Photolysis Dynamics Laser Chemistry Lab

This paper proposes a new type of roaming mechanism. We find a signature of trajectory with chaotic behavior in the action-angle diagram of the H + H2 reaction on a LEP surface, namely the trajectory is found to be very sensitive to the initial angle variable which corresponds to the phase of the H2 vibration. The trajectory pattern switches from the direct to the complex forming mechanism, and vice versa, in the angle range (0 ∼ π). In the complex forming angle range, trajectories switch from reactive to non-reactive randomly and suddenly, as the result, we cannot predict the collision pattern from the initial conditions. Therefore, we may classify such trajectory as a new type of roaming with chaotic behavior, and it is different from the ordinary trajectory with deterministic behavior. This chaotic behavior could be due cooperative nearby network interaction (CNN effect). We also suggest that the KPP (Kolmogorow-Petrovsky-Piskounov) equation is useful to estimate the density gradient of the activated reagents, so that one can evaluate the branching ratio to various exit channels, such as triple fragmentation, tight transition state, or the roaming channel with the aid of the present classical trajectory calculation. © 2017 Author(s).

This paper aims at discussing new facets on stereodynamical behaviors in chemical reactions, i.e. the effects of molecular orientation and alignment on reactive processes. Further topics on macroscopic processes involving deviations from Arrhenius behavior in the temperature dependence of chemical reactions and chirality effects in collisions are also discussed. © 2015 AIP Publishing LLC.

We present here a physico-mathematical picture for explaining the unexpectedly large decoherence cross-section (almost 10 times larger than its gas-kinematic cross-section) recently observed by Ureña and coworkers in their scattering experiment involving a coherent NO beam with Ar gas. The present topological picture consists of a stereographic projection and the cusp catastrophe theory of Thom, and we find that this model enables us to clarify the origin of the collisional decoherence. From the view of the stereographic projection, we can naturally introduce the wave property originating from the singular point at the “North pole” on the circumference S1 coordinate corresponding to a critical point for the collisional decoherence (condition 1). This picture also predicts the sudden changes of wave-phase collapse due to network interaction in the many-body system (condition 2). Thus it is hoped that the model proposed by Ureña et al. based on the dipole-induced dipole interaction in the NO + Ar system could be modified through this picture by including interactions with many Ar atoms in the environment. One way to fill the gap between the single-pair interaction picture and the multiple interaction one would be to employ theoretical calculations by use of the density matrix theory with and without adding the second Ar atom to the NO–Ar system. The cusp catastrophe theory reinforces the necessity of some cooperative network interaction between the coherent NO molecule and many neighboring Ar atoms and provides a qualitative scenario in which the whole system leads to a sudden change of the collisional decoherence of NO as a function of the control parameters (a, b). At this stage, the present physico-mathematical picture cannot give any specific values of the decoherence distance by the theory itself, but it clearly provides us a new topological concept for clarifying the origin of collisional decoherence which is strongly connected with the complexity of the system. Thus it gives us a global guide map toward further clarification of the collisional decoherence phenomenon with the aid of more sophisticated quantum mechanical calculations in the future. © 2016 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

This perspective article aims at accounting for the versatility of some current experimental investigations for exploring novel paths in chemical reactions. It updates a previous one [Phys. Chem. Chem. Phys., 2005, 5, 291] and is limited to work by the authors. The use of advanced molecular beam techniques together with a combination of modern tools for specific preparation, selection and detection permits us to discover new trends in reactivity in the gas phase as well as at interfaces. We specifically discuss new facets of stereodynamics, namely the effects of molecular orientation and alignment on reactive and photodissociation processes. Further topics involve roaming paths and triple fragmentation in photodissociation probed by imaging techniques, chirality effects in collisions and deviations from Arrhenius behavior in the temperature dependence of chemical reactions. © the Partner Organisations 2014.

