J. J. Lin

The O(1D)+H2 reaction at 56 meV collision energy: A comparison between quantum mechanical, quasiclassical trajectory, and crossed beam results

Quantum mechanical and quasiclassical trajectory reactive scattering calculations have been performed for the O(1D)+H2 (v=0,j=0) reaction on the Dobbyn–Knowles ab initio 1 1A′ and 1 1A″ potential energy surfaces (PES) at the mean collision energy Ecol=56 meV (1.3 kcal/mol) of a crossed beam experimental study based on H-atom Rydberg “tagging” time-of-flight detection. Novel data from this latter experiment are presented and compared with the theoretical results at the level of state-resolved integral and differential cross sections and product recoil energy distributions. A good overall agreement with small discrepancies is found between the experimental data and the results of the two theoretical approaches. The main conclusion of the present work is that the contribution of the ground state 1 1A′ PES to the global reactivity accounts for the experimental observations and that, at the title collision energy, the participation of the 1 1A″ PES in the reaction is negligible for all practical purposes.

Multiple dynamical pathways in the O(1D)+CH4 reaction: A comprehensive crossed beam study

In this report, the O(1D)+CH4 reaction has been reinvestigated using universal crossed molecular beam methods. Angular resolved time-of-flight spectra have been measured for various reaction channels of the title reaction: OH+CH3, H+H2COH/H3CO, and H2+HCOH/H2CO. Different product angular distributions have been observed for these product channels, indicating that these reaction channels occur via distinctive dynamical pathways. This study provides an excellent example of multiple dynamical pathways in a single chemical reaction, which opens enormous opportunities in investigating the dynamics of complicated chemical reactions that are important in combustion and atmospheric chemistry, and also provides a link between kinetics studies and dynamical research.

Dissociation dynamics of the water molecule on the à 1B1 electronic surface

Photodissociation of H2O, D2O, and HOD on the A 1B1′ surface through 157.6 nm excitation has been studied using the H(D) atom Rydberg tagging time-of-flight technique. Vibrational state distribution has been measured for the OH/OD product from the photodissociation of the H2O, D2O, and HOD molecules. Comparisons of our results with previous theoretical calculations and experimental results obtained using the laser induced fluorescence (LIF) technique have been made. Experimental results in this work indicate that the relative populations for vibrationally excited OH(v⩾2) products measured using LIF 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 rather good agreement with previous theoretical calculations. However, our results indicate that the calculated vibrational populations for the higher vibrational states of OH are still somewhat overest...

Photodissociation dynamics of H2O at 121.6 nm: Effect of parent rotational excitation on reaction pathways

Photodissociation dynamics of H2O at 121.6 nm through the B 1A1′ state have been studied using the high-resolution H atom Rydberg tagging technique. Experimental evidences show two different dissociation pathways to form the ground OH (X,v=0) products: dissociation through the B–X conical intersection, and dissociation through B–A Coriolis coupling. By preparing the H2O molecules at higher rotational temperatures, dissociation through the B–A Coriolis coupling pathway can be enhanced.

COMPETING ATOMIC AND MOLECULAR HYDROGEN PATHWAYS IN THE PHOTODISSOCIATION OF METHANOL AT 157 NM

Photofragment translational spectra at m/e=1(H), 2(H2,D), 3(HD), and 4(D2) have been obtained for CH3OH, CH3OD, and CD3OH at 157 nm excitation. Analysis of the time-of-flight spectra reveals two different atomic H loss channels: hydroxyl H elimination, and methyl H elimination. While the hydroxyl H elimination seems to be a single fast process, the methyl H loss exhibits clearly two significantly different mechanisms: one fast and one slow. Experimental results also show two molecular hydrogen elimination channels: three-center elimination from the methyl group, which displays two different micropathways, and four-center elimination involving hydrogen atoms on both the C and O sites. The relative branching of the atomic versus molecular hydrogen elimination channels was found to be 1:0.15. These results present a uniquely clear picture of methanol photodissociation at 157 nm, and thus provide an excellent case for quantitative theoretical investigations.

