Ryohei Nakata

Laser photochemical ablation of CdWO4 studied with the time‐of‐flight mass spectrometric technique

Pulsed laser ablation of CdWO4 at 266 nm is studied with a quadrupole mass spectrometric (QMS) time‐of‐flight method. Ablation threshold, energy distribution, and angular distribution of the ablated species as well as nonlinearity of the ablated species mass intensity are elucidated as a function of laser fluence. Ablated species of O+2, Cd+, Cd2+, W+, and WO+ translate with energies strongly depending on the fragment mass, meaning that they are confined in a space with the same velocity distribution. Ablated species detected with the QMS filament off show a Gaussian distribution for their translation energy, which is interpreted by the Franck–Condon electron excitation mechanism. A simple model is proposed based on a photochemical bond breaking to explain the observed threshold and nonlinearity of the ablated species. Nonlinearity can be explained by photofragmentation of CdWO4 cluster ions and the successively occurring volume expansion. The latter will be the main cause for the desorption of ion specie...

Laser silicon ablation studied by time-of-flight mass spectrometry : generation of Si ions and neutrals and their translational properties

Laser pulses of Nd:YAG (532 and 266 nm) were irradiated on a Si target to investigate the ablation process. Monoatomic ions and neutrals were simultaneously ablated, with ions having higher most probable kinetic energy (MPKE) and broader velocity distribution than neutrals. The two beams showed close ablation fluence threshold, but different exponential relationships of the intensity of the ablated species vs laser fluence, with the 266 nm beam having a higher increasing rate than that of the 532 nm beam. The 266 nm beam also resulted in higher kinetic energy and a higher fraction of ions in the ejected particle stream. The time-of-flight (TOF) results fitted the shifted Maxwell-Boltzmann distribution well. The laser Si ablation mechanism was discussed.

ESR Study of Manganese Complexes in Heavily Doped CaF2 Crystals

In heavily manganese doped CaF 2 , three kinds of Mn 2+ centers and exchange coupled pairs other than the usual Mn 2+ centers in cubic crystal fields were found at room temperature. The spin Hamiltonian parameters are (I) g =2.00, | D |=740(G), E =36(G), A =80(G), a s =12(G) and a p =2∼3(G); (II) g =2.00, | D |=938(G), E =20(G), A =67(G), a s =11.2(G) and a p =1∼2(G); (III) g =2.00, | D |=1250(G), E =0(G), A =64(G) and a [111] =15.1(G), respectively.

ERR of Off-Center Eu2+ Ions with Resolved Structures in RbCl

In RbCl: Eu 2+ crystals, there are found three groups of EPR spectra with an intensity ratio of 2: 1: 1, which are attributed to off-center displacements of Eu 2+ ions along [111] axis from normal lattice sites. The spectra of the off-center Eu 2+ ions can be described by combining the spin Hamiltonian in an orthorhombic field and the perturbation terms depending on the magnetic field as follows; \(\mathscr{H}{=}g\beta\textbf{\itshape HS}+\sum_{l{=}1}^{3}\sum_{m{=}0}^{l}\text{B\)_2l^2m O _2l^2m\(}+\textbf{\itshape AIS}+u\beta[S_{x}^{3}H_{x}+S_{y}^{3}H_{y}+S_{z}^{3}H_{z}-\frac{1}{5}(\textbf{\itshape SH})[3S(S+1)-1]]+U[S_{x}^{3}I_{x}+S_{y}^{3}I_{y}+S_{z}^{3}I_{z}-\frac{1}{5}(\textbf{\itshape SI})[3S(S+1)-1]]\), where g =1.995, b 2 0 =294.6, b 2 2 =-139.8, A 151 =-31.3, u =-0.2∼0.9, U =0∼-0.05 (10 -4 cm -1 ) at room temperature. The displacement is estimated from the angles between the principal axes and [110] directions to be 0.082 a ( a : anion to cation distance), and the induced dipole moment to be 2.57 ...

ESR study of europium ions in hydrolyzed CaF2: Eu crystals

Abstract Oxidation and reduction processes of europium ions in CaF 2 were studied by optical and ESR methods. Europium ions can be readily oxidized from the divalent to the trivalent states by heat-treatment above 600°C in a moist air atmosphere. The conversion can be well approximated by first order kinetics with an activation energy of 0.75 eV. The reverse conversion from the trivalent to the divalent states can be accomplished by exposure to ionizing X-ray radiation. However, those reduced europium ions are thermally unstable and bleached completely by heating to 200°C. The decay process by low temperature annealing also follows first order kinetics with an activation energy of 0.60 eV. The conversions between the two states are explained by electron transfer.

