Laser Ablation Molecular Isotopic Spectrometry for Molecules Formation Chemistry in Femtosecond-Laser Ablated Plasmas

FISI: frequency domain integration sequential imaging at 1.26×1013 frames per second and 108 lines per millimeter

High spatial resolution on the image plane (intrinsic spatial resolution) has always been a problem for ultrafast imaging. Single-shot ultrafast imaging methods can achieve high spatial resolution on the object plane through amplification systems but with low intrinsic spatial resolutions. We present frequency domain integration sequential imaging (FISI), which encodes a transient dynamic by an inversed 4f (IFF) system and decodes it using optical spatial frequencies recognition (OFR), which overcomes the limitation of the spatial frequencies recognition algorithm. In an experiment on the process of an air plasma channel, FISI achieved shadow imaging of the channel with a framing rate of 1.26×10¹³ fps and an intrinsic spatial resolution of 108 lp/mm (the spatial resolution on the image plane). Owing to its excellent framing time and high intrinsic spatial resolution, FISI can probe both repeatable and unrepeatable ultrafast phenomena, such as laser-induced damage, plasma physics, and shockwave interactions in living cells with high quality.

Improved plasma stoichiometry recorded by laser ablation ionization mass spectrometry using a double-pulse femtosecond laser ablation ion source

RATIONALE

Femtosecond (fs) laser ablation ion sources have allowed for improved measurement capabilities and figures of merit of laser ablation based spectroscopic and mass spectrometric measurement techniques. However, in comparison to longer pulse laser systems, the ablation plume from fs lasers is observed to be colder, which favors the formation of polyatomic species. Such species can limit the analytical capabilities of a system due to isobaric interferences. In this contribution, a double-pulse femtosecond (DP-fs) laser ablation ion source is coupled to our miniature Laser Ablation Ionization Mass Spectrometry (LIMS) system and its impact on the recorded stoichiometry of the generated plasma is analyzed in detail.

METHODS

A DP-fs laser ablation ion source (temporal delays of +300 to - 300 ps between pulses) is connected to our miniature LIMS system. The first pulse is used for material removal from the sample surface and the second for post-ionization of the ablation plume. To characterize the performance, parametric double- and single-pulse studies (temporal delays, variation of the pulse energy, voltage applied on detector system) were conducted on three different NIST SRM alloy samples (SRM 661, 664 and 665).

RESULTS

At optimal instrument settings for both the double-pulse laser ablation ion source and the detector voltage, relative sensitivity coefficients were observed to be closer (factor of ~2) to 1 compared with single-pulse measurements. Furthermore, the optimized settings worked for all three samples, meaning no further optimization was necessary when changing to another alloy sample material during this study.

CONCLUSIONS

The application of a double-pulse femtosecond laser ablation ion source resulted in the recording of improved stoichiometry of the generated plasma using our LIMS measurement technique. This is of great importance for the quantitative chemical analysis of more complex solid materials, e.g., geological samples or metal alloys, especially when aiming for standard-free quantification procedures for the determination of the chemical composition.

Unraveling Spatio-Temporal Chemistry Evolution in Laser Ablation Plumes and Its Relation to Initial Plasma Conditions

The chemistry evolution in a laser ablation plume depends strongly on its initial physical conditions. In this article, we investigate the impact of plasma generation conditions on the interrelated phenomena of expansion dynamics, plasma chemistry, and physical conditions. Plasmas are produced from a uranium metal target in air using nanosecond, femtosecond, and femtosecond filament-assisted laser ablation. Time-resolved two-dimensional spectral imaging was performed to evaluate the spatio-temporal evolution of atoms, diatoms, polyatomic molecules, and nanoparticles in situ. Emission spectral features reveal that molecular formation occurs at early times in both femtosecond and filament ablation plumes, although with different temporal decays. In contrast, molecular formation is found to occur at much later times in nanosecond plasma evolution. Spectral modeling is used to infer temporal behavior of plasma excitation temperature. We find U atoms and UO molecules co-exist in ultrafast laser-produced plasmas even at early times after plasma onset owing to favorable temperatures for molecular formation. Regardless of irradiation conditions, plume emission features showed the presence of higher oxides (i.e., U_xO_y), although with different temporal histories. Our study provides insight into the impact of plasma generation conditions on chemistry evolution in plasmas produced from traditional focused femtosecond, nanosecond, and filament-assisted laser ablation.