Experimental studies on reaction dynamics by use of molecular beams and oriented molecular beams are reviewed in order for looking closer to chemical reactions as well as photodissociations at the molecular level. We discuss about versatility and usefulness of the electrostatic hexapole sate-selector as a non-destructive selector for molecular structure analysis. Some experimental evidences on novel reaction dynamics in photodissociation and stereodynamics are presented followed by concluding remarks and future perspectives for controlling chemical reactions from the point of view of green chemistry, by manipulating molecular orientation without any catalyst nor by applying any external forces like intense electromagnetic field. © 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

A new reaction scheme is proposed to account for roaming and chaotic behaviors in collisional and photo-initiated molecular-beam reactions, where nonadiabatic dynamics plays a key role and the collapse of superposition of wave functions is considered to be important in the beginning of the present scheme. Since the feature of molecular orbitals of reagents is crucial in reaction, we showed how to map out the spatial distribution of the relevant HOMO molecular orbitals of CH3Cl in the impact of fast electrons. We identified by experiment that the multiple overlap of nearby molecular orbitals affects even the vibrational motion of adjacent molecule DCl of the transient [ClDCl] chemical species. We also showed dynamical steric effects in the HBr + OH four-atom reaction as a manifestation of the nonadiabatic dynamics in complex systems. The roaming mechanism in the photo-initiated reaction of methyl formate is clarified in detail by experiment as well as the QCT trajectory calculation, where the conical intersection region plays an essential role. We suggest that two types of roaming trajectories coexist, i.e., deterministic and chaotic roaming trajectories based on classical trajectory calculations. To clarify the nonadiabatic dynamics in the roaming mechanism for non-collinear three-dimensional (3D) collisions, a new model of the 3D Polanyi rule is proposed as the extension of the well-established 2D Polanyi rule. In the 3D Polanyi rule, it is expected that the curvature and torsion of Frenet–Serret formulas in three-dimensional space would provide us key concepts in understanding reaction dynamics. © 2018, Accademia Nazionale dei Lincei.

We report a ratiometric fluorescent metal ion sensor based on the mechanism of fluorescence resonance energy transfer (FRET) between synthesized 15-crown-5-ether capped CdSe/ZnS quantum dots (QDCE) and 15-crown-5-ether attached rhodamine B (RBCE) in pH 8.3 buffer solution. Fluorescence titration with different metal ions in pH 8.3 buffer solution of the QDCE-RBCE conjugate showed a decrease and an increase in the fluorescence intensity for QDCE and RBCE moieties respectively due to FRET from QDCE to RBCE. This sensor system shows excellent selectivity towards K+ ions resulting in increasing efficiency of FRET. Energy transfer efficiency depends on the affinity between metal ions and crown ether functionalized with QDCE/RBCE. The detailed analysis of FRET was explored. This water soluble ratiometric sensor system can act as a good FRET probe for sensing applications especially in biological systems. © The Royal Society of Chemistry 2015.

The inhibition of platelet activation is considered a potential therapeutic strategy for the treatment of arterial thrombotic diseases; therefore, maintaining platelets in their inactive state has garnered much attention. In recent years, nanoparticles have emerged as important players in modern medicine, but potential interactions between them and platelets remain to be extensively investigated. Herein, we synthesized a new type of carbon dot (CDOT) nanoparticle and investigated its potential as a new antiplatelet agent. This nanoparticle exerted a potent inhibitory effect in collagen-stimulated human platelet aggregation. Further, it did not induce cytotoxic effects, as evidenced in a lactate dehydrogenase assay, and inhibited collagen-activated protein kinase C (PKC) activation and Akt (protein kinase B), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (MAPK) phosphorylation. The bleeding time, a major side-effect of using antiplatelet agents, was unaffected in CDOT-treated mice. Moreover, our CDOT could reduce mortality in mice with ADP-induced acute pulmonary thromboembolism. Overall, CDOT is effective against platelet activation in vitro via reduction of the phospholipase C/PKC cascade, consequently suppressing the activation of MAPK. Accordingly, this study affords the validation that CDOT has the potential to serve as a therapeutic agent for the treatment of arterial thromboembolic disorders. © 2020 by the authors.

