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 molecular frame) to be determined. In the present work, the degree of alignment that is necessary in order for this to be achieved in practice is explored. Alignment is commonly parametrised in experiments by a single parameter, , which is insufficient to enable predictive calculations to be performed. Here it is shown that, if the full distribution of molecular axes takes a Gaussian form, this single parameter can be used to determine the complete set of alignment moments needed to characterise the distribution. In order to demonstrate the degree of alignment that is required in order to approach the molecular frame, the set of alignment moments corresponding to a few chosen values of are used to project a model molecular frame photoelectron angular distribution into the laboratory frame. These calculations show that needs to approach 0.9 in order to avoid significant blurring to be caused by averaging

Switched wave packets: A route to nonperturbative quantum control

The dynamic Stark effect due to a strong nonresonant but nonionizing laser field provides a route to quantum control via the creation of novel superposition states. We consider the creation of a field-free "switched" wave packet through adiabatic turn-on and sudden turn-off of a strong dynamic Stark interaction. There are two limiting cases for such wave packets. The first is a Raman-type coupling, illustrated by the creation of field-free molecular axis alignment. An experimental demonstration is given. The second case is that of dipole-type coupling, illustrated by the creation of charge localization in an array of quantum wells

All normal dispersion nonlinear fibre supercontinuum source characterization and application in hyperspectral stimulated Raman scattering microscopy

Hyperspectral stimulated Raman scattering (SRS) microscopy is a powerful label-free, chemical-specific technique for biomedical and mineralogical imaging. Usually, broad and rapid spectral scanning across Raman bands is required for species identification. In many implementations, however, the Raman spectral scan speed is limited by the need to tune source laser wavelengths. Alternatively, a broadband supercontinuum source can be considered. In SRS microscopy, however, source noise is critically important, precluding many spectral broadening schemes. Here we show that a supercontinuum light source based on all normal dispersion (ANDi) fibres provides a stable broadband output with very low incremental source noise. We characterized the noise power spectral density of the ANDi fibre output and demonstrated its use in hyperspectral SRS microscopy applications. This confirms the viability and ease of implementation of ANDi fibre sources tier broadband SRS imaging. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Dynamics of excited-state proton transfer systems via time-resolved photoelectron spectroscopy

The use of time-resolved photoelectron spectroscopy for analyzing excited state intramolecular proton transfer (ESIPT) and internal conversion dynamics in a model system was investigated. The photoelectron spectra of both the excited state enol and keto tautomers were presented as a function of pump laser wavelength and pump-probe time delay. It was found that the internal conversion dynamics in o-hydroxybenzaldehyde (OHBA) was influenced by interactions with a close-lying n??* state.open958

Quantum beat photoelectron imaging spectroscopy of Xe in the VUV

Time resolved pump probe measurements of Xe, pumped at 133 nm and probed at 266 nm, are presented. The pump pulse prepared a long lived hyperfine wave packet in the Xe 5p5 2P amp; 8728;1 2 6s2[1 2] amp; 8728;1 manifold E 77185cm amp; 8722;1 9.57eV . The wave packet was monitored via single photon ionization and velocity map photoelectron images were measured. The images provide angle and time resolved data which, when obtained over a large time window 900 ps , constitute a precision quantum beat spectroscopy measurement of the hyperfine state splittings. Additionally, analysis of the full photoelectron image stack provides a quantum beat imaging modality, in which the Fourier components of the photoelectron images correlated with specific beat components can be obtained. This may also permit the extraction of isotope resolved photoelectron images in the frequency domain, in cases where nuclear spins hence beat components can be uniquely assigned to specific isotopes as herein , and also provides phase information relating to the ionization dynamics. The information content of both raw and inverted image stacks is investigated, suggesting the utility of the Fourier analysis methodology in cases where images cannot be inverte

Excited state non-adiabatic dynamics of pyrrole:A time-resolved photoelectron spectroscopy and quantum dynamics study

The dynamics of pyrrole excited at wavelengths in the range 242-217 nm are studied using a combination of time-resolved photoelectron spectroscopy and wavepacket propagations performed using the multi-configurational time-dependent Hartree method. Excitation close to the origin of pyrrole's electronic spectrum, at 242 and 236 nm, is found to result in an ultrafast decay of the system from the ionization window on a single timescale of less than 20 fs. This behaviour is explained fully by assuming the system to be excited to the A2(\u3c0\u3c3 17) state, in accord with previous experimental and theoretical studies. Excitation at shorter wavelengths has previously been assumed to result predominantly in population of the bright A1(\u3c0\u3c0 17) and B2(\u3c0\u3c0 17) states. We here present time-resolved photoelectron spectra at a pump wavelength of 217 nm alongside detailed quantum dynamics calculations that, together with a recent reinterpretation of pyrrole's electronic spectrum [S. P. Neville and G. A. Worth, J. Chem. Phys. 140, 034317 (2014)], suggest that population of the B1(\u3c0\u3c3 17) state (hitherto assumed to be optically dark) may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum. The B1(\u3c0\u3c3 17) state is found to decay on a timescale of less than 20 fs by both N-H dissociation and internal conversion to the A2(\u3c0\u3c3 17) state.Peer reviewed: YesNRC publication: Ye

