Ultrafast Dissociation Dynamics of the Sensitive Explosive Ethylene Glycol Dinitrate.

Ethylene glycol dinitrate (EGDN) is a nitrate ester explosive widely used in military ordnance and missile systems. This study investigates the decomposition dynamics of the EGDN cation using a comprehensive approach that combines femtosecond time-resolved mass spectrometry (FTRMS) experiments with electronic structure and molecular dynamics computations. We identify three distinct dissociation time scales for the metastable EGDN cation of approximately 40-60 fs, 340-450 fs, and >2 ps.

Understanding Selectivity in Product Distributions from Laser Ablation of Organic Liquids.

Pulsed laser ablation in organic solvents is widely used to produce oxide-free metal and metal carbide nanoparticles, often with carbon coatings resulting from laser-induced reactions in the organic solvent. To gain insight into how the molecular structure of the solvent affects these reaction pathways, this work investigates ablation of the CH isomers -hexane, 2-methylpentane, and 3-methylpentane through characterization of the gas and liquid products with mass spectrometry. Ablation of each CH isomer produces a distinct distribution of product molecular weights and isomers.

Photodissociation Dynamics of the Highly Stable -Nitroaniline Cation.

-Nitroaniline (ONA) is a model for the insensitive high explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) that shares strong hydrogen bonding character between adjacent nitro and amino groups. This work reports femtosecond time-resolved mass spectrometry (FTRMS) measurements and theoretical calculations that explain the high stability of the ONA cation compared with related nitroaromatic molecules. Ab initio calculations found that the lowest-lying electronic excited state of the ONA cation, D, lies more than 2 eV above the ground state, and the energetic barriers to rearrangement and dissociation reactions exceed this D energy.

Enhancement of Metal Nanostructure Deposition on Silicon Laser-Induced Periodic Surface Structures by Galvanic Replacement.

We report a chemically motivated, single-step method to enhance metal deposition onto silicon laser-induced periodic surface structures (LIPSSs) using reactive laser ablation in liquid (RLAL). Galvanic replacement (GR) reactions were used in conjunction with RLAL (GR-RLAL) to promote the deposition of Au and Cu nanostructures onto a Si LIPSS. To increase the deposition of Au, sacrificial metals Cu, Fe, and Zn were used; Fe and Zn also enhanced the deposition of Cu.

Experimental and Theoretical Analysis of Tricyclic Antidepressants by Ultraviolet Picosecond Laser Desorption Post-Ionization Mass Spectrometry.

Imipramine class tricyclic antidepressants have low ionization efficiencies that make them difficult to detect by using secondary ion mass spectrometry. Ultraviolet picosecond laser desorption postionization (ps-LDPI-MS) is examined here for the detection of four tricyclic antidepressants: imipramine, desipramine, amitriptyline, and clomipramine. About 30 ps laser pulses at either 213 nm (5.

How Pulse Width Affects Laser Ablation of Organic Liquids.

Laser synthesis in liquids is often carried out in organic solvents to prevent oxidation of metals during nanoparticle generation and to produce tailored carbon-based nanomaterials. This work investigates laser ablation of neat organic liquids acetone, ethanol, -hexane, and toluene with pulse widths ranging from 30 fs to 4 ps through measurements of reaction kinetics and characterization of the ablation products with optical spectroscopy and mass spectrometry. Increasing the pulse width from 30 fs to 4 ps impacts both the reaction kinetics and product distributions, suppressing the formation of solvent molecule dimers and oxidized molecules while enhancing the yields of gaseous molecules, -hybridized carbons, and fluorescent carbon dots.

Understanding photochemical pathways of laser-induced metal ion reduction through byproduct analysis.

Laser-induced reduction of metal ions is attracting increasing attention as a sustainable route to ligand-free metal nanoparticles. In this work, we investigate the photochemical reactions involved in reduction of Ag and [AuCl] upon interaction with lasers with nanosecond and femtosecond pulse duration, using strong-field ionization mass spectrometry and spectroscopic assays to identify stable molecular byproducts. Whereas Ag in aqueous isopropyl alcohol (IPA) is reduced through plasma-mediated mechanisms upon femtosecond laser excitation, low-fluence nanosecond laser excitation induces electron transfer from IPA to Ag.

Coulomb Explosion Dynamics of Multiply Charged -Nitrotoluene Cations.

This work explores Coulomb explosion (CE) dissociation pathways in multiply charged cations of -nitrotoluene (PNT), a model compound for nitroaromatic energetic molecules. Experiments using strong-field ionization and mass spectrometry indicate that metastable cations PNT and PNT undergo CE to produce NO and NO. The experimentally measured kinetic energy release from CE upon formation of NO and NO agrees qualitatively with the kinetic energy release predicted by computations of the reaction pathways in PNT and PNT using density functional theory (DFT).

Coherent Control of Molecular Dissociation by Selective Excitation of Nuclear Wave Packets.

