A new supersonic molecular beam-surface scattering apparatus permits eigenstate-resolved measurements of gas-surface reactivity. Infrared light from a narrow-bandwidth tunable laser intersects a supersonic molecular beam and prepares an ensemble of molecules in a single rotational and vibrational quantum state. The energized molecules, with their well-defined translational, vibrational, and rotational energies, pass into an ultrahigh vacuum chamber and impinge on a single crystal metal surface where their reactivity is quantified. The apparatus provides independent control over translational, vibrational, and rotational degrees of freedom and permits highly detailed studies of gas-surface reactivity. In this article we describe the design and characterization of our apparatus and illustrate its use to study the dissociative chemisorption of methane on Ni(100).
A simple adapter converts a manual motion feedthrough device into its motorized equivalent.
A new experimental technique uses state-resolved infrared laser excitation to probe a polyatomic molecule's dissociative chemisorption dynamics with previously unattainable detail. Methane molecules excited to v = 1 of the v(3) C-W stretching vibration are up to 1600 times more reactive on a clean Ni(100) surface than are molecules in the ground vibrational state. Over a translational energy range of 27 to 54 kJ/mol, their absolute reaction probability increases from 3 x 10(-1) to 6 x 10(-3), which indicates that v(3) is responsible only in part for the vibrational activation reported in previous studies.
Many commonly used PC-based data acquisition counter boards are specified to count regularly occurring events at rates up to 7 MHz. Boards with this frequency limit begin to systematically and significantly underestimate the rate of randomly occurring events occurring at average frequencies much lower than the specified 7 MHz rate due to the dead time of the counter. This report highlights the origin of this error and describes an easily constructed prescaler circuit that permits accurate event counting at rates up to, and even exceeding, the nominal speed of the counter.
