Spin Selection Rules Research Articles

Experimental investigation of the optical spin-selection rules in bulk Si and Ge/Si quantum dots

We study the relationship between the circular polarization of photoluminescence and the magnetic field-induced spin-polarization of the recombining charge carriers in bulk Si and Ge/Si quantum dots. First, we quantitatively compare experimental results on the degree of circular polarization of photons resulting from phonon-assisted radiative transitions in intrinsic and doped bulk Si with calculations which we adapt from recently predicted spin-dependent phonon-assisted transition probabilities in Si. The excellent agreement of our experiments and calculations quantitatively verifies these spin-dependent transition probabilities and extends their validity to weak magnetic fields. Such magnetic fields can induce a luminescence polarization of up to 3%/T. We then investigate phononless transitions in Ge/Si quantum dots as well as in degenerately doped Si. Our experiments systematically show that the sign of the degree of circular polarization of luminescence resulting from phononless transitions is opposite to the one associated with phonon-assisted transitions in Si and with phononless transitions in direct band gap semiconductors. This observation implies qualitatively different spin-dependent selection rules for phononless transitions, which seem to be related to the confined character of the electron wave function.

Heavy quark spin selection rules in meson-antimeson states

In the heavy quark limit, we discuss the spin of the charm-anticharm pair $J_{c\bar{c}}$ in an S-wave meson-antimeson molecule or resonance. One finds two cases that $J_{c\bar{c}}$ cannot be 0: (a) $J=|j_2-j_4|-1$ or $J=j_2+j_4+1$ where $J$ is the total spin of the system and $j_2$ ($j_4$) is the total angular momentum of the light degree of freedom in a charmed meson (antimeson); (b) $J^C=1^+,2^-,3^+,...$, if the two different mesons belong to the same doublet. The decays to spin-singlet charmonium states are suppressed when one of the two conditions is satisfied. We discuss constrained decay properties for selected systems.

ORTHO-PARA SELECTION RULES IN THE GAS-PHASE CHEMISTRY OF INTERSTELLAR AMMONIA

The ortho–para chemistry of ammonia in the cold interstellar medium is investigated using a gas-phase chemical network. Branching ratios for the primary reaction chain involved in the formation and destruction of ortho- and para-NH3 were derived using angular momentum rules based on the conservation of the nuclear spin. We show that the "anomalous" ortho-to-para ratio of ammonia (∼0.7) observed in various interstellar regions is in fact consistent with nuclear spin selection rules in a para-enriched H2 gas. This ratio is found to be independent of temperature in the range 5–30 K. We also predict an ortho-to-para ratio of ∼2.3 for NH2. We conclude that a low ortho-to-para ratio of H2 naturally drives the ortho-to-para ratios of nitrogen hydrides below the statistical values.

Coulomb matrix elements for the impact ionization process in nanocrystals: An envelope function approach

We propose a method for calculating Coulomb matrix elements between exciton and biexciton states in semiconductor nanocrystals based on the envelope function formalism. We show that such a calculation requires proper treatment of the Bloch parts of the carrier wave functions which, in the leading order, leads to spin selection rules identical to those holding for optical interband transitions. Compared to the usual (intraband) Coulomb couplings, the resulting matrix elements are additionally scaled by the ratio of the lattice constant to the nanocrystal radius. As a result, the Coulomb coupling between exciton and biexciton states scale as 1/R^2. We present also some statistical estimates of the distribution of the coupling magnitudes and energies of the coupled states The number of biexciton states coupled to exciton states form a certain energy range shows a power-law scaling with the ratio of the coupling magnitude to the energy separation. We estimate also the degree of mixing between exciton and biexciton states. The amount of biexciton admixture to exciton states at least 1 eV above the multiple exciton generation threshold can reach 80% but varies strongly with the nanocrystal size.

