Extensive theoretical and experimental studies have shown the hydrogen exchange reaction H+H2 --> H2+H to occur predominantly through a 'direct recoil' mechanism: the H--H bonds break and form concertedly while the system passes straight over a collinear transition state, with recoil from the collision causing the H2 product molecules to scatter backward. Theoretical predictions agree well with experimental observations of this scattering process. Indirect exchange mechanisms involving H3 intermediates have been suggested to occur as well, but these are difficult to test because bimolecular reactions cannot be studied by the femtosecond spectroscopies used to monitor unimolecular reactions. Moreover, full quantum simulations of the time evolution of bimolecular reactions have not been performed. For the isotopic variant of the hydrogen exchange reaction, H+D2 --> HD+D, forward scattering features observed in the product angular distribution have been attributed to possible scattering resonances associated with a quasibound collision complex. Here we extend these measurements to a wide range of collision energies and interpret the results using a full time-dependent quantum simulation of the reaction, thus showing that two different reaction mechanisms modulate the measured product angular distribution features. One of the mechanisms is direct and leads to backward scattering, the other is indirect and leads to forward scattering after a delay of about 25 femtoseconds.

The termolecular association reaction OH + NO 2 + M was studied in a low-pressure discharge flow reactor, and both HONO 2 and HOONO products were detected by infrared cavity ringdown spectroscopy (IR-CRDS). The absorption spectrum of the fundamental ν 1 band of the cis-cis isomer of HOONO (pernitrous or peroxynitrous acid) was observed at 3306 cm -1 , in good agreement with matrix isolation studies and ab initio predictions. The rotational contour of this band was partially resolved at 1 cm -1 resolution and matched the profile predicted by ab initio calculations. The integrated absorbances of the ν 1 bands of the cis-cis HOONO and HONO 2 products were measured as a function of temperature and pressure. These were converted to product branching ratios by scaling the experimentally observed absorbances with ab initio integrated cross sections for HOONO and HONO 2 computed at the CCSD(T)/cc-pVTZ level. The product branching ratio for cis-cis HOONO to HONO 2 was 0.075 ( 0.020(2σ) at room temperature in a 20 Torr mixture of He/Ar/N 2 buffer gas. The largest contribution to the uncertainty is from the ab initio ratio of the absorption cross sections, computed in the double harmonic approximation, which is estimated to be accurate to within 20%. The branching ratio decreased slightly with temperature over the range 270 to 360 K at 20 Torr. Although transperp HOONO was not observed, its energy was computed at the CCSD(T)/cc-pVTZ level to be E 0 ) +3.4 kcal/mol relative to the cis-cis isomer. Statistical rate calculations showed that the conformers of HOONO should reach equilibrium on the time scale of this exeriment. These results suggested that essentially all isomers had converted to cis-cis HOONO; thus, the reported branching ratio is a lower bound for and may represent the entire HOONO yield.

Differential cross section polarization moments: Location of the D-atom transfer in the transition-state region for the reactions Cl+C 2 D 6 →DCl (v ′ =0,J ′ =1)+ C 2 D 5 and Cl+CD 4 →DCl (v ′ =0,J ′ =1)+ CD 3 A 1:4 mixture of HBr and D 2 is expanded into a vacuum chamber, fast H atoms are generated by photolysis of HBr ca. 210 nm, and the resulting HD (vЈ, JЈ) products are detected by (2ϩ1) resonance-enhanced multiphoton ionization ͑REMPI͒ in a Wiley-McLaren time-of-flight spectrometer. The photoloc technique allows a direct inversion of HD (vЈ, JЈ) core-extracted time-of-flight profiles into differential cross sections for the HϩD 2 →HD(vЈϭ1, JЈϭ1,5,8)ϩD reactions at collision energies ca. 1.7 eV. The data reveal a systematic trend from narrow, completely backward scattering for HD (vЈϭ1, JЈϭ1) toward broader, side scattering for HD (vЈϭ1, JЈϭ8). A calculation based on the line of centers model with nearly elastic specular scattering accounts qualitatively for the observations.

