~ ' - Y "S.~ ,6 ,~:" ,~ ~. ~ .i ~, ELSEVIER 11 October 1996 Chemical Physics Letters 261 (1996) 145-154 CHEMICAL PHYSICS LETTERS Toward understanding the role of Stark effects when probing the nuclear hyperfine states of atomic hydrogen K. Thomas Lorenz, Kenneth A. Cowen z, Patrick E. Fleming, Michael G. Mathews, Michael F. Herman, Brent Koplitz Department of Chemistry, Tulane University, New Orleans, LA 70118, USA Received 12 April 1996; in final form 2 July 1996 Abstract A variation on velocity-aligned Doppler spectroscopy is used to probe the nuclear hyperfine states of atomic hydrogen resulting from the 193 nm photodissociation of HBr. By utilizing 1 + 1 resonance-enhanced ionization through the Lyman-c~ transition along with a novel experimental approach, one can observe the onset of Stark effects that come about because of the modest electric field present in the ionization region of the mass spectrometer. A straightforward theoretical treatment is used to address the observed behavior. 1. Introduction By virtue of its simplicity and overall importance, atomic hydrogen has the well-earned distinction of being the single most scrutinized element in physics and chemistry. In atomic physics, the case of hydrogen is central to our understanding of proton/electron interactions as well as the effects of electric and magnetic fields on these interactions. When viewing the hydrogen atom as a reactive species, however, the more subtle features of the quantum mechanical nature of the H atom often become masked. The focus of this Letter is one of these subtle interactions: the Stark effect in atomic hydrogen in very weak electric fields. Recently, we implemented a variation on velocity-aligned Doppler spectroscopy (VADS) in order to probe the nuclear hyperfine states of atomic hydrogen as the product states resulting from a photochemical reaction: the 193 nm photodissociation of HBr [1]. The detection method incorporated in the experiment involves 1 + 1 resonance ionization through Lyman-ec [2-4] and is applied in the ionization region of a time-of-flight mass spectrometer. During the course of the most recent experiments, it became apparent that subtle Stark effects could be observed even under modest extraction voltages. Since the electric field strength within the ionization region is easily varied, the spectral resolution inherent in the modified VADS method allows one to readily observe the "turning on" of interactions that accompany the increasing electric field. i Present address: Tosoh SMD Inc., 3600 Gantz Road, Grove City, OH 43123, USA. 0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0009-261 4(96)00908-6