Photodissociation Dynamics of the Highly Stable ortho-Nitroaniline Cation.

0rtho-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, D1, lies more than 2 eV above the ground state, and the energetic barriers to rearrangement and dissociation reactions exceed this D1 energy. These theoretical results were confirmed by FTRMS pump-probe measurements showing that (1) fragment ions represented less than 30% of the total ion yield when a 1014 W cm-2, 1300 nm, 20 fs pump pulse was used to ionize ONA; and (2) 3.1 eV (400 nm) photons were required to induce dissociation of the ONA cation. Stronger coupling between the ground D0 and excited D4 states of the ONA cation at the geometry of neutral ONA resulted in a transient enhancement of fragment ion yields at <300 fs pump-probe delay times, prior to relaxation of the ONA cation to its optimal geometry.

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. We show that the deposited metal content, surface morphology, and metal crystallite size can be tuned based on the difference in electrochemical potentials of the deposited and sacrificial metal. Compared to the Au and Cu reference samples, GR more than doubled the metal content on the LIPSS and reduced metal crystallite sizes by up to 20%. The ability to tune the metal content and crystalline domain size simultaneously makes GR-RLAL a potentially useful approach in the manufacturing of functional metal-LIPSS materials such as surface-enhanced Raman spectroscopy substrates.

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 (5.8 eV) or 355 nm (3.5 eV) are used for desorption of samples under vacuum, 7.9 eV (157 nm) fluorine laser pulses are used for post-ionization, and the ions so formed are detected by time-of-flight mass spectrometry. Detection of imipramine by 213 nm ps-LDPI-MS shows less fragmentation than either 355 nm ps-LDPI-MS or prior results from 800 nm fs-LDPI-MS. Ionization energies of imipramine, desipramine, amitriptyline, and clomipramine are predicted using density functional theory calculations and used to explain the corresponding ps-LDPI-MS data for these four compounds as resulting from single-photon ionization. The experimental observation of low-mass amine-containing fragments with calculated ionization energies below 7.9 eV is attributed mostly to dissociation during laser desorption, followed by single-photon ionization of the neutral fragments rather than the more traditional mechanism of unimolecular dissociation following single-photon ionization of the parent molecule.

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, n-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, sp-hybridized carbons, and fluorescent carbon dots. The observed trends are explained in the context of established ionization mechanisms and cavitation bubble dynamical processes that occur during ultrashort pulsed laser ablation of liquid media. The results of this work have important implications both for controlling the formation of carbon shells around metal nanoparticles during the ablation of solid targets in liquid and producing carbon nanomaterials directly from the ablation of organic liquids without a solid target.

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 [AuCl4]- 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+. Both nanosecond and femtosecond laser excitation of aqueous [AuCl4]- produce reactive chlorine species by Au-Cl bond homolysis. Formation of numerous volatile products by IPA decomposition during both femtosecond and nanosecond laser excitation of [AuCl4]- is attributed to enhanced optical breakdown by the Au nanoparticle products of [AuCl4]- reduction. These mechanistic insights can inform the design of laser synthesis procedures to improve control over metal nanoparticle properties and enhance byproduct yields.

Coulomb Explosion Dynamics of Multiply Charged para-Nitrotoluene Cations.

This work explores Coulomb explosion (CE) dissociation pathways in multiply charged cations of para-nitrotoluene (PNT), a model compound for nitroaromatic energetic molecules. Experiments using strong-field ionization and mass spectrometry indicate that metastable cations PNT2+ and PNT3+ undergo CE to produce NO2+ and NO+. The experimentally measured kinetic energy release from CE upon formation of NO2+ and NO+ agrees qualitatively with the kinetic energy release predicted by computations of the reaction pathways in PNT2+ and PNT3+ using density functional theory (DFT). Both DFT computations and mass spectrometry identified additional products from CE of highly charged PNTq+ cations with q > 3. The dynamical timescales required for direct CE of PNT2+ and PNT3+ to produce NO2+ were estimated to be 200 and 90 fs, respectively, using ultrafast disruptive probing measurements.

