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).

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).

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.

Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl)2 at 248 nm obtaining a primary Cl2 elimination channel. A ratio of vibrational population is estimated to be 1:(0.12 ± 0.03):(0.011 ± 0.003) for the v = 0, 1, and 2 levels. The quantum yield of Cl2 molecular channel is obtained to be 0.8 ± 0.4 initiated from the X̃ 1Ag ground state surface (COCl)2 via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl)2 + hν → 2CO + Cl2) and secondary reactions (dominated by Cl + COCl → Cl2 + CO). The former is estimated to share a yield of >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl2 is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ring-down time to 70 μs, thus allowing for the contribution from 17% up to 66% of the total Cl2 yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, the primary Cl2 dissociation channel eliminated from (COCl)2 is observed for the first time. © 2017 American Chemical Society.

This Note aims to clarify the source of CO in photodissociation of acetyl cyanide (CH3COCN) at 308 nm. From the theoretical aspects, a new pathway via isomerization transition state (TS) at 391 ± 8 kJ/mol is found leading to the CO + CH3NC products. An amount of 60% reactant molecules at 300 K is estimated to successfully surpass the average TS barrier lying above the excitation energy by 3.5 kJ/mol. Further, a prior distribution method is conducted to characterize the vibrational energy distribution of CO on a statistical basis. The pathway to CH3NC + CO yields a vibrational branching ratio (v = 0:v = 1:v = 2:v = 3∼0.63:0.25:0.093:0.032) in excellent agreement with the observation (0.62:0.25:0.09:0.05). © 2013 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.

Electron transfer (ET) kinetics of CdSe/ZnS core/shell quantum dots (QDs) on bare coverslips and a TiO 2 nanoparticle-coated thin film has been investigated at the single-molecule level. The QDs prepared have three different diameters of 3.6, 4.6, and 6.4 nm. The trajectories of fluorescence intensity are acquired with respect to the arrival time. The on-time events and subsequent fluorescence lifetimes are shorter with decreasing size. Given the lifetime measurements for QDs on glass and TiO 2, the rate constant of ET from QDs to TiO 2 may be determined to be 1.3×10 7, 6.0×10 6, and 4.7×10 6 s -1 for the increasing sizes of the QDs. The plot of on-time probability density versus arrival time is characterized by power-law statistics in the short time region and a bending tail in the long time region. Marcus's ET model is employed to satisfactorily fit the bending tail behavior and to further calculate the ET rate constants. The theoretical counterparts for the different sizes are 1.4×10 7, 6.4×10 6, and 1.9×10 6 s -1, showing good agreement with the experimental results. Going dotty: Electron transfer kinetics of CdSe/ZnS core/shell quantum dots (QDs) on bare coverslips and on TiO 2 nanoparticle coated thin films have been investigated at the single-molecule level. As the size of the QDs changes, the shift in the valence band (VB) energy is less significant than the shift in the conduction band (CB) energy. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

Time-resolved Fourier transform infrared emission spectroscopy is employed in the photolysis of propionaldehyde (CH3CH2CHO) at 248 nm to characterize the role of the roaming pathway. High-resolution spectra of CO are analyzed to yield a single Boltzmann rotational distribution for each vibrational level (ν = 1-4) with small rotational and large vibrational energy disposals. A roaming saddle point is found containing two far separated moieties of HCO and CH3CH2 with a weak interaction between them. Quasiclassical trajectory calculations on this configuration yield the CO energy flow behavior, consistent with the findings. The rate constant along the roaming pathway is evaluated to be larger by >1-2 orders of magnitude than those along tight transition state or three-body dissociation pathways. This work implies that the roaming mechanism plays an increasingly important role in aliphatic aldehydes as the molecular size becomes larger. © 2013 American Chemical Society.

Molecular orientation techniques are becoming available in the study of elementary chemical processes, in order to highlight those structural and dynamical properties that would be concealed by random rotational motions. Recently successful orientation was achieved for asymmetric-top and chiral molecules of much larger complexity than hitherto. In this work, we report and discuss the correlation between the vectors' photofragment recoil velocity v, transition dipole moment μ, and permanent dipole moment d in a dissociation experiment on hexapole oriented 2-bromobutane, photoinitiated by a linearly polarized laser. The sliced ion images of the Br∗(2P1/2) and Br(2P3/2) photofragments were acquired at 234.0 and 254.1 nm, respectively, by a (2 + 1) resonance-enhanced multiphoton ionization technique. A detailed analysis of the sliced ion images obtained at a tilting angle 45° of laser polarization provides information on the correlation of the three vectors, which are confined by two polar angles α and χ and one azimuthal angle φμd in the recoil frame. The sliced ion images of Br fragments eliminated individually from the enantiomers at 254.1 nm yield an asymmetric factor close to zero; for this reason the photofragment angular distributions do not show significant differences. The elimination of the Br∗ fragment at 234.0 nm is mainly correlated with a parallel transition, giving rise to a large anisotropy parameter of 1.85, and thus can be considered as a single state excitation. The resulting recoil frame angles are optimized to 163° ± 8° and 164° ± 1° for α and χ, respectively, whereas φμd is approaching 0° for the best fit. Since for the present molecule, the three vectors have an only slight spatial arrangement, the photofragment angular distributions of the two enantiomers do not show appreciable differences. Theoretical and computational simulations provide us the basis to state that oriented enantiomers can be discriminated on-the-fly in photodissociation processes even initiated by non-circularly polarized light, provided that the three vectors encountered above have specific three-dimensional arrangements. The fact that Br fragment elimination involves a multi-potential dissociation carries uncertainties in theoretical estimates of the vector direction. Therefore, this work represents a preliminary but significant step on the road to chiral discrimination on-the-fly, which is shown to be best propitiated in molecules where vectors are far from having degenerate mutual angular directions. © 2019 the Owner Societies.

Investigations were carried out on the carbon-dots (C-dots) based fluorescent off - on (Fe 3 €‰+ €‰ - S 2 O 3 2 ') and on - off (Zn 2 €‰+ €‰ - PO 4 3 ') sensors for the detection of metal ions and anions. The sensor system exhibits excellent selectivity and sensitivity towards the detection of biologically important Fe 3 €‰+ €‰, Zn 2 €‰+ €‰ metal ions and S 2 O 3 2 ', PO 4 3 ' anions. It was found that the functional group on the C-dots surface plays crucial role in metal ions and anions detection. Inspired by the sensing results, we demonstrate C-dots based molecular logic gates operation using metal ions and anions as the chemical input. Herein, YES, NOT, OR, XOR and IMPLICATION (IMP) logic gates were constructed based on the selection of metal ions and anions as inputs. This carbon-dots sensor can be utilized as various logic gates at the molecular level and it will show better applicability for the next generation of molecular logic gates. Their promising properties of C-dots may open up a new paradigm for establishing the chemical logic gates via fluorescent chemosensors.