Semiempirical theory of Carver rates in alkali/noble-gas systems

Abstract

During an encounter between a noble-gas atom and an alkali-metal atom, the electron density at the alkali nucleus is altered, resulting in a collisional change in the alkali's hyperfine coupling (i.e., AI.S-->(A+deltaA)I.S). In the case of binary encounters, this process has been termed the Carver mechanism. The short-lived collisional change in hyperfine coupling can have very noticeable effects: it plays an important role in the loss of nuclear spin polarization in high magnetic fields, and it can be one of the dominant line broadening mechanisms for alkali hyperfine transitions (e.g., in atomic clocks). Unfortunately, though there have been measurements of the Carver relaxation rate, to date there has been little theoretical analysis of the Carver mechanism, in large part due to the very difficult problem of computing deltaA. In the present work, the author develops a theory of the Carver rate based on a semiempirical expression for deltaA(r)/A, where r is the internuclear alkali/noble-gas separation, and validates the theory by comparing to experiment. This model is then used to compute Carver and Bouchiat relaxation rates (i.e., the three-body sticking-collision analog of the Carver rate) in diverse/alkali-noble gas systems. The main conclusion of this work is that Carver rates vary by orders of magnitude across the alkalies, and in general will likely only play a significant role for Rb and Cs noble-gas systems.

Nonlinear Pressure Shifts of Alkali-Metal Atoms in Inert Gases

“…A radial dependence similar to this would be consistent with the temperature dependence of the pressure shift measured by Bean and Lambert [2]. A similar radial dependence was predicted by Camparo [7]. The phase of the density-matrix element of a free Rb atom evolves at the Bohr frequency !…”

Temperature and pressure shift of the Cs clock transition in the presence of buffer gases: Ne,N2, Ar

“…The Cs ground-state hyperfine structure shift in the presence of buffer gases is useful for the study of alkali-metal noble gas interactions and the physics of collisions [3,6,13,41]. It also has applications in vapor-cell atomic clocks, where the use of buffer gases leads to an adverse temperature sensitivity and degrades the long-term frequency stability.…”

Optical pumping and spectroscopy of Cs vapor at high magnetic field

“…The nuclear-spin projection I z , however, increases by more than an order of magnitude. The magnitude of nuclear polarization induced in the vapor is roughly proportional to the rate of hyperfine-shift damping induced by alkali-metal-atom-buffer-gas collisions based on a measured rate for Cs-N 2 [23] and a theoretical estimate for Cs-He [24].…”

“…The hyperfine-shift interaction has a characteristic rate hfs , also known as the Carver rate [24,25] h), the nuclear and electronic polarizations had opposite sign, while for the allowed transitions, they had the same sign. As the hyperfine-shift damping rate hfs increased, the electronspin projection S z increased by about 50%, but the nuclear-spin projection I z increased by more than an order of magnitude.…”