Multiscale Visualization of Colloidal Particle Lens Array Mediated Plasma Dynamics for Dielectric Nanoparticle Enhanced Femtosecond Laser-Induced Breakdown Spectroscopy

A multiscale visualization of silica colloidal particle lens array (CPLA) assisted laser ablation of copper is investigated. The distributed holes on a crater of CPLA-deposited Cu (CPLA-Cu) show a near-field effect by the silica nanoparticles (NPs), and the plasma emission signal of CPLA-Cu is 3-5 times as strong as that of Cu. Time-resolved plasma expansion, shockwave propagation, plasma plume emission, and nanoparticle distribution are observed and analyzed for ablations on both Cu and CPLA-Cu substrates. The initial expansion of plasma generated on CPLA-Cu is faster than that of pristine Cu. The shockwave of CPLA-Cu is rounder and its plasma plume is wider than those of Cu. The nanoparticle distribution shows a strong lateral collision during plume ejection for CPLA-Cu. Plasma characterization shows the increased plasma temperature is the key reason for femtosecond laser-induced breakdown spectroscopy (fs-LIBS) signal enhancement. This work demonstrates the signal enhancement effect of dielectric NPs on fs-LIBS and provides insights into hydrodynamics of the fs laser-induced plasma generated on CPLA-deposited substrate.

Collision-mediated ultrafast decay of N2 fluorescence during fs-laser-induced filamentation

We investigate experimentally spatiotemporal characteristics of fluorescence emission from fs-laser-induced filaments in air. Emissions accompanying the transitions of N₂ (C³Π_u-B³Π_g) and N 2+ (B²Σu+-X²Σg+) are dominant. The decay dynamics of fluorescence from different radial positions and longitudinal sections of a filament column are obtained along with high resolution spectra. A decay curve contains two exponential components: a fast one (with a decay time constant ∼10s ps), and a slow one (∼sub-ns). The lifetime of the N ₂ fluorescence is about three orders shorter than its spontaneous emission lifetime, indicating that most of the N ₂ molecules in the excited state (C³Π_u) are de-excited through collision. Different de-excitation mechanisms of N ₂ (C³Π_u) molecules contributing to fluorescence decay constants, e.g., the e ^--N₂, N ₂-N₂, and O ₂-N₂ collisions, are elucidated. We analyze the variations of decay constants together with corresponding fluorescence intensities, and obtain temperature distributions by fitting band spectra of N ₂ molecules and N 2+ ions with a synthetic spectral model. Our results suggest that the fast and slow decay processes originate from the e ^--N₂ and O ₂-N₂ collisions, respectively.

Compressed Ultrafast Spectral-Temporal Photography

Acquiring ultrafast and high spectral resolution optical images is key to measure transient physical or chemical processes, such as photon propagation, plasma dynamics, and femtosecond chemical reactions. At a trillion Hz frame rate, most ultrafast imaging modalities can acquire only a limited number of frames. Here, we present a compressed ultrafast spectral-temporal (CUST) photographic technique, enabling both an ultrahigh frame rate of 3.85 trillion Hz and a large frame number. We demonstrate that CUST photography records 60 frames, enabling precisely recording light propagation, reflection, and self-focusing in nonlinear media over 30 ps. CUST photography has the potential to further increase the frame number beyond hundreds of frames. Using spectral-temporal coupling, CUST photography can record multiple frames with a subnanometer spectral resolution with a single laser exposure, enabling ultrafast spectral imaging. CUST photography with high frame rate, high spectral resolution, and high frame number in a single modality offer a new tool for observing many transient phenomena with high temporal complexity and high spectral precision.

Time-resolved resonance fluorescence spectroscopy for study of chemical reactions in laser-induced plasmas

Identification of chemical intermediates and study of chemical reaction pathways and mechanisms in laser-induced plasmas are important for laser-ablated applications. Laser-induced breakdown spectroscopy (LIBS), as a promising spectroscopic technique, is efficient for elemental analyses but can only provide limited information about chemical products in laser-induced plasmas. In this work, time-resolved resonance fluorescence spectroscopy was studied as a promising tool for the study of chemical reactions in laser-induced plasmas. Resonance fluorescence excitation of diatomic aluminum monoxide (AlO) and triatomic dialuminum monoxide (Al₂O) was used to identify these chemical intermediates. Time-resolved fluorescence spectra of AlO and Al₂O were used to observe the temporal evolution in laser-induced Al plasmas and to study their formation in the Al-O₂ chemistry in air.