Dynamics of Above-Threshold Ionization and Laser-Assisted Electron Scattering inside Helium Nanodroplets.

Laser-assisted electron scattering (LAES) is a fundamental three body interaction process that enables energy transfer between electrons and photons in the presence of matter. Here, we focus on the multiscattering regime of electrons generated by above-threshold ionization (ATI) of In atoms inside a high-density nanostructure, helium nanodroplets (HeN) of ∼40 Šradius. The stochastic nature of the multiscattering regime results in photoelectron spectra independent of laser polarization. Numerical simulations via tunnel-type ionization followed by applying the Kroll-Watson approximation for LAES are in agreement with experimental spectra and yield a mechanistic description of electron generation and the LAES energy modulation processes. We find a negligible influence of the electron start position inside the helium droplet on the simulated electron energy spectrum. Further, our simulations shine light on the interplay of electron time of birth, number of LAES gain/loss events, and final kinetic energy; early ionization leads to the largest number of scattering events and thereby the highest electron kinetic energy.

Photodissociation of [Ar-N2]+ induced by near-IR femtosecond laser fields by ion-trap time-of-flight mass spectrometry.

Photodissociation of [Ar-N2]+ induced by a near-IR (800 nm) femtosecond laser pulse is investigated using ion-trap time-of-flight mass spectrometry. The intra-complex charge transfer proceeding in the course of the decomposition of the electronically excited Ar+(2P3/2)⋯N2(X1Σg +), prepared by the photoexcitation of the electronic ground Ar(1S0)⋯N2 +(X2Σg +), is probed by the ion yields of Ar+ and N2 +. The yield ratio γ of N2 + with respect to the sum of the yields of Ar+ and N2 + is determined to be γ = 0.62, which is much larger than γ ∼ 0.2 determined before when the photodissociation is induced by a nano-second laser pulse in the shorter wavelength region between 270 and 650 nm. This enhancement of γ at 800 nm and the dependence of γ on the excitation wavelength are interpreted by numerical simulations, in which the adiabatic population transfer from Ar+(2P3/2)⋯N2(X1Σg +) to Ar(1S0)⋯N2 +(X2Σg +) at the avoided crossings is accompanied by the vibrational excitation in the N2 +(X2Σg +) moiety followed by the intra-complex vibrational energy transfer from the N2 +(X2Σg +) moiety to the intra-complex vibrational mode leading to the dissociation.

Observation of laser-assisted electron scattering in superfluid helium.

Laser-assisted electron scattering (LAES), a light-matter interaction process that facilitates energy transfer between strong light fields and free electrons, has so far been observed only in gas phase. Here we report on the observation of LAES at condensed phase particle densities, for which we create nano-structured systems consisting of a single atom or molecule surrounded by a superfluid He shell of variable thickness (32-340 Å). We observe that free electrons, generated by femtosecond strong-field ionization of the core particle, can gain several tens of photon energies due to multiple LAES processes within the liquid He shell. Supported by Monte Carlo 3D LAES and elastic scattering simulations, these results provide the first insight into the interplay of LAES energy gain/loss and dissipative electron movement in a liquid. Condensed-phase LAES creates new possibilities for space-time studies of solids and for real-time tracing of free electrons in liquids.

Absolute carrier-envelope-phase dependences of single and double ionization of methanol in a near-IR few-cycle laser field.

We investigate the carrier-envelope phase (CEP) dependences of the single and double ionization processes of methanol (CH3OH) in an intense near-IR few-cycle laser field (2.1 × 1014 W/cm2) by the asymmetry in the ejection direction of CH3 + for the non-hydrogen migration channels and CH2 + for the hydrogen migration channels created through the C-O bond breaking after the ionization. Based on the absolute CEP values at the laser-molecule interaction point, calibrated by the method using intense few-cycle circularly polarized laser pulses [Fukahori et al., Phys. Rev. A 95, 053410-1-053410-14 (2017)], we confirm that methanol cations are produced by tunnel ionization and methanol dications are produced by the recollisional double ionization. We obtain the phase offset for the double ionization accompanying no hydrogen migration to be 1.85π as the absolute CEP at which the extent of the asymmetry becomes maximum. We interpret the phase shift of 0.85π from the phase offset of 1.0π for the tunnel ionization, estimated by a tunnel ionization model incorporating the chemical bond asymmetry, as the corresponding time delay associated with the electron recollisional ionization. The positive phase shift of 0.13π for the single ionization in the non-hydrogen migration channel is interpreted as the additional time (165 as) with which a methanol cation can be excited electronically prior to the decomposition. The additional phase shift of 0.22π for the single ionization in the hydrogen migration channel is interpreted as the additional time (280 as) required for a methanol cation to be excited electronically leading to the hydrogen migration prior to the decomposition.

Decomposition of cyclohexane ion induced by intense femtosecond laser fields by ion-trap time-of-flight mass spectrometry.

