Spin Selection Rules Research Articles

Nuclear spin selection rule in the photochemical reaction of CH3 in solid parahydrogen

Photolysis of a methyl radical CH3 in solid parahydrogen produces a methane molecule CH4 via the reaction between an intermediate singlet methylene CH21 and a parahydrogen molecule H2. Conservation of nuclear spin during the reaction has been investigated by the intensity distribution of the rotation-vibration spectrum of methane produced by the reaction. It was found that the population of each nuclear spin state of methane just after the reaction was different from that of the statistical ratio, which indicates that a nuclear spin selection rule does exist in the reaction. However, the observed population was significantly different from the theoretically predicted ratio. The discrepancy between the experiment and the theory may indicate a breakdown of the nuclear spin conservation during the reaction, if the reaction mechanism in solid parahydrogen is the same as in the gas phase.

Japan's program for back-end of nuclear fuel cycle flexibility is the key

The excited states generated in an organic light-emitting diode (OLED) by recombination of opposite charges may be singlet or triplet states, the latter being formed in ratios as high as 3 : 1 over the singlets. Such triplet states are normally non-emissive in a purely organic device, due to the spin selection rule. This chapter describes how phosphorescent complexes of heavy metal ions, particularly iridium(III) and platinum(II), can be used as dopants to induce emission from the triplet states, exploiting the high spin-orbit coupling constants of these elements. Design strategies for preparing highly luminescent metal complexes are discussed, together with methods for tuning the emission colour through rational structural modification. Examples of some representative blue, red and white-emitting systems are presented.

Transfer of spin orientation into electric current in quantum wells

A monopolar spin polarization has been achieved in quantum well structures applying terahertz circularly polarized radiation. The spin polarization results in a directed motion of free carriers in the plane of a quantum well perpendicularly to the direction of light propagation. Due to spin selection rules the direction of the current is determined by the helicity of the light and can be reversed by switching the helicity from right- to left handed.

Spin Selection Rules in Radiative and Nonradiative Recombination Processes in Bulk Crystals and in Thin Films of II-VI Compounds

The origin of a fast component of Mn 2+ intra-shell photoluminescence (PL) decay is discussed. We demonstrate that efficient spin-dependent interactions between localised spins of Mn ions and spins of free carriers result in a variation of formation and decay rates of PL emissions in diluted magnetic semiconductors and affect spin selection rules for radiative recombination transitions.

Spin-Dependent Energy Transfer from Exciton States into the Mn2+(3d5) Internal Transitions

Wide band-gap (Zn,Cd,Mn)Se/ZnSe quantum well structures were grown by MBE. Q-discs have been prepared of these parent structures by electron beam lithography followed by dry and wet-chemical etching. With increasing magnetic field a decrease of the energy transfer from excitonic states into the localised internal Mn 2+ (3d 5 ) states has been observed. The effect is caused by the spin dependence of the energy transfer. The underlying mechanism is discussed, highlighting the importance of bound exciton states for fulfilling the spin selection rule.

Wavelength selective modulation in femtosecond pump–probe spectroscopy and its application to heme proteins

We demonstrate novel lock-in detection techniques, using wavelength selective modulation of ultrafast pump and probe laser pulses, to discriminate between vibrational coherence and electronic population decay signals. The technique is particularly useful in extracting low frequency oscillations from the monotonically decaying background, which often dominates the signal in resonant samples. The central idea behind the technique involves modulating the red and/or blue wings of the laser light spectrum at different frequencies, ΩR and ΩB, followed by a lock-in detection at the sum or difference frequency, ΩR±ΩB. The wavelength selective modulation and detection discriminates against contributions to the pump–probe signal that arise from degenerate electric field interventions (i.e., only field interactions involving different optical frequencies are detected). This technique can be applied to either the pump or the probe pulse to enhance the off-diagonal terms of the pump induced density matrix, or to select the coherent components of the two-frequency polarizability. We apply this technique to a variety of heme-protein samples to reveal the presence of very low-frequency modes (∼20 cm−1). Such low-frequency modes are not observed in standard pump–probe experiments due to the dominant signals from electronic population decay associated with resonant conditions. Studies of the diatomic dissociation reaction of myoglobin (MbNO→Mb+NO), using wavelength selective modulation of the pump pulse, reveal the presence of an oscillatory signal corresponding to the 220 cm−1 Fe–His mode. This observation suggests that the spin selection rules involving the ferrous iron atom of the heme group may be relaxed in the NO complex. Mixed iron spin states associated with adiabatic coupling in the MbNO sample could explain the fast time scales and large amplitude that characterize the NO geminate recombination.