Angular resolved EPMA analysis of thin films prepared by pulsed laser ablation of CdWO4(010)

Abstract Thin films prepared by focused 266 nm pulsed laser ablation of CdWO 4 (010) single crystals were analyzed with an electron probe micro analysis (EPMA) as a function of the desorbing angle θ from the normal surface, which corresponds to the position of the films. The spatial distribution of the ablated particles and the chemical composition of the films were studied by focusing the effects of O 2 gas pressure and the substrate-target distance. These results were interpreted by the comparison with the size of the ablation plume determined by the photograph. The thin films with nearly the stoichiometric composition showed a photoluminescence similar to the single crystal.

ESR studies of Mn2+ centres in Sr1−xBaxF2:Mn crystals

Abstract A new orthorhombic Ma 2+ centre was observed by ESR in Sr 1− x Ba x F 2 :Mn crystals at room temperature. Least square fitting of the angular dependence of the ESR lines to the spin-Hamiltonian revealed that the spin-Hamiltonian parameters are g = 2.001, B 2 0 = −18.29, B 2 2 = −45.02, B 4 0 = 0.016, B 4 2 = −0.042, B 4 4 = −0.058 and A = −100.4 (Gauss). A probable model for the new ESR centre is a cubic Mn 2+ perturbed by a divalent barium ion placed at the nearest neighbour strontium site along the [110] direction. The spin-Hamiltonian parameters B 2 0 and B 2 2 are compared with theoretically estimated values using the polarizable point charge (PPI) and the superposition (SP) models.

ESR Study of New Eu2+-Complex in Additively Colored KCl: Eu Crystals

A new ESR spectrum due to Eu 2+ ions was found in additively colored KCl: Eu crystals as well as KOH doped KCl: Eu crystals at room temperature. The angular dependence of the new spectrum has a tetragonal symmetry around [001] axis and could be analyzed by a spin Hamiltonian \begin{aligned} \mathscr{H}=\textbf{\itshape g}\beta\textbf{\itshape H}\cdot\textbf{\itshape S}+b^{0}_{2}O^{0}_{2}+b^{0}_{4}O^{0}_{4}+A_{\text{i}}\textbf{\itshape S}\cdot\textbf{\itshape I} \end{aligned} with S =7/2 and I =5/2. The spin Hamiltonian parameters were determined as g =1.995, | b 0 2 |=1189(G), b 0 4 =-2.3(G), A 151 =30.1(G) and A 153 =13.5(G). Both ESR and optical absorption studies suggest that the new Eu 2+ complex is originated from the association of the Eu 2+ ion with an O 2- ion at [001] site which was produced by the decomposition of an OH - ion.

EPR Study of Characteristics of Fe3+ Ions with Cubic Symmetry in CaF2 Crystals

CaF2 single crystals containing Fe3+ ions with cubic symmetry are prepared by co-doping alkali ions as charge compensators, and they are studied at room temperature with an x-band EPR spectrometer. The spectrum observed is described by a spin Hamiltonian, \mathscrH=gβHS+(a/6)[Sx4+Sy4+Sz4-(1/5)S(S+1)(3S2+3S-1)], where S=5/2 and Sz is along [001] direction. The spin parameters are determined as g=2.005±0.0005 and |a|=(37.3±0.4)×10-4 cm-1. The EPR signals are stable at room temperature, but they decay appreciably rapidly at high temperatures above 550°C. The activation energies for the decay are determined as ranging from 1.1 to 2.9 eV, which are dependent upon the kinds of doped alkali ions and also upon the atmospheres around the crystals during the heat treatments.

ESR studies of Mn2+Ba2+ Centres in Ca1 − xBaxF2:Mn

Abstract An orthorhombic Mn2+ centre was observed with x-band ESR at room temperature in Ca1 − xBaxF2:Mn crystals. A least-squares fit of the angular dependence of the ESR fields to an orthorhombic spin Hamiltonian gave values of g = 2.001, B20 = −20.59, B22 = −63.10, B40 = −0.016, B44 = −0.016, B44 = 0.028 and A = −102.7 (Gauss). The observed ESR centre is a typical cubic Mn2+ centre perturbed by a divalent barium ion replaced at the nearest neighbour divalent cation site to the Mn2+ ion along the [100] direction. The theoretical spin Hamitonian parameters of B20 and B22 were estimated to be B20 = −12.58 and B22 = −45.34 (Gauss) with a polarizable point ion (PPI) model. By using lattice sites of eight displaced fluorine ions around the Mn2+ ion and R 0 = 2.3085 A calculated with the PPI model, the superposition (SP) parameters were determined to be t = 5.1 and B 2 (R 0 ) = −77.32 (Gauss).