Origin of the Energy Barrier to Chemical Reactions ofO2on Al(111): Evidence for Charge Transfer, Not Spin Selection

Dissociative adsorption of molecular oxygen on the Al(111) surface exhibits mechanistic complexity that remains surprisingly poorly understood in terms of the underlying physics. Experiments clearly indicate substantial energy barriers and a mysteriously large number of adsorbed single oxygen atoms instead of pairs. Conventional first principles quantum mechanics (density functional theory) predicts no energy barrier at all; instead, spin selection rules have been invoked to explain the barrier. In this Letter, we show that correct barriers arise naturally when embedded correlated electron wave functions are used to capture the physics of the interaction of O(2) with the metal surface. The barrier originates from an abrupt charge transfer (from metal to oxygen), which is properly treated within correlated wave function theory but not within conventional density functional theory. Our potential energy surfaces also identify oxygen atom abstraction as the dominant reaction pathway at low incident energies, consistent with measurements, and show that charge transfer occurs in a stepwise fashion.

Teaching the Spin Selection Rule: An Inductive Approach

In the group exercise described, students are guided through an inductive justification for the spin conservation selection rule (ΔS = 0). Although the exercise only explicitly involves various states of helium, the conclusion is one of the most widely applicable selection rules for the interaction of light with matter, applying, in various ways, to atoms and molecules of all sizes. Connections are made among several concepts routinely covered in physical chemistry courses including the Pauli Principle, orthonormal wave functions, overlap integrals, atomic term symbols, multiplicity, radiative lifetimes, fluorescence, and phosphorescence. Detailed student directions are included in the Supporting Information.

Valley–spin blockade and spin resonance in carbon nanotubes

The manipulation and readout of spin qubits in quantum dots have been successfully achieved using Pauli blockade, which forbids transitions between spin-triplet and spin-singlet states. Compared with spin qubits realized in III-V materials, group IV materials such as silicon and carbon are attractive for this application because of their low decoherence rates (nuclei with zero spins). However, valley degeneracies in the electronic band structure of these materials combined with Coulomb interactions reduce the energy difference between the blocked and unblocked states, significantly weakening the selection rules for Pauli blockade. Recent demonstrations of spin qubits in silicon devices have required strain and spatial confinement to lift the valley degeneracy. In carbon nanotubes, Pauli blockade can be observed by lifting valley degeneracy through disorder, but this makes the confinement potential difficult to control. To achieve Pauli blockade in low-disorder nanotubes, quantum dots have to be made ultrasmall, which is incompatible with conventional fabrication methods. Here, we exploit the bandgap of low-disorder nanotubes to demonstrate robust Pauli blockade based on both valley and spin selection rules. We use a novel stamping technique to create a bent nanotube, in which single-electron spin resonance is detected using the blockade. Our results indicate the feasibility of valley-spin qubits in carbon nanotubes.

Generation of Greenberger-Horne-Zeilinger state of distant diamond nitrogen-vacancy centers via nanocavity input-output process

An alternative scheme is proposed for the generation of an N-qubit Greenberger-Horne-Zeilinger (GHZ) state with distant nitrogen-vacancy (N-V) centers confined in spatially separated photonic crystal (PC) nanocavities via input-output process of photon. The GHZ state is produced by the phase shift brought by the input-output photon. The certain polarized photon transmitted from a PC nanocavity side-coupled a waveguide can obtain different phase shifts due to the different spin states in diamond N-V centers and the optical spin selection rule. Our calculations show that the proposed scheme can work well with a large cavity damping rate which ensures the efficient output of photon.

Effect of DNA Binding on Geminate CO Recombination Kinetics in CO-sensing Transcription Factor CooA

