The importance of molecular axis alignment and symmetry-breaking in photoelectron elliptical dichroism.

Photoelectron angular distributions (PADs) produced from the photoionization of chiral molecules using elliptically polarized light exhibit a forward/backward asymmetry with respect to the optical propagation direction. By recording these distributions using the velocity-map imaging (VMI) technique, the resulting photoelectron elliptical dichroism (PEELD) has previously been demonstrated as a promising spectroscopic tool for studying chiral molecules in the gas phase. The use of elliptically polarized laser pulses, however, produces PADs (and consequently, PEELD distributions) that do not exhibit cylindrical symmetry about the propagation axis. This leads to significant limitations and challenges when employing conventional VMI acquisition and data processing strategies. Using novel photoelectron image analysis methods based around Hankel transform reconstruction tomography and machine learning, however, we have quantified-for the first time-significant symmetry-breaking contributions to PEELD signals that are of a comparable magnitude to the symmetric terms in the multiphoton ionization of (1R,4R)-(+)- and (1S,4S)-(-)-camphor. This contradicts any assumptions that symmetry-breaking can be ignored when reconstructing VMI data. Furthermore, these same symmetry-breaking terms are expected to appear in any experiment where circular and linear laser fields are used together. This ionization scheme is particularly relevant for investigating dynamics in chiral molecules, but it is not limited to them. Developing a full understanding of these terms and the role they play in the photoionization of chiral molecules is of clear importance if the potential of PEELD and related effects for future practical applications is to be fully realized.

Photochemical carbon-sulfur bond cleavage in thioethers mediated via excited state Rydberg-to-valence evolution.

Time-resolved photoelectron imaging and supporting ab initio quantum chemistry calculations were used to investigate non-adiabatic excess energy redistribution dynamics operating in the saturated thioethers diethylsulfide, tetrahydrothiophene and thietane. In all cases, 200 nm excitation leads to molecular fragmentation on an ultrafast (<100 fs) timescale, driven by the evolution of Rydberg-to-valence orbital character along the S-C stretching coordinate. The C-S-C bending angle was also found to be a key coordinate driving initial internal conversion through the excited state Rydberg manifold, although only small angular displacements away from the ground state equilibrium geometry are required. Conformational constraints imposed by the cyclic ring structures of tetrahydrothiophene and thietane do not therefore influence dynamical timescales to any significant extent. Through use of a high-intensity 267 nm probe, we were also able to detect the presence of some transient (bi)radical species. These are extremely short lived, but they appear to confirm the presence of two competing excited state fragmentation channels - one proceeding directly from the initially prepared 4p manifold, and one involving non-adiabatic population of the 4s state. This is in addition to a decay pathway leading back to the S0 electronic ground state, which shows an enhanced propensity in the 5-membered ring system tetrahydrothiophene over the other two species investigated.

Spectroscopic application of few-femtosecond deep-ultraviolet laser pulses from resonant dispersive wave emission in a hollow capillary fibre.

We exploit the phenomenon of resonant dispersive wave (RDW) emission in gas-filled hollow capillary fibres (HCFs) to realize time-resolved photoelectron imaging (TRPEI) measurements with an extremely short temporal resolution. By integrating the output end of an HCF directly into a vacuum chamber assembly we demonstrate two-colour deep ultraviolet (DUV)-infrared instrument response functions of just 10 and 11 fs at central pump wavelengths of 250 and 280 nm, respectively. This result represents an advance in the current state of the art for ultrafast photoelectron spectroscopy. We also present an initial TRPEI measurement investigating the excited-state photochemical dynamics operating in the N-methylpyrrolidine molecule. Given the substantial interest in generating extremely short and highly tuneable DUV pulses for many advanced spectroscopic applications, we anticipate our first demonstration will stimulate wider uptake of the novel RDW-based approach for studying ultrafast photochemistry - particularly given the relatively compact and straightforward nature of the HCF setup.

Ultraviolet Excitation Dynamics of Nitrobenzenes.

Time-resolved photoelectron imaging was used to investigate nonadiabatic processes operating in the excited electronic states of nitrobenzene and three methyl-substituted derivatives: 3,5-, 2,6-, and 2,4-dimethylnitrobenzene. The primary goal was evaluating the dynamical impact of the torsional angle between the NO2 group and the benzene ring plane-something previously implicated in mediating the propensity for branching into different photodissociation pathways (NO vs NO2 elimination). Targeted, photoinitiated release of NO radicals is of interest for clinical medicine applications, and there is a need to establish basic structure-dynamics-function principles in systematically varied model systems following photoexcitation. Within our 200 ps experimental detection window, we observed no significant differences in the excited-state lifetimes exhibited by all species under study using a 267 nm pump and ionization with an intense 400 nm probe. In agreement with previous theoretical predictions, this suggests that the initial energy redistribution dynamics within the singlet and triplet manifolds are driven by motions localized predominantly on the NO2 group. Our findings also imply that both NO and NO2 elimination occur from a vibrationally hot ground state on extended (nanosecond) timescales, and any variations in NO vs NO2 branching upon site-selective methylation are due to steric effects influencing isomerization prior to dissociation.

Improved insights in time-resolved photoelectron imaging.

Time-resolved photoelectron imaging (TRPEI) is a highly differential technique for the detailed study of non-adiabatic energy redistribution dynamics operating in the electronically excited states of molecules following the absorption of ultraviolet light. This Perspective briefly reviews the main elements of the TRPEI method but also seeks to address some of its limitations. With the help of various examples drawn from our own recent work, we illustrate some of the challenges commonly encountered during the analysis and interpretation of experimental data and introduce some initial thoughts on approaches to help deal with them. We also discuss some novel methods that aim to expand the capabilities and utility of the TRPEI technique by extending the observation window along the photochemical reaction coordinate(s) and improving the temporal resolution. Given the widespread use of TRPEI and related ultrafast spectroscopies, we anticipate that this Perspective will be of broad interest to a sizeable research community. Furthermore, we hope it will also serve as a useful overview for those engaging with this topic for the first time.