Following photodissociation of acetaldehyde (CH3CHO) at 308 nm, the CO(v = 1-4) fragment is acquired using time-resolved Fourier-transform infrared emission spectroscopy. The CO(v = 1) rotational distribution shows a bimodal feature; the low- and high-J components result from H-roaming around CH3CO core and CH3-roaming around CHO radical, respectively, in consistency with a recent assignment by Kable and co-workers (Lee et al., Chem. Sci. 5, 4633 (2014)). The H-roaming pathway disappears at the CO(v 2) states, because of insufficient available energy following bond-breaking of H + CH3CO. By analyzing the CH4 emission spectrum, we obtained a bimodal vibrational distribution; the low-energy component is ascribed to the transition state (TS) pathway, consistent with prediction by quasiclassical trajectory calculations, while the high-energy component results from H- and CH3-roamings. A branching fraction of H-roaming/CH3-roaming/TS contribution is evaluated to be (8% ± 3%)/(68% ± 10%)/(25% ± 5%), in which the TS pathway was observed for the first time. The three pathways proceed concomitantly along the electronic ground state surface. © 2015 AIP Publishing LLC.

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The multiple sensing capabilities of molybdenum disulfide quantum dots (MoS2 QDs) towards metal ions were scrutinized by tuning their surface functional groups. The MoS2 QDs surface was individually modified with thiol-containing capping agents to form carboxylic-, amine- and thiol-functionalized MoS2 QDs (MoS2/COOH, MoS2/NH2 and MoS2/SH) by the facile hydrothermal method. Each as-prepared QDs exhibits strong excitation wavelength dependent fluorescence behavior. The design of MoS2 QDs based metal ion sensor was implemented based on the fluorescence turn-on mechanism. These MoS2/COOH, MoS2/NH2 and MoS2/SH QDs sensors exhibit superior performance towards the highly selective detection of Co2+, Cd2+ and Pb2+ ions, respectively, due to the varied association of each functional group towards metal ions. The resultant detection limit of Co2+, Cd2+ and Pb2+ was evaluated to be 54.5, 99.6 and 0.84 nM, respectively, and the related fluorescence turn-on mechanism is verified unambiguously. The binding energies were calculated for QDs with metal ions pairs and the results lent support to the determined sensitivity. The as-prepared QDs were also successfully demonstrated to detect the above metal ions in real water samples. While becoming potential candidates in the chemosensors based on the fluorescence probe, these surface modified MoS2 QDs can offer an excellent sensing capability for specific metal ions with extremely high selectivity.

A clear and positive correlation between the CO2 concentration and the blood-sugar level has been observed via a noninvasive and time-dependent monitoring of CO2 concentration from human breath, which is carried out by using a homemade gas chromatography (GC)/milli-whistle compact analyzer. The time-dependent sampling of the CO2 concentration correlated between 5.0 to 5.6% (1% = 104 ppm) in accordance with blood-sugar level variations of 80 to 110 mg/dL. The analytical method results in a rapid, continuous and non-invasive determination of blood-sugar level via measurement of the CO2 concentration exhaled from the lungs.

Evanescent wave-cavity ring-down spectroscopy (EW-CRDS) is employed to characterize micellization of anionic surfactants and the related capability of removing cationic substance off the silica surface. Crystal violet (CV +) cationic dye is used as a molecular probe to effectively determine critical hemimicelle concentration (HMC) of surfactants on the surface. The HMC results are 1×10 -2, 4×10 -3, 8×10 -4, and 2.5×10 -4mol/L for sodium sulfate salts with a carbon-chain length of C-10, C-12, C-14, and C-16, respectively. A stronger hydrophobic interaction results in a less concentration required to undergo micellization. The HMC values on the surface are about half of those in solution. When NaCl solution is added, the electrolyte helps reduce the electrostatic repulsion between the anionic sulfate heads to facilitate the surfactant aggregation, and thus, the subsequent HMC is reduced. Furthermore, the probable phase change for dye-surfactant interactions on the surface at the concentration below HMC is observed, and the desorption rates of CV + are measured as a function of concentration and carbon-chain length of surfactants above HMC. Given each surfactant concentration at its respective HMC, the corresponding desorption rates are along the order of C-12<C-14<C-16<-C-10. The trend may be realized by two competing factors of hemimicelle size and number density. The consequences help with understanding how to apply surfactant in the chromatographic separation. © 2012 Elsevier Inc.