Carbon monoxide oxidation activator (CooA) proteins are heme-based CO-sensing transcription factors. Here we study the ultrafast dynamics of geminate CO rebinding in two CooA homologues, Rhodospirillum rubrum (RrCooA) and Carboxydothermus hydrogenoformans (ChCooA). The effects of DNA binding and the truncation of the DNA-binding domain on the CO geminate recombination kinetics were specifically investigated. The CO rebinding kinetics in these CooA complexes take place on ultrafast time scales but remain non-exponential over many decades in time. We show that this non-exponential kinetic response is due to a quenched enthalpic barrier distribution resulting from a distribution of heme geometries that is frozen or slowly evolving on the time scale of CO rebinding. We also show that, upon CO binding, the distal pocket of the heme in the CooA proteins relaxes to form a very efficient hydrophobic trap for CO. DNA binding further tightens the narrow distal pocket and slightly weakens the iron-proximal histidine bond. Comparison of the CO rebinding kinetics of RrCooA, truncated RrCooA, and DNA-bound RrCooA proteins reveals that the uncomplexed and inherently flexible DNA-binding domain adds additional structural heterogeneity to the heme doming coordinate. When CooA forms a complex with DNA, the flexibility of the DNA-binding domain decreases, and the distribution of the conformations available in the heme domain becomes restricted. The kinetic studies also offer insights into how the architecture of the heme environment can tune entropic barriers in order to control the geminate recombination of CO in heme proteins, whereas spin selection rules play a minor or non-existent role.

Optically pump-induced athermal and nonresonant refractive index changes in the reference Cr-doped laser materials: Cr:GSGG and ruby

The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with Cr3+ and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the Cr3+ ion in its excited and ground states, Δαp. The Cr3+ optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to Δαp has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths ∼3 orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of Δαp in Cr3+-doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in Cr:Al2O3 (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its E2 and T24 emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before. On the other hand, Cr:Al2O3 (ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the E2–T24 energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative Δαp value, which is contrary to all refractive index variation (including Z-scan) measurements.

K-shell Auger lifetime variation in doubly ionized Ne and first row hydrides

We consider 1s Auger decay in doubly (core-core and core-valence) ionized Ne and in the isoelectronic first row element hydrides. We show theoretically that the presence of the spectator inner valence vacancy leads to Auger lifetime variation of up to about a factor of 2, relative to the Auger lifetimes in the singly ionized species. The origin of this effect is traced to spin selection rules. Implications on the modelling of the radiation damage in strong x-ray fields are discussed.

Transport Spectroscopy of an Impurity Spin in a Carbon Nanotube Double Quantum Dot

We make use of spin selection rules to investigate the electron spin system of a carbon nanotube double quantum dot. Measurements of the electron transport as a function of the magnetic field and energy detuning between the quantum dots reveal an intricate pattern of the spin state evolution. We demonstrate that the complete set of measurements can be understood by taking into account the interplay between spin-orbit interaction and a single impurity spin coupled to the double dot. The detection and tunability of this coupling are important for quantum manipulation in carbon nanotubes.

X‐ray, ESR, and quantum mechanics studies unravel a spin well in the cofactor‐less urate oxidase

Urate oxidase (EC 1.7.3.3 or UOX) catalyzes the conversion of uric acid using gaseous molecular oxygen to 5-hydroxyisourate and hydrogen peroxide in absence of any cofactor or transition metal. The catalytic mechanism was investigated using X-ray diffraction, electron spin resonance spectroscopy (ESR), and quantum mechanics calculations. The X-ray structure of the anaerobic enzyme-substrate complex gives credit to substrate activation before the dioxygen fixation in the peroxo hole, where incoming and outgoing reagents (dioxygen, water, and hydrogen peroxide molecules) are handled. ESR spectroscopy establishes the initial monoelectron activation of the substrate without the participation of dioxygen. In addition, both X-ray structure and quantum mechanic calculations promote a conserved base oxidative system as the main structural features in UOX that protonates/deprotonates and activate the substrate into the doublet state now able to satisfy the Wigner's spin selection rule for reaction with molecular oxygen in its triplet ground state.

Throwing light on dark states of α-oligothiophenes of chain lengths 2 to 6: radical anion photoelectron spectroscopy and excited-state theory

In this work, we apply photodetachment photoelectron spectroscopy (PD-PES) on radical anions to access the lowest excited electronic states of neutral α-oligothiophenes nT (n = 2-6, where n denotes the number of thiophene rings) in the gas phase. Besides electron affinities, the spectra provide the energies of the T(1) and T(2) states which are otherwise difficult to investigate in neutral molecules due to spin selection rules. The assignment of the spectra is assisted by quantum chemical calculations using a combined density functional theory and multi-reference configuration interaction approach. For all α-oligothiophenes investigated in this work, the T(2) state is situated below S(1). In the gas phase, the S(1) state energies lie higher than in non-polar solution (0.2 to 0.4 eV). The geometry optimizations show that the S(0) state and especially the excited states gain planarity with increasing chain length. A non-planar structure or out-of-plane vibrational activity is needed to allow an efficient intersystem crossing (ISC) dynamics from S(1) to T(2), followed by internal conversion to T(1). Our theoretical calculations predict that in 6T a doubly excited state becomes nearly isoenergetic to S(1). This state is not observed by PD-PES, which is explained by the analysis of the calculated contributing electron configurations.