Product state(s)-resolved differential cross section of the reaction O ( 1 D)+ HD→OH (v,j)+ D The photoloc technique with core extraction of the nascent product laboratory speed distribution in a Wiley-McLaren time-of-flight spectrometer has been used to measure differential cross sections for the reaction HϩD 2˜H D (vЈϭ2, JЈϭ0,3,5)ϩD at collision energies ϳ1.55 eV. We find that the peak of each angular distribution shifts from complete backward scattering toward side scattering as the rotational excitation of the product increases. We found the same trend in our previous study of HϩD 2˜H D (vЈϭ1, JЈϭ1,5,8)ϩD at ϳ1.70 eV. We conclude that the same type of correlation exists between impact parameter and rotational quantum number in both product vibrational manifolds. Further analysis of the HD (vЈϭ2, JЈ) differential cross section data reveals, however, a clear tendency of this vibrational manifold to scatter sideways at lower JЈ than HD(vЈϭ1, JЈ). Within the framework of a line-of-centers model with nearly elastic specular scattering, this result implies that smaller impact parameters lead to more vibrationally excited products.

In this study, we develop a vegetation monitoring framework which is applicable at a planetary scale, and is based on the BACI (Before-After, Control-Impact) design. This approach utilizes Google Earth Engine, a state-of-the-art cloud computing platform. A web-based application for users named EcoDash was developed. EcoDash maps vegetation using Enhanced Vegetation Index (EVI) from Moderate Resolution Imaging Spectroradiometer (MODIS) products (the MOD13A1 and MYD13A1 collections) from both Terra and Aqua sensors from the years 2000 and 2002, respectively. to detect change in vegetation, we define an EVI baseline period, and then draw results at a planetary scale using the web-based application by measuring improvement or degradation in vegetation based on the user-defined baseline periods. We also used EcoDash to measure the impact of deforestation and mitigation efforts by the Vietnam Forests and Deltas (VFD) program for the Nghe An and Thanh Hoa provinces in Vietnam. Using the period before 2012 as a baseline, we found that as of March 2017, 86% of the geographical area within the VFD program shows improvement, compared to only a 24% improvement in forest cover for all of Vietnam. Overall, we show how using satellite imagery for monitoring vegetation in a cloud-computing environment could be a cost-effective and useful tool for land managers and other practitioners

Scattering of the HD(v′=3,j′=0) product from the H+D2 reaction is measured as a function of angle and collision energy from 1.39 to 1.85 eV. The plot of the cross section vs angle and energy is believed to be the first fully experimental plot of its kind reported for this benchmark reaction. Changes in the differential cross section (DCS) are observed in this collision energy range, including a forward-scattering component that peaks at about 1.64 eV and is a strong function of collision energy. This feature has been assigned to result from a barrier resonance, but its full interpretation is presently unsettled. These changes in the DCS do not manifest themselves as variations in the integral cross section (ICS), which varies less than 25% over the energy range measured. Comparisons of the DCSs and the ICS with quantum mechanical calculations show quantitative agreement, although some aspects of the DCS near 1.54 eV are not fully satisfactory.

A 212.8 nm laser initiates the reaction H+D2→HD+D in a mixture of HBr and D2. A second laser state-selectively ionizes the HD(v′=3,j′) reaction product, allowing a determination of the speed distribution and the relative cross section in a velocity-sensitive time-of-flight mass spectrometer. From these measurements we construct differential and integral cross sections for H+D2→HD(v′=3,j′=0–7)+D at 1.64±0.05 eV collision energy. Although the integral cross sections do not show any unusual features, the differential cross sections reveal forward-scattered features that have not been observed in crossed-beam experiments. An analysis of the scattering features in HD(v′=3,j′=1–4) suggests that these states are dominated by classical hard-sphere scattering. This hard-sphere (direct recoil) mechanism, however, cannot account for the dominant forward scattering observed in HD(v′=3,j′=0).

We investigate the sensitivity of photoinitiated experiments to forward-scattering features by direct comparison of experimental angular distributions with quantum-mechanical calculations as well as by forward-convolution of theoretical and model center-of-mass differential cross sections. We find that the experimental sensitivity to forward-scattering angles depends on the instrumental velocity resolution as well as on the kinematics of the detected product channel. Explicit comparison is made between experimental HD(vЈϭ1,2; jЈ) center-of-mass angular distributions at collision energies Ϸ1.6 eV ͑deduced from time-of-flight profiles using a single-laser, photolysis-probe approach͒ and quantum-mechanical calculations on the BKMP2 potential energy surface. The comparison takes into account the contributions from both slow and fast H atoms from the photolysis of HBr. We find that the contribution of the slow H atoms, which is the major source of experimental uncertainty, does not greatly affect the extraction of the angular distribution from the experimental time-of-flight profile for a specific HD(vЈ, jЈ) state. Except for HD(vЈϭ1, jЈϭ8) and HD(vЈϭ2, jЈϭ0), for which either slow H atoms or the presence of a narrow forward-scattering peak make the analysis more uncertain, the agreement between experiment and theoretical predictions is excellent.