Jim J. Lin

Application of time-sliced ion velocity imaging to crossed molecular beam experiments

A three-dimensional (3D) ion velocity imaging method was developed to measure the product velocity distributions in crossed molecular beam experiments. While maintaining conventional two-dimension velocity mapping, the third velocity component was mapped linearly to the ion time of flight. A weak extraction field was used to spread the ion turnaround time to several hundred nanoseconds, which permits good resolution for selection of the longitudinal velocity. A fast gated (⩾5 ns) intensified charge coupled device camera was used to record time-sliced ion images. Calibration of the apparatus was done by measuring O+ images from the multiphoton dissociation/ionization of O2. The resolution in velocity achieved was about 1% (Δv/v) in slicing through the center of a Newton sphere. The overall performance was examined by observing product ion images from the F+CD4→DF+CD3 reaction. To detect CD3+ with kinetic energy release of about 1 eV, 50 ns time slicing provides sufficient velocity resolution, such that res...

Photodissociation of D2O at 121.6 nm: A state-to-state dynamical picture

Photodissociation dynamics of H2O at 121.6 nm have been studied using the H atom Rydberg “tagging” time-of-flight technique and by quasiclassical trajectory (QCT) calculations. Product kinetic energy distributions and angular distributions have been measured. From these distributions, rovibronic distributions of the OH radical product as well as the state resolved angular anisotropy parameters were determined. The dissociation energy D00(H–OH) is determined to be 41151±5 cm−1. Two clear alternations in the OH(X,v=0) rotational distribution have been observed, with each alternation corresponding to an oscillation in the anisotropy distribution. These oscillations had been attributed to the dynamical interference between the two conical intersection pathways. Further theoretical modeling in this work strongly supports this argument. Very highly vibrationally excited OH(X) products (up to v=9) have also been observed. These are ascribed to interconversion of H–O–H bending (H–H vibration) and O–H vibration in...

Crossed-beam scattering of F+CD4→DF+CD3(νNK): The integral cross sections

The title reaction was investigated in a crossed-beam experiment. A (2+1) resonance-enhanced multiphon ionization technique was used to interrogate the internal-state distributions of the CD3 product at three different collision energies. Only the ν2 (umbrella) mode excitation was observed. Its distribution changes from a monotonically declined distribution at low energy to a slightly inverted one at higher collision energy. Although the rotational excitations of CD3 were small, a strong preference for K=0 was found, indicative of the dominance of the tumbling rotation motion of the CD3 product. The vibration-resolved excitation functions were also measured for ν2=0–3. A reaction barrier of 0.5 kcal/mol was deduced.

Probing the effect of the H2 rotational state in O(1D)+H2→OH+H: Theoretical dynamics including nonadiabatic effects and a crossed molecular beam study

Theoretical estimates of reactive cross sections for O(1D)+H2(X,v=0,j)→OH(X)+H(2S), with H2 rotational quantum numbers j=0 and 1, are obtained for a range of collision energies, Ecol. Crossed molecular beam measurements are also used to infer the ratio, r1,0, of the j=1 and 0 cross sections at Ecol=0.056 eV. The theory indicates that the 1 1A′ potential surface is the most important one. However, the 2 1A′ and 1 1A″ surfaces can also contribute. Adiabatic dynamics on the 1 1A″ surface, particularly at Ecol above its 0.1 eV barrier to reaction plays a role. The 2 1A′ surface, while not correlating with ground electronic state products, can still lead to products via nonadiabatic interactions with the 1 1A′ surface. Many quantum dynamics and quasiclassical classical trajectory calculations are carried out. Accurate, ab initio based potential energy surfaces are employed. Quantum cross sections are based on helicity decoupled wave packet calculations for several values of total angular momentum. Nonadiabatic...

Kinetics of a Criegee intermediate that would survive high humidity and may oxidize atmospheric SO2