Photodissociation dynamics of propyne at 157 nm

Photodissociation of propyne at 157 nm has been investigated using photofragment translational spectroscopy. Detailed investigation of various photofragments from the deuterated compounds CD3CCH and CH3CCD, as well as the unlabeled propyne provides a uniquely clear picture of an inherently complex process. Hydrogen atom elimination processes from both the CH3 group and the C≡C–H group have been clearly observed. H atom elimination from the methyl group appears to be a single dynamical process, while ethynyl H elimination shows two distinctive dynamical pathways with a ratio of 0.30 (fast): 0.43 (slow). The relative contribution of the atomic hydrogen elimination from the two different sites was determined to be 0.73 (ethynyl): 0.27 (methyl). Molecular hydrogen elimination processes have also been observed, but with a much smaller yield compared to the atomic hydrogen elimination (1:9.6). Comparison of the H2 HD and D2 photoproducts from various deuterated propyne molecules shows that the molecular hydroge...

Photodissociation of hydrogen sulfide at 157.6 nm: Observation of SH bimodal rotational distribution

Photodissociation of the H2S molecule at 157.6 nm was studied experimentally using the Rydberg tagging technique. Translational energy distributions of the H-atom product from the H2S photodissociation were measured, and the SH(X 2Π)+H(2S) channel was found to be the dominant dissociation process. Spin-orbit and rovibrational state distributions were also obtained for the SH product, which was found to be both vibrationally and rotationally excited. An intriguing bimodal rotational distribution in the lowest two vibrational states, v=0 and 1, has been clearly observed for the SH product, indicating that there are two distinctive dissociation mechanisms involved in the photodissociation of H2S at 157 nm excitation.

Mass analyzed threshold ionization spectroscopy of p-methoxyaniline cation and influence of the OCH3 substituent

Abstract We report the mass analyzed threshold ionization (MATI) spectra of p -methoxyaniline recorded via the 0 0 vibrationless and several vibrational levels in the S 1 state. The adiabatic ionization energies (IE) of this molecule is determined to be 57 445±5 cm −1 , which is red shifted from that of aniline by about 4826 cm −1 . This large energy shift is attributed by the electron-donating nature of the OCH 3 group, supported by the ab initio calculations. In addition, the presence of the methoxy substituent also influences the vibrational frequencies depending upon the pattern.

Mass analyzed threshold ionization spectroscopy of N-methylaniline, N-ethylaniline, and N,N-dimethylaniline cations: Influence of N-alkyl substitution on the ionization energy and molecular vibration

We have applied two-color resonant two-photon mass analyzed threshold ionization spectroscopy to investigate some ionic properties of N-alkylanilines. The respective adiabatic ionization energies of N-methylaniline (NMA), N-ethylaniline (NEA), and N,N-dimethylaniline (DMA) are determined to be 59 822, 59 204, and 58 018 cm−1 with an uncertainty of about 5 cm−1. This indicates that the longer alkyl chain gives rise to a larger redshift in the IE due to a stronger interaction between the alkyl group and the nitrogen atom in the ionic state. Because the alkyl group gives rise to an increase in the electron density around the nitrogen atom of the neutral species, the IE of DMA is lower than that of NMA. In addition, the N-alkyl substitution also influences the frequency of the internal motion of the cations. However, the frequency variation is dependent upon the vibrational pattern and the extent of the coupling between the N-alkyl group and ring vibrations.

Mass analyzed threshold ionization spectroscopy of N-deuterium substituted indoline cation: isotope effect on the electronic transition, ionization and molecular vibration

Abstract The resonant two-photon ionization (R2PI) and mass analyzed threshold ionization (MATI) spectra of N-deuterium substituted indoline (indoline-d1) have been recorded. The origin of the S1←S0 transition and the adiabatic ionization energy (IE) are found to be 32254±1 and 58284±5 cm −1 , corresponding to the isotope shifts of 38 and 15 cm −1 , respectively. Comparing these data with those of N-deuterated aniline suggests that the five-membered ring is responsible for the observed blue shifts in the electronic energy and IE of indoline-d1. The present results also show that the N-deuteration can lead to a reduction in the frequencies of the characteristic C2-puckering, butterfly, ring twisting, and N-inversion vibrations of indoline.