Publicaties – Dr. Arno Vredenborg

Abstract:

We compare different tilted‑pulse‑front pumping schemes for single‑cycle THz generation in LiNbO₃ crystals both theoretically and experimentally in terms of conversion efficiency. The conventional setup with a single lens suffers from strong chromatic aberrations for sub‑50 fs pump pulses. A new arrangement using two concave mirrors in a Keplerian telescope largely compensates these aberrations and yields a six‑fold increase of the THz energy for a 35‑fs pump. The measured THz field strength is 60 kV cm^‑1 for 0.5 µJ pump pulses. Detailed ray‑tracing simulations and experimental data are presented.

Abstract:

In 1997 it was predicted that an electronically excited atom or molecule placed in a loosely bound chemical system (such as a hydrogen‑bonded or van‑der‑Waals‑bonded cluster) could efficiently decay by transferring its excess energy to a neighbouring species that would then emit a low‑energy electron. This intermolecular Coulombic decay (ICD) process has since been shown to be a common phenomenon. Here we show experimentally that resonant Auger decay can indeed trigger ICD in dimers of both molecular nitrogen and carbon monoxide. By using ion and electron momentum spectroscopy to measure simultaneously the charged species created in the resonant‑Auger‑driven ICD cascade, we find that ICD occurs in less time than the 20 fs it would take for individual molecules to undergo dissociation. Our experimental confirmation of this process and its efficiency may trigger renewed efforts to develop resonant X‑ray excitation schemes for more localized and targeted cancer radiation therapy.

Abstract:

We review the capabilities of a reaction‑microscope (COLTRIMS) instrument equipped with velocity‑map imaging and ultrafast pulse shaping. The device records the full three‑dimensional momentum vectors of electrons and ions in coincidence, enabling reconstruction of molecular‑frame photoelectron angular distributions (MFPADs) and recoil‑frame PADs (RFPADs). Recent applications include non‑adiabatic alignment of van‑der‑Waals dimers, control of multichannel fragmentation in CF₃I, and photoelectron circular dichroism (PECD) for chiral molecules. The combination of coincidence imaging with adaptive pulse shaping opens new routes to steer and interrogate ultrafast photochemical dynamics.

Abstract:

We demonstrate that the weakly bound Ar₂ dimer can be non‑adiabatically aligned with a short 35‑fs, 1400‑nm pump pulse. Field‑free revivals of ⟨cos²φ⟩ are observed up to ≈1 ns, from which a rotational constant B₀ = 0.05756 ± 0.00004 cm⁻¹ is extracted. Alignment‑dependent measurements of frustrated tunnelling ionisation and bond‑softening‑induced dissociation reveal that both processes are strongly enhanced when the dimer axis is parallel to the laser polarisation.

Abstract:

We have studied the two‑site double ionization of argon dimers by ultrashort laser pulses leading to fragmentation into two singly charged fragments. Contrary to the expectations from a pure Coulomb explosion following rapid removal of one electron from each atom, we find three distinct peaks in the kinetic‑energy‑release distribution. The most abundant channel results from tunnel ionization at one site followed by charge‑enhanced tunnel ionization of the second atom. The second mechanism, which leads to a higher KER we identify as sequential tunnel ionization of both atoms accompanied by excitation. The third mechanism, present only with linear polarization, is most likely a frustrated triple ionization, where the third electron does not escape but is trapped in a Rydberg state.

Abstract:

We employ Coulomb explosion imaging to directly map the ground‑state wave function of the dimer, trimer, and tetramer of argon and neon. The technique was pioneered in accelerator‑based experiments by Varger and collaborators. The laser‑based Coulomb explosion requires a short intense laser pulse so that the pulse duration is short enough to avoid nuclear motion during the pulse. From the measured momentum vectors of all fragments we retrieve the internuclear distances and bond angles. For the argon trimer the dominant configuration is an equilateral triangle with an average edge length of 3.8 Å; for neon the trimer is considerably more floppy. The tetramer adopts a triangular‑pyramid structure. The results agree well with high‑level ab‑initio calculations and demonstrate that Coulomb explosion imaging can be used to obtain structural information on weakly bound van‑der‑Waals clusters.

Abstract:

We observe multiply frustrated tunnelling ionisation‑induced dissociation of Ar₂ by intense linearly polarised femtosecond laser pulses. By measuring the kinetic‑energy release (KER) and angular distribution of the Coulomb explosion of up to eight‑fold ionised argon dimers we trace the recapture of up to two electrons to Rydberg states of the highly charged compound at the end of the laser pulse. The Rydberg electron preferentially localises at the atom with the higher charge state. A weak time‑delayed probe pulse confirms the recapture dynamics on a picosecond time scale.

