Fine-structure States Research Articles

Ab initio study of the spectroscopy of AgBr: A CASSCF+Averaged Coupled Pair Functional approach to the lowest excited states including spin-orbit couplings

The nine lowest-lying singlet and triplet (X (1)Sigma(+), 2 (1)Sigma(+), 3 (1)Sigma(+), (3)Sigma(+), 1 (3,1)Pi, 2 (3)Pi, and (3,1)Delta) electronic states of AgBr were studied through state-specific Complete Active Space Self-Consistent Field with 16 active electrons in 12 orbitals followed by extensive Averaged Coupled Pair Functional and CIPT2 calculations with large optimized valence basis sets. The spin-orbit effects were included to obtain the Omega fine-structure states arising from the |Lambda S Sigma> parents. Even before the inclusion of the spin-orbit effects, the 2 (1)Sigma(+) and 3 (1)Sigma(+) states present shallow minima near the equilibrium geometry of the ground state. The 2 (1)Sigma(+) state has another minimum around 8.0 a.u. and is attractive up to 20 a.u. The lowest (3,1)Pi states were found to be totally repulsive while the (3,1)Delta states present deep minima around 4.8 a.u. Most of the calculated spectroscopic constants for the ground and B states are slightly improved with respect to the previous theoretical study using the much smaller CASSCF(16,10) reference wave functions [M. Guichemerre et al., Chem. Phys. 280, 71 (2002)]. The observed B<--X transition is confirmed as arising from the singlet-to-singlet 0(+)(2 (1)Sigma(+))<--0(+)(X (1)Sigma(+)) excitation around 31 900 cm(-1). However, at variance with the previous theoretical prediction, the C(Omega=0(+)) state is dominated around the equilibrium geometry of the ground state by the third (1)Sigma(+) state with a small contribution from the 2 (3)Pi state around 43,500 cm(-1); thus the X-C excitation is now explained as arising also from a singlet-to-singlet spin-allowed transition.

Time-averaging within the excited state of the nitrogen-vacancy centre in diamond

The emission intensity of diamond samples containing negatively charged nitrogen-vacancy centres are measured as a function of magnetic field along the ⟨111⟩ direction for various temperatures. At low temperatures the responses are sample and stress dependent and can be modelled in terms of the previous understanding of the 3E excited state fine structure which is strain dependent. At room temperature the responses are largely sample and stress independent, and modelling involves invoking a strain independent excited state with a single zero field spin-level splitting of 1.42 GHz. The change in behaviour is attributed to a temperature dependent averaging process over the components of the excited state orbital doublet. It decouples orbit and spin and at high temperature the spin levels become independent of any orbit splitting. One significant implication of this averaging is that it simplifies the development of room temperature applications.

Exciton Fine Structure and Spin Relaxation in Semiconductor Colloidal Quantum Dots

Quantum dots (QDs) have discrete quantum states isolated from the environment, making QDs well suited for quantum information processing. In semiconductor QDs, the electron spins can be coherently oriented by photoexcitation using circularly polarized light, creating optical orientation. The optically induced spin orientation could serve as a unit for data storage and processing. Carrier spin orientation is also envisioned to be a key component in a related, though parallel, field of semiconductor spintronics. However, the oriented spin population rapidly loses its coherence by interaction with the environment, thereby erasing the prepared information. Since long-lasting spin orientation is desirable in both areas of investigation, spin relaxation is the central focus of investigation for optimization of device performance. In this Account, we discuss a topic peripherally related to these emerging areas of investigation: exciton fine structure relaxation (EFSR). The radiationless transition occurring in the exciton fine structure not only highlights a novel aspect of QD exciton relaxation but also has implications for carrier spin relaxation in QDs. We focus on examining the EFSR in connection with optical spin orientation and subsequent ultrafast relaxation of electron and hole spin densities in the framework of the exciton fine structure basis. Despite its significance, the study of exciton fine structure in colloidal QDs has been hampered by the experimental challenge arising from inhomogeneous line broadening that obscures the details of closely spaced fine structure states in the frequency domain. In this Account, we show that spin relaxation occurring in the fine structure of CdSe QDs can be probed by a time-domain nonlinear polarization spectroscopy, circumventing the obstacles confronted in the frequency-domain spectroscopy. In particular, by combining polarization sequences of multiple optical pulses with the unique optical selection rules of semiconductors, fast energy relaxation among the QD exciton fine structure states is selectively measured. The measured exciton fine structure relaxation, which is a nanoscale analogue of molecular radiationless transitions, contains direct information on the relaxation of spin densities of electron and hole carriers, that is, spin relaxation in QDs. From the exciton fine structure relaxation rates measured for CdSe nanorods and complex-shaped nanocrystals using nonlinear polarization spectroscopy, we elucidated the implications of QD size and shape on the QD exciton properties as well, for example, size- and shape-scaling laws governing exciton spin flips and how an exciton is delocalized in a QD. We envision that the experimental development and the discoveries of QD exciton properties presented in this Account will inspire further studies toward revealing the characteristics of QD excitons and spin relaxation therein, for example, spin relaxation in QDs made of various materials with different electronic structures, spin relaxation under an external perturbation of QD electronic states using magnetic fields, and spin relaxation of separated electrons and holes in type-II QD heterostructures.