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 p-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-1 and the in-plane ring-stretching mode at 650 cm-1. Both vibrational wave packets are observed as oscillations in parent and fragment ion yields in the mass spectrum upon optical excitation. Excitation with 650 nm selectively fragments the PNT cation into C 7 H 7 + , whereas excitation with 400 nm selectively produces C 5 H 5 + and C 3 H 3 + . In both cases the ion yield oscillations result from torsional wave packet excitation, but 650 and 400 nm excitation produce oscillations with opposite phases. Ab initio calculations of the ground and excited electronic potential energy surfaces of PNT cation along the C-C-N-O dihedral angle reveal that 400 nm excitation accesses an allowed transition from D0 to D6 at 0° dihedral angle, whereas 650 nm excitation accesses a strongly allowed transition from D0 to D4 at a dihedral angle of 90°. This ability to access different electronic excited states at different locations along the potential energy surface accounts for the selective fragmentation observed with different probe wavelengths. The ring-stretching mode, only observed using 800 nm excitation, is attributed to a D0 to D2 transition at a geometry with 90° dihedral angle and elongated C-N bond length. Collectively, these results demonstrate that strong field ionization induces multimode coherent excitation and that the vibrational wave packets can be excited with specific photon energies at different points on their potential energy surfaces to induce selective fragmentation.

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, PO2C2H7+ and PO2CH4+, oscillate approximately out of phase with the parent ion, but with a slight phase shift relative to each other. We use electronic structure theory and nonadiabatic surface hopping dynamics to understand the underlying dynamics. The results show that while the cation oscillates on the ground state along the P=O bond stretch coordinate, the probe excites population to higher electronic states that can lead to fragments PO2C2H7+ and PO2CH4+. The computational results combined with the experimental observations indicate that the two conformers of DMMP that are populated under experimental conditions exhibit different dynamics after being excited to the higher electronic states of the cation leading to different dissociation products. These results highlight the potential usefulness of these pump-probe measurements as a tool to study conformer-specific dynamics in molecules of biological interest.

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 D2 state of NM+ upon strong-field ionization provides the necessary energy to initiate NNR and subsequent dissociation into NO+. Both pump-probe measurements and molecular dynamics simulations are consistent with the completion of NNR within 500 fs of ionization with dissociation into NO+ and OCH3 occurring ∼30 fs later. Pump-probe measurements indicate that NO+ formation is in competition with the direct dissociation of NM+ to CH3+ and NO2. Electronic structure calculations indicate that a strong D0 → D1 transition can be excited at 650 nm when the C-N bond is stretched from its equilibrium value (1.48 Å) to 1.88 Å. On the other hand, relaxation of the NM+ cation after ionization into D0 occurs in less than 50 fs and results in observation of intact NM+. Direct dissociation of the equilibrium NM+ to produce NO2+ and CH3 can be induced with 650 nm excitation via a weakly allowed D0 → D2 transition.

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 ortho-, para-, and meta-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 m/z 137. Upon measurement of reference dynamics of each individual isomer, the molar fractions of binary and ternary mixtures can be predicted to within ∼5 and ∼7% accuracy, respectively.

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. Time-resolved measurements using a probe wavelength of 650 nm, nearly resonant with the strong transition, result in enhanced ion yield oscillation amplitudes as compared to excitation with the nonresonant 800 nm wavelength. Analogous behavior is found for the closely related molecules 2- and 4-nitrotoluene. These results demonstrate that computational chemistry can predict the best choice of probe wavelength in time-resolved measurements of vibrational coherent states in molecular cations.

