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The microwave spectrum of acrolein has been studied in the 19–37 kmc region. A series of R-type transitions was identified for the ground and three excited torsional states of the s-trans isomer and these spectra were successfully fitted in the rigid rotor approximation. Using the ground state moments and additional electron diffraction data a reasonable though not unique structure has been obtained. The torsional frequency was found to be 200±50 cm—1 from relative intensity measurements, and a lower limit of about 2300 cal was estimated for the barrier hindering trans-cis conversion. The dipole moment for the ground state was found to be 3.11±0.04 d, oriented about 14° from the carbonyl group. The results obtained and the absence of a strong s-cis spectrum establish the s-trans isomer as the predominant form.

Fully angular-resolved kinetic-energy distributions of alkali and halogen atoms emitted due to electron-stimulated desorption have been measured for (100) and (110) alkali-halide surfaces. An unexpected strong directional emission of nonthermal halogen atoms along the (100) axis of the investigated crystals has been found which is in contradiction to the predictions of the previously proposed "Pooley model." We propose a new model involving diffusion of hot holes to the surface, and sudden localization of the excited hole at the surface of the crystal.PACS numbers: 79.20. Kz, 7i.35.+z, 78.55.Fv, 82.65.My Since the early work of Townsend and Kelly [1] and Palmberg and Rhodin [2], it is known that electron irradiation of alkali-halide surfaces results in the efficient desorption of alkali and halogen atoms. Several attempts have been made to construct a model of electron-stimulated desorption (ESD) in alkali halides [3][4][5][6][7][8]. In particular, the "Pooley model" has been applied as an explanation of ESD from alkali halides by Townsend [3,6]. The main idea of this model is that nonradiative decay of the bulk self-trapped exciton followed by a focused replacement sequence along the closely packed (110) chains of the halogen atoms is responsible for halogen desorption. Thus, this bulk-exciton model predicts strong directional emission of halogen atoms along the (110) directions of the crystal. Despite a rather wide acceptance of the Pooley model, we believed that a comprehensive experimental test of its predictions had not yet been performed. Early attempts to measure angular dependence of emitted species were done by means of a collector method [3,6,9]. Unfortunately this technique does not provide any information about either mass or charge of the different desorbing species that arrive at the collected deposit and stick with various probabilities. In addition, resputtering of the collector deposit by reflected primary electrons, strongly focused along surface crystallographic directions [10], is likely to influence such results.The first time-of-flight measurements for ESD of alkali halides were performed by Overeijnder et al. [11] for compressed powder samples. The only published energy distributions for halogen atoms desorbed from singlecrystal material were obtained by Postawa and coworkers [12,13]. In the work we report here, for the first time a direct correlation has been measured among the direction of ejection, the mass and translational energy of a desorbed particle, and the crystallographic structure of a well-characterized insulator surface. In particular, we present a direct comparison between angle-resolved energy spectra of bromine atoms taken for (100) and (110) surfaces of KBr. The data demonstrate that independent of the surface orientation the directional ejection of nonthermal halogen atoms takes place along the (100) axis of the bombarded crystals rather than along the (110) direction as predicted by the Pooley model. Our data strongly indicate the need for a new theoretical approach t...

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