Conversion of spin into directed electric current in quantum wells.

A nonequilibrium population of spin-up and spin-down states in quantum well structures has been achieved applying circularly polarized radiation. The spin polarization results in a directed motion of free carriers in the plane of a quantum well perpendicular to the direction of light propagation. Because of the spin selection rules the direction of the current is determined by the helicity of the light and can be reversed by switching the helicity from right to left handed. A microscopic model is presented which describes the origin of the photon helicity driven current. The model suggests that the system behaves as a battery which generates a spin polarized current.

Reacting polymers with highly correlated initial conditions

We propose and theoretically study an experiment designed to measure short time polymer reaction kinetics in melts or dilute solutions. The photolysis of groups centrally located along chain backbones, one group per chain, creates pairs of spatially highly correlated macroradicals. We calculate time-dependent rate coefficients $\kappa(t)$ governing their first order recombination kinetics, which are novel on account of the far-from-equilibrium initial conditions. In dilute solutions (good solvents) reaction kinetics are intrinsically weak, despite the highly reactive radical groups involved. This leads to a generalised mean field kinetics in which the rate of radical density decay $-\ndot \twid S(t)$ where $S(t)\twid t^{-(1+g/3)}$ is the {\em equilibrium} return probability for 2 reactive groups, given initial contact. Here $g\approx 0.27$ is the correlation hole exponent for self-avoiding chain ends. For times beyond the longest coil relaxation time $\tau$, $-\ndot \twid S(t)$ remains true, but center of gravity coil diffusion takes over with rms displacement of reactive groups $x(t)\twid t^{1/2}$ and $S(t)\twid 1/x^3(t)$. At the shortest times ($t\lsim 10^{-6}$ sec), recombination is inhibited due to spin selection rules and we find $\ndot \twid t S(t)$. In melts, kinetics are intrinsically diffusion-controlled, leading to entirely different rate laws. During the regime limited by spin selection rules, the density of radicals decays linearly, $n(0)-n(t)\twid t$. At longer times the standard result $-\ndot\twid dx^3(t)/dt$ (for randomly distributed ends) is replaced by $\ndot\twid d^2x^3(t)/dt^2$ for these correlated initial conditions. The long time behavior, $t>\tau$, has the same scaling form in time as for dilute solutions.

Photon upconversion properties of Ni2+ in magnetic and nonmagnetic chloride host lattices.

Near-infrared to visible upconversion luminescence in Ni2+:CsCdCl3, Ni2+:CsMnCl3, and Ni2+:RbMnCl3 is presented and analyzed. In all three materials upconversion occurs via a sequence of ground-state absorption/excited-state absorption processes, which are both formally spin-forbidden transitions. Consequently, in the diamagnetic Ni2+:CsCdCl3 they are weak, and the efficiency of the upconversion process is relatively low. This is in clear contrast to the isostructural Ni2+:RbMnCl3 where the spin selection rule relaxes because of Ni(2+)-Mn2+ exchange interactions, leading to an intensity enhancement of the spin-flip transitions involved in the Ni2+ upconversion mechanism. This results in an exchange-induced enhancement of the upconversion rate in Ni2+:RbMnCl3 relative to Ni2+:CsCdCl3 by 2 orders of magnitude after two-color excitation into the maxima of the ground-state and excited-state absorption bands. In Ni2+:CsMnCl3 the Ni(2+)-Mn2+ exchange interaction does not play a significant role. This is due to the different Ni(2+)-Cl(-)-Mn2+ bridging geometry relative to Ni2+:RbMnCl3. In contrast to Ni2+:CsCdCl3 and Ni2+:RbMnCl3 where the upconversion luminescence occurs from Ni2+, in Ni2+:CsMnCl3 the upconverted energy is emitted from Mn2+ in the visible spectral region. This leads to an enhanced visible upconversion luminescence in Ni2+:CsMnCl3, relative to the other two samples where Ni2+ near-infrared inter-excited-state emissions compete with the visible upconversion luminescence.

