UKRmol+: A suite for modelling electronic processes in molecules interacting with electrons, positrons and photons using the R-matrix method

High-order harmonic generation in polyatomic molecules generally involves multiple channels of ionization. Their relative contribution can be strongly influenced by the presence of resonances, whose assignment remains a major challenge for high-harmonic spectroscopy. Here we present a multi-modal approach for the investigation of unaligned polyatomic molecules, using ​SF6 as an example. We combine methods from extreme-ultraviolet spectroscopy, above-threshold ionization and attosecond metrology. Fragment-resolved above-threshold ionization measurements reveal that strong-field ionization opens at least three channels. A shape resonance in one of them is found to dominate the signal in the 20–26 eV range. This resonance induces a phase jump in the harmonic emission, a switch in the polarization state and different dynamical responses to molecular vibrations. This study demonstrates a method for extending high-harmonic spectroscopy to polyatomic molecules, where complex attosecond dynamics are expected.

/pdf/multi-channel-electronic-and-vibrational-dynamics-in-3tonj2p1jj.pdf

Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation

In high-order harmonic generation, resonant harmonics (RH) are sources of intense, coherent extreme-ultraviolet radiation. However, intensity enhancement of RH only occurs for a single harmonic order, making it challenging to generate short attosecond pulses. Moreover, the mechanism involved behind such RH was circumstantial, because of the lack of direct experimental proofs. Here, we demonstrate the exact quantum paths that electron follows for RH generation using tin, showing that it involves not only the autoionizing state, but also a harmonic generation from dressed-AIS that appears as two coherent satellite harmonics at frequencies ±2Ω from the RH (Ω represents laser frequency). Our observations of harmonic emission from dressed states open the possibilities of generating intense and broadband attosecond pulses, thus contributing to future applications in attosecond science, as well as the perspective of studying the femtosecond and attosecond dynamics of autoionizing states.

/pdf/high-order-harmonic-generation-from-the-dressed-autoionizing-2udktom92k.pdf

High-order harmonic generation from the dressed autoionizing states.

Resonant enhancement of high harmonic generation can be obtained in plasmas containing ions with strong radiative transitions resonant with harmonic orders. The mechanism for this enhancement is still debated. We perform the first temporal characterization of the attosecond emission from a tin plasma under near-resonant conditions for two different resonance detunings. We show that the resonance considerably changes the relative phase of neighboring harmonics. For very small detunings, their phase locking may even be lost, evidencing strong phase distortions in the emission process and a modified attosecond structure. These features are well reproduced by our simulations, allowing their interpretation in terms of the phase of the recombination dipole moment.

http://iopscience.iop.org/article/10.1088/1367-2630/15/1/013051/pdf

Phase distortions of attosecond pulses produced by resonance-enhanced high harmonic generation

https://iopscience.iop.org/article/10.3367/UFNe.2015.12.037670/pdf

Attosecond electromagnetic pulses: generation, measurement, and application. Generation of high-order harmonics of an intense laser field for attosecond pulse production

We investigate high-order harmonic generation from laser-irradiated systems that support a shape resonance. From the numerical solution of the time-dependent Schr\"odinger equation, we calculate the harmonic spectra and the time-frequency analysis of the harmonic intensity and phase. The analysis reveals the separate contributions of the short and long trajectories as well as the resonance. A range of harmonics is strongly enhanced by the presence of the resonance irrespective of the pulse length. The signature of the resonance remains significant after coherent summation over intensities as a simple method to simulate macroscopic effects. The time-frequency analysis supports the recently proposed four-step mechanism of the enhanced harmonic generation process.

/pdf/high-order-harmonic-generation-in-the-presence-of-a-1b7b93nfuy.pdf

High-order harmonic generation in the presence of a resonance

We report studies of high-order harmonic generation in laser-produced manganese plasmas using sub-4-fs drive laser pulses. The measured spectra exhibit resonant enhancement of a small spectral region of about 2.5 eV width around the 31st harmonic (~50eV). The intensity contrast relative to the directly adjacent harmonics exceeds one order of magnitude. This finding is in sharp contrast to the results reported previously for multi-cycle laser pulses [Physical Review A 76, 023831 (2007)]. Theoretical modelling suggests that the enhanced harmonic emission forms an isolated sub-femtosecond pulse.

Isolated sub-fs XUV pulse generation in Mn plasma ablation.

Collisions of low energy electrons with molecules are important for understanding many aspects of the environment and technologies. Understanding the processes that occur in these types of collisions can give insights into plasma etching processes, edge effects in fusion plasmas, radiation damage to biological tissues and more. A radical update of the previous expert system for computing observables relevant to these processes, Quantemol-N, is presented. The new Quantemol Electron Collision (QEC) expert system simplifyies the user experience, improving reliability and implements new features. The QEC graphical user interface (GUI) interfaces the Molpro quantum chemistry package for molecular target setups, and the sophisticated UKRmol+ codes to generate accurate and reliable cross-sections. These include elastic cross-sections, super elastic cross-sections between excited states, electron impact dissociation, scattering reaction rates, dissociative electron attachment, differential cross-sections, momentum transfer cross-sections, ionization cross sections, and high energy electron scattering cross-sections. With this new interface we will be implementing dissociative recombination estimations, vibrational excitations for neutrals and ions, and effective core potentials in the near future.

https://www.mdpi.com/2218-2004/7/4/97/pdf

Quantemol Electron Collisions (QEC): An enhanced expert system for performing electron molecule collision calculations using the R-matrix method

The ring-opening dynamics of 1,3-cyclohexadiene (CHD) following UV excitation is studied using a model based on quantum molecular dynamics simulations with the ab initio multiconfigurational Ehrenfest (AI-MCE) method coupled to the Dyson orbital approach for photoionisation cross sections. Time-dependent photoelectron spectra are calculated for probe photon energies in the range 2-15 eV. The calculations demonstrate the value of universal high-energy probes, capable of tracking the dynamics in full, but also the utility of more selective lower-energy probes with a more restricted ionisation range. The predicted signal, especially with the universal probes, becomes highly convoluted due to the contributions from multiple reaction paths, rendering interpretation challenging unless complementary measurements and theoretical comparisons are available.

/pdf/effects-of-probe-energy-and-competing-pathways-on-time-1jh3knzx37.pdf

Effects of probe energy and competing pathways on time-resolved photoelectron spectroscopy: the ring-opening of 1,3-cyclohexadiene

High-harmonic generation and its time-frequency analysis are studied numerically for a system with a shape resonance. Resonant enhancement occurs for long or short laser pulses. In a gas the resonance remains significant after intensity averaging.

/pdf/high-order-harmonic-generation-in-the-presence-of-a-10yhia0mij.pdf

Maria Tudorovskaya

Papers

High-order harmonic generation in the presence of a resonance

Isolated sub-fs XUV pulse generation in Mn plasma ablation.

Quantemol Electron Collisions (QEC): An enhanced expert system for performing electron molecule collision calculations using the R-matrix method

Effects of probe energy and competing pathways on time-resolved photoelectron spectroscopy: the ring-opening of 1,3-cyclohexadiene

High-Order Harmonic Generation in the Presence of a Resonance