We report on pump-probe control schemes to manipulate fragmentation product yields in -nitrotoluene (PNT) cation. Strong field ionization of PNT prepares the parent cation in the ground electronic state, with coherent vibrational excitation along two normal modes: the C-C-N-O torsional mode at 80 cm and the in-plane ring-stretching mode at 650 cm. Both vibrational wave packets are observed as oscillations in parent and fragment ion yields in the mass spectrum upon optical excitation.

Conformer-Specific Dissociation Dynamics in Dimethyl Methylphosphonate Radical Cation.

The dynamics of the dimethyl methylphosphonate (DMMP) radical cation after production by strong field adiabatic ionization have been investigated. Pump-probe experiments using strong field 1300 nm pulses to adiabatically ionize DMMP and a 800 nm non-ionizing probe induce coherent oscillations of the parent ion yield with a period of about 45 fs. The yields of two fragments, POCH and POCH, oscillate approximately out of phase with the parent ion, but with a slight phase shift relative to each other.

Ultrafast Dynamics of Nitro-Nitrite Rearrangement and Dissociation in Nitromethane Cation.

We report new insights into the ultrafast rearrangement and dissociation dynamics of nitromethane cation (NM) using pump-probe measurements, electronic structure calculations, and ab initio molecular dynamics simulations. The "roaming" nitro-nitrite rearrangement (NNR) pathway involving large-amplitude atomic motion, which has been previously described for neutral nitromethane, is demonstrated for NM. Excess energy resulting from initial population of the electronically excited D state of NM upon strong-field ionization provides the necessary energy to initiate NNR and subsequent dissociation into NO.

Quantitative Analysis of Nitrotoluene Isomer Mixtures Using Femtosecond Time-Resolved Mass Spectrometry.

Discrimination of isomers in a mixture is a subject of ongoing interest in biology, pharmacology, and forensics. We demonstrate that femtosecond time-resolved mass spectrometry (FTRMS) effectively quantifies mixtures of -, -, and -nitrotoluenes, the first two of which are common explosive degradation products. The key advantage of the FTRMS approach to mixture quantification lies in the ability of the pump-probe laser control scheme to capture distinct fragmentation dynamics of each nitrotoluene cation isomer on femtosecond timescales, thereby allowing for discrimination of the isomers using only the signal of the parent molecular ion at / 137.

Using computational chemistry to design pump-probe schemes for measuring nitrobenzene radical cation dynamics.

The electronic potential energy surfaces of the nitrobenzene cation obtained from time-dependent density functional theory and coupled cluster calculations are used to predict the most efficient excitation wavelength for femtosecond time-resolved mass spectrometry measurements. Both levels of theory identify a strongly-coupled transition from the ground state of the nitrobenzene cation with a geometry-dependent oscillator strength, reaching a maximum at 90° C-C-N-O dihedral angle with a corresponding energy gap of ∼2 eV. These results are consistent with the experimental observation in the nitrobenzene cation of a coherent superposition of vibrational states: a vibrational wave packet.

Synthesis of Air-Stable Cu Nanoparticles Using Laser Reduction in Liquid.

We report the synthesis of air-stable Cu nanoparticles (NPs) using the bottom-up laser reduction in liquid method. Precursor solutions of copper acetlyacetonate in a mixture of methanol and isopropyl alcohol were irradiated with femtosecond laser pulses to produce Cu NPs. The Cu NPs were left at ambient conditions and analyzed at different ages up to seven days.

Deposition of Cubic Copper Nanoparticles on Silicon Laser-Induced Periodic Surface Structures via Reactive Laser Ablation in Liquid.

We report the deposition of cubic copper nanoparticles (Cu NPs) of varying size and particle density on silicon laser-induced periodic surface structures via reactive laser ablation in liquid (RLAL) using intense femtosecond laser pulses. Two syntheses were compared: (1) simultaneous deposition, wherein a silicon wafer was laser-processed in aqueous Cu(NO) solution and (2) sequential deposition, wherein the silicon wafer was laser-processed in water and then exposed to aqueous Cu(NO). Only simultaneous deposition resulted in high Cu loading and cubic Cu NPs deposited on the surface.

Mechanism of Gold-Silver Alloy Nanoparticle Formation by Laser Coreduction of Gold and Silver Ions in Solution.

Photochemical reduction of aqueous Ag and [AuCl] into alloy Au-Ag nanoparticles (Au-Ag NPs) with intense laser pulses is a green synthesis approach that requires no toxic chemical reducing agents or stabilizers; however size control without capping agents still remains a challenge. Hydrated electrons produced in the laser plasma can reduce both [AuCl] and Ag to form NPs, but hydroxyl radicals (OH·) in the plasma inhibit Ag NP formation by promoting the back-oxidation of Ag into Ag. In this work, femtosecond laser reduction is used to synthesize Au-Ag NPs with controlled compositions by adding the OH· scavenger isopropyl alcohol (IPA) to precursor solutions containing KAuCl and AgClO.

Dissociation of Singly and Multiply Charged Nitromethane Cations: Femtosecond Laser Mass Spectrometry and Theoretical Modeling.

