Collision Dynamics of Argon Cluster Ions, Ar^+_n (n=3-23) : Molecular Dynamics Simulation Based on Diatomics-in-Molecules Method

The molecular dynamics method combined with a quantum mechanical calculation has been used to simulate the collision between an argon atom and an argon cluster ion, Ar^+_n(n=3-23), which contains a given amount of internal energy. Two pathways were observed; (i) Evaporation after collisional energy transfer to the internal degrees of freedom vs. (ii) fusion via complex formation. The total reaction cross sections were compared with those experimentally obtained. It is found that the branching fractions of the evaporation and the fusion depend critically on the internal energy and the impact parameter

Optical pumping by a laser pulse traveling in a cavity

We have developed a general method to perform optical pumping by a pulsed laser with the aid of an optical cavity ͑cavity-assisted optical pumping͒. Optical pumping is achieved by repetitive interaction of a single laser pulse with a target material in the cavity. This method is demonstrated for manganese ions, Mn + , stored in a linear radio-frequency ion trap; about 10 8 ions are spin polarized by a 5-ns laser pulse via 7 P J ← 7 S 3 ͑J =3 or 4͒ transition in the ultraviolet region. The linewidth of the pulsed light source is broad enough to transfer the populations of lower hyperfine levels to the highest one ͑F =11/ 2͒ in the 7 S 3 ground state; the nuclear spin is polarized as well. DOI: 10.1103/PhysRevA.77.033417 PACS number͑s͒: 32.80.Xx, 32.10.Fn, 37.10.Ty, 37.20.ϩj Optical pumping ͓1͔ is a powerful technique for spin polarization widely used since the first idea of Kastler in 1950 ͓2͔. It is operated by repeated cycles of absorption of circularly polarized light and spontaneous emission back to the initial state. This cycle transfers angular momentum of photons to target atoms. The atoms eventually reach a nonstatistical population distribution, where only one of the magnetic sublevels is populated. Spin-polarized atoms and nuclei thus produced have a variety of applications ͓3-5͔: Highly precise spectroscopy especially with double resonance techniques ͓6,7͔, spin-exchange collisions ͓8͔, manipulation and statecontrol of atoms ͓9͔, sensitive magnetometry ͓10͔, and so forth. Although the advent of tunable lasers greatly expanded the application of optical pumping, the light source has been limited to continuous-wave ͑cw͒ lasers. This is due to the long interaction time needed to repeat the pumping cycles until the spin-polarization process is completed; the time scale is typically longer than several microseconds. Therefore, standard nanosecond laser pulses are not suitable for the light source. Recently, several schemes have been proposed for pulsed lasers to generate spin-polarized atomic ions without relying on optical pumping ͓11͔. The elaborate schemes, however, have been applied only to alkaline-earth elements; the ground-state atoms in 1 S 0 are excited by a circularly polarized laser pulse to 3 P 1 ͑M J = +1͒, and further ionization results in spin-polarized ions in the 2 S 1/2 ground state. The advantage of pulsed lasers over cw ones, particularly in the tunability in short wavelengths, urges us to develop a new method with broader applicability. In this paper, we present "cavity-assisted optical pumping," which allows us to perform optical pumping by a laser pulse with the aid of an optical cavity. It is shown that repeated interaction of a single laser pulse produces highdegree spin polarization of target materials in the cavity. This method provides a general technique for using pulsed lasers in a manner similar to cw light sources. The broad linewidth inherent to pulsed lasers enables nuclear spins to be polarized as well by exciting transitions split by hyperfine structures. In the experiment, we have created a spin-polarized ensemble of about 10 8 ions of manganese, Mn + , stored in a linear ion trap. Mn + has a nuclear and an electron spin of I =5/ 2 and J = 3, respectively, in the ground state ͑ 7 S 3 ͒ with an electronic configuration of 3d 5 4s 1 . We have observed spin polarization of Mn + ions in the highest angular momentum of F = J + I =11/ 2; the ions are forced to populate in the sublevel of the magnetic quantum number M = +11/ 2 by interaction with a laser pulse of + circular polarization

Breathing Vibration of Ar Clusters Analyzed by Molecular Dynamics Calculation : Cluster-Shape Dependence of the Mode-Separation

The vibrational motion of Ar clusters (Ar_n, n=20 and 30) having isomers in a variety of shapes was simulated by use of the molecular dynamics method and the mode-separation of the breathing vibration from the quadruple spheroidal vibration was investigated. It was found that these modes of highly spherical isomers were almost fully separated from each other, while coupling between these modes were significant in non-spherical isomers. The relation between the cluster shape and the mode-separation was elucidated

Experimental and theoretical studies of clusters

ix, 281 p. : ill. ; 24 cm

Molecular Dynamics Simulation of Dynamic Solvation Effect in the Collision of I_2^-(CO_2)_n Cluster with Si Surface

Cluster-surface collision induced dissociation of an I_2^- molecule initially embedded in a I_2^-(CO_2)_n cluster was investigated. Molecular dynamics simulation which provides a microscopic description for energy acquisition in the cluster-surface impact. The trajectory calculations using a realistic Si surface model indicate that the collisions of I_2^-(CO_2)_n with a Si surface can be treated as perfectly elastic ones. The dissociation probability of I_2^- was computed for I_2^-(CO_2)_n with n=0-6 on the basis of a hard-wall model. The size dependence of the dissociation probability ascribable to the wedge effect by a CO_2 molecule located halfway between the I atoms, which is consistent with experimental result by Yasumatsu et al