Nobuaki Yonekura

Simultaneous production of spin-polarized ions/electrons based on two-photon ionization of laser-ablated metallic atoms

We demonstrate the simultaneous production of spin-polarized ions/electrons using two-color, two-photon ionization of laser-ablated metallic atoms. Specifically, we have applied the developed technique to laser-ablated Sr atoms, and found that the electron-spin polarization of Sr+ ions, and accordingly, the spin polarization of photoelectrons is 64%±9%, which is in good agreement with the theoretical prediction we have recently reported [T. Nakajima and N. Yonekura, J. Chem. Phys. 117, 2112 (2002)]. Our experimental results open up a simple way toward the construction of a spin-polarized dual ion/electron source.

Electron spin-polarized alkaline-earth ions produced by multiphoton ionization

We propose two realistic schemes for producing electron spin-polarized alkaline-earth ions. One of the schemes is based on one-photon resonant two-photon ionization via fine structure manifolds of triplet states, while the other is based on one-photon resonant one-photon ionization. For both schemes we find that the ionization must take place from a triplet state to produce spin-polarized photoions. Photoions produced from a singlet state are always unpolarized regardless of the laser polarization and a magnetic sublevel from which ionization takes place. We carry out specific analysis for both schemes with Sr atoms, and find 85%–90% and 66% polarizations for the former and the latter schemes, respectively, if the wavelengths and polarizations of lasers are judiciously chosen. In particular, if all the produced photoions are in the ground s state, we find an exact one-to-one correspondence between the spin-polarization of photoions and ejected photoelectrons.

Electron-spin polarization of photoions produced through photoionization from the laser-excited triplet state of Sr

We report the detailed experimental study on the production of electron-spin-polarized Sr+ ions through one-photon resonant two-photon ionization via laser-excited 5s5p 3P1 (MJ = +1) of Sr atoms produced by laser-ablation. We have experimentally confirmed that the use of laser-ablation for the production of Sr atoms prior to photoionization does not affect the electron-spin polarization. We have found that the degree of electron-spin polarization is 64+/-9%, which is in good agreement with our recent theoretical prediction. As we discuss in detail, we infer, from a simple analysis, that photoelectrons, being the counterpart of electron-spin-polarized Sr+ ions, have approximately the same degree of electron-spin polarization. Our experimental results demonstrate that the combined use of laser-ablation technique and pulsed lasers for photoionization would be a compact and effective way to realize a pulsed source for spin-polarized ions and electrons for the studies of various spin-dependent dynamics in chemical physics.

Collisional relaxation of the 5d6p states of laser-ablated Ba in He gas at room temperature

The laser-ablation method has been applied for the study of collisional relaxation of the laser-excited Ba atom in He gas at room temperature. When He buffer gas is introduced collisional relaxation processes take place, and as a result depolarization and quenching are seen in the laser-induced fluorescence signal. Both depolarization and quenching rates are determined by solving a set of rate equations and fitting the responsible parameters to the time-resolved fluorescence signal. We have studied such collisional relaxations for the 5d6p J = 1 states of Ba, i.e. 5d6p , , and . It turns out that the quenching is negligible for and in a few hundred Pa of He gas at room temperature, but the quenching for is found to be relatively fast and the corresponding cross section has been found to be . The depolarization cross sections of , , and are found to be , , and , respectively, in He gas at room temperature (295 K).

The 3d←3p endothermic collisional population transfer of Li in He and Ar at 298 K

The 3p state of Li was excited in He and Ar buffer gases at room temperature (298 K) and the time profiles of sensitized fluorescence from the 3s and 3d states were measured. The 3d←3p endothermic population transfer rates determined from the pressure dependence of the time profiles were 6.5×10−11 cm3 s−1 for He and less than 10−3 cm3 s−1 for Ar. The origin of this large difference between He and Ar is discussed in terms of non-adiabatic transitions between the 3p and 3d molecular states of the Li–He and Li–Ar molecules.

Photoionization Characteristics of Sr into 5skl Continua through the Spin-Resolved Ion Detection by Laser-Induced Fluorescence

We study the characteristics of one-photon resonant two-photon ionization of Sr through the spin-resolved detection of photoions by laser-induced fluorescence. In particular, the dependence of spin polarization on the wavelength and polarization of the ionization laser is experimentally investigated. By changing the polarization of the ionization laser from parallel linear to perpendicular linear with respect to the quantization axis defined by the circularly polarized pump laser, a significant difference is found in the degree of spin polarization, whereas the dependence on the laser wavelength at three different wavelengths (308, 266, and 248 nm) turns out to be very small. To understand the experimental results, we have also undertaken a theoretical analysis, which predicts consistent spin polarization with our experimental results for both parallel and perpendicular linear polarization for a given ratio of bound-free transition moment into the 5sks and 5skd continua. The ratio of the bound-free transition probabilities from the bound 5s5 p3P1 state to the 5sks and 5skd continua is evaluated.

Intramolecular Rotational Diffusion Crossover in Supercooled Liquids

Intramolecular rotational diffusion processes of molecular rotors and phenylmethanes in supercooled van der Waals liquids and associated liquids have been studied by measuring temperature dependences of fluorescence intensity of the molecules. Crossovers have been observed in the dependences for all the liquids, of which the temperatures are close to MCT critical temperatures.