Extracting sub-cycle electronic and nuclear dynamics from high harmonic spectra

Abstract

We present a new methodology for measuring few-femtosecond electronic and nuclear dynamics in both atoms and polyatomic molecules using multidimensional high harmonic generation (HHG) spectroscopy measurements, in which the spectra are recorded as a function of the laser intensity to form a two-dimensional data set. The method is applied to xenon atoms and to benzene molecules, the latter exhibiting significant fast nuclear dynamics following ionization. We uncover the signature of the sub-cycle evolution of the returning electron flux in strong-field ionized xenon atoms, implicit in the strong field approximation but not previously observed directly. We furthermore extract the nuclear autocorrelation function in strong field ionized benzene cations, which is determined to have a decay of $$\tau _0 = 4 \pm 1$$ τ 0 = 4 ± 1 fs, in good agreement with the $$ \tau _0 = 3.5$$ τ 0 = 3.5 fs obtained from direct dynamics variational multi-configuration Gaussian calculations. Our method requires minimal assumptions about the system, and is applicable even to un-aligned polyatomic molecules.

Backward Flux Re-Deposition Patterns during Multi-Spot Laser Ablation of Stainless Steel with Picosecond and Femtosecond Pulses in Air

“…From the time-resolved plasma emission (10 ps), backward re-deposition with single spot ablation likely starts about~100 ns after ablation and from the pump-probe observations, (600 fs) continues over 10 µs and longer. The debris power laws yield R deb ∝ E 0.3-0.5 for both single spot and jets but the much larger jet radii if applied to R D~f R yields f~0.7-0.8 using Equation (2) inferring that an additional energetic process is involved during the plasma plume interactions.…”

“…The interaction of intense ultrafast laser radiation with atoms, gases and solid materials has led to remarkable scientific advances such as High Harmonic Generation for attosecond spectroscopy [1,2], the use of filamentation for atmospheric probing [3,4], two-photon microscopy [5,6] and intraocular fs-LASIK in eye surgery [7]. In particular, laser ablation (LA) of materials with ps and sub-ps temporal pulse length has advanced materials analysis in areas such as Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) [8][9][10] and Laser-Induced Breakdown Spectroscopy (LIBS) [11,12].…”

Quantum coherence in molecular photoionization

“…Thus, the nuclear autocorrelation function C aa (t) (eqn (24c)) modulates the high-harmonics yield in molecules, 67 as beautifully demonstrated in PACER (''Probing attosecond dynamics by chirp-encoded recollisions'') experiments. 68,69 It has been recently shown that the shorttime autocorrelation functions can be extracted directly from the intensity-dependent HHG spectra for medium-sized molecules, 70 without resorting to elaborate theory modeling. These femtosecond nuclear dynamics in turn give access to the local potential-energy surface parameters of the transiently-created cations, 67,71 despite their fleeting, sub laser-cycle existence.…”

Femtosecond intramolecular rearrangement of the CH3NCS radical cation

“…While first demonstrated in smaller systems, decades of studies on strong-field rescattering and highharmonic generation (HHG) have shown that these phenomena is commonplace during strongfield processes involving polyatomic molecules. [7][8][9][10][11][12][13][14][15] Femtosecond pulses of 800 nm central wavelength and 1×10 14 W cm -2 peak intensity can tunnel ionize large polyatomic molecules with ionization potentials (IPs) ranging from 8 to 10 eV, creating high-energy electrons that can deposit much of their energy back into the molecule upon rescattering. The maximum kinetic energy of the rescattering electrons is proportional to the laser intensity, approximately 3.2 Up, 16 where Up is the ponderomotive energy.…”