Selection rules for nuclear spin modifications in ion-neutral reactions involving H3+

We present experimental evidence for nuclear spin selection rules in chemical reactions that have been theoretically anticipated by Quack [M. Quack, Mol. Phys. 34, 477 (1997)]. The abundance ratio of ortho-H3+ (I=3/2) and para-H3+ (I=1/2), R=[o-H3+]/[p-H3+], has been measured from relative intensities of their infrared spectral lines in hydrogen plasmas using para-H2 and normal-H2 (75% o-H2 and 25% p-H2). The observed clear differences in the value of R between the p-H2 and n-H2 plasmas demonstrate the spin memory of protons even after ion-neutral reactions, and thus the existence of selection rules for spin modifications. Both positive column discharges and hollow cathode discharges have been used to demonstrate the effect. Experiments using pulsed plasmas have been conducted in the hollow cathode to minimize the uncertainty due to long-term conversion between p-H2 and o-H2 and to study the time dependence of the o-H3+ to p-H3+ ratio. The observed R(t) has been analyzed using simultaneous rate equations assuming the nuclear spin branching ratios calculated from Quack’s theory. In p-H2 plasmas, the electron impact ionization followed by the ion-neutral reaction H2++H2→H3++H produces pure p-H3+, but the subsequent reaction between p-H3+ and p-H2 scrambles protons. While the proton hop reaction (rate constant kH) maintains the purity of p-H3+, the hydrogen exchange reaction (rate constant kE) produces o-H3+ and acts as the gateway for nuclear spin conversion. The value of R(t), therefore, depends critically on the ratio of their reaction rates α=kH/kE. From observed values of R(t), the ratio has been determined to be α=2.4. This is in approximate agreement with the value reported by Gerlich using isotopic species.

Spin Fractionalization of an Even Number of Electrons in a Quantum Dot

We find that Kondo resonant conductance can occur in a quantum dot in the Coulomb blockade regime with an even number of electrons N. The contacts are attached to the dot in a pillar configuration, and a magnetic field B( perpendicular) along the axis is applied. B( perpendicular) lifts the spin degeneracy of the dot energies. Usually, this prevents the system from developing the Kondo effect. Tuning B( perpendicular) to the value B(*) where levels with different total spin cross restores both the degeneracy and the Kondo effect. We analyze a dot charged with N = 2 electrons. Coupling to the contacts is antiferromagnetic due to a spin selection rule and, in the Kondo state, the charge is unchanged while the total spin on the dot is S = 1/2.

Sequential magnetotunneling in a vertical quantum dot tuned at the crossing to higher spin states

We have calculated the linear magnetoconductance across a vertical parabolic Quantum Dot with a magnetic field in the direction of the current. Gate voltage and magnetic field are tuned at the degeneracy point between the occupancies N=2 and N=3, close to the Singlet-Triplet transition for N=2. We find that the conductance is enhanced prior to the transition by nearby crossings of the levels of the 3 particle dot. Immediately after it is depressed by roughly 1/3, as long as the total spin S of the 3 electron ground state doesn't change from S=1/2 to S=3/2, due to spin selection rule. At low temperature this dip is very sharp, but the peak is recovered by increasing the temperature.

X-ray emission and photoelectron spectra of Pr0.5Sr0.5MnO3

Abstract The results of measurements of X-ray photoelectron (XPS), X-ray emission (XES) and X-ray absorption spectra and LSDA band structure calculations of Pr0.5Sr0.5MnO3 are presented. The excitation energy dependence of Mn L2,3 and O Kα X-ray emission spectra of Pr0.5Sr0.5MnO3 is measured using tunable synchrotron radiation. Comparing XPS and XES measurements and the LSDA band structure calculations, one concludes that the Mn 3d states are localized near the top of the valence band. Some differences in the Mn 3d-distribution in the upper part of the XPS valence band and Mn L3 XES are due to spin selection rules.

