Brian D. Bean

Observation and interpretation of a time-delayed mechanism in the hydrogen exchange reaction

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.

Asymmetric Photolysis with Elliptically Polarized Light

Previous investigations have shown the efficacyof right-(RCPL) and left-(LCPL) circularly polarized light inpromoting the asymmetric photolysis of racemic organic substratesand producing measurable enantiomeric excesses (e.e.s) whenphotolysis is incomplete. Synchrotron radiation, polychromaticand having out-of-plane components which are elliptically andultimately circularly polarized, has been suggested as auniversal source of RCPL and LCPL on a cosmic scale. The moreprevalent right-(REPL) and left-(LEPL) elliptically polarizedcomponents have never been investigated for similar capabilities.The present study, using a 212.8 nm laser beam to mimic thesynchtrotron radiation, explores the potential of REPL and LEPLin this context and finds a qualitative trend indicating thateach induces asymmetric photolysis in the same sense as RCPL andLCPL, but to a lesser degree.

State-resolved differential and integral cross sections for the reaction H+D2→HD(v′=3,j′=0–7)+D at 1.64 eV collision energy

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).

Measurement of the HD(v′=2,J′=3) product differential cross section for the H+D2 exchange reaction at 1.55±0.05 eV using the photoloc technique

We describe a time-of-flight apparatus that uses core extraction to determine nascent product laboratory velocity distributions from which differential cross sections may be deduced. We emphasize the characterization of the instrument, the reaction conditions, and the calibration procedure. For this purpose, we have measured H-atom velocity distributions from HBr photolysis, as well as the H2(v′=4,J′=1) velocity distribution arising from the H+HBr reaction under quasi-monoenergetic collision conditions at 1.9 eV. Collisional energy spread and reagent internal state distributions were determined from the rotational and translational temperatures of the HBr photolytic precursor and the D2 diatomic reagent. The differential cross section for H+D2→HD(v′=2,J′=3)+D at 1.55±0.05 eV is presented and found to peak near 145°±10° with an approximate full width at half maximum (FWHM) of 40°.

Forward scattering in the H+D2→HD+D reaction: Comparison between experiment and theoretical predictions

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(v′=1,2;j′) center-of-mass angular distributions at collision energies ≈1.6 eV (deduced from time-of-flight profiles using a single-laser, photolysis-probe approach) 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...

Differential cross sections for H+D2→HD (v′=2, J′=0,3,5)+D at 1.55 eV

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 H+D2→HD (v′=2, J′=0,3,5)+D at collision energies ∼1.55 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 H+D2→HD (v′=1, J′=1,5,8)+D at ∼1.70 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 (v′=2, J′) differential cross section data reveals, however, a clear tendency of this vibrational manifold to scatter sideways at lower J′ than HD(v′=1, J′). Within the framework of a line-of-centers model with nearly elastic specular scattering, this result implies that smaller impact parameters lead to more vib...

Differential cross sections for H+D2→HD(v′=1, J′=1,5,8)+D at 1.7 eV

A 1:4 mixture of HBr and D2 is expanded into a vacuum chamber, fast H atoms are generated by photolysis of HBr ca. 210 nm, and the resulting HD (v′, J′) products are detected by (2+1) resonance-enhanced multiphoton ionization (REMPI) in a Wiley–McLaren time-of-flight spectrometer. The photoloc technique allows a direct inversion of HD (v′, J′) core-extracted time-of-flight profiles into differential cross sections for the H+D2→HD(v′=1, J′=1,5,8)+D reactions at collision energies ca. 1.7 eV. The data reveal a systematic trend from narrow, completely backward scattering for HD (v′=1, J′=1) toward broader, side scattering for HD (v′=1, J′=8). A calculation based on the line of centers model with nearly elastic specular scattering accounts qualitatively for the observations.

Measurement of the cross section for H+D2→HD(v′=3,j′=0)+D as a function of angle and energy

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.

Kinetics of n-Butoxy and 2-Pentoxy Isomerization and Detection of Primary Products by Infrared Cavity Ringdown Spectroscopy

The primary products of n-butoxy and 2-pentoxy isomerization in the presence and absence of O(2) have been detected using pulsed laser photolysis-cavity ringdown spectroscopy (PLP-CRDS). Alkoxy radicals n-butoxy and 2-pentoxy were generated by photolysis of alkyl nitrite precursors (n-butyl nitrite or 2-pentyl nitrite, respectively), and the isomerization products with and without O(2) were detected by infrared cavity ringdown spectroscopy 20 μs after the photolysis. We report the mid-IR OH stretch (ν(1)) absorption spectra for δ-HO-1-C(4)H(8)•, δ-HO-1-C(4)H(8)OO•, δ-HO-1-C(5)H(10)•, and δ-HO-1-C(5)H(10)OO•. The observed ν(1) bands are similar in position and shape to the related alcohols (n-butanol and 2-pentanol), although the HOROO• absorption is slightly stronger than the HOR• absorption. We determined the rate of isomerization relative to reaction with O(2) for the n-butoxy and 2-pentoxy radicals by measuring the relative ν(1) absorbance of HOROO• as a function of [O(2)]. At 295 K and 670 Torr of N(2) or N(2)/O(2), we found rate constant ratios of k(isom)/k(O(2)) = 1.7 (±0.1) × 10(19) cm(-3) for n-butoxy and k(isom)/k(O(2)) = 3.4(±0.4) × 10(19) cm(-3) for 2-pentoxy (2σ uncertainty). Using currently known rate constants k(O(2)), we estimate isomerization rates of k(isom) = 2.4 (±1.2) × 10(5) s(-1) and k(isom) ≈ 3 × 10(5) s(-1) for n-butoxy and 2-pentoxy radicals, respectively, where the uncertainties are primarily due to uncertainties in k(O(2)). Because isomerization is predicted to be in the high pressure limit at 670 Torr, these relative rates are expected to be the same at atmospheric pressure. Our results include corrections for prompt isomerization of hot nascent alkoxy radicals as well as reaction with background NO and unimolecular alkoxy decomposition. We estimate prompt isomerization yields under our conditions of 4 ± 2% and 5 ± 2% for n-butoxy and 2-pentoxy formed from photolysis of the alkyl nitrites at 351 nm. Our measured relative rate values are in good agreement with and more precise than previous end-product analysis studies conducted on the n-butoxy and 2-pentoxy systems. We show that reactions typically neglected in the analysis of alkoxy relative kinetics (decomposition, recombination with NO, and prompt isomerization) may need to be included to obtain accurate values of k(isom)/k(O(2)).