Spectrally resolved multi-channel contributions to the harmonic emission in N₂

Abstract

When generated in molecules, high-order harmonics can be emitted through different ionization channels. The coherent and ultrafast electron dynamics occurring in the ion during the generation process is directly imprinted in the harmonic signal, i.e. in its amplitude and spectral phase. In aligned N 2 molecules, we find evidence for a fast variation of this phase as a function of the harmonic order when varying the driving laser intensity. Basing our analysis on a three-step model, we find that this phase variation is a signature of transitions from a single-to a multi-channel regime. In particular, we show that significant nuclear dynamics may occur in the ionization channels on the attosecond timescale, affecting both the amplitude and the phase of the harmonic signal.

Precise Access to the Molecular-Frame Complex Recombination Dipole through High-Harmonic Spectroscopy

“…This cutoff extension is consistent with the onset of significant contributions from the HOMO-1, which has a π orbital symmetry, giving it the highest probability perpendicular to the molecular axis for both recombination and strong-field ionization, and whose ionization potential energy (I p = 17.2 eV) is higher than that of the HOMO (I p = 15.58 eV), giving it a slightly higher effective cutoff [22,24]. This extension and reversal in the cutoff, attributed to the HOMO-1, has previously been observed in N 2 at 0.8 µm, albeit at a much lower energy [30,31].…”

“…When the harmonic process is driven in the tunneling ionization regime (the Keldysh parameter [17] is γ = 0.62 for our experimental conditions) without ground-state depletion, the associated recombination dipole is equivalent to the time-reversed photoionization dipole [18][19][20]. HHS has been previously applied to N 2 [21,22] in the context of molecular tomography [21,23], although driven with Ti:Sapphire lasers (0.8 µm wavelength). In such conditions, HHS is plagued by the intricate contributions of the two highest-occupied molecular orbitals (noted HOMO and HOMO-1 respectively) [24], preventing access to precise molecular dipoles over an extended spectral range.…”

“…mainly in the cutoff region [21][22][23]30]. To this end, we compare the harmonic spectral intensity measured for ten alignment angles between 0…”

Full Characterization of a Molecular Cooper Minimum Using High-Harmonic Spectroscopy

“…The applicability of the SFA emission time to HHG in polyatomic molecules is nontrivial; along with distortion of the GD from the CM, interference between multiple ionization centers [30] or multiple orbitals can affect the GD [31,32]. Furthermore, an implicit assumption in the SFA calculation is that the electron, following tunneling ionization, is unaffected by the ionic potential.…”

Full-dimensional treatment of short-time vibronic dynamics in a molecular high-order-harmonic-generation process in methane

“…High-order-harmonic spectroscopy (HHS) [7], based on the celebrated high-order-harmonic-generation (HHG) process [8,9], is one of the most powerful and versatile tools in the arsenal of attosecond science. Among other examples [10,11], it has been successfully employed to image the "molecular orbitals" [3,[12][13][14] and resolve multiple final states in strong-field ionization [15,16]; follow evolution of bound electronic [17][18][19][20][21][22][23][24][25], rotational [26][27][28], and nuclear [16,29] wave packets; monitor electron correlation in atoms [30]; measure molecular chirality [31,32]; resolve the time tunneling electron emerges from underneath the barrier [33]; probe strongly driven electrons in the continuum [18,[34][35][36]; and control attosecond emission from molecules [37,38].…”