Relaxation Mechanisms in Optical Pumping

Abstract

The distribution of population in the ground-state sublevels of an optically pumped alkali-metal vapor has been found to be strongly dependent on the mechanisms assumed responsible for relaxation. Two modes of optical pumping zero mixing and complete mixing in the excited state, and three modes of alkali relaxation, uniform, Zeeman, and electron randomization, are considered. Calculations of the sublevel relative populations are made in terms of experimentally measurable parameters for all six combinations of pumping and relaxation modes. Optical-pumping transient signals are then used to demonstrate that alkali relaxation due to wall collisions is of the low-frequency "Zeeman" variety, while relaxation due to alkali-rare-gas collisions takes place through randomization of the spin of the alkali valence electron. Equations for the rate of change of the electronic spin polarization are derived for all cases, and are found to be nonexponential, in contrast with the case of an alkali of zero nuclear spin.

Response of a Bell–Bloom Magnetometer to a Magnetic Field of Arbitrary Direction

“…Considering a vapor cell which contains 133 Cs atoms and buffer gas in an external static magnetic field , where is the unit vector along the z 0 -axis, the 133 Cs atomic spins, which can be described by a spin polarization vector , will precess about the z 0 -axis from a macroscopic view. When the density of 133 Cs atoms is low, so that the atomic system is not in the spin-exchange relaxation free regime [ 8 ], considering the spin relaxation due to some relaxation mechanisms [ 26 ], and neglecting the spin polarization of 133 Cs atoms in the lower ground-state hyperfine level, since it is much smaller than that of 133 Cs atoms in the upper ground-state hyperfine level in the general case and under our experimental condition [ 8 , 27 , 28 ], the evolution of , which is represented as in the laboratory reference frame x 0 y 0 z 0 , satisfies the following Bloch equation [ 27 , 28 ]: …”

“…The pump light attempts to improve the spin polarization of 133 Cs atoms along the y 0 and z 0 axes [ 28 , 29 ]. Meanwhile, the absorption of pump light by the 133 Cs atoms randomizes the direction of atomic spins, relaxing the transverse and longitudinal spin components [ 26 , 28 ]. Adding the influences of optical pumping, Equation (1) becomes …”

Coherent population trapping under periodic polarization modulation: Appearance of the CPT doublet

“…For vapor-phase atoms described by a density matrix ρ, S z equals Tr[ρS z ] [29] and to compute the temporal variation of S z under polarization modulation, we will assume uniform relaxation of the ground-state Zeeman sublevels [33]. While this is a fair assumption in situations where relaxation is dominated by diffusion, in our experiments relaxation more likely proceeds though electron randomization arising from Rb/N 2 collisions [23].…”

A Rubidium Clock Model,

“…Thus, one *does not complicate the physical picture by differentiating among uniform, electron randomization and Zeeman relaxation mechanisms. 26 [At the present time., there appears to be no compelling reason for considering relaxation more rigorously (see Ref. 20).]…”