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

Time-resolved photoelectron angular distributions from nonadiabatically aligned CO2 molecules with SX-FEL at SACLA

Weperformed time-resolved photoelectron spectroscopy of valence orbitals of alignedCO2 molecules using the femtosecond soft x-ray free-electron laser and the synchronized near-infrared laser. By properly ordering the individual single-shot ion images, we successfully obtained the photoelectron angular distributions (PADs) of theCO2 molecules aligned in the laboratory frame (LF). The simulations using the dipole matrix elements due to the time dependent density functional theory calculations well reproduce the experimental PADs by considering the axis distributions of the molecules. The simulations further suggest that, when the degrees of alignment can be increased up to \ue1 cos2 q\uf1 > 0.8, themolecular geometries during photochemical reactions can be extracted fromthe measured LFPADs once the accurate matrix elements are given by the calculations

Electrochemical control and protonation of the strontium iron oxide SrFeOy by using proton-conducting electrolyte

To electrochemically control structural and transport properties of oxygen-deficient perovskite SrFeOy (2.5 ≦ y ≦ 3) (SFO) epitaxial films, we employed electric-field-effect transistor structures in which the proton-conducting solid electrolyte Nafion is used as a gate insulator. When a positive gate voltage (VGS) is applied and protons are injected toward the film channel layer, the SFO films are electrochemically reduced, leading to increases in the channel resistance. On the other hand, when a negative VGS is applied and protons are removed, the SFO films are oxidized, and as a result, the channel resistances decrease. In addition, we found that the electrochemically reduced SFO films accommodate protons, forming the proton-containing oxide HxSrFeO₂.₅ whose proton concentration is determined by elastic recoil detection analysis to be x ∼ 0.11. Our results indicate the usefulness of the proton-conducting solid electrolyte for electrochemically controlling transition metal oxides and for exploring proton-containing oxides

Effects of electronic stopping power on fast-ion-induced secondary ion emission from methanol microdroplets

The formation processes of secondary ions in liquid materials were studied for methanol microdroplets bombarded by carbon ions with incident energies of 0.4–4.0 MeV, where the corresponding electronic stopping power ranged 300–800 eV nm−1. Positive and negative secondary ions including molecular fragments, methanol clusters, and reaction products were investigated, and each ion yield was examined as a function of electronic stopping power Se. We observed different Se-dependence on the emission yields between positive and negative ions. For positive cluster ions [(CH3OH)n + H]+ (n = 2−10), the yield nonlinearly increases and follows the power-law Seα with α = 3. For negative secondary ions, the value of α varies according to secondary ion species or ion mass: α ≈ 0 for fragments with small mass (CH−, CH2−, and OH−), α = 0.5–1.5 for reaction products with medium mass(C2−, C2H−, C2HO−, and C2H5O−), and α = 1.2−1.5 for clusters with large mass [(CH3OH)n – H]− (n = 1−25). The latter finding implies that the value of α is a quantity related to the electronic energy density depending on the distance from the ion trajectory

Fast Heavy-Ion-Induced Anion–Molecule Reactions on the Methanol Droplet Surface

To gain insight into complex ion–molecule reactions induced by MeV-energy heavy ion irradiation of condensed matter, we performed a mass spectrometric study of secondary ions emitted from methanol microdroplet surfaces by 2.0 MeV C²⁺. We observed positive and negative secondary ions, including fragments, clusters, and reaction products. We found that a wider variety of negative ions than positive ions (such as C₂H⁻, C₂HO⁻, C₂H₅O⁻, and C₂H₃O₂⁻) were formed. We performed measurements for deuterated methanol (CH₃OD) droplets to identify the hydrogen elimination site of the intermediates involved in the reactions and to reveal the mechanism that generates various negative reaction product ions. Comparing the results of CH₃OD with CH₃OH droplets, we propose that the primary formation mechanism is association reactions of anion and neutral fragments, such as CH₃O⁻ + CO → C₂H₃O₂⁻. Quantum chemical calculations confirmed that the reactions can proceed with no barrier. This study provides new insights into the importance of rapid anion–molecule reactions among fragments as the mechanism that generates complex molecular species in fast heavy-ion-induced reactions

Effects of radical scavengers on aqueous solutions exposed to heavy-ion irradiation using the liquid microjet technique

The effects of the radical scavenger ascorbic acid on water radiolysis are studied by fast heavy-ion irradiation of aqueous solutions of ascorbic acid, using the liquid microjet technique under vacuum. To understand the reaction mechanisms of hydroxyl radicals in aqueous solutions, we directly measure secondary ions emitted from solutions with different ascorbic acid concentrations. The yield of hydronium secondary ions is strongly influenced by the reaction between ascorbic acid and hydroxyl radicals. From analysis using a simple model considering chemical equilibria, we determine that the upper concentration limit of ascorbic acid with a radical scavenger effect is approximately 70 μM

Relation between biomolecular dissociation and energy of secondary electrons generated in liquid water by fast heavy ions

In this work, we measured and simulated the dissociation of biomolecules in liquid water induced by secondary electrons ejected from water molecules during fast heavy-ion irradiation. We calculated the energy spectra of secondary electrons generated along carbon ion tracks in liquid water in the Bragg peak region. The calculation was done using the Particle and Heavy Ion Transport code System (PHITS) in carbon track structure mode. This mode enables simulation of inelastic collisions along a carbon ion track based on the cross sections considered in the Monte Carlo code KURBUC. To understand the biomolecular dissociation processes in our previous MeV-SIMS experiments with microdroplet targets of glycine solution, we calculated the collision spectra of secondary electrons produced near liquid surfaces using PHITS. Furthermore, we examined the relationship between the secondary electron energy and formation of positive and negative glycine fragments. The results showed that the formation of methylene amine cations is caused by secondary electrons with energies of 13–100 eV. The formation of glycine-related negative ions such as cyanide anion, formate anion, and deprotonated glycine was found to be caused by low-energy (less than 13 eV) secondary electrons. These ions are known products of dissociative electron attachment

Effects of molecular axis orientation of MeV diatomic projectiles on secondary ion emission from biomolecular targets

We report first experimental results on the orientation effect of fast diatomic molecular projectiles on secondary ion emission processes for 3.6-MeV C2+ traversing a self-supporting target of phenylalanine (Phe) film evaporated on a carbon foil. Coincidence measurements were performed on the image of fragments resulting from the Coulomb explosion of C2+ projectiles and the time-of-flight mass spectrometry of secondary ions emitted from the Phe film. We compared the secondary ion yield for C2+ projectiles with parallel and perpendicular orientations to the beam direction. The parallel orientation was found to enhance the secondary ion yield by a factor of approximately 1.1 compared with that for the perpendicular orientation. This enhancement corresponds to an increase in the average charge of Coulomb-exploded fragments. This finding implies that there is a linear correlation between the electronic energy deposition causing secondary ion production and the effective charge number of the incident molecular ions which interact with biomolecular targets