Spin Exchange In Collisions Research Articles

Complex cross sections of spin exchange in collisions of sodium and potassium atoms in the ground state

Complex cross sections of the spin exchange are calculated for the first time for interacting Na and K atoms based on the data on the singlet (X1Σ+) and triplet (a3Σ+) potentials that describe the interaction of these alkali-metal atoms in the ground state. The obtained cross sections allow one to theoretically consider the polarization transfer processes and calculate the relaxation times and the magnetic resonance frequency shifts caused by Na-K spin-exchange collisions.

Chemiionization and spin exchange in collisions of triplet metastable atoms in a mixture of helium isotopes

Cross sections of spin exchange and chemiionization in collisions of two triplet metastable helium atoms are calculated for different isotopes of helium. The data on the spin exchange cross sections for the systems studied are reported for the first time.

Experimental determination of the rate constant for spin exchange in collisions of polarized metastable helium atoms with ground-state cesium atoms

The rate constant for spin exchange in a system consisting of a metastable helium atom and an alkali-metal atom is determined. An experiment on optical orientation of atoms established that the rate constant for spin exchange in a collision of a metastable 23S1 helium atom with a cesium atom in the 62S1/2 ground state equals (2.8±0.8)×10−9 cm3 s−1. The rate constant for chemoionization of cesium atoms by metastable helium atoms was determined at the same time to be (1.0±0.3)×109 cm3s−1.

Rotational energy transfer within the Ã2A1state of NH2: absolute rate coefficients and the influence of spin exchange in collisions with H atoms

Experiments are described which concern rotationally inelastic events occurring within the (0, 9, 0) vibronic level of the Ã2A1state of NH2. Continuous-wave (c. w.) dye-laser excitation and dispersion of the result­ing fluorescence allows the observation of specific rotational state to state processes. It is demonstrated experimentally that hydrogen atoms are the dominant agent in causing these inelastic events. Absolute rate coeffi­cients for state to state transfer are quoted and are shown where relevant to agree with the principle of microscopic reversibility. Propensity rules are clearly evident, and it is found that transitions with ΔK= 0 and 2 occur with very similar rates. The rate data are interpreted in terms of the restricted distorted-wave Born model. This model is found to give a semi-quantitative fit to the experimental data. In order to explain anomalies in rates among levels of para symmetry a collisionally induced diabatic mechanism is proposed in which high levels of the ground X~2B1state of NH2are involved. The electronic spectrum of NH2possesses resolvable doublet structure. A series of experiments are described in which the relative populations within collisionally populated spin doublets were measured. These results are interpreted in terms of the theory set out in the preceding paper (Dixon & Field 1978) for encounters in which both collision partners are open-shell. We show that our findings point to a marked spin correlation in the interaction potential. The rate data allow us to draw conclusions regarding the anisotropy of the potential of interaction of H and NH2. The analysis of the spin branch­ing data indicates the strength of the anisotropy of the triplet and singlet surface to be approximately in the ratio 1:4. Such conclusions are found to be consistent with the physical picture that emerges from a qualitative knowledge of surfaces of the NH3system. The general relevance of our data to laser modelling and of spin exchange to interstellar maser action is noted.

Spin Exchange in Collisions between Atoms

Spin Exchange in Collisions between Atoms

Spin Exchange in Collisions between Atoms

Polarization phenomena in collisions between one-electron (alkali-like) atoms are calculated with density-matrix techniques. The kinetic energies for which this treatment is valid are limited from above by the use of the Born-Oppenheimer approximation, and from below by the assumed degeneracy of the hyperfine states. The basic mechanism for polarization changes is the electron-exchange effect, as suggested by Purcell and Field and by Wittke and Dicke. The special situation of unpolarized targets is treated in some detail. In this case, there are two numbers which characterize the scattering through any angle: the absolute values of certain direct and exchange scattering amplitudes, ${F}_{d}$ and ${F}_{x}$. All cross sections must be expressible in terms of $|{F}_{d}|$ and $|{F}_{x}|$, and this is explicitly carried out in two cases of special interest. One refers to the case in which only the electron-spin polarization is measured, and the other to measurements of the polarization of the hyperfine states of the scattered atom. Effects of the identity of the atoms are also studied in detail.