Dissociation pathways of singly- and multiply charged gas-phase nitromethane cations were investigated with strong-field laser photoionization mass spectrometry and density functional theory computations. There are multiple isomers of the singly charged nitromethane radical cation, several of which can be accessed by rearrangement of the parent CH-NO structure with low energy barriers. While direct cleavage of the C-N bond from the parent nitromethane cation produces NO and CH, rearrangement prior to dissociation accounts for fragmentation products including NO, CHOH, and CHNO.

Fabrication of Gold-Silicon Nanostructured Surfaces with Reactive Laser Ablation in Liquid.

Laser processing is an emerging technique capable of synthesizing metal-silicon composite surfaces for various applications. However, little is known about the chemical composition of these laser-processed surfaces, and the reaction mechanisms leading to their formation are poorly understood. In this work, we report the formation of gold-silicon nanostructured surfaces through reactive laser ablation in liquid.

Laser-assisted synthesis of gold-graphene oxide nanocomposites: effect of pulse duration.

Laser photoreduction of metal ions onto graphene oxide (GO) is a facile, environmentally friendly method to produce functional metal-GO nanocomposites for a variety of applications. This work compares Au-GO nanocomposites prepared by photoreduction of [AuCl] in aqueous GO solution using laser pulses of nanosecond (ns) and femtosecond (fs) duration. The presence of GO significantly accelerates the [AuCl] photoreduction rate, with a more pronounced effect using ns laser pulses.

Homocoupling and Heterocoupling of Grignard Perfluorobenzene Reagents via Aryne Intermediates: A DFT Study.

Perfluorobenzenes are reactive species with the lowest magnesium metalation barriers among all hexahalobenzenes. This fact makes them good candidates for the study of heterocoupling reactions of the Grignard type. In this work, we investigated a number of pathways for both heterocoupling and homocoupling reactions and estimated the solvated energy barrier heights.

Kinetic Control of [AuCl] Photochemical Reduction and Gold Nanoparticle Size with Hydroxyl Radical Scavengers.

Laser-induced photochemical reduction of aqueous [AuCl] is a green synthesis approach requiring no chemical reducing agents or stabilizers; but size control over the resulting gold nanoparticles remains a challenge. Under optical breakdown conditions producing hydrated electrons (e) and hydroxyl radicals (OH) through decomposition of water, [AuCl] reduction kinetics follow an autocatalytic rate law, which is governed by rate constants: nucleation rate , dependent on e; and growth rate , dependent on the OH recombination product, HO. In this work, we add the hydroxyl radical scavengers isopropyl alcohol and sodium acetate to limit HO formation.

Coherent Vibrational and Dissociation Dynamics of Polyatomic Radical Cations.

The ultrafast dynamics of polyatomic radical cations contribute to important processes including energy transfer in photovoltaics, electron transfer in photocatalysis, radiation-induced DNA damage, and chemical reactions in the upper atmosphere and space. Probing these dynamics in the gas phase is challenging due to the rapid dissociation of polyatomic radical cations following electron removal, which arises from excess electronic excitation of the molecule during the ionization process. This Concept article introduces the reader to how the pump-probe technique of femtosecond time-resolved mass spectrometry (FTRMS) can overcome this challenge to capture coherent vibrational dynamics on the femtosecond timescale in polyatomic radical cations and enable the analysis of their dissociation pathways.

Probing Coherent Vibrations of Organic Phosphonate Radical Cations with Femtosecond Time-Resolved Mass Spectrometry.

Organic phosphates and phosphonates are present in a number of cellular components that can be damaged by exposure to ionizing radiation. This work reports femtosecond time-resolved mass spectrometry (FTRMS) studies of three organic phosphonate radical cations that model the DNA sugar-phosphate backbone: dimethyl methylphosphonate (DMMP), diethyl methylphosphonate (DEMP), and diisopropyl methylphosphonate (DIMP). Upon ionization, each molecular radical cation exhibits unique oscillatory dynamics in its ion yields resulting from coherent vibrational excitation.

Conserved Vibrational Coherence in the Ultrafast Rearrangement of 2-Nitrotoluene Radical Cation.

2-Nitrotoluene (2-NT) is a good model for both photolabile protecting groups for organic synthesis and the military explosive 2,4,6-trinitrotoluene (TNT). In addition to the direct C-NO bond-cleavage reaction that initiates detonation in TNT, 2-NT undergoes an H atom attack reaction common to the photolabile 2-nitrobenzyl group, which forms the aci-nitro tautomer. In this work, femtosecond pump-probe measurements with mass spectrometric detection and density functional theory (DFT) calculations demonstrate that the initially prepared vibrational coherence in the 2-NT radical cation (2-NT) is preserved following H atom attack.

Nucleation and growth of gold nanoparticles initiated by nanosecond and femtosecond laser irradiation of aqueous [AuCl].

Irradiation of aqueous [AuCl4]- with 532 nm nanosecond (ns) laser pulses produces monodisperse (PDI = 0.04) 5 nm Au nanoparticles (AuNPs) without any additives or capping agents via a plasmon-enhanced photothermal autocatalytic mechanism. Compared with 800 nm femtosecond (fs) laser pulses, the AuNP growth kinetics under ns laser irradiation follow the same autocatalytic rate law, but with a significantly lower sensitivity to laser pulse energy.