Abstract:

We have measured the two‑site double ionisation of Ar₂ by ultrashort laser pulses leading to fragmentation into two singly charged fragments. Three distinct peaks in the kinetic‑energy‑release (KER) distribution are observed. The most abundant channel results from tunnel ionisation at one site followed by charge‑enhanced tunnel ionisation of the second atom. A second mechanism, identified as sequential tunnel ionisation of both atoms accompanied by excitation, yields a higher KER. The third mechanism, present only for linear polarisation, is most likely a frustrated triple ionisation in which the third electron is trapped in a Rydberg state.

Abstract:

We report on photoelectron–photoion coincidence imaging after single‑colour (400 nm) multiphoton excitation of CF₃I at laser intensities of ≈10¹² W cm⁻². The dominant ionic species are the parent CF₃I⁺ and the CF₃⁺ fragment (four‑photon excitation). The electron kinetic‑energy spectrum shows two main peaks (1.94 eV and 1.30 eV) corresponding to ionisation into the spin‑orbit split ground and excited states of CF₃I⁺. Additional weaker peaks arise from five‑photon processes. Analysis of the electron–ion energy correlation reveals that the excess energy is partitioned between the emitted electron and the kinetic energy of the fragments, allowing us to assign the underlying ionisation and dissociation pathways.

Abstract:

We report on femtosecond molecular‑dynamics experiments in NO₂ using a novel photoelectron‑photoion coincidence imaging apparatus. By varying the pump‑probe delay we obtain full three‑dimensional energy‑ and angle‑resolved information on the photofragments. The dominant product in the single‑color 400 nm multiphoton excitation of NO₂ is NO⁺, which accounts for 95 % of the events. The coincidence electron and ion images reveal a five‑photon excitation (≈15.5 eV) that produces slow electrons and ions, and a series of well‑defined electron‑energy bands (≈0.1–2 eV) that correlate with distinct dissociation pathways.

Abstract:

A new time‑resolved photoelectron‑photoion coincidence imaging machine was built in Amsterdam. Using a two‑color pump‑probe scheme (400 nm + 266 nm) we assign several multiphoton pathways that lead to NO⁺ + O + e. The dominant processes are a 1 + 2 photon (12.39 eV) excitation that yields very slow electrons and NO⁺ fragments, and a 3 + 1 photon (13.93 eV) excitation that populates fast‑dissociating Rydberg states (b³A₂ and a³B₂) and produces electron‑energy bands at ≈0.3, 0.65, 1.4 eV. Transient peaks (≈1.2 eV) are attributed to a 2 + 2 photon (15.48 eV) pathway. The recoil‑frame photoelectron angular distributions (RFPADs) show marked anisotropies that differ for the two electronic states.

Abstract:

We report on the construction and performance of a novel photoelectron‑photoion coincidence imaging apparatus for femtosecond time‑resolved molecular‑beam experiments. The instrument employs open electron and ion lenses that can be pulsed to provide optimal extraction fields for velocity‑map imaging of both species. Small‑pore (5 µm) microchannel plates and fast timing electronics yield an electron time‑of‑flight (TOF) resolution of τ = 18 ps (≈0.12 % of a typical 15 ns TOF) and an energy resolution of ΔE/E ≈ 3.5 % for electrons near 2 eV. The ion detector achieves a mass resolution of m/Δm ≈ 4150. Representative coincidence images of Xe, NO and NO₂ demonstrate the capability of the setup for studying ultrafast photodynamics.

Abstract:

More than 250 rotationally resolved vibrational bands of the A²B₂–X²A₁ electronic system of ¹⁵NO₂ have been recorded in the 14 300–18 000 cm⁻¹ region using a high‑resolution jet‑cooled fluorescence setup. The majority of the bands are assigned and rotational constants extracted. An exceptionally strong band at 14 851 cm⁻¹ is highlighted as a benchmark transition for atmospheric remote‑sensing. Statistical analysis of the polyad structure shows that the internal dynamics of ¹⁵NO₂ differ markedly from its ¹⁴NO₂ counterpart because of vibronic mixing.

Abstract:

A spectroscopic setup designed for high‑resolution spectroscopy of jet‑cooled molecules is described. The system combines a piezo‑valve jet source with time‑gated fluorescence detection, yielding an extremely low gas consumption (≈0.025 mg per scan) and high detection efficiency. The setup is demonstrated on the A²B²–X²A¹ transition of ¹⁵N¹⁶O₂. More than 300 rovibronic bands are recorded; a particularly strong band at 14 850 cm⁻¹ is identified as a useful benchmark for atmospheric remote‑sensing of the isotopologue.