Xueming Yang

Performance of the vacuum ultraviolet high-resolution and high-flux beamline for chemical dynamics studies at the Advanced Light Source

At the Advanced Light Source an undulator beamline, with an energy range from 6 to 30 eV, has been constructed for chemical dynamics experiments. The higher harmonics of the undulator are suppressed by a novel, windowless gas filter. In one branchline high-flux, 2% bandwidth radiation is directed toward an end station for photodissociation and crossed molecular-beam experiments. A photon flux of 1016 photon/s has been measured at this end station. In a second branchline a 6.65 m off-plane Eagle monochromator delivers narrow bandwidth radiation to an end station for photoionoization studies. At this second end station a peak flux of 3×1011 was observed for 25 000 resolving power. This monochromator has achieved a resolving power of 70 000 using a 4800 grooves/mm grating, one of the highest resolving powers obtained by a vacuum ultraviolet monochromator.

A differentially pumped harmonic filter on the Chemical Dynamics Beamline at the Advanced Light Source

A differentially pumped rare gas cell has been developed to suppress undulator harmonics on the Chemical Dynamics Beamline at the Advanced Light Source. Greater than 104 suppression of the harmonics has been demonstrated with no measurable (<5%) attenuation of the fundamental. The overall design is presented, and vacuum and optical performance are reported.

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...

Universal crossed molecular beams apparatus with synchrotron photoionization mass spectrometric product detection

Vacuum ultraviolet radiation was generated from an undulator at the Advanced Light Source Synchrotron facility and used for photoionization detection of reaction products in a new universal crossed molecular beams machine. A description of the machine and its performance is presented. Initial experiments on the photodissociation of methylamine (CH3 NH2), ozone (O3), oxalyl chloride [ (OCCl)2] as well as the reactive scattering of Cl with C3 H8 show many of the advantages of photoionization in comparison to electron impact ionization, which has been exclusively used in such instruments in the past. “Momentum matching” of reaction products is much more easily accomplished than in electron impact studies due to suppression of dissociative ionization. The tunability of the vacuum ultraviolet radiation can be used to suppress background from residual gases especially when it is desired to detect free radical reaction products. Even when the tunability cannot be used to suppress background, the fact that little...

A study of the structure and dynamics of the hydronium ion by high resolution infrared laser spectroscopy. III. The ν3 band of D3O

The results of an extensive study of the ν3 (asymmetric stretch) vibration of the H3O+ ion are reported. A total of 86 and 41 transitions were measured with a precision of 0.002 cm−1 for the s–s and a–a inversion subbands, respectively, and least‐squares fit to an accurate Hamiltonian. Twenty‐four parameters were determined in the analysis. The ν3 frequencies are 3535.974(11) and 3519.397(25) cm−1 for the s–s and a–a inversion components, respectively.

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...

A fully state-and angle-resolved study of the H+HD→D+ H2 reaction: Comparison of a molecular beam experiment to ab initio quantum reaction dynamics

We present the results of a joint experimental and theoretical investigation of the reaction dynamics of the H+HD→D+H2 chemical reaction. The experiment was performed using a crossed molecular beam apparatus that employed the Rydberg-atom time-of-flight detection scheme for the product D atom. The photolysis of a HI precursor molecule produced a beam source of hot H atoms, which, when crossed with a cold HD beam, yielded two well-defined center-of-mass collision energies, EC=0.498 and 1.200 eV. The resolution of the experiment was sufficient to allow the measurement of the rovibrationally state-resolved differential cross section from the ground state of the HD reagent. The reaction was modeled theoretically using a converged coupled channel scattering calculation employing the BKMP2 potential energy surface: The S matrix was computed on a grid of 56 energies in the range EC=0.245–1.551 eV. It is found that the experimental and theoretical state-to-state differential cross sections are in quantitative agr...

UV Photodissociation Dynamics of allyl radical by photofragment translational spectroscopy

Photodissociation of the allyl radical, CH2CHCH2, has been studied using the method of molecular beam photofragment translational spectroscopy following excitation to the C(2 2B1) and A(1 2B1) states by 248 and 351 nm photons. Two different primary channels have been detected following 248 nm excitation: H-atom loss (84%) and CH3 elimination (16%). From the product translational energy distribution and polarization dependence studies, dissociation processes from the ground-state C3H5 potential energy surface are inferred for both wavelengths. At 248 nm there may also be a contribution to the H-atom loss channel from predissociation by a higher electronically excited state. Rice–Ramsperger–Kassel–Marcus (RRKM) calculations show that the formation of cyclopropene is not important, while formation of allene and methylacetylene from dissociation of 1- and 2-propenyl radicals are important reaction pathways at both wavelengths. Translational energy distributions peaking well away from zero provide evidence fo...

THE VIBRATIONAL DISTRIBUTION OF THE OH PRODUCT FROM H2O PHOTODISSOCIATION AT 157 NM : DISCREPANCIES BETWEEN THEORY AND EXPERIMENT

Photodissociation of H2O at 157 nm has been studied using the H atom Rydberg tagging time-of-flight technique. Vibrational state distribution has been measured for the OH product from H2O photodissociation. Comparisons with previous theoretical calculations and experimental results by laser-induced fluorescence (LIF) measurements have been made. The results in this work indicate that the relative populations for the high vibrationally excited OH(v≧2) products measured by the LIF technique are significantly underestimated, suggesting that LIF as a technique to quantitatively measure vibrational distributions of reaction product OH is seriously flawed. The experimental results presented here are in fairly good agreement with previous theoretical calculations, even though the calculated vibrational populations for the higher vibrational states of OH are still somewhat overestimated.

Photodissociation of ozone at 193 nm by high‐resolution photofragment translational spectroscopy

The photodissociation of ozone has been studied at 193 nm using high resolution photofragment translational spectroscopy. The results show six distinct peaks in the time‐of‐flight spectra for the O2 product and its momentum‐matched O atom counterpart. The translational energy distributions determined from the time‐of‐flight spectra reveal the production of a range of electronic states of the photofragments. The product electronic states were identified based on the translational energy distributions, with the aid of state‐resolved imaging experiments by Houston and co‐workers. The results reveal the production of a substantial yield of highly excited triplet states of O2, recently suggested to play an important role in the stratospheric ozone balance. In addition, peaks corresponding to O2(a 1Δg) and O2(b 1Σg+) were observed, the latter confirming a previous report [A. A. Turnipseed et al., J. Chem. Phys. 95, 3244 (1991)]. Evidence was seen for a small contribution from the triple dissociation O3→3O(3P), ...