Three-dimensional Doppler, polarization-gradient, and magneto-optical forces for atoms and molecules with dark states

We theoretically investigate the damping and trapping forces in a three-dimensional magneto-optical trap (MOT), by numerically solving the optical Bloch equations. We focus on the case where there are dark states because the atom is driven on a ”type-II" system where the angular momentum of the excited state, F', is less than or equal to that of the ground state, F. For these systems we find that the force in a three-dimensional light field has very different behaviour to its one-dimensional counterpart. This differs from the more commonly used “type-I" systems (F' = F +1) where the 1D and 3D behaviours are similar. Unlike type-I systems where, for red-detuned light, both Doppler and sub-Doppler forces damp the atomic motion towards zero velocity, in type-II systems in 3D, the Doppler force and polarization gradient force have opposite signs. As a result, the atom is driven towards a non-zero equilibrium velocity, v₀, where the two forces cancel. We find that v₀² scales linearly with the intensity of the light and is fairly insensitive to the detuning from resonance. We also discover a new magneto-optical force that alters the normal MOT force at low magnetic fields and whose influence is greatest in the type-II systems. We discuss the implications of these findings for the laser cooling and magneto-optical trapping of molecules where type-II transitions are unavoidable in realising closed optical cycling transitions

Three-dimensional Doppler, polarization-gradient, and magneto-optical forces for atoms and molecules with dark states

We theoretically investigate the damping and trapping forces in a three-dimensional magneto-optical trap (MOT), by numerically solving the optical Bloch equations. We focus on the case where there are dark states because the atom is driven on a "type-II" system where the angular momentum of the excited state, $F'$, is less than or equal to that of the ground state, $F$. For these systems we find that the force in a three-dimensional light field has very different behaviour to its one dimensional counterpart. This differs from the more commonly used "type-I" systems ($F'=F+1$) where the 1D and 3D behaviours are similar. Unlike type-I systems where, for red-detuned light, both Doppler and sub-Doppler forces damp the atomic motion towards zero velocity, in type-II systems in 3D, the Doppler force and polarization gradient force have opposite signs. As a result, the atom is driven towards a non-zero equilibrium velocity, $v_{0}$, where the two forces cancel. We find that $v_{0}^{2}$ scales linearly with the intensity of the light and is fairly insensitive to the detuning from resonance. We also discover a new magneto-optical force that alters the normal MOT force at low magnetic fields and whose influence is greatest in the type-II systems. We discuss the implications of these findings for the laser cooling and magneto-optical trapping of molecules where type-II transitions are unavoidable in realising closed optical cycling transitions.Comment: 20 pages, 7 figures. Revised version to correct several small typographical errors and clarify the discussion on page 9. Labeling of figure 1 and colours in figure 5 also changed, and additional information provided for equations 13 and 1

Accelerator measurement of the energy spectra of neutrons emitted in the interaction of 3-GeV protons with several elements

The application of time of flight techniques for determining the shapes of the energy spectra of neutrons between 20 and 400 MeV is discussed. The neutrons are emitted at 20, 34, and 90 degrees in the bombardment of targets by 3 GeV protons. The targets used are carbon, aluminum, cobalt, and platinum with cylindrical cross section. Targets being bombarded are located in the internal circulating beam of a particle accelerator

Reframing \u27the problem\u27: students from low socioeconomic status backgrounds transitioning to university

At the heart of this book are people enrolling at university for the first time and entering into the broad variety of social relations and contexts entailed in their ‘coming to know’ at, of and through university

Laser cooling and magneto-optical trapping of molecules analyzed using optical Bloch equations and the Fokker-Planck-Kramers equation

We study theoretically the behavior of laser-cooled calcium monofluoride (CaF) molecules in an optical molasses and magneto-optical trap (MOT), and compare our results to recent experiments. We use multilevel optical Bloch equations to estimate the force and the diffusion constant, followed by a Fokker-Planck-Kramers equation to calculate the time evolution of the velocity distribution. The calculations are done in three dimensions, and we include all the relevant energy levels of the molecule and all the relevant frequency components of the light. Similar to simpler model systems, the velocity-dependent force curve exhibits Doppler and polarization-gradient forces of opposite signs. We show that the temperature of the MOT is governed mainly by the balance of these two forces. Our calculated MOT temperatures and photon scattering rates are in broad agreement with those measured experimentally over a wide range of parameters. In a blue-detuned molasses, the temperature is determined by the balance of polarization-gradient cooling, and heating due to momentum diffusion, with no significant contribution from Doppler heating. In the molasses, we calculate a damping rate similar to the measured one, and steady-state temperatures that have the same dependence on laser intensity and applied magnetic field as measured experimentally, but are consistently a few times smaller than measured. We attribute the higher temperatures in the experiments to fluctuations of the dipole force which are not captured by our model. We show that the photon scattering rate is strongly influenced by the presence of dark states in the system, but that the scattering rate does not go to zero even for stationary molecules because of the transient nature of the dark states

Isolation and characterization of cGMP phosphodiesterase from bovine rod outer segments

Journal ArticleA simple purification method has been developed for isolation of bovine cGMP phosphodiesterase from photoreceptor rod outer segments. The enzyme is peripherally membrane bound in its native state and is present in relatively high concentrations. In the bovine photoreceptors its molar ratio to rhodopsin can be estimated to be as great as 1:40 and not lower than 1: 170. The isolated enzyme is purified to homogeneity as demonstrated by gel electrophoresis under native and denaturing conditions and analytical ultracentrifuga- Con. The core enzyme has a molecular weight of approximately 170,000, as demonstrated by sucrose gradient centrifugation and analytical ultracentrifugation, and is composed of two major subunits, 88,000 (CY) and 84,000 (p), augmented by a small subunit of 13,000 (y). In its purified state, the enzyme is activated neither by light nor GTP in contrast to the native membranebound enzyme. As isolated, the molar activity of the enzyme is 45 mol of cGMP hydrolized s-' mol-' with a Km = 150 PM (cGMP) and Km > 4 XnM (CAMP). Protamine activates the enzyme to 360 mol of cGMP see-' mol-' @-fold) and limited digestion by trypsin activates it to as great as 2100 mol of cGMP s-l mol-' @O-fold)