Stable kilohertz rate molecular beam laser ablation sources

A stable kilohertz (kHz) rate laser ablation/desorption supersonic molecular beam source for use in kHz rate laser experiments was discussed. The source was based based upon strong nonresonant interaction of a dithering laser focus with a rotating and translating solid rod. The kHz laser ablation of a high temperature refractory metal (niobium) for use in studied of metal clusters was also demonstrated. The kHz laser desorption and jet cooling of an involatile biomolecule (the DNA based guanine) for use in spectroscopic and dynamical studies was described.open151

Excited state wavepacket dynamics in NO 2 probed by strong-field ionization

We present an experimental femtosecond time-resolved study of the 399 nm excited state dynamics of nitrogen dioxide using channel-resolved above threshold ionization (CRATI) as the probe process. This method relies on photoelectron-photoion coincidence and covariance to correlate the strongfield photoelectron spectrum with ionic fragments, which label the channel. In all ionization channels observed, we report apparent oscillations in the ion and photoelectron yields as a function of pumpprobe delay. Further, we observe the presence of a persistent, time-invariant above threshold ionization comb in the photoelectron spectra associated with most ionization channels at long time delays. These observations are interpreted in terms of single-pump-photon excitation to the first excited electronic X˜ 2A1 state and multi-pump-photon excitations to higher-lying states. The short time delay (<100 fs) dynamics in the fragment channels show multi-photon pump signatures of higherlying neutral state dynamics, in data sets recorded with higher pump intensities. As expected for pumping NO2 at 399 nm, non-adiabatic coupling was seen to rapidly re-populate the ground state following excitation to the first excited electronic state, within 200 fs. Subsequent intramolecular vibrational energy redistribution results in the spreading of the ground state vibrational wavepacket into the asymmetric stretch coordinate, allowing the wavepacket to explore nuclear geometries in the asymptotic region of the ground state potential energy surface. Signatures of the vibrationally “hot” ground state wavepacket were observed in the CRATI spectra at longer time delays. This study highlights the complex and sometimes competing phenomena that can arise in strong-field ionization probing of excited state molecular dynamics

Dissociative photoionization of the NO molecule studied by photoelectron-photon coincidence technique

Low-energy photoelectron–vacuum ultraviolet (VUV) photon coincidences have been measured using synchrotron radiation excitation in the inner-valence region of the nitric oxide molecule. The capabilities of the coincidence set-up were demonstrated by detecting the 2s−1 → 2p−1 radiative transitions in coincidence with the 2s photoelectron emission in Ne. In NO, the observed coincidence events are attributed to dissociative photoionization with excitation, whereby photoelectron emission is followed by fragmentation of excited NO+ ions into O+ + N* or N+ + O* and VUV emission from an excited neutral fragment. The highest coincidence rate occurs with the opening of ionization channels which are due to correlation satellites of the 3σ photoionization. The decay time of VUV photon emission was also measured, implying that specific excited states of N atoms contribute significantly to observed VUV emission

Steady and Time-Resolved Photoelectron Spectra Based on Nuclear Ensembles

Semiclassical methods to simulate both steady and time-resolved photoelectron spectra are presented. These approaches provide spectra with absolute band shapes and vibrational broadening beyond the Condon approximation, using an ensemble of nuclear configurations built either via distribution samplings or nonadiabatic dynamics simulations. Two models to account for the electron kinetic energy modulation due to vibrational overlaps between initial and final states are discussed. As illustrative examples, the steady photoelectron spectra of imidazole and adenine and the time- and kinetic-energy-resolved photoelectron spectrum of imidazole were simulated within the frame of time-dependent density functional theory. While for steady spectra only electrons ejected with maximum allowed kinetic energy need to be considered, it is shown that to properly describe time-resolved spectra, electrons ejected with low kinetic energies must be considered in the simulations as well. The results also show that simulations based either on full computation of photoelectron cross section or on simple Dyson orbital norms provide results of similar quality