Artificial Neural Networks for Noise Removal in Data-Sparse Charged Particle Imaging Experiments.

We present the first demonstration of artificial neural networks (ANNs) for the removal of Poissonian noise in charged particle imaging measurements with very low overall counts. The approach is successfully applied to both simulated and real experimental image data relating to the detection of photoions/photoelectrons in unimolecular photochemical dynamics studies. Specific examples consider the multiphoton ionization of pyrrole and (S)-camphor. Our results reveal an extremely high level of performance, with the ANNs transforming images that are unusable for any form of quantitative analysis into statistically reliable data with an impressive similarity to benchmark references. Given the widespread use of charged particle imaging methods within the chemical dynamics community, we anticipate that the use of ANNs has significant potential impact - particularly, for example, when working in the limit of very low absorption/photoionization cross-sections, or when attempting to reliably extract subtle image features originating from phenomena such as photofragment vector correlations or photoelectron circular dichroism.

Short-wavelength probes in time-resolved photoelectron spectroscopy: an extended view of the excited state dynamics in acetylacetone.

Femtosecond pulses of light in the vacuum ultraviolet (VUV) spectral region permit extended observation of non-adiabatic dynamics in gas-phase molecules. When used as a probe in time-resolved photoelectron spectroscopy, such pulses project deeply into the ionization continuum and allow the evolution of excited state population to be monitored across multiple potential energy surfaces. When compared with longer-wavelength probes, this often provides a more complete view along the reaction coordinate(s) connecting photoreactants to photoproducts. Here we report the use of 160 nm VUV light to interrogate the excited state dynamics operating in acetylacetone following 267 nm excitation. Multiple non-adiabatic processes (internal conversion and intersystem crossing) were observed on timescales ranging from a few femtoseconds to hundreds of picoseconds. Our quantitative results are in excellent agreement with earlier studies that individually sampled smaller sub-sections of the total reaction coordinate. Furthermore, we also observe additional dynamical signatures not previously reported elsewhere. Overall, our findings provide a good illustration of the need to use short-wavelength VUV probes to obtain the most comprehensive picture possible in photoionization-based studies of photochemical dynamics.

Vacuum ultraviolet excited state dynamics of small amides.

Time-resolved photoelectron spectroscopy in combination with ab initio quantum chemistry calculations was used to study ultrafast excited state dynamics in formamide (FOR), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA) following 160 nm excitation. The particular focus was on internal conversion processes within the excited state Rydberg manifold and on how this behavior in amides compared with previous observations in small amines. All three amides exhibited extremely rapid (<100 fs) evolution from the Franck-Condon region. We argue that this is then followed by dissociation. Our calculations indicate subtle differences in how the excited state dynamics are mediated in DMA/DMF as compared to FOR. We suggest that future studies employing longer pump laser wavelengths will be useful for discerning these differences.

Ultrafast Molecular Spectroscopy Using a Hollow-Core Photonic Crystal Fiber Light Source.

We demonstrate, for the first time, the application of rare-gas-filled hollow-core photonic crystal fibers (HC-PCFs) as tunable ultraviolet light sources in femtosecond pump-probe spectroscopy. A critical requirement here is excellent output stability over extended periods of data acquisition, and we show this can be readily achieved. The time-resolved photoelectron imaging technique reveals nonadiabatic dynamical processes operating on three distinct time scales in the styrene molecule following excitation over the 242-258 nm region. These include ultrafast (<100 fs) internal conversion between the S2(ππ*) and S1(ππ*) electronic states and subsequent intramolecular vibrational energy redistribution within S1(ππ*). Compact, cost-effective, and highly efficient benchtop HC-PCF sources have huge potential to open up many exciting new avenues for ultrafast spectroscopy in the ultraviolet and vacuum ultraviolet spectral regions. We anticipate that our initial validation of this approach will generate important impetus in this area.

Relative detection sensitivity in ultrafast spectroscopy: state lifetime and laser pulse duration effects.

We present a numerical modelling study employing a kinetic model based on rate equations to investigate the role of excited state lifetime and laser pulse duration on effective relative detection efficiency in time-resolved pump-probe spectroscopy. The work begins to address the critical outstanding problem of photochemical branching ratio determination when excited state population evolves via competing relaxation pathways in molecular systems. Our findings reveal significant differences in detection sensitivity, which can exceed an order of magnitude under typical experimental conditions for excited state lifetimes ranging between 10 fs and 1 ps. We frame our discussion within the widely used approach of ultrafast photoionization for interrogating excited state populations, but our overall treatment may be readily extended to consider a broader range of experimental methodologies and timescales.

Caveats in the interpretation of time-resolved photoionization measurements: A photoelectron imaging study of pyrrole.

We report time-resolved photoelectron imaging studies of gas-phase pyrrole over the 267-240 nm excitation region, recorded in conjunction with a 300 nm probe. Of specific interest is the lowest-lying (3s/πσ*) state, which exhibits very weak oscillator strength but is thought to be excited directly at wavelengths ≤254 nm. We conclude, however, that the only significant contribution to our photoelectron data at all wavelengths investigated is from non-resonant ionization. Our findings do not rule out (3s/πσ*) state excitation (as appears to be confirmed by supporting time-resolved ion-yield measurements) but do potentially highlight important caveats regarding the use and interpretation of photoreactant ionization measurements to interrogate dynamical processes in systems exhibiting significant topological differences between the potential energy surfaces of the neutral and cation states.