K . Suominen

Theories for cold atomic collisions in light fields

I describe the current theories for cold collisions between laser-cooled and trapped neutral atoms in light fields. The emphasis is on the dynamical nature of the problem, i.e. on the collisional loss of atoms from the trap or collisional heating of the atomic cloud. The recent work on optical shielding of collisions is also reviewed.

Molecular excitation by chirped pulses

We study numerically the excitation process of a molecular wavepacket when the laser pulse frequency undergoes time-dependent change (chirp). We show that the wave dynamical effects during the excitation process can strongly affect the validity of the standard area theorem. We demonstrate that, when these wave dynamical effects can be neglected, the excitation process can be well described with analytically solvable models in terms of a modified area theorem.

The adiabatic limit of level crossing models

Abstract In this paper we consider simple models describing population transfer at level crossings. Special attention is devoted to the adiabatic limit, its description within the framework of the Landau-Zener model and the asymptotic methods introduced by Dykhne and Davis and Pechukas. In the latter the singularity structure of the adiabatic quasi-energies play a decisive role, and our treatment explores this fact to view the emergence of the adiabatic result. We discuss a model which reduces to the Landau-Zener case in the adiabatic limit, and another model with hyperbolic functions, which gives results differing qualitatively for all choices of the parameters. An exceptional point of the model reduces its solution to the case treated by Rosen and Zener. The full model has been solved analytically by Demkov and Kunike, and the approach to the adiabatic limit can be followed exactly. It turns out that the correct adiabatic result derives, in all cases, from the inclusion of a pair of singularities in the Dykhne-Davis-Pechukas treatment; only in the weak coupling limit is the Landau-Zener result relevant.

Laser-induced dispersion in a three-level system

A theoretical calculation of the polarization changes of a weak probe laser beam as it traverses a strongly pumped gaseous atomic medium is presented. The medium is chosen to be a three-level cascade (J = 0–J = 1–J = 0) system. The polarization of the probe changes from linear to elliptical owing to the asymmetrical population transfer induced by a strong, circularly polarized pumping laser beam. We study this phenomenon as a function of laser intensities, laser detunings, and the relaxation parameters of the medium. We compare the predictions of the theory with results obtained from experimental studies of the neon 1s4(J = 1)–2p4(J = 2)–3s2(J = 1) system.

Quantum and Semiclassical Calculations of Cold Atom Collisions in Light Fields

We derive and apply an optical Bloch equation (OBE) model for describing collisions of ground and excited laser cooled alkali atoms in the presence of near-resonant light. Typically these collisions lead to loss of atoms from traps. We compare the results obtained with a quantum mechanical complex potential treatment, semiclassical Landau-Zener models with decay, and a quantum time-dependent Monte Carlo wave packet (MCWP) calculation. We formulate the OBE method in both adiabatic and diabatic representations. We calculate the laser intensity dependence of collision probabilities and find that the adiabatic OBE results agree quantitatively with those of the MCWP calculation, and qualitatively with the semiclassical Landau-Zener model with delayed decay, but that the complex potential method or the traditional Landau-Zener model fail in the saturation limit.

NONADIABATIC DYNAMICS IN EVAPORATIVE COOLING OF TRAPPED ATOMS BY A RADIO-FREQUENCY FIELD

Magnetically trapped neutral atoms can be cooled with the evaporation technique. This is typically done by using a radio-frequency (rf) field that adiabatically couples trapped and untrapped internal atomic states for atoms with kinetic energies above a value set by the field frequency. The rf field can also induce nonadiabatic changes of internal atomic spin states

Parabolic level crossing models

(F,M)

Weisskopf - Wigner model for wavepacket excitation

that lead to heating and enhanced loss of atoms. In this paper we use wave-packet simulations to show that the evaporation process can induce these nonadiabatic transitions, which change the internal spin state of doubly spin-polarized (2,2) trapped atoms. We also verify the validity of a multistate Landau-Zener model in describing the nonadiabatic dynamics. In addition, we calculate exchange relaxation rate coefficients for collisions between atoms in the

Laser induced polarization properties of a neon three-level system

(2,M)

Cold atoms see the light

states of