ChemInform Abstract: Photoelectrochemical Reduction of p-Halonitrobenzenes.

A detailed and comparative mechanistic study of the photoelectrochemical dehalogenation of the four p-halonitrobenzenes, X-C6H4-NO2(X = F, Cl, Br or I) in acetonitrile solution is reported. In the case of p-iodonitrobenzene, iodide loss is accomplished by electrochemical reduction alone. The formation of I– is shown to take place via an ECE process with the ultimate generation of the radical anion of nitrobenzene. Dual photo- and electro-chemical activation of p-chloronitrobenzene and p-bromonitrobenzene leads to halide loss through a photo-ECE mechanism. This proceeds via absorption of light by the radical anions, [X-C6H4-NO2]˙–, which is followed by fragmentation forming the ˙C6H4NO2 aryl radical. The latter is shown to react with the solvent system forming nitrobenzene which is further reduced at the electrode with the generation of [C6H4-NO2]˙–. The aryl radical is demonstrated to be able to undergo (partial) recombination with added Br– or Cl–. The effectiveness of different electronic transitions in the radical ions, [X-C6H4-NO2]˙–(X = Br, Cl) towards dehalogenation are compared; both radical anions-exhibit transitions centred near 330 and 470 nm in the near UV–VIS part of the spectrum. For the chloro-compound only the former band is effective in stimulating chloride release, whereas for the bromo-compound the excitation of either band causes bromide loss. The long wavelength band of the latter is quantified as being some 5.6 times more effective towards fragmentation on a per photon absorbed basis and this is rationalised using spin selection rules. No loss of fluoride from [F-C6N4-NO2]˙– was observed at any wavelength in the visible region of the spectrum.

Mechanisms of manganese-assisted non-radiative recombination in Cd(Mn)Se/Zn(Mn)Sequantum dots

Mechanisms of non-radiative recombination of electron–hole complexes inCd(Mn)Se/Zn(Mn)Se quantum dots accompanied by interconfigurational excitations ofMn2 + ions are analyzed within the framework of the single-electron model of deep 3d levels insemiconductors. In addition to the mechanisms caused by Coulomb and exchangeinteractions, which are related because of the Pauli principle, another mechanism due tosp–d mixing is considered. It is shown that the Coulomb mechanism reducesto long-range dipole–dipole energy transfer from photoexcited quantum dots toMn2 + ions. The recombination due to the Coulomb mechanism is allowed for any states ofMn2 + ions and e–h complexes. In contrast, short-range exchange and sp–d recombinationsare subject to spin selection rules, which are the result of strong lh–hh splittingof hole states in quantum dots. Estimates show that efficiency of the sp–dmechanism can considerably exceed that of the Coulomb mechanism. Thephonon-assisted recombination and processes involving the upper excited states ofMn2 + ions are studied. The increase in PL intensity of an ensemble of quantum dotsin a magnetic field perpendicular to the sample growth plane observed earlieris analyzed as a possible manifestation of the spin-dependent recombination.

Coupling of bonding and antibonding electron orbitals in double quantum dots by spin-orbit interaction

We perform a systematic exact diagonalization study of spin-orbit coupling effects for stationary few-electron states confined in quasi-two-dimensional double quantum dots. We describe the spin-orbit-interaction induced coupling between bonding and antibonding orbitals and its consequences for magneto-optical absorption spectrum. The spin-orbit coupling for odd electron numbers (one, three) opens avoided crossings between low energy excited levels of opposite spin orientation and opposite spatial parity. For two electrons the spin-orbit coupling allows for low-energy optical transitions that are otherwise forbidden by spin and parity selection rules. We demonstrate that the energies of optical transitions can be significantly increased by an in-plane electric field but only for odd electron numbers. Occupation of single-electron orbitals and effects of spin-orbit coupling on electron distribution between the dots are also discussed.