Excited-state spectroscopy of single NV defects in diamond using optically detected magnetic resonance

Using pulsed optically detected magnetic resonance techniques, we directly probe electron-spin resonance transitions in the excited-state of single nitrogen-vacancy (NV) color centers in diamond. Unambiguous assignment of excited state fine structure is made, based on changes of NV defect photoluminescence lifetime. This study provides significant insight into the structure of the emitting 3E excited state, which is invaluable for the development of diamond-based quantum information processing.

Ultraviolet photodissociation of the SD radical in vibrationally ground and excited states

Ultraviolet (UV) photodissociation dynamics of the SD radical in vibrationally ground and excited states (X (2)Pi(3/2), v'' = 0-5) are investigated in the photolysis wavelength region of 220 to 244 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The UV photodissociation dynamics of SD (X (2)Pi(3/2)) from v'' = 0-5 are similar to each other and to that of SH studied previously. The anisotropy parameter of the D-atom product is approximately -1; the spin-orbit branching fractions of the S((3)P(J)) products are essentially constant, with an average S((3)P(2)) : S((3)P(1)) : S((3)P(0)) = 0.51 : 0.37 : 0.12. The UV photolysis of SD is a direct dissociation from the repulsive (2)Sigma(-) state following the perpendicular (2)Sigma(-)-X (2)Pi excitation. The S((3)P(J)) product fine-structure state distributions approach that in the sudden limit dissociation on the single repulsive (2)Sigma(-) curve, but they are also affected by nonadiabatic couplings among the repulsive (4)Sigma(-), (2)Sigma(-), and (4)Pi states. A bond dissociation energy D(0)(S-D) = 29 660 +/- 25 cm(-1) is obtained.

Relaxation in the Exciton Fine Structure of Semiconductor Nanocrystals

A spectroscopic study is reported of relaxation in the exciton fine structure of CdSe nanorods measured using ultrafast cross-polarized heterodyned third-order transient grating (CPH-3TG) spectroscopy. The CPH-3TG spectroscopy probes the dynamics of population transfer between states accompanying exciton spin flip (radiationless transitions in the exciton fine structure associated with changes in the sign of the total angular momentum). Analysis of the data enabled elucidation of pathways for exciton fine structure relaxation (EFSR) involving all of the exciton fine structure states. The mechanism and origin of the EFSR dynamics in CdSe nanorods are discussed in the exciton state picture with an analogy to molecular radiationless transition processes such as internal conversion and intersystem crossing. Key conclusions are that the fast transitions are between states with total angular momentum of ±1 and ∓2 and that the rate of this transition follows an inverse diameter to the fourth power size dependence, which originates from the Dresselhaus spin−orbit coupling effect.

Photodissociation of Molecular Bromine in Solid H2 and D2: Spectroscopy of the Atomic Bromine Spin−Orbit Transition

We report 355 nm photodissociation studies of molecular bromine (Br2) trapped in solid parahydrogen (pH2) and orthodeuterium (oD2). The product Br atoms are observed via the spin-orbit transition ((2)P(1/2)<-- (2)P(3/2)) of atomic bromine. The quantum yield (Phi) for Br atom photoproduction is measured to be 0.29(3) in pH2 and 0.24(2) in oD2, demonstrating that both quantum solids have minimal cage effects for Br2 photodissociation. The effective Br spin-orbit splitting increases when the Br atom is solvated in solid pH2 (+1.1%) and oD2 (+1.5%); these increases are interpreted as evidence that the solvation energy of the Br ground fine structure state ((2)P(3/2)) is significantly greater than the excited state ((2)P(1/2)). Molecular bromine induced H2 infrared absorptions are detected in the Q1(0) and S1(0) regions near 4150 and 4486 cm(-1), respectively, which allow the relative Br2 concentration to be monitored as a function of 355 nm photolysis.

