Influence of Vibrational Excitation and Nuclear Dynamics in Multiphoton Photoelectron Circular Dichroism of Fenchone.

We report photoelectron circular dichroism of -(+)-fenchone enantiomers recorded with state-state vibrational level resolution using picosecond laser (2 + 1) resonance enhanced multiphoton ionization via 3s and 3p Rydberg intermediate states. The 3p state decays to the 3s state on a picosecond time scale so that, above the 3p Rydberg excitation threshold, ionization of vibrationally hot 3s states competes with direct 3p ionization. Complex vibronic dynamics of the 3p → 3s internal conversion weaken the Rydberg Δ = 0 propensity rule in both the 3p and 3s ionization channels.

Identifying complex Fermi resonances in -difluorobenzene using zero-electron-kinetic-energy (ZEKE) spectroscopy.

The vibrations of the ground state cation ( ) of -difluorobenzene (DFB) have been investigated using zero-electron-kinetic-energy (ZEKE) spectroscopy. A comprehensive set of ZEKE spectra were recorded via different vibrational levels of the S state (<0 + 1300 cm). The adiabatic ionization energy for DFB was measured as 73 869 ± 5 cm.

Accessing the molecular frame through strong-field alignment of distributions of gas phase molecules.

A rationale for creating highly aligned distributions of molecules is that it enables vector properties referenced to molecule-fixed axes (the ) to be determined. In the present work, the degree of alignment that is necessary for this to be achieved in practice is explored. Alignment is commonly parametrized in experiments by a single parameter, [Formula: see text], which is insufficient to enable predictive calculations to be performed.

Probing the origins of vibrational mode specificity in intramolecular dynamics through picosecond time-resolved photoelectron imaging studies.

We have studied the intramolecular dynamics induced by selective photoexcitation of two near-isoenergetic vibrational states in Sp-fluorotoluene using picosecond time-resolved photoelectron imaging. We find that similar dynamics ensue following the preparation of the 131 and 7a1 states that lie at 1990 cm and 2026 cm, and that these dynamics are mediated by a single strongly coupled doorway state in each case. However, the lifetimes differ by a factor of three, suggesting an influence of the vibrational character of the modes involved.

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.

The 700-1500 cm⁻¹ region of the S₁ (ùB₂) state of toluene studied with resonance-enhanced multiphoton ionization (REMPI), zero-kinetic-energy (ZEKE) spectroscopy, and time-resolved slow-electron velocity-map imaging (tr-SEVI) spectroscopy.

We report (nanosecond) resonance-enhanced multiphoton ionization (REMPI), (nanosecond) zero-kinetic-energy (ZEKE) and (picosecond) time-resolved slow-electron velocity map imaging (tr-SEVI) spectra of fully hydrogenated toluene (Tol-h8) and the deuterated-methyl group isotopologue (α3-Tol-d3). Vibrational assignments are made making use of the activity observed in the ZEKE and tr-SEVI spectra, together with the results from quantum chemical and previous experimental results. Here, we examine the 700-1500 cm(-1) region of the REMPI spectrum, extending our previous work on the region ≤700 cm(-1).

Critical influences on the rate of intramolecular vibrational redistribution: a comparative study of toluene, toluene-d3 and p-fluorotoluene.

The intramolecular vibrational redistribution (IVR) dynamics following the excitation of a mode in the first electronically excited states of toluene, toluene-d3 and p-fluorotoluene that has predominantly C-CH3 stretching character and an internal energy of ~1200 cm(-1) have been compared using picosecond time-resolved photoelectron imaging spectroscopy as a probe. Temporal changes in the intensities of spectral features in each molecule have enabled IVR lifetimes of 12, 15 and 50 ps, respectively, to be determined. Our measurements show that doorway states are critical in mediating the IVR dynamics in toluene and toluene-d3, and we deduce that these doorway states, which are assigned in the course of this work, are also instrumental in reducing the IVR lifetimes of these molecules relative to p-fluorotoluene.

A generic π* shape resonance observed in energy-dependent photoelectron angular distributions from two-colour, resonant multiphoton ionization of difluorobenzene isomers.

We present new evidence for the existence of a near threshold π* shape resonance as a common feature in the photoionization of each isomer of difluorobenzene. Experimentally, this is revealed by significant changes in the anisotropy of the photoelectron angular distributions (PADs) following the ionization of the optically aligned S1 state of these molecules at varying photon energies. Continuum multiple scattering Xα calculations reproduce this behaviour well, and allow the visualisation of the continuum shape resonances.

Elucidating quantum number-dependent coupling matrix elements using picosecond time-resolved photoelectron spectroscopy.

We measure quantum beating patterns of photoelectron intensity caused by intramolecular vibrational energy redistribution following the excitation of a low-lying ring breathing state in S(1) parafluorotoluene. Analysis of the beating patterns reveals an exceptional sensitivity to details of the evolving wave packet which is found to contain two incoherent components, one of which rapidly dephases. This analysis enables the determination of coupling matrix elements, which are shown to depend strongly on torsional and rotational quantum numbers.