X-ray emission and photoelectron spectra ofPr0.5Sr0.5MnO3

The results of measurements of x-ray photoelectron (XPS), x-ray emission (XES), and x-ray absorption spectra and local spin-density approximation band structure (LSDA) calculations of ${\mathrm{Pr}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ are presented. The excitation energy dependence of Mn ${L}_{2,3}$ and O $K\ensuremath{\alpha}$ x-ray emission spectra of ${\mathrm{Pr}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ is measured using tunable synchrotron radiation. The XES measurements yielded no photon energy dependence for the O $K\ensuremath{\alpha}$ spectra, but the Mn ${L}_{2,3}$ spectra yielded inelastic scattering losses of 2 and 6 eV, corresponding to features in the structure of the occupied part of the valence band. Comparing XPS and XES measurements with LSDA band-structure calculations, one concludes that the electronic structure of the compound consists mainly of Mn $3d$ and O $2p$ states. States of $3d$ character localized at the Mn site predominate near the top of the valence band (VB). Some differences in the Mn $3d$ distribution in this part of the XPS valence band and Mn ${L}_{3}$ XES with d symmetry due to spin-selection rules that govern the Mn ${L}_{3}$ XES. In addition, the Mn $3d$ states distribution is hybridized with the O $2p$ part of the VB. Mn ${L}_{3}$ XES spectra were determined relative to the Fermi energy by assuming normal x-ray emission begins from the lowest level of the ${p}^{5}{d}^{n+1}L$ intermediate state (which is the Mn $2p$ ionizatation threshold). From the local spin-density approximation, the orbital character of the Mn $3d$ electrons can be assigned ${e}_{g}$ symmetry at the top of the valence band ${T}_{2g}$ in the central part of the VB, and equal contributions of ${e}_{g}$ and ${t}_{2g}$ states at the bottom of the valence band.

Energy-resolved collision-induced dissociation of Aln+ clusters (n=2–11) in the center of mass energy range from few hundred meV to 10 eV

Energy-resolved collision-induced dissociation (CID) of Aln+ (n=2–11) in collision with argon is presented for the energy ranges from few hundred meV to 10 eV in the center of mass frame. The experiments were carried out with a recently constructed secondary ion tandem mass spectrometer, that is described in detail. The collision energy dependence is measured for the total and the partial dissociation cross sections, and the dissociation thresholds for the individual processes are estimated. The release of Al+ is found to be the dominating channel for n<8. For n>8, the cross section for the release of Al+ and Al are comparable. The release of more than one neutral atom from the larger clusters (n>6) is found to be in good agreement with sequential atom loss. In the case of the smaller clusters, on the other hand, fission is the energetically favorable process. The closed shell cluster, Al7+ (20 valence electrons), is found to be exceptionally stable and the adiabatic ionization potential of Al7 is found to be lower than that of the monomer. The stability of Al7+ is further reflected in the dissociation dynamics of the next neighbor, Al8+. The high stability of Al7+ as well as the dissociation dynamics of Al8+ are treated in the simple frame of the electronic shell model. Unlike Al7+, Al3+ (with 8 valence electrons) shows no sign of increased stability, and the dissociation dynamics seems to be controlled by the spin selection rules, rather than the energetics. In the present work, general trends and the dissociation dynamics of individual clusters are discussed. Qualitative information on the development of the geometric and electronic structure, with increasing cluster size, is deduced and discussed in terms of a transition from a covalent to a metallic character. Finally, this work is compared to earlier theoretical and experimental approaches to Aln+ clusters.

Vertically coupled double quantum dots in magnetic fields

Ground-state and excited-state properties of vertically coupled double quantum dots are studied by exact diagonalization. Magic-number total angular momenta that minimize the total energy are found to reflect a crossover between electron configurations dominated by intra-layer correlation and ones dominated by inter-layer correlation. The position of the crossover is governed by the strength of the inter-layer electron tunneling and magnetic field. The magic numbers should have an observable effect on the far infra-red optical absorption spectrum, since Kohn's theorem does not hold when the confinement potential is different for two dots. This is indeed confirmed here from a numerical calculation that includes Landau level mixing. Our results take full account of the effect of spin degrees of freedom. A key feature is that the total spin, $S$, of the system and the magic-number angular momentum are intimately linked because of strong electron correlation. Thus $S$ jumps hand in hand with the total angular momentum as the magnetic field is varied. One important consequence of this is that the spin blockade (an inhibition of single-electron tunneling) should occur in some magnetic field regions because of a spin selection rule. Owing to the flexibility arising from the presence of both intra-layer and inter-layer correlations, the spin blockade is easier to realize in double dots than in single dots.

