Yuan T. Lee

Molecular Beam Reactive Scattering Apparatus with Electron Bombardment Detector

A crossed beams apparatus for study of thermal energy neutral-neutral reactions is described. The detector comprises a high efficiency (∼0.1%) electron bombardment ionizer, quadrupole mass filter, scintillation ion counter, and gated scalers synchronized with the beam modulation. The ionizer is nested within three chambers, each pumped by a separate ion pump and the innermost attached to a liquid nitrogen trap. The design is such that molecules which pass through the ionizer without being ionized fly on into another differentially pumped region before hitting a surface. The entire detector unit, including pumps and trap, is mounted on a rotatable platform (angular range ∼140°) which forms the lid of the scattering chamber. The beam sources also comprise modular units, mounted in differentially pumped side chambers which insert into ports in the scattering chamber. Angular distributions of reaction products have been measured for several reactions of Cl, Br, or D atoms with halogen molecules or hydrogen ha...

Earth System Science for Global Sustainability: Grand Challenges

Progress in understanding and addressing both global environmental change and sustainable development requires better integration of social science research. Tremendous progress has been made in understanding the functioning of the Earth system and, in particular, the impact of human actions (1). Although this knowledge can inform management of specific features of our world in transition, societies need knowledge that will allow them to simultaneously reduce global environmental risks while also meeting economic development goals. For example, how can we advance science and technology, change human behavior, and influence political will to enable societies to meet targets for reductions in greenhouse gas emissions to avoid dangerous climate change? At the same time, how can we meet needs for food, water, improved health and human security, and enhanced energy security? Can this be done while also meeting the United Nations Millennium Development Goals of eradicating extreme poverty and hunger and ensuring ecosystem integrity?

Local modes of benzene and benzene dimer, studied by infrared–ultraviolet double resonance in a supersonic beam

We used rotational cooling of molecules to ∼5 K by supersonic expansion and state‐selective, multilevel saturation spectroscopy to obtain high‐resolution spectra of the fundamental and first and second overtone transitions of C–H stretching modes in ground‐electronic‐state benzene and its dimer. Greatly reduced linewidths (<3 cm−1 FWHM) in the rich spectra show that previously reported spectra have suffered from inhomogeneous congestion. Our observed spectral widths indicate that the vibrational lifetimes of the C–H stretches are at least a few ps, even at the energy of the second overtone (8800 cm−1). The ‘‘local mode’’ picture appears to apply when at least three quanta of C–H stretching motion are present. Spectra of the dimer are similar to those of the monomer but show a red shift of a few cm−1, the appearance of combination bands involving van der Waals vibrational modes, some intensity changes, and a broadening of spectral features that increases with the vibrational energy. The dimer’s predissocia...

Intermolecular Potentials from Crossed‐Beam Differential Elastic Scattering Measurements. IV. Ar+Ar

High‐resolution low‐energy elastic differential cross sections have been measured for Ar–Ar over a wide angular range. Rainbow structure as well as symmetry oscillations at wide angels were resolved. These results are used to evaluate 16 different Ar–Ar potentials proposed previously and also to construct a new potential. Vibrational energy levels and the second virial coefficient calculated for the new potential are compared with experimental results.

Molecular Beam Kinetics: Reactions of Deuterium Atoms with Halogen Molecules

Angular distributions and velocity distributions have been measured for deuterium halides reactively scattered from crossed beams of D atoms and five halogen molecules, Cl2, Br2, I2, ICl, and IBr. The D beam was obtained either from a tungsten furnace at ∼2800 ° K or a microwave discharge at ∼350 ° K, the halogen beam from a supersonic nozzle source at ∼300–350 ° K. A small computer was employed as a multiscalar to record the time of flight spectrum of the product at each observation angle. Data of satisfactory quality (signal‐to‐noise ratio > lim ∼ 8) were obtained with rather short counting times (> lim ∼ 1000 sec) for reactive scattering signals of a few counts per second. Contour maps of the differential reaction cross sections are derived from an approximate transformation to the center‐of‐mass coordinate system. The main results found with D atoms at ∼2800°K are as follows: (1) In the Cl2 reaction, most of the DCl recoils into the backward hemisphere with respect to the incoming D atoms; the angular...

Ozone photolysis: A determination of the electronic and vibrational state distributions of primary products

A high resolution molecular beam measurement of the energy spectra of O3 photolysis products was performed at 266 nm. (AIP)

Evidence for stepwise dissociation dynamics in acetone at 248 and 193 nm

The technique of molecular beam photofragment translational spectroscopy has been used to study the dissociation of acetone following S1←S0 (248 nm) and S2←S0 (193 nm) excitation. Excitation at 248 nm resulted in the production of CH3 and CH3CO with 14.2±1.0 kcal/mole on average of the available energy appearing as translation of the photofragments. Comparison of the measured 〈ET〉 with values reported at 266 nm suggest that the energy partitioning is dominated by the exit barrier caused by an avoided crossing on the potential energy surface. A substantial fraction (30±4%) of the nascent acetyl radicals from the primary dissociation contain sufficient energy to undergo spontaneous secondary decomposition. From the onset of the truncation of the CH3CO P(ET) a threshold of 17.8±3.0 kcal/mole for the dissociation of the acetyl radical has been determined in agreement with recent results on the photodissociation of acetyl chloride. The translational energy release in the dissociation of CH3CO closely matches t...

Infrared multiphoton dissociation of RDX in a molecular beam

Infrared multiphoton dissociation (IRMPD) of hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX) in a molecular beam has been performed in order to investigate the mechanism of RDX thermal decomposition. A beam of molecules was crossed by a pulsed TEA CO2 laser and velocity distributions of the various products were measured by the time‐of‐flight (TOF) technique as a function of the laboratory angle using a mass spectrometric detector. The dissociation channels, their branching ratios, and the translational energy distributions of the products were determined. In contrast to the conventional view of simple bond rupture through loss of NO2 as the dominant primary channel in RDX decomposition, it was found that the dominant primary channel is concerted symmetric triple fission to produce three CH2N2O2 fragments which subsequently undergo secondary concerted dissociation to produce HCN, H2CO, HONO (or HNO2), and N2O. A total of two primary and four secondary dissociation channels were observed. Concerted reactions predominate over simple bond rupture not only in the number of channels (four vs two) but also in the amount of products. A fair amount of translational energy release through concerted reaction channels was observed, which is significant for an explanation of the energies of RDX decomposition.

Simple bond rupture reactions in multiphoton dissociation of molecules

A molecular beam study was carried out of the infrared multiphoton dissociation of seven molecules dissociating by a simple bond rupture. Translational energy distributions for the dissociation products were obtained, and the results can be described very well by a statistical (RRKM) theory of unimolecular reactions. A general discussion of the main features of the multiphoton dissociation process is given.

The vibrational predissociation spectroscopy of hydrogen cluster ions

Although molecular ion spectroscopy has progressed rapidly[1], there has been very little spectroscopy done of weakly bound ionic clusters[2,3]. These clusters play an important role in atmospheric chemistry and have been studied extensively in thereto-chemical and kinetic experiments over the past decade[4]. Much of our understanding of the dynamics and structure of cluster ions to date has relied on the results of ab initio quantum calculations. Stimulated by the theoretical work, we have begun experiments on the infrared spectroscopy of cluster ions, and have recently made the first spectroscopic observation of the protonated hydrogen clusters H n + (n=5,7,9,11,13 and 15).