Atomic halogen elimination from halogen-related compounds plays a vital role in the depletion of the ozone layer and is well investigated. However, the probabilities for elimination of molecular halogens and hydrogen halides are rarely scrutinised. We develop distinct method for the investigation of each kind of fragment. Velocity-mapping ion-imaging was employed to study the atomic halogen elimination from alkyl halides and aryl halides, focusing on the fractions of the translational energy release, the quantum yields of the atomic fragments, transition probability for curve crossing, competitive halogen-related bond fission, and anisotropy parameters to understand their dynamical complexity. Cavity ring-down absorption spectroscopy was implemented to investigate the molecular halogen fragments dissociated from the aliphatic halides and acyl halides for their optical spectra, vibrational branches, quantum yields, and the dissociation mechanisms. Time-resolved Fourier transform infrared emission spectroscopy was employed to confine the primary products of hydrogen halide elimination from acyl halides in the presence of Ar gas. It is, for the first time, to overview these existing small halogen-related fragments eliminated from halogen-containing compounds. The detailed characterisation of these fragments should unveil complicated halogen-related dissociation mechanisms which may supplement the current knowledge and help with the photochemical assessment of halogen-related environmental issue. © 2020 Informa UK Limited, trading as Taylor & Francis Group.

In this work, an asymmetric top molecule 2-bromobutane has been successfully oriented by using hexapole state selector combined with orientation field, followed by detection of the bromine atomic photofragment distribution in the photolysis. The photofragment is produced in both the ground Br (2P3/2) and the excited Br (2P1/2) electronic states and both channels are studied by the slice imaging technique, revealing new features in the stereodynamic vectorial properties with respect to previous investigations on non-oriented molecules. © 2017 Author(s).

The photodissociation channels of methyl formate have been extensively investigated by two different advanced experimental techniques, ion imaging and Fourier-Transform-Infrared emission spectroscopy, combined with quantum chemical calculations and molecular dynamics simulations. Our aim is to characterize the role of alternative routes to the conventional transition-state mediated pathway: the roaming and the triple fragmentation processes. The photolysis experiments, carried out at a range of laser wavelengths in the vicinity of the triple fragmentation threshold, beside the simulation of large bunches of classical trajectories with different initial conditions, have shown that both mechanisms share a common path that involves a conical intersection during the relaxation process from the electronic excited state S1 to the ground state S0. © 2015 AIP Publishing LLC.

By using time-resolved Fourier-transform infrared emission spectroscopy, the HCO fragment dissociated from acetaldehyde (CH3CHO) at 248 nm is found to partially decompose to H and CO. The fragment yields are enhanced by the Ar addition that facilitates the collision-induced internal conversion. The channels to CH2CO + H2 and CH3CO + H are not detected significantly. The rotational population distribution of CO, after removing the Ar collision effect, shows a bimodal feature comprising both low- and high-rotational (J) components, sharing a fraction of 19% and 81%, respectively, for the vibrational state v = 1. The low-J component is ascribed to both roaming pathway and triple fragmentation. They are determined to have a branching ratio of <0.13 and >0.06, respectively, relative to the whole v = 1 population. The CO roaming is accompanied by a highly vibrational population of CH4 that yields a vibrational bimodality. © 2014 AIP Publishing LLC.

An alternative to the transition state (TS) pathway, the roaming route, which bypasses the minimum energy path but produces the same molecular products, was recently found in photodissociation dynamics. This account describes signatures of roaming in photodissociation of the carbonyl compounds, specifically methyl formate and aliphatic aldehydes. Methyl formate was promoted to the excited state, followed by internal conversion via a conical intersection. Then, the energetic precursor dissociated to fragments which proceeded along either TS or roaming path. In contrast to the lack of a roaming saddle point found in methyl formate, the structure of the roaming saddle point for each of a series of aliphatic aldehydes comprises two moieties that are weakly bound at a distance. As its size increases, the energy difference between the TS barrier and the roaming saddle point increases and the roaming pathway becomes increasingly dominant. Experimentally, the rotational-level dependence of the roaming route was measured with ion imaging, while the vibrational-state dependence was observed with time-resolved Fourier-transform infrared emission spectroscopy. The roaming signature was verified theoretically by quasi-classical trajectory (QCT) calculations. As an alternative to the QCT method, a multi-center impulsive model was developed to simulate the roaming scalar and vector properties. © 2018 Informa UK Limited, trading as Taylor & Francis Group.

