High-harmonic transient grating spectroscopy of NO2 electronic relaxation

Toward Complete All-Optical Intensity Modulation of High-Harmonic Generation from Solids

Optical modulation of high-harmonics generation in solids enables the detection of material properties, such as the band structure, and promising new applications, such as super-resolution imaging in semiconductors. Various recent studies have shown optical modulation of high-harmonics generation in solids, in particular, suppression of high-harmonics generation has been observed by synchronized or delayed multipulse sequences. Here we provide an overview of the underlying mechanisms attributed to this suppression and provide a perspective on the challenges and opportunities regarding these mechanisms. All-optical control of high-harmonic generation allows for femtosecond, and in the future possibly subfemtosecond, switching, which has numerous possible applications: These range from super-resolution microscopy to nanoscale controlled chemistry and highly tunable nonlinear light sources.

Ultrafast time-resolved polarization-dependent investigations on the dynamics in the Ã2B2 state of NO2 molecules

We perform time-resolved polarization-dependent study on ultrafast dynamics in the à 2 B 2  state of NO 2 . A linearly-polarized 400 nm femtosecond laser is used to resonantly pump NO 2 to its first excited state à 2 B 2 , and the time-dependent ionic yields produced via strong field ionization at 800 nm are measured under different laser polarizations. The yield ratios measured with the lasers perpendicular and parallel to each other first decrease then increase as the wave packet evolves on the excited state, with a minimum ratio at the delay time of 180 fs, which can be attributed to the evolution time in the à 2 B 2 state. The behavior of the time-resolved ionization in elliptically polarized laser field is also investigated and discussed. Our study indicates that the time-resolved polarization-dependent studies will shed some light on and pave the way to understand ultrafast dynamics in molecular excited states.

Coupled nuclear and electron dynamics in the vicinity of a conical intersection

Ultrafast optical techniques allow us to study ultrafast molecular dynamics involving both nuclear and electronic motion. To support interpretation, theoretical approaches are needed that can describe both the nuclear and electron dynamics. Hence, we revisit and expand our ansatz for the coupled description of the nuclear and electron dynamics in molecular systems (NEMol). In this purely quantum mechanical ansatz, the quantum-dynamical description of the nuclear motion is combined with the calculation of the electron dynamics in the eigenfunction basis. The NEMol ansatz is applied to simulate the coupled dynamics of the molecule NO₂ in the vicinity of a conical intersection (CoIn) with a special focus on the coherent electron dynamics induced by the non-adiabatic coupling. Furthermore, we aim to control the dynamics of the system when passing the CoIn. The control scheme relies on the carrier envelope phase of a few-cycle IR pulse. The laser pulse influences both the movement of the nuclei and the electrons during the population transfer through the CoIn.

Aurore: A platform for ultrafast sciences

We present the Aurore platform for ultrafast sciences. This platform is based on a unique 20 W, 1 kHz, 26 fs Ti:sapphire laser system designed for reliable operation and high intensity temporal contrast. The specific design ensures the high stability in terms of pulse duration, energy, and beam pointing necessary for extended experimental campaigns. The laser supplies 5 different beamlines, all dedicated to a specific field: attosecond science (Aurore 1), ultrafast phase transitions in solids (Aurore 2 and 3), ultrafast luminescence in solids (Aurore 4), and femtochemistry (Aurore 5). The technical specifications of these five beamlines are described in detail, and examples of the recent results are given.

Ultrafast imaging of laser-controlled non-adiabatic dynamics in NO2 from time-resolved photoelectron emission

Imaging and controlling the ultrafast conical intersection dynamics in NO₂ using the latest advances in attosecond and light-synthesizer technology.

Ab initio calculation of femtosecond-time-resolved photoelectron spectra of NO2 after excitation to the A-band

We present calculations of time-dependent photoelectron spectra of NO₂ after excitation to the A-band for comparison with extreme-ultraviolet (XUV) time-resolved photoelectron spectroscopy. We employ newly calculated potential energy surfaces of the two lowest-lying coupled ²A' states obtained from multi-reference configuration-interaction calculations to propagate the photo-excited wave packet using a split-step-operator method. The propagation includes the nonadiabatic coupling of the potential surfaces as well as the explicit interaction with the pump pulse centered at 3.1 eV (400 nm). A semiclassical approach to calculate the time-dependent photoelectron spectrum arising from the ionization to the eight energetically lowest-lying states of the cation allows us to reproduce the static experimental spectrum up to a binding energy of 16 eV and enables direct comparisons with XUV time-resolved photoelectron spectroscopy.

Excited state wavepacket dynamics in NO2 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 strong-field 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 pump-probe 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 X̃ 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 higher-lying 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.

Role of electronic correlations in photoionization of NO2 in the vicinity of the 2A1/2B2 conical intersection

We present the first ab initio multi-channel photoionization calculations for NO₂ in the vicinity of the ²A₁/²B₂ conical intersection, for a range of nuclear geometries, using our newly developed set of tools based on the ab initio multichannel R-matrix method.

The role of Rydberg states in photoionization of NO2 and (NO+, O-) ion pair formation induced by one VUV photon

We report a combined experimental and theoretical study of photoionization (PI) of the NO2 molecule into the NO2(+) (X (1)Σg(+)) ground state and the photodissociation of NO2 into the NO(+)((1)Σ(+)) + O(-)((2)P) ion pair. These processes were induced by 10.9 eV-13 eV synchrotron radiation and the products were detected using electron-ion or O(-)-NO(+) coincident momentum spectroscopy. The results demonstrate the strong influence of [R(∗)(4b2)(-1), nlα(i), v2(')] Rydberg states vibrationally resolved in the v2(') bending modes for both processes. In particular, we emphasize two regions around 11.5 eV and 12.5 eV that were studied in more detail for their relevance to 400 nm multiphoton ionization induced by femtosecond pulses. The photoelectron energy spectra and asymmetry parameters support the existence of two PI mechanisms, as probed with the help of fixed-nuclei frozen-core Hartree-Fock calculations. We found significant deviations from Franck-Condon ionization predictions which may be assigned to vibronic coupling of NO2(∗) states such as that induced by a conical intersection. The limited agreement between theory and experiment, even for the non-resonant processes, indicates the need for calculations that go beyond the approximations used in the current study. Ion pair formation leads to strong vibrational and rotational excitation of the NO(+)((1)Σ(+),v) product, with an ion fragment angular anisotropy depending on both the v2(') bending quantum number of the excited parent molecule and the v vibrational level of the fragment.