Collisional Deactivation of Ba 5d7p 3D1 by Noble Gases

Collisional deactivation of the 5d7p (3)D1 state of Ba by noble gases is studied by time- and wavelength-resolved fluorescence techniques. A pulsed, frequency-doubled dye laser at 273.9 nm excites the 5d7p (3)D1 level from the ground state, and fluorescence at 364.1 and 366.6 nm from the 5d7p (3)D1 --> 6s5d (3)D1 and 5d7p (3)D1 --> 6s5d (3)D2 transitions, respectively, is monitored in real time to obtain the deactivation rate constants. At 835 K these rate constants are as follows: He, (1.69 +/- 0.08) x 10(-9) cm(3) s(-1); Ne, (3.93 +/- 0.14) x 10(-10) cm(3) s(-1); Ar, (4.53 +/- 0.15) x 10(-10) cm(3) s(-1); Kr, (4.64 +/- 0.13) x 10(-10) cm(3) s(-1); Xe, (5.59 +/- 0.22) x 10(-10) cm(3) s(-1). From time-resolved 5d7p (3)D1 emission in the absence of noble gas and from the intercepts of the quenching plots, the lifetime of this state is determined to be 100 +/- 1 ns. Using time- and wavelength-resolved Ba emission with a low background pressure of noble gas, radiative lifetimes of several near-resonant states are determined from the exponential rise of the fluorescence signals. These results are as follows: 5d6d (3)D3, 28 +/- 3 ns; 5d7p (3)P1, 46 +/- 2 ns; 5d6d (3)G3, 21.5 +/- 0.8 ns; 5d7p (3)F3, 48 +/- 1 ns. Integrated fluorescence signals are used to infer the relative rate constants for population transfer from the 5d7p (3)D1 state to eleven near-resonant fine structure states.

Emission Spectrum of a Dressed Exciton-Biexciton Complex in a Semiconductor Quantum Dot

The photoluminescence spectrum of a single quantum dot was recorded as a secondary resonant laser optically dressed either the vacuum-to-exciton or the exciton-to-biexciton transitions. High-resolution polarization-resolved measurements using a scanning Fabry-Pérot interferometer reveal splittings of the linearly polarized fine-structure states that are nondegenerate in an asymmetric quantum dot. These splittings manifest as either triplets or doublets and depend sensitively on laser intensity and detuning. Our approach realizes complete resonant control of a multiexcitonic system in emission, which can be either pulsed or continuous wave, and offers direct access to the emitted photons.

Inner-shell2pphotoionization and Auger decay of atomic silicon

A detailed experiment on 2p photoionization and subsequent Auger decay of atomic silicon is presented. Fine-structure-resolved photoelectron and Auger electron spectra are interpreted with the aid of large-scale multiconfiguration calculations. Energy separation and the relative cross sections of the 2p ionized fine-structure states of Si+ are given. The complete 2p Auger electron spectrum of Si is interpreted, and the intensity distribution to individual doubly ionized final states is studied.

Controlling the Optical Properties of Inorganic Nanoparticles

AbstractThe sophistication with which we can now prepare and characterize inorganic nanoparticles has inspired new areas of research into the fundamental properties and applications of these fascinating nanoscale systems. In this article some of the recent ideas concerning control of their optical properties are examined and explained, focusing on semiconductor nanocrystals. It is known that the optical properties of nanocrystals can be size‐tunable, but it is less obvious how shape matters. To explain how size as well as shape matters, the electronic structure of nanocrystals is sketched in relatively simple terms, leading to an introduction to deeper concepts at the heart of spectroscopy such as the exciton fine structure. The exciton fine structure states, although obscured by inhomogeneous line broadening, dictate selection rules for optical excitation. These viewpoints are compared and contrasted to well‐established principles in molecular spectroscopy that provide inspiration as well as perspective. The control of optical poperties is founded on our ability to prepare good quality colloidal particles. Recent advances in nanocrystal shape control are described. The current status of heterostructures is examined, with an emphasis on charge separation in CdSe–CdTe nanorods.

Occupation of fine-structure states in electron capture and transport

We study the population of the fine-structure states in excited-state manifolds of hydrogenic ${\text{Ar}}^{17+}$ ions resulting from charge transfer in collisions of 13.6 MeV/amu ${\text{Ar}}^{18+}$ ions with thin carbon foils or ${\text{CH}}_{4}$ and ${\text{N}}_{2}$ gases. Experimental information on excited states populations is determined by x-ray spectroscopy of individual fine-structure components of the Lyman series allowing for sensitive probes of relativistic effects during the primary charge transfer and the subsequent transport. We show that the ratio of the populations of the $2{p}_{1/2}$ and $2{p}_{3/2}$ states provides clear evidence for spin dependent relativistic effects at moderately relativistic speeds.

