO molecule were measured by femtosecond time-resolved photoelectron imaging with (1 + 1(')) resonance enhanced multiphoton ionization via the A state. High-precision measurements of the anisotropy parameters of LF-PADs were performed for the photoelectron kinetic energy from 0.03 to 1.05 eV as a function of the pump-probe delay time. The revival feature of the rotational wave packet on the A state was clearly observed in the time dependence of the photoelectron anisotropy parameters. By approximating the phase shifts of the photoelectron partial waves by the quantum defects in the high-lying Rydberg states using the multichannel quantum defect theory, the energy-dependent photoionization transition dipole moments were determined, for the first time, from time-dependent LF-PADs measured by time-resolved photoelectron spectroscopy.

Reevaluation of the use of photoelectron angular distributions as a probe of dynamical processes: strong dependence of such distributions from s1 paradifluorobenzene on photoelectron kinetic energy

Photoelectron angular distributions (PADs) have been measured following the excitation of the S1 origin band in paradifluorobenzene using a range of ionizing wavelengths and for resolved ion vibrational states. The PADs show a dramatic sensitivity to the photoelectron kinetic energy over an energy range of at least 1 eV from threshold, and almost no sensitivity to any prepared intermediate state alignment. This has important consequences for those studies of intramolecular dynamics that use PADs. We suggest that the observed behavior is caused by a shape resonance in the continuum.

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.

Femtosecond time-resolved photoelectron spectroscopy of polyatomic molecules

Femtosecond time-resolved photoelectron spectroscopy is emerging as a useful technique for investigating excited state dynamics in isolated polyatomic molecules. The sensitivity of photoelectron spectroscopy to both electronic configurations and vibrational dynamics makes it well suited to the study of ultrafast nonadiabatic processes. We review the conceptual interpretation of wavepacket dynamics experiments, emphasizing the role of the final state. We discuss the advantages of the molecular ionization continuum as the final state in polyatomic wavepacket experiments and show how the electronic structure of the continuum can be used to disentangle electronic from vibrational dynamics. We illustrate these methods with examples from diatomic wavepacket dynamics, internal conversion in polyenes and polyaromatic hydrocarbons, excited state intramolecular proton transfer, and azobenzene photoiosomerization dynamics.

Time-resolved photoelectron angular distributions: concepts, applications, and directions

The use of photoelectron angular distributions (PADs) as a probe in short-pulse, pump-probe scenarios is reviewed. We focus on concepts, on the insight that can be gained through theoretical analysis, on applications, and on future opportunities. Time-resolved PADs are sensitive to both the time-evolving rotational composition of wavepackets and their time-evolving electronic symmetry. The former feature renders this observable a potential probe of molecular structure, intensity effects, and rotational perturbations. The latter feature renders the PAD a potential probe of radiationless transitions.

Photoelectron imaging on time-dependent molecular alignment created by a femtosecond laser pulse

Rotational wave packet revivals on an excited electronic state have been measured by femtosecond time-resolved photoelectron imaging for the first time. The first full revival at 82 ps of S1 (n,pi*) pyrazine was clearly observed in the time dependencies of the photoelectron intensity and the photoelectron angular distribution (PAD). The PAD, measured for laser aligned pyrazine, clearly reflects the different characters of pi* and 3s molecular orbitals.

Coincidence spectroscopy

The application of coincidence techniques to the study of the reaction dynamics of isolated molecules is reviewed. Coincidence spectroscopy is a powerful approach for carrying out a number of measurements. At its most basic level, coincidence techniques can identify the source of a specific signal, as in the well-known photoelectron-photoion coincidence approach used for several years. By carrying out coincidence experiments in an increasingly differential manner, correlated energy and angular distributions of reaction products may be recorded. Completely energy- and angle-resolved measurements of photoelectrons and ionic or neutral products can reveal molecular-frame photoelectron and photofragment angular distributions and aid in the characterization of dissociative states of molecules and ions. Recent work in this area is reviewed, including examples from studies of dissociative photodetachment, dissociative photoionization, time-resolved studies of dissociative photoionization, and three-body dissociation processes.

Time-resolved photoelectron spectroscopy of molecules and clusters

Time-resolved photoelectron spectroscopy (TRPES) has become a powerful new tool in studying the dynamics of molecules and clusters. It has been applied to processes ranging from energy flow in electronically excited states of molecules to electron solvation dynamics in clusters. This review covers experimental and theoretical aspects of TRPES, focusing on studies of neutral and negatively charged species.