Atmospheric halogen chemistry has drawn much attention, because the halogen atom (X) playing a catalytic role may cause severe stratospheric ozone depletion. Atomic X elimination from X-containing hydrocarbons is recognized as the major primary dissociation process upon UV-light irradiation, whereas direct elimination of the X2 product has been seldom discussed or remained a controversial issue. This account is intended to review the detection of X2 primary products using cavity ring-down absorption spectroscopy in the photolysis at 248 nm of a variety of X-containing compounds, focusing on bromomethanes (CH2Br2, CF2Br2, CHBr2Cl, and CHBr3), dibromoethanes (1,1-C 2H4Br2 and 1,2-C2H 4Br2) and dibromoethylenes (1,1-C2H 2Br2 and 1,2-C2H2Br2), diiodomethane (CH2I2), thionyl chloride (SOCl 2), and sulfuryl chloride (SO2Cl2), along with a brief discussion on acyl bromides (BrCOCOBr and CH2BrCOBr). The optical spectra, quantum yields, and vibrational population distributions of the X2 fragments have been characterized, especially for Br2 and I2. With the aid of ab initio calculations of potential energies and rate constants, the detailed photodissociation mechanisms may be comprehended. Such studies are fundamentally important to gain insight into the dissociation dynamics and may also practically help to assess the halogen-related environmental variation. This journal is © the Partner Organisations 2014.

Evanescent wave cavity ring-down absorption spectroscopy is applied to investigate thermodynamics, kinetics, orientation of the substrates on the surface, probe critical hemimicelle concentration of surfactants, and examine interaction and binding kinetics of DNA strands. © 2014 OSA.

Carbonyl compounds studied are confined to acetyl halide (CH3COCl), acetyl cyanide (CH3COCN), acetyl sulfide (CH3COSH), acetaldehyde (CH3CHO), and methyl formate (HCOOCH3). They are asymmetrically substituted, but do not follow the well-known Norrish type I reactions. Each compound ejected in an effusive beam at about 300 K is commonly excited to the 1(n, π∗)CO lower state; that is, a nonbonding electron on O of the C=O group is promoted to the antibonding orbital of π∗CO. The photolysis experiments are conducted in the presence of Ar gas and the corresponding fragments are detected using time-resolved Fourier-transform Infrared (FTIR) emission spectroscopy. The enhancement of the collision-induced internal conversion or intersystem crossing facilitates the dissociation channels via highly vibrational states of the ground singlet (So) or triplet (T1) potential energy surfaces. In this manner, an alternative nonadiabatic channel is likely to open yielding different products, even if the diabatic coupling strength is strong between the excited state and the neighboring state. For instance, the photodissociation of CH3COCl at 248 nm produces HCl, CO, and CH2 fragments, in contrast to the supersonic jet experiments showing dominance of the Cl fragment eliminated from the excited state. If the diabatic coupling strength is weak, dissociation proceeds mainly through internal conversion, such as the cases of CH3COCN and CH3COSH. The photodissociation of CH3COCN at 308 nm has never been reported before, while for CH3COSH matrix-isolated photodissociation was conducted that shows a distinct spectral feature from the current FTIR method. The CH3CHO and HCOOCH3 molecules belong to the same type of carbonyl compounds, in which the molecular products, CO + CH4 and CO + CH3OH, are produced through both transition state and roaming pathways. Their products are characterized differently between molecular beam and current FTIR experiments. For instance, the photodissociation of HCOOCH3 at 248 nm yields CO with the vibrational state v ≥ 4, in contrast to the molecular beam experiments producing CO at v = 1. The photodissociation of CH3CHO at 308 nm intensifies a low energy component in the CH4 vibrational distribution, thus verifying the transition state pathway for the first time. © the Owner Societies 2016.

