reporting their measurements on excitation cross sections in helium, also calculated K= 0. 89+ 0. 04. We obtained K= 0. 89 + 0.03 by neglecting the electric field perturbation of the ground state; inclusion of this correction resulted in K = 0. 92 + 0. 03 as reported in Sec. II.Their observed transmission curve for 2'So atoms corresponds to a value of K well below ours; this is due to poorly known velocity distribution and plate separation, neither of which was important to their main work.It was suggested to us that metastable He quenching could serve as a useful method for measuring electric fields in a beam apparatus, since often other considerations of the experiment make it difficult to know the effective plate separation accurately. ' Having measured the transmission of Eq.(6) with K= 0. 926+. 020 gives the field strength uniquely if the velocity distribution is known.We would like to thank B. Schiff for sending us a report of his and Pekeris's work [Schiff and Pekeris (unpublished)].Using their newly calculated oscillator strength between the m'So and n'P& states in Eq. (3), for m = 1 and 2 and n =4 and 5, instead of the corresponding f values of Table I, we arrived at essentially the same values for E. J. Burger and A. Lurio recently measured the oscillator strength between the 1 80 and n P& state, for n=2 and 3. Their results are in excellent agreement with the

This article describes observations made during a recent series of single-mode lasercom experiments in which high-rate data transmission was demonstrated between a small aircraft and a ground station separated by distances up to 80 km. A significant result of the subsequent data analysis was the discovery of near-unity correlations between the signal fluctuations observed by power monitors at the two ends of the link. This evidence of reciprocity is presented, along with the description of a preliminary concept for utilizing this channel state information to improve link performance.

We present an overview of an air-to-ground laser communications demonstration performed at MIT Lincoln Laboratory. Error-free communication at 2.5 Gb/s was demonstrated along a 25-km slant path between a 1-in transmit aperture on an aircraft at 12 kft altitude and ground terminal with 4 separate 1-cm receivers. Power fluctuations from turbulence-induced scintillation are mitigated in the spatial domain by use of the multiple ground receivers and in the time domain by the use of forward error correction and interleaving. The optical terminals are monitored by multiple high-rate sensors which allow us to quantify total system performance.

The effect of nuclear mass on the g factor of the bound electron has been determined by measuring G=gj(R)/gj{D) to a precision pf 3x 10" 11 using a pulsed double-mode hydrogen maser. The result, G = 1 + (7.22 ±0.03) x 10" 9 , is in excellent agreement with the theory of Grotch and Hegstrom.The past several years have witnessed lively theoretical and experimental activity on the question of the interaction of atomic hydrogen with a magnetic field. 1 The problem is important not only because of the intrinsic interest in understanding the nature of electromagnetic interactions of composite systems, but because it bears on the validity of the Zeeman theory of hydrogen which plays an important role in understanding the hydrogen fine structure.The magnetic moment of the bound electron provides the most sensitive test for the theory of hydrogenlike atoms in external fields. The following result has been derived by Grotch and Hegstrom 2 for the g factor of hydrogenic atoms in the IS state:where g e is the free-electron g factor and M is the nuclear mass. This result has been derived independently by Faustov 3 and Close and Osborn 4 using different calculational techniques. The leading correction term (Za) 2 /3, the well-known Breit correction, 5 has so far not been observed directly because of the technical problem of comparing free and bound electrons. Consequently, interest has centered on the mass-dependent terms which can be observed by measuring the isotope shift of the bound-electron g factor. For hydrogen and deuterium we have, with G=gj(H, lS)/gj(D, IS),