The reaction Y ؉ CH4 3 HYCH3 3 YCH2 ؉ H2 is initiated by C-H insertion involving a 20 ؎ 3 kcal/mol potential energy barrier. The reaction is studied in crossed molecular beams under two different conditions with nearly the same total energy. One experiment is carried out at a collision energy of 15.1 kcal/mol with one quantum of CH 4 antisymmetric (3) stretching vibrational excitation (8.63 kcal/mol), the other at a collision energy of 23.8 kcal/mol. The reaction cross-section for C-H stretch excited methane (s) is found to be at least a factor of 2.2 times larger than for ground-state methane ( g) at the same total energy. mode-specific chemistry ͉ potential energy barrier ͉ reaction dynamics T he concepts of early and late potential energy barriers made it possible to rationalize in simple, intuitive terms the roles of reactant translational and vibrational energy in promoting atom ϩ diatom reactions (1). The observation of mode-and bond-specific effects in gas phase reactions such as Cl ϩ CH 4 3 HCl ϩ CH 3 and Cl ϩ H 2 O 3 HCl ϩ OH have illustrated that the dynamics of polyatomic systems involving multiple vibrational degrees of freedom can also be highly sensitive to the reactant vibrational state (2, 3).In a recent study, Yan and coworkers provided the first direct comparison of C-H reactant vibrational energy to reactant translational energy in promoting the Cl ϩ CHD 3 3 HCl ϩ CD 3 abstraction reaction (4). Although C-H antisymmetric vibrational excitation enhanced reactivity, it was found to be somewhat less effective than an equivalent amount of reactant translational energy. However, CHD 3 bending excitation induced by thermal excitation was somewhat more effective in promoting reaction than an equivalent amount of translational energy. For gaseous polyatomic systems, different forms of reactant energy may not be equivalent in facilitating passage through the transition state for atom transfer (2-5).The dissociative adsorption of methane (CH 4 ) on a metal surface is the rate-limiting step in the steam re-forming of methane, used to produce Ϸ9 million tons of hydrogen annually in the United States. It is well established that reactant translational and vibrational excitation are both effective in promoting this activated process (6). Significant mode-and bondspecific effects have been observed for this class of reaction. Smith and coworkers showed that antisymmetric CH 4 vibrational excitation ( 3 ) is somewhat more effective than an equivalent amount of translational energy in promoting reaction on a Ni(111) surface (7), in contrast to earlier work on Ni(100) (6) and Pt(111) (8), where translational energy was more effective in promoting reaction. Juurlink et al. (9) demonstrated that CH 4 overtone bending excitation (3 4 ) is much less effective than 3 on Ni(100) and Ni(111), despite the higher energy of 3 4 . Maroni and coworkers (10) found that CH 4 symmetric excitation ( 1 ) is about an order of magnitude more effective than antisymmetric stretching ( 3 ) in promoting reaction on Ni(100) at nearly...

Using two or three commercial pulsed nanosecond dye lasers pumped by a single 30 Hz Nd:YAG laser, generation of 0.10 mJ pulses at 125 nm (6 × 10(13) photons∕pulse) has been demonstrated by resonance enhanced four-wave mixing of collimated (unfocussed) laser beams in mercury (Hg) vapor. Phase matching at various vacuum ultraviolet (VUV) wavelengths is achieved by tuning one laser in the vicinity of the 6 (1)S0 → 6 (3)P1 resonance near 253.1 nm. A number of different mixing schemes are characterized. Our observations using broadband lasers (~0.15 cm(-1) bandwidths) are compared to previous calculations pertaining to four-wave mixing of low intensity narrowband laser beams. Prospects for further increases in pulse energies are discussed. We find that VUV tuning curves and intensities are in good agreement with theoretical predictions. The utility of the VUV light source is demonstrated by "soft universal" single-photon VUV ionization in crossed molecular beam studies and for generation of light at 130.2 nm for oxygen atom Rydberg time-of-flight experiments.

We describe the assembly of piezoelectric actuators suitable for use in Proch-Trickl pulsed gas valves employed in pulsed molecular beam experiments. In their simplest form, these actuators have performance equal to and resistance to chemical attack far superior to the recently discontinued commercial actuator around which the valve was originally designed. New actuators have been designed employing several different dual piezo configurations. The dual piezo design reliably produces shorter pulses than the original actuator, resulting in reduced gas consumption and lower average source chamber pressures for a given backing pressure and pulse rate. By electrically isolating the actuator assembly, active adjustment of the closed position may be achieved by simple addition of a continuous voltage bias.