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. TEM analysis indicates a broad size distribution of spherical NPs surrounded by a carbon matrix, with the majority of the NPs less than 10 nm and small numbers of large particles up to ∼100 nm in diameter. XRD collected over seven days confirmed the presence of fcc-Cu NPs, with some amorphous Cu2O, indicating the stability of the zero-valent Cu phase. Raman, FTIR, and XPS data for oxygen and carbon regions put together indicated the presence of a graphite oxide-like carbon matrix with oxygen functional groups that developed within the first 24 h after synthesis. The Cu NPs were highly active towards the model catalytic reaction of para-nitrophenol reduction in the presence of NaBH4.

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(NO3)2 solution and (2) sequential deposition, wherein the silicon wafer was laser-processed in water and then exposed to aqueous Cu(NO3)2. Only simultaneous deposition resulted in high Cu loading and cubic Cu NPs deposited on the surface. The solution pH, Cu(NO3)2 concentration, and sample translation rate were varied to determine their effects on the size, morphology, and density of Cu NPs. Solution pH near ∼6.8 maximized Cu deposition. The Cu(NO3)2 concentration affected the Cu NP morphology but not the size or Cu loading. The sample translation rate most significantly affected the Cu loading, particle size, and particle density. The observed synthesis parameter dependence of these Cu NP properties resembles results by electrodeposition to grow Cu NPs on silicon surfaces, which suggests that Cu NP deposition by RLAL follows a mechanism similar to electrodeposition.

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

Photochemical reduction of aqueous Ag+ and [AuCl4]- 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 [AuCl4]- and Ag+ to form NPs, but hydroxyl radicals (OH·) in the plasma inhibit Ag NP formation by promoting the back-oxidation of Ag0 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 KAuCl4 and AgClO4. With sufficient IPA concentration, varying the precursor ratio enabled control over the Au-Ag NP composition and produced alloy NPs with average sizes less than 10 nm and homogeneous molar compositions of Au and Ag. By investigating the kinetics of Ag+ and [AuCl4]- coreduction, we find that the reduction of [AuCl4]- into Au-Ag NPs occurs before most of the Ag+ is incorporated, giving us insight into the mechanism of Au-Ag NP formation.

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 [AuCl4]- in aqueous GO solution using laser pulses of nanosecond (ns) and femtosecond (fs) duration. The presence of GO significantly accelerates the [AuCl4]- photoreduction rate, with a more pronounced effect using ns laser pulses. This difference is rationalized in terms of the stronger interaction of the 532 nm laser wavelength and long pulse duration with the GO. Both the ns and fs lasers produce significant yields of sub-4 nm Au nanoparticles attached to GO, albeit with different size distributions: a broad 5.8 ± 1.9 nm distribution for the ns laser and two distinct distributions of 3.5 ± 0.8 and 10.1 ± 1.4 nm for the fs laser. Despite these differences, both Au-GO nanocomposites had the same high catalytic activity towards p-nitrophenol reduction as compared to unsupported 4-5 nm Au nanoparticles. These results point to the key role of GO photoexcitation in catalyzing metal ion reduction and indicate that both ns and fs lasers are suitable for producing functional metal-GO nanocomposites.

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. Silicon wafers were immersed in pH-controlled solutions of KAuCl4 and processed with ultrashort laser pulses. Gold deposition on the silicon wafers was found to depend on the pH of the precursor solution: neutral solutions (pH ∼6.3) resulted in much higher gold deposition than acidic or basic solutions. Laser processing of silicon wafers in water followed by immersion in the KAuCl4 solution resulted in lower gold deposition. X-ray photoelectron spectroscopy and depth profiling showed the existence of both gold (Au0) and gold-silicide (AuxSi) phases on the surfaces. Under both types of processing conditions, the gold atomic fraction and gold-silicide content increased with depth to at least 150 nm into the surface of the silicon wafer, although significantly more gold and gold-silicide were formed when the silicon was ablated in KAuCl4 solution as compared to immersion in KAuCl4 after ablation in water. Based on these data and existing literature on laser processing of silicon, we propose mechanisms that explain the observed gold penetration depth and its deposition dependence on solution pH. The mechanistic understanding gained in this work may be useful for synthesizing a variety of metal-silicon composite surfaces through laser processing to prepare functional materials such as catalysts and surface-enhanced Raman spectroscopy substrates.