Decomposition of cyclohexane cations induced by intense femtosecond laser fields at the wavelength of 800 nm is investigated by ion-trap time-of-flight mass spectrometry in which cyclohexane cations C6H12 (+) stored in an ion trap are irradiated with intense femtosecond laser pulses and the generated fragment ions are recorded by time-of-flight mass spectrometry. The various fragment ion species, C5Hn (+) (n = 7, 9), C4Hn (+) (n = 5-8), C3Hn (+) (n = 3-7), C2Hn (+) (n = 2-6), and CH3 (+), identified in the mass spectra show that decomposition of C6H12 (+) proceeds efficiently by the photo-irradiation. From the laser intensity dependences of the yields of the fragment ion species, the numbers of photons required for producing the respective fragment ions are estimated.

Light-Dressing Effect in Laser-Assisted Elastic Electron Scattering by Xe.

The light-dressing effect in Xe atoms was identified in laser-assisted elastic electron scattering (LAES) signals. In the angular distribution of LAES signals with energy shifts of ±â„ω recorded by the scattering of 1 keV electrons by Xe in an intense nonresonant laser field, a peak profile appeared at small scattering angles (<0.5°). This peak was interpreted as evidence of the light dressing of Xe atoms induced by an intense laser field on the basis of a numerical simulation in which the light-dressing effect is included.

Laser-assisted electron diffraction for femtosecond molecular imaging.

We report the observation of laser-assisted electron diffraction (LAED) through the collision of 1 keV electrons with gas-phase CCl4 molecules in a femtosecond near-infrared laser field. In the angular distribution of the scattered electrons with the energy shifts of ±â„ω, we observed clear diffraction patterns which reflect the geometrical structure of the molecules at the moment of laser irradiation. Our results demonstrate that ultrafast nuclear dynamics of molecules can be probed by LAED with the high temporal (<10 fs) and spatial (∼0.01 Å) resolutions.

Hydrogen scrambling in ethane induced by intense laser fields: statistical analysis of coincidence events.

Two-body Coulomb explosion processes of ethane (CH(3)CH(3)) and its isotopomers (CD(3)CD(3) and CH(3)CD(3)) induced by an intense laser field (800 nm, 1.0 × 10(14) W/cm(2)) with three different pulse durations (40 fs, 80 fs, and 120 fs) are investigated by a coincidence momentum imaging method. On the basis of statistical treatment of the coincidence data, the contributions from false coincidence events are estimated and the relative yields of the decomposition pathways are determined with sufficiently small uncertainties. The branching ratios of the two body decomposition pathways of CH(3)CD(3) from which triatomic hydrogen molecular ions (H(3)(+), H(2)D(+), HD(2)(+), D(3)(+)) are ejected show that protons and deuterons within CH(3)CD(3) are scrambled almost statistically prior to the ejection of a triatomic hydrogen molecular ion. The branching ratios were estimated by statistical Rice-Ramsperger-Kassel-Marcus calculations by assuming a transition state with a hindered-rotation of a diatomic hydrogen moiety. The hydrogen scrambling dynamics followed by the two body decomposition processes are discussed also by using the anisotropies in the ejection directions of the fragment ions and the kinetic energy distribution of the two body decomposition pathways.

Apparatus for laser-assisted electron scattering in femtosecond intense laser fields.

An apparatus for observation of laser-assisted electron scattering (LAES) in femtosecond intense laser fields was developed. The unique apparatus has three essential components, i.e., a photocathode-type ultrashort pulsed-electron gun, a toroidal-type electron energy analyzer enabling simultaneous detection of energy and angular distributions of scattered electrons with high efficiency, and a high repetition-rate data acquisition system combined with a high power 5 kHz Ti:sapphire laser system. These advantages make extremely weak femtosecond-LAES signals distinguishable from the huge elastic scattering signals. A precise method for securing a spatial overlap between three beams, that is, an atomic beam, an electron beam, and a laser beam, and synchronization between the electron and laser pulses is described. As a demonstration of this apparatus, an electron energy spectrum of the LAES signals with 1.4 × 10(12) W/cm(2), 795 nm, 50 fs laser pulses was observed, and the detection limit and further improvements of the apparatus are examined.

Observation of laser-assisted electron-atom scattering in femtosecond intense laser fields.

Laser-assisted electron scattering (LAES) in a femtosecond near-infrared intense laser field was observed through the scattering of 1 keV electrons by xenon atoms. The intensities, angular distributions, and laser polarization dependence of the observed LAES signals for the energy gain (+ℏω) and energy loss (-ℏω) processes were interpreted well by numerical simulations. As an application of this femtosecond-LAES process, a new method of time-resolved gas electron diffraction for probing geometrical change of molecules with high precision is proposed.

Pendular-state spectroscopy of the S(1)-S(0) electronic transition of 9-cyanoanthracene.

Fluorescence excitation spectra of the S(1)-S(0) origin band of 9-cyanoanthracene have been observed under a uniform electric field up to 200 kV/cm to explore pendular-state spectrum of an asymmetric-top molecule close to the strong field limit. The observed spectra exhibit distinct evolution of the band contour as a function of the applied electric field, which are much different from each other for different excitation configurations. An approximate method suitable for spectrum simulations of large asymmetric-top molecules in a pendular condition is developed for the analysis of the experimental results. The comparison of the observed and simulated spectra shows that the spectra are well ascribed in terms of the pendular-state selection rules, which have recently been derived from theoretical consideration of the pendular-limit representation of energy levels and spectra.