In one-photon dissociation of gaseous acetyl chloride at 248 nm, time-resolved Fourier-transform infrared emission spectroscopy is used to detect the fragments of HCl, CO, and CH2 in the presence of Ar or O 2. The high-resolution spectra of HCl and CO are analyzed to yield the corresponding internal energy deposition of 8.9 ± 1.1 and 6.2 ± 0.9 kcal/mol. The presence of the CH2 fragment is verified by detecting the CO2 product resulting from the reaction of CH 2 and the added O2. The probability of the HCl formation via a hot Cl reaction with the precursor is examined to be negligible by performing two experiments, the CH3COCl pressure dependence and the measurement of Br2 with Cl reaction. The HCl elimination channel under the Ar addition is verified to be slowed by 2 orders of magnitude, as compared to the Cl elimination channel. The observed fragments are proposed to dissociate on the hot ground electronic state via collision-induced internal conversion. A two-body dissociation channel is favored leading to HCl and CH2CO, followed by secondary dissociation. © 2010 American Chemical Society.

By employing time-resolved Fourier transform infrared emission spectroscopy, the fragments HCl (v=1-3), HBr (v=1), and CO (v=1-3) are detected in one-photon dissociation of 2-bromopropionyl chloride (CH3CHBrCOCl) at 248 nm. Ar gas is added to induce internal conversion and to enhance the fragment yields. The time-resolved high-resolution spectra of HCl and CO were analyzed to determine the rovibrational energy deposition of 10.0A ±0.2 and 7.4A ±0.6 kcal mol-1, respectively, while the rotational energy in HBr is evaluated to be 0.9A ±0.1 kcal mol-1. The branching ratio of HCl(v>0)/HBr(v>0) is estimated to be 1:0.53. The bond selectivity of halide formation in the photolysis follows the same trend as the halogen atom elimination. The probability of HCl contribution from a hot Cl reaction with the precursor is negligible according to the measurements of HCl amount by adding an active reagent, Br2, in the system. The HCl elimination channel under Ar addition is verified to be slower by two orders of magnitude than the Cl elimination channel. With the aid of ab initio calculations, the observed fragments are dissociated from the hot ground state CH3CHBrCOCl. A two-body dissociation channel is favored leading to either HCl+CH3CBrCO or HBr+CH2CHCOCl, in which the CH 3CBrCO moiety may further undergo secondary dissociation to release CO. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this study, Liquidambar formosana tree leaves have been used as a renewable biomass precursor for preparing porous carbons (PCs). The PCs were produced by pyrolysis of natural waste of leaves after 10% KOH activation under a nitrogen atmosphere and characterized by a variety of state-of-the-art techniques. The PCs possess a large surface area, micro-/mesoporosity, and functional groups on its surface. A glassy carbon electrode modified with high PCs was explored as an efficient binder-free electrocatalyst material for the voltammetric determination of nitro isomers such as 3-nitroaniline (3-NA) and 4-nitroaniline (4-NA). Under optimal experimental conditions, the electrochemical detection of 3-NA and 4-NA was found to have a wide linear range of 0.2-115.6 and 0.5-120 μM and a low detection limit of 0.0551 and 0.0326 μM, respectively, with appreciable selectivity. This route not only enhanced the benefit from biomass wastes but also reduced the cost of producing electrode materials for electrochemical sensors. Additionally, the sensor was successfully applied in the determination of nitro isomers even in the presence of other common electroactive interference and real samples analysis (beverage and pineapple jam solutions). Therefore, the proposed method is simple, rapid, stable, sensitive, specific, reproducible, and cost-effective and can be applicable for real sample detection. © 2019 American Chemical Society.

Photodissociation of CH2BrI was investigated in search of unimolecular elimination of BrI via a primary channel using cavity ring-down absorption spectroscopy (CRDS) at 248 nm. The BrI spectra were acquired involving the first three ground vibrational levels corresponding to A3Π1 ← X1Σ+ transition. With the aid of spectral simulation, the BrI rotational lines were assigned. The nascent vibrational populations for v′′ = 0, 1, and 2 levels are obtained with a population ratio of 1:(0.58 ± 0.10):(0.34 ± 0.05), corresponding to a Boltzmann-like vibrational temperature of 713 ± 49 K. The quantum yield of the ground state BrI elimination reaction is determined to be 0.044 ± 0.014. The CCSD(T)//B3LYP/MIDI! method was employed to explore the potential energy surface for the unimolecular elimination of BrI from CH2BrI.