Significance Ozonolysis of alkenes produces highly reactive Criegee intermediates. Whereas water dimer efficiently scavenges the simplest Criegee intermediate CH2OO in the troposphere, this study clear demonstrates that water vapor does not react with dimethyl substituted Criegee intermediate (CH3)2COO, at least not fast enough to significantly consume (CH3)2COO in the troposphere. On the other hand, (CH3)2COO reacts with SO2 three times faster than CH2OO does, indicating Criegee intermediates of a structure similar to (CH3)2COO are potential candidates for an efficient oxidant in the atmospheric SO2 oxidation. Criegee intermediates are thought to play a role in atmospheric chemistry, in particular, the oxidation of SO2, which produces SO3 and subsequently H2SO4, an important constituent of aerosols and acid rain. However, the impact of such oxidation reactions is affected by the reactions of Criegee intermediates with water vapor, because of high water concentrations in the troposphere. In this work, the kinetics of the reactions of dimethyl substituted Criegee intermediate (CH3)2COO with water vapor and with SO2 were directly measured via UV absorption of (CH3)2COO under near-atmospheric conditions. The results indicate that (i) the water reaction with (CH3)2COO is not fast enough (kH2O < 1.5 × 10−16 cm3s−1) to consume atmospheric (CH3)2COO significantly and (ii) (CH3)2COO reacts with SO2 at a near–gas-kinetic-limit rate (kSO2 = 1.3 × 10−10 cm3s−1). These observations imply a significant fraction of atmospheric (CH3)2COO may survive under humid conditions and react with SO2, very different from the case of the simplest Criegee intermediate CH2OO, in which the reaction with water dimer predominates in the CH2OO decay under typical tropospheric conditions. In addition, a significant pressure dependence was observed for the reaction of (CH3)2COO with SO2, suggesting the use of low pressure rate may underestimate the impact of this reaction. This work demonstrates that the reactivity of a Criegee intermediate toward water vapor strongly depends on its structure, which will influence the main decay pathways and steady-state concentrations for various Criegee intermediates in the atmosphere.

The UV absorption spectrum of the simplest Criegee intermediate CH2OO

SO2 scavenging and self-reaction of CH2OO were utilized for the decay of CH2OO to extract the absorption spectrum of CH2OO under bulk conditions. Absolute absorption cross sections of CH2OO at 308.4 and 351.8 nm were obtained from laser-depletion measurements in a jet-cooled molecular beam. The peak cross section is (1.23 ± 0.18) × 10(-17) cm(2) at 340 nm.

Observation of a reactive resonance in the integral cross section of a six-atom reaction: F+CHD3

The title reaction was investigated under crossed-beam conditions at collisional energies ranging from about 0.4 to 7.5 kcal/mol. Product velocity distributions were measured by a time-sliced, velocity-map imaging technique to explicitly account for the density-to-flux transformation factors. Both the state-resolved, pair-correlated excitation functions and vibrational branching ratios are presented for the two isotopic product channels. An intriguing resonance tunneling mechanism occurring near the reaction threshold for the HF+CD3 product channel is surmized, which echoes the reactive resonances found previously for the F+HD-->HF+D reaction and more recently for the F+CH4 reaction.

New low background crossed molecular beam apparatus: Low background detection of H2

A low background and almost hydrocarbon free (∼1×10−14 Torr) molecular beam apparatus with an improved universal detector, based on electron bombardment ionization, has been constructed for crossed molecular beam research. Extremely high vacuum (∼1×10−12 Torr) for the detector’s ionization region is achieved using multiple ultrahigh vacuum pumps. In addition to a home-made liquid nitrogen cryopump and a turbomolecular pump, a two stage cryogenic He cold head (∼10 K) is used to pump the detector’s ionization region. Using this arrangement, the H2 background in the detector can be reduced by about two orders of magnitude in comparison with previously built similar instruments. Therefore, the signal-to-noise for detecting H2 product detection sensitivity is substantially enhanced, making experimental studies of H2 elimination channels in photodissociation processes much easier. Backgrounds at m/e=28 (CO+), 16 (CH4+,O+), 15 (CH3+), 14 (CH2+), and 13 (CH+) in the ionization detection region are also significan...

Insights into dynamics of the F+CD4 reaction via product pair correlation

To unravel the “extra-atom” complexity of the title reaction, we exploit an experimental approach which, by taking advantage of the correlated information of coincident product pairs, allows us to peel off judiciously the intrinsic complications of a six-atom reaction, extracting the underlying backbone of three-atom dynamics. Examining the collisional energy dependencies of the pair-correlated attributes for a given state(s) of CD3 products from the title reaction, several of major observations can qualitatively be understood, whereas others await further theoretical investigations. An intriguing possibility for the existence of reactive resonances in this six-atom reaction is surmised.

Mode-correlated product pairs in the F+CHD3→DF+CHD2 reaction

The title reaction was investigated at three different collision energies in a pulsed, crossed-beam apparatus. The (2+1) REMPI spectra of the CHD2 products revealed, in addition to the anticipated 4mn vibronic bands, a hitherto unobserved feature. The new feature was shown and assigned to the 311 band. A time-sliced ion velocity imaging technique was applied to map out the coincident DF attributes of the two product states 42 and 31, whose energy levels lie nearly degenerate. Remarkably similar results were found for the two states in every aspect at all three collision energies. A simple model of Fermi-coupled states was proposed to rationalize this, at first sight, surprising finding. Implications to collisional processes which involve mixed molecular basis states in general are outlined. Possible quantum interference phenomenon is suggested.