Transport properties of quantum dots

AbstractLinear and nonlinear transport through a quantum dot that is weakly coupled to ideal quantum leads is investigated in the parameter regime where charging and geometrical quantization effects coexist. The exact eigenstates and spins of a finite number of correlated electrons confined within the dot are combined with a rate equation. The current is calculated in the regime of sequential tunneling. The analytic solution for an Anderson impurity is given. The phenomenological charging model is compared with the quantum mechanical model for interacting electrons. The current‐voltage characteristics show Coulomb blockade. The excited states lead to additional fine‐structure in the current voltage characteristics. Asymmetry in the coupling between the quantum dot and the leads causes asymmetry in the conductance peaks which is reversed with the bias voltage. The spin selection rules can cause a ‘spin blockade’ which decreases the current when certain excited states become involved in the transport. In two‐dimensional dots, peaks in the linear conductance can be suppressed at low temperatures, when the total spins of the corresponding ground states differ by more than 1/2. In a magnetic field, an electron number parity effect due to the different spins of the many‐electron ground states is predicted in addition to the vanishing of the spin blockade effect. All of the predicted features are consistent with recent experiments.

Absolute differential cross sections for electron-impact excitation of CO near threshold: I. The valence states of CO

Absolute differential cross sections for the electronic excitation of valence states in CO have been measured in the energy range from threshold to 3.7 eV. In particular, a newly developed experimental technique to control the analyser's efficiency allowed us to measure absolute angular dependences of those states for scattering angles between and . The transitions from the ground state to the and states are forbidden by spin selection rules. It is found that the shape resonance located at 1.8 eV partly decays into these valence states, which are located several eV above the resonance. The decay occurs by ejection of a core electron whose spin is antiparallel to that of the incident electron causing a sharp rise of the cross sections just at threshold. Three new core-excited shape resonances were found, one for each of , and symmetry. By comparison with recently calculated cross sections their electron configurations have been classified. Resonant excitation via these three resonances or via the low-lying resonance dominates the cross sections of the two spin forbidden transitions in the detected energy range. The excitation of the optically allowed transition, however, is dominated by non-resonant processes.

Measurement of absolute differential cross sections for vibrational excitation of O2 by electron impact

New measurements of absolute differential cross sections (DCSS) for vibrational excitation of O2, with improved sensitivity and resolution, are presented. The 90 degrees DCSS are given as a function of energy up to 16 eV for elastic scattering and vibrational excitation of up to v=7. A weak continuous, probably non-resonant, background scattering is found in the v=1 cross section in addition to the well known sharp 2 Pi g resonances dominating vibrational excitation in the energy range up to 2.5 eV. The sharp 2 Pi g resonances interfere coherently with the background scattering both in the v=1 and in the elastic channels. New absolute values for the energy-integrated DCSS for 2 Pi g resonances are given. They exhibit oscillatory Franck-Condon factors in the v=4-7 exit channels. The broad resonance peaking at 9 eV, dominating vibrational DCSS at higher energies, causes excitation of high vibrational levels (up to the dissociation limit) of the X 3 Sigma g- ground state, but not of the a1 Delta g and b1 Sigma g+ electronically excited states, supporting its assignment as the 4 Sigma u- resonance, the selectivity being caused by spin selection rules. The excitation of high vs indicates a relatively narrow autodetachment width for this resonance. The angular dependence at 9 eV is, however, not a simple psigma wave as would be expected for a simple sigma u* shape resonance. Preliminary experiments with a free jet cooled sample, concerned with rotational broadening of the 2 Pi g resonance, are reported.

Hot-electron impact cross section of impurities in semiconductors

This paper is devoted to the discussion of inelastic scattering of a high-energy conduction electron incident on an impurity in a semiconductor. On the one hand the rate of inelastic collisions of a hot electron samples the high-energy tail of the electron distribution, and thus may serve as a test of high-field transport theories. On the other hand the excited impurity may release its energy radiatively, and this is used in solid-state displays. We adopt a description of the impurity which emphasizes the structure of the excitable shell in a solid-state environment. Then we describe the direct and exchange paths between the initial and the final state, which we relate to spin selection rules. The scattering amplitudes are calculated to first order, with emphasis on the functional structure of each term. A crude computation of impact cross sections near the excitation threshold is performed, giving a rough magnitude of 10-18 cm2 which may be enhanced by exchange effects for a valence shell containing many electrons. More importantly, taking three examples we qualitatively discuss the factors affecting the cross section according to the structure of the impurity valence shell, and its location in the host lattice. The conclusions are of direct interest to device designers wanting to optimize the impact yield of hot-electron electroluminescence devices.