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 CH3-NO2 structure with low energy barriers. While direct cleavage of the C-N bond from the parent nitromethane cation produces NO2+ and CH3+, rearrangement prior to dissociation accounts for fragmentation products including NO+, CH2OH+, and CH2NO+. Extensive Coulomb explosion in fragment ions observed at high laser intensity indicates that rapid dissociation of multiply charged nitromethane cations produces additional species such as CH2+, H+, and NO22+.  On the basis of analysis of Coulomb explosion in the mass spectral signals and pathway calculations, sufficiently intense laser fields can remove four or more electrons from nitromethane.

Kinetic Control of [AuCl4]- Photochemical Reduction and Gold Nanoparticle Size with Hydroxyl Radical Scavengers.

Laser-induced photochemical reduction of aqueous [AuCl4]- 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 (eaq-) and hydroxyl radicals (OH•) through decomposition of water, [AuCl4]- reduction kinetics follow an autocatalytic rate law, which is governed by rate constants: nucleation rate k1, dependent on eaq-; and growth rate k2, dependent on the OH• recombination product, H2O2. In this work, we add the hydroxyl radical scavengers isopropyl alcohol and sodium acetate to limit H2O2 formation. Higher scavenger concentrations both lowered k2 values and produced smaller gold nanoparticles with Gaussian size distributions and remarkably narrow mass-weighted size distributions. With sufficiently high scavenger concentrations, the mean nanoparticle size could be tuned from 3.8 to 6.1 nm with polydispersity indices below 0.08. Both the higher surface area-normalized catalytic activity of the gold nanoparticles synthesized in the presence of scavengers, and FTIR measurements, indicate no capping ligands on the nanoparticle surfaces. These results demonstrate that the size distributions of "naked" gold nanoparticles produced by photochemical [AuCl4]- reduction can be effectively tuned by controlling the reaction kinetics.

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. DMMP has particularly well-resolved 45 fs ( 732 ± 28 cm - 1 ) oscillations with a weak feature at 610⁻650 cm - 1 , while DIMP exhibits bimodal oscillations with a period of ∼55 fs and two frequency features at 554 ± 28 and 670⁻720 cm - 1 . In contrast, the oscillations in DEMP decay too rapidly for effective resolution. The low- and high-frequency oscillations in DMMP and DIMP are assigned to coherent excitation of the symmetric O⁻P⁻O bend and P⁻C stretch, respectively. The observation of the same ionization-induced coherently excited vibrations in related molecules suggests a possible common excitation pathway in ionized organophosphorus compounds of biological relevance, while the distinct oscillatory dynamics in each molecule points to the potential use of FTRMS to distinguish among fragment ions produced by related molecules.

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-NO2 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. Strong-field adiabatic ionization is used to prepare 2-NT+, which can overcome a modest 0.76 eV energy barrier to H atom attack to form the aci-nitro tautomer as soon as ∼20-60 fs after ionization. Once formed, the aci-nitro tautomer spontaneously loses -OH to form C7H6NO+, which exhibits distinctly faster oscillations in its ion yield (290 fs period) as compared to the 2-NT+ ion (380 fs period). The fast oscillations are attributed to the coherent torsional motion of the aci-nitro tautomer, which has a significantly faster computed torsional frequency (86.9 cm-1) than the 2-NT+ ion (47.9 cm-1). Additional DFT calculations identify reaction pathways leading to the formation of the dissociation products C7H6NO+, C7H7+, and C6H6N+. Collectively, these results reveal a rich picture of coherently and incoherently driven dissociation pathways in 2-NT+.