Optically pumped atoms with velocity- and spin-changing collisions at low gas pressures

Journal Article · Thu Apr 15 00:00:00 EDT 2010 · Physical Review. A

DOI:https://doi.org/10.1103/PHYSREVA.81.042703· OSTI ID:21408619

Morgan, Steven W; Happer, William ^[1]

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1. Princeton University Department of Physics, Princeton, New Jersey 08544 (United States)

We discuss optical pumping when (a) the collision rates of optically pumped atoms with atoms or molecules of the background gas are small enough that individual velocity groups can be preferentially excited by a monochromatic light beam, (b) the collision rates are still fast enough to partially transfer the spin polarization to other velocity groups, and (c) there are nonnegligible losses of polarization due to collisional spin relaxation and Larmor precession. These conditions lead to a strong correlation between the velocity and the spin polarization of the atoms - that is, to 'spin-tagging' of the different velocity groups. This regime is similar to that of optically pumped {sup 23}Na atoms of the Earth's upper atmosphere, but it is seldom encountered in laboratory experiments. For cooling and trapping experiments, the collision rates with background gas are negligible. For gas-cell experiments the velocity-changing rates are normally so fast compared to spin relaxation or Larmor precession rates that the atoms have a Maxwellian velocity distribution with negligible correlation between the spin polarization and the velocity. We analyze the limiting cases of strong and weak collisions, which change the velocity by a large or small fraction, respectively, of the mean thermal velocity. The Keilson-Storer model [J. Keilson and A. E. Storer, Q. Appl. Math. 10, 243 (1952)] is used to discuss strong collisions, with memory parameter {alpha}=0 and weak collisions with {alpha}{yields}1. For weak collisions, the physics can be modeled by coupled Fokker-Planck equations, identical to those for forced diffusion in a harmonic-oscillator potential well. In this limit there are solutions analogous to the quantum mechanical coherent states of a harmonic oscillator.

• OSTI ID: 21408619
• Journal Information: Physical Review. A, Vol. 81, Issue 4; Other Information: DOI: 10.1103/PhysRevA.81.042703; (c) 2010 The American Physical Society; ISSN 1050-2947
• Country of Publication: United States
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
74 ATOMIC AND MOLECULAR PHYSICS
ANNIHILATION OPERATORS
ATOMS
COLLISIONS
CORRELATIONS
FOKKER-PLANCK EQUATION
HARMONIC OSCILLATORS
LARMOR PRECESSION
MATHEMATICAL SOLUTIONS
MONOCHROMATIC RADIATION
OPTICAL PUMPING
POLARIZATION
QUANTUM MECHANICS
RELAXATION
SODIUM 23
SPIN
SPIN ORIENTATION
VELOCITY
ANGULAR MOMENTUM
DIFFERENTIAL EQUATIONS
ELECTROMAGNETIC RADIATION
EQUATIONS
ISOTOPES
LIGHT NUCLEI
MATHEMATICAL OPERATORS
MECHANICS
NUCLEI
ODD-EVEN NUCLEI
ORIENTATION
PARTIAL DIFFERENTIAL EQUATIONS
PARTICLE PROPERTIES
PRECESSION
PUMPING
QUANTUM OPERATORS
RADIATIONS
SODIUM ISOTOPES
STABLE ISOTOPES