Intramolecular vibrational dynamics in S1 p-fluorotoluene. I. Direct observation of doorway states.

Picosecond time-resolved photoelectron spectroscopy is used to investigate intramolecular vibrational redistribution (IVR) following excitation of S(1) 18a(1) in p-fluorotoluene (pFT) at an internal energy of 845 cm(-1), where ν(18a) is a ring bending vibrational mode. Characteristic oscillations with periods of 8 ps and 5 ps are observed in the photoelectron signal and attributed to coupling between the initially excited zero-order bright state and two doorway states. Values for the coupling coefficients connecting these three vibrational states have been determined.

Photoionization dynamics of ammonia (B(1)E''): dependence on ionizing photon energy and initial vibrational level.

In this article we present photoelectron spectra and angular distributions in which ion rotational states are resolved. This data enables the comparison of direct and threshold photoionization techniques. We also present angle-resolved photoelectron signals at different total energies, providing a method to scan the structure of the continuum in the near-threshold region.

Deducing anharmonic coupling matrix elements from picosecond time-resolved photoelectron spectra: application to S(1) toluene at low vibrational energy.

The 6a(1) + 10b(1)16b(1) Fermi resonance in S(1) toluene is studied through picosecond time-resolved photoelectron spectroscopy. Our time and energy resolution, together with the necessary stability to monitor dynamics for many hundreds of picoseconds, enable new and unexpected insight into the dynamics and identity of the prepared wavepacket, and the determination of the coupling matrix elements responsible for those dynamics. In particular we are able to determine the influence of the torsional motion of the methyl group on the dynamics; this motion has long been implicated as an effective accelerator of IVR processes.

Rotationally resolved photoelectron angular distributions from a nonlinear polyatomic molecule.

We present, for the first time, rotationally resolved photoelectron images resulting from the ionization of a polyatomic molecule. Photoelectron angular distributions pertaining to the formation of individual rotational levels of NH3+ have been extracted from the images and analyzed to enable a complete determination of the radial dipole matrix elements and relative phases that describe the ionization dynamics. This determination leads to the deduction of significantly different dynamics from those extracted in previous studies which lacked either angular information or rotational resolution.

Applications of slow electron velocity map imaging to the study of spectroscopy and dynamics in small aromatic molecules.

Slow electron velocity map imaging provides a means of performing relatively high resolution photoelectron spectroscopy while still maintaining many of the advantages of imaging techniques. Here, we describe its application to the spectroscopy and dynamics of some substituted toluene molecules and show it to be a versatile technique whose resolution can approach that of zero kinetic energy (ZEKE) photoelectron spectroscopy, and provides a good match to the bandwidth of transform limited 1 ps laser pulses. We provide a series of comparisons of the results obtained with different ionizing wavelengths and use these to help understand the advantages and limitations of the technique.

Complete determination of the photoionization dynamics of a polyatomic molecule. II. Determination of radial dipole matrix elements and phases from experimental photoelectron angular distributions from A1Au acetylene.

We present a fit to photoelectron angular distributions (PADs) measured following the photoionization of rotationally selected A1Au state acetylene. In the case of the 4(1)2Sigmau- vibronic state of the ion, we are able to use this fit to make a complete determination of the radial dipole matrix elements and phases connecting the prepared level to each photoelectron partial wave. We have also investigated other Renner-Teller subbands with a view to disentangling geometrical and dynamical contributions to the resulting PADs.

Complete determination of the photoionization dynamics of a polyatomic molecule. I. Experimental photoelectron angular distributions from A1Au acetylene.

Angle-resolved photoelectron spectra from rotationally selected A1Au state acetylene have been recorded using velocity-map imaging. Several Renner-Teller split vibrational bands have been observed and assigned, showing good agreement with previous zero kinetic energy photoelectron (ZEKE) work [S. T.

Progress in understanding the intramolecular vibrational redistribution dynamics in the S(1) state of para-fluorotoluene.

We employ zero-kinetic-energy (ZEKE) photoelectron spectroscopy with nanosecond laser pulses to study intramolecular vibrational redistribution (IVR) in S(1) para-fluorotoluene. The frequency resolution of the probe step is superior to that obtained in any studies on this molecule to date. We focus on the behavior of the 13(1) (C-CH(3) stretch) and 7a(1) (C-F stretch) vibrational states whose dynamics have previously received significant attention, but with contradictory results.

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.

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.

Photoelectron spectroscopy of S1 toluene: I. Photoionization propensities of selected vibrational levels in S1 toluene.

Laser photoelectron spectra have been obtained following the preparation of eight vibrational states in S(1) toluene. For four of the vibrational states (up to approximately 550 cm(-1) excess energy) excitation and ionization with nanosecond laser pulses give rise to photoelectron spectra with well-resolved vibrational peaks. For the other states (>750 cm(-1) excess energy) the photoelectron spectra show a loss of structure when nanosecond pulses are used, as a result of intramolecular dynamics [see Whiteside et al.

An unusual pi* 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.

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.

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.