CuF-Chemiluminescent Reaction and Plasma Switching by Laser Ablation in the Gaseous Cu−CF4System and Their Interactions with Magnetic Field†

In view of new metal catalysis reactions, CuF-chemiluminescent and plasma reactions in a gaseous Cu-CF(4) system are studied as well as their interactions with a magnetic field. As for the chemiluminescent reaction, the spectroscopic analysis of the CuF chemiluminescence in the reaction of CF(4) with Cu emitted by laser ablation with a fundamental beam of a Nd(3+):YAG laser indicates that the rotational, vibrational, and translational temperatures of the product CuF are rather similar, suggesting the simple heating mechanism. In this mechanism, the high translational energy of the reactant Cu atom will be used to go over the early barrier. The mechanism is evidenced by experimentally observing that the reaction barrier decreases with an increase in the translational energy of the reactant Cu by an increase in the laser power for ablation. On the other hand, in the further study, we find that Cu emitted by laser ablation can switch the plasma in an electric field less than that necessary for the direct discharge in dc-plasma and call it PLASLA (plasma switching by laser ablation). In PLASLA, the plasma is formed by the first ablation, quenched by the second ablation, formed by the third ablation, quenched by the fourth ablation, and so on. Thus PLASLA will be a promising new metal catalysis process for materials science. In particular, the cooperation of the ablation laser with the laser for photoreactions will be significant as a new technique. The studies on the mechanism of PLASLA with metals of Cu, Al, Ag, Zn, Co, Ni, Ti, Mo, and W indicate that the metals are classified into three groups. It is of great interest that the classification agrees with the classification by their electronic configurations. PLASLA is found to be formed rather easily and stable with Al, Ag, Cu, and Zn of group I. Species such as C, C(+), C(2+), and C(2) are observed in the time-resolved spectra of PLASLA luminescence at 0.5 mus after laser ablation, suggesting carbon polymers as the final product. In fact, the TOF (time-of-flight) mass spectrometric analysis of the product material confirms it. Furthermore, PLASLA and the CuF-chemiluminescent reaction are studied in a magnetic field. In the chemiluminescent reaction, we propose that the field switches the reaction paths by mixing the singlet and triplet potential surfaces at level crossing by breaking the spin selection rule. The field affects PLASLA through the MHD (magneto-hydrodynamic) processes such as E x B drifts and cyclotron circulation, as well as the singlet-triplet mixing.

Distributed quantum information processing via quantum dot spins

We propose a scheme to engineer a non-local two-qubit phase gate between two remote quantum-dot spins. Along with one-qubit local operations, one can in principal perform various types of distributed quantum information processing. The scheme employs a photon with linearly polarisation interacting one after the other with two remote quantum-dot spins in cavities. Due to the optical spin selection rule, the photon obtains a Faraday rotation after the interaction process. By measuring the polarisation of the final output photon, a non-local two-qubit phase gate between the two remote quantum-dot spins is constituted. Our scheme may has very important applications in the distributed quantum information processing.

Communications: Development and characterization of a source of rotationally cold, enriched para-H3+

In an effort to develop a source of H(3)(+) that is almost entirely in a single quantum state (J=K=1), we have successfully generated a plasma that is enriched to approximately 83% in para-H(3)(+) at a rotational temperature of 80 K. This enrichment is a result of the nuclear spin selection rules at work in hydrogenic plasmas, which dictate that only para-H(3)(+) will form from para-H(2), and that para-H(3)(+) can be converted to ortho-H(3)(+) by subsequent reaction with H(2). This is the first experimental study in which the H(2) and H(3) (+) nuclear spin selection rules have been observed at cold temperatures. The ions were produced from a pulsed solenoid valve source, cooled by supersonic expansion, and interrogated via continuous-wave cavity ringdown spectroscopy.