Molecular Imaging by Strong-Field Induced Tunnelling, Scattering and recombination
Intense laser pulses generate high-order harmonics when focused in a gas jet. High harmonic spectroscopy consists in measuring <br />the emitted XUV radiation to retrieve structural information on the generating gas. This signal can be used to follow photochemical <br />processes triggered by a pump pulse, or to extract attosecond dynamics of the hole created by the laser. High harmonic <br />spectroscopy is a very promising technique and its application to polyatomic molecules raise new challenges, both from an <br />experimental and theoretical point of view. MISFITS aims at tackling these challenges.
In order to measure complex dynamics of polyatomic molecules, MISFITS will use the following methods: : improvement of temporal resolution to
measure faster dynamics, calculations of high harmonic emission by semi-classical methods taking into account complex reaction
dynamics, and theoretical and experimental study of the influence of the intense laser field on the molecular structure, on the
femtosecond and attosecond timescales.
-Complete characterization of the influence of a shape resonance in high-order harmonics generation: Ferré et al., Nature Communications 2015.
- Generation of circularly polarized extreme ultraviolet light pulses: Ferré et al., Nature Photonics 2015
- New technique to measure the amplitude and phase of high harmonics using molecular excitation as an optical gate: FROMAGE (Ferré et al., PRL 2016)
The next step is the investigation of the dynamical influence of a strong laser field on a polyatomic molecule by attosecond transient absorption spectroscopy.
1. A table-top ultrashort light source in the extreme-ultraviolet for time-resolved circular dichroism experiments
A. Ferré, C. Handschin, M. Dumergue, F. Burgy, A. Comby, D. Descamps, B. Fabre, G. A. Garcia, R. Géneaux, L. Merceron, E. Mével, L. Na
When an intense laser pulse is focused into a gaseous target, high-order harmonics of the fundamental frequency can be emitted. High harmonic spectroscopy (HHS) consists in extracting structural properties of molecules from this extremely non-linear response to the laser field. From the harmonic emission, photochemical processes induced by a pump pulse in the neutral molecule can be revealed, and attosecond hole dynamics induced in the cation by the probe laser field can even be extracted. All these aspects make HHS a very appealing technique to complement conventional ultrafast spectroscopy. While it was long restricted to simple linear molecules, HHS is being extended to more complex systems. This extension raises many challenges, both experimentally and theoretically, which the MISFITS project aims to tackle.
A serious bottleneck in HHS is the interpretation of the extreme sensitivity observed experimentally, which requires a very close interaction with theory and the development of new accurate models. The MISFITS project combines theoreticians with complementary expertises. The CELIA team will take particular care in modeling the harmonic emission from polyatomic molecules, including the influence of the molecular potential. This model will be coupled to state-of-the-art molecular dynamics calculations performed at ISM, providing a complete theoretical support for the experiments.
A second important issue lies in the use of a strong laser pulse as a probe, which can significantly perturb the molecule both vibrationally and electronically. To investigate experimentally the extent of these effects, we will implement original optical detection techniques based on spectro- and spatio-temporal mapping to extract the evolution of the harmonic signal within the probe pulse duration. These tools will allow us to disentangle the influence of the probe from that of the pump on a femtosecond timescale.
The strong-field probe pulses can also influence the electron dynamics on a much faster, attosecond timescale, by inducing polarizations, Stark-shifts and even non-adiabatic electron dynamics. Quantifying such effects is extremely challenging, due to both their subtlety and ultrashort timescale. We will implement an original high-sensitivity transient absorption spectroscopy experiment, in which the instantaneous response of a molecule to a strong field will be probed by a single attosecond pulse. By varying the electric field amplitude, we will be able to reveal the limit at which the absorption is altered. This will set the threshold at which HHS should be affected, absorption and emission of an extreme ultra-violet photon being microreversible processes. Scanning the delay between the attosecond pulse and the laser field will reveal possible non-adiabatic dynamics of the electron cloud. This will be the first experiment to tackle the attosecond strong field effects in molecules by absorption spectroscopy.
After having completely characterized and simulated HHS, we will use it to explore unresolved molecular dynamics involving vibronic couplings. Molecules possessing pi-electrons play an essential role in photochemistry and photophysics due to their conductive properties. In particular, conical intersections in such systems offer an efficient route for internal conversion to the electronic ground state. We will investigate polycyclic aromatic hydrocarbons, which represent 10 to 20% of the carbon in the universe and are ubiquitous species in flame and fuel chemistry. We will also focus our attention on the photoreactivity of azides, with the formation of radical cyclic N3. Last, we will study multielectron effects in organosulfur tetrathiafulvalene, an heterocyclic pi compound.
By developing these different aspects, MISFITS will open the way to systematic HHS of ultrafast complex processes in polyatomic molecules, including an in-depth knowledge of the inherent attosecond strong-field effects.
Baptiste Fabre (Centre Lasers Intenses et Applications)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
CELIA Centre Lasers Intenses et Applications
ISM Institut des Sciences Moléculaires
Help of the ANR 486,962 euros
Beginning and duration of the scientific project: September 2014 - 48 Months