The distribution in center‐of‐mass energy caused by the thermal motion of the target gas molecules has been rigorously derived for the case of a monoenergetic particle beam interacting with target molecules having an isotropic Maxwellian velocity distribution corresponding to temperature T°K. Provided the nominal c.m. energy E0 exceeds a few kT, the distribution has a full width at half‐maximum (FWHM) of W1/2=(11.1γk T E0)1/2. where γ=m/(m+M), m and M being the projectile and target masses. This is identical to the width derived previously in a one‐dimensional approximate treatment by Bethe and Placzek. The exact and approximate distributions differ significantly, however, in shape and mean energy, particularly at low values of E0/γkT. The Doppler width, W1/2, is shown to significantly affect the appearance curve of the products of endothermic reactions involving heavy particles. Convolution integrals are derived for a number of idealized forms of the cross section for such reactions. In the extreme case ...

Doppler Broadening in Beam Experiments

An electron monochromator for use in an axial magnetic field is described. Electrons are injected parallel to the magnetic field and an electric field is applied in a perpendicular direction. The electrons thus describe trochoids and drift in a direction perpendicular to both the electric and magnetic fields and disperse according to their initial velocities. An electron energy width at half‐maximum of 0.020 eV can be obtained, with a transmitted current of about 10−9 A.

Characteristics of the Trochoidal Electron Monochromator

The experimental methods used to characterize molecular anions differ substantially depending on the stability of the species. The application of laser spectroscopic methods, discussed elsewhere in this issue, is generally limited to anions possessing hound electronic ground states or to those with lifetimes against autodetachment of the electron in excess of a few microseconds. For a great many molecules, including such important prototypes as ethylene, butadiene, benzene, naphthalene, formaldehyde, and acetylene, the ground states of the anions are known to be unstable in the gas phase with lifetimes less than lo-'* s, typically in the neighborhood of s. Even for those molecules with hound ground-state anions, nearly all of the excited anion states lie in the continuum and hence decay by electron detachment. Thus from consideration of numbers alone, the manifold of temporary anion states

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Temporary Anion States of Polyatomic Hydrocarbons

Study of triplet states and short-lived negative ions by means of electron impact spectroscopy

At the very early time of irradiation, ballistic secondary electrons are produced as the most abundant of the radiolytic species directly within DNA or its environment. Here, we demonstrate the propensity of such low-energy (<3 eV) electrons to damage DNA bases via an effective loss of hydrogen located at the specific nitrogen positions. Since this site is directly implicated in the bonding of nucleobases within DNA and since dehydrogenation of the nucleic acid bases has been observed to be the predominant dissociative channel, the present findings foreshadow significant implications for the initial molecular processes leading to genotoxicity in living cells following unwanted or intended exposure to ionizing radiation (e.g., sunbathing, air travel, radiotherapy, etc.).

G. J. Schulz

Papers

Trochoidal Electron Monochromator