Ultrafast Exciton Fine Structure Relaxation Dynamics in Lead Chalcogenide Nanocrystals

The rates of fine structure relaxation in PbS, PbSe, and PbTe nanocrystals were measured on a femtosecond time scale as a function of temperature with no applied magnetic field by cross-polarized transient grating spectroscopy (CPTG) and circularly polarized pump-probe spectroscopy. The relaxation rates among exciton fine structure states follow trends with nanocrystal composition and size that are consistent with the expected influence of material dependent spin-orbit coupling, confinement enhanced electron-hole exchange interaction, and splitting between L valleys that are degenerate in the bulk. The size dependence of the fine structure relaxation rate is considerably different from what is observed for small CdSe nanocrystals, which appears to result from the unique material properties of the highly confined lead chalcogenide quantum dots. Modeling and qualitative considerations lead to conclusions about the fine structure of the lowest exciton absorption band, which has a potentially significant bearing on photophysical processes that make these materials attractive for practical purposes.

Quantum beats of fine-structure states in InP quantum dots

Different types of quantum beats were experimentally observed in the photoluminescence kinetics of semiconductor nanostructures with InP quantum dots characterized by strong inhomogeneous broadening of the optical transitions. Specific types of beats were selected by varying the magnitude and orientation of the magnetic field, the applied bias voltage, and the frequencies of the exciting and detected light. Parameters of the fine structure of electrons and holes in the system under study and characteristic relaxation times of the spin coherence are determined from the experimental data.

Decoherence suppression in a resonant driving field

Resonant radio frequency (rf) control fields have been employed to suppress decoherence in single quantum bits (qubits) encoded in the probability amplitudes of np fine-structure states in Li Rydberg atoms. As described previously [1], static electric-field tuning of the spin and orbital angular momentum composition of the fine-structure eigenstates enables qubit storage in an approximate decoherence-free subspace in which phase errors due to small stray electric and magnetic fields are strongly suppressed. In addition, it was found that sequences of short electric field pulses could be utilized in a ‘bang-bang’ dynamic decoupling scheme to improve coherence times. We now show that a continuous resonant rf field can also suppress decoherence in this system. The rf-dressed fine-structure states form a more robust basis in which the energy splitting between the component qubit levels is locked to the drive frequency, and decoherence is essentially eliminated. Measurements of the operational range of rf frequency and field strength required to achieve decoherence suppression are in agreement with the predictions of a two-level model.

Linearly polarized ‘fine structure’ of the bright exciton state in individual CdSe nanocrystal quantum dots

We report polarization-resolved, low-temperature, high-spectral-resolution studies of the photoluminescence from individual CdSe nanocrystal quantum dots (NQDs). The spectra reveal a `fine structure' splitting of the bright exciton state in a subset of the studied NQDs. The two fine structure states are spectrally separated by an energy of up to $3\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ depending on NQD size and are characterized by linearly (and orthogonally) polarized emission dipoles. The average splitting scales approximately with inverse NQD volume, consistent with an anisotropic electron-hole exchange interaction which can result from a breakdown of cylindrical symmetry in some of the NQDs.

A vacuum ultraviolet laser photoionization and pulsed field ionization study of nascent S(P2,1,3) and S(D21) formed in the 193.3nm photodissociation of CS2

The photoionization efficiency (PIE) and pulsed field ionization-photoion (PFI-PI) spectra for sulfur atoms S(3P2,1,0) and S(1D2) resulting from the 193.3 nm photodissociation of CS2 have been measured using tunable vacuum ultraviolet (vuv) laser radiation in the frequency range of 82 750-83 570 cm(-1). The PIE spectrum of S(3P2,1,0) near their ionization threshold exhibits steplike structures. On the basis of the velocity-mapped ion-imaging measurements, four strong autoionizing peaks observed in the PIE measurement in this frequency range have been identified to originate from vuv excitation of S(1D2). The PFI-PI measurement reveals over 120 previously unidentified new Rydberg lines. They have been assigned as Rydberg states [3p3(4S composite function nd3 D composite function (n=17-64)] converging to the ground ionic state S+(4S composite function) formed by vuv excitations of S(3P2,1,0). The converging limits of these Rydberg series have provided more accurate values, 82 985.43+/-0.05, 83 162.94+/-0.05, and 83 559.04+/-0.05 cm(-1) for the respective ionization energies of S(3P0), S(3P1), and S(3P2) to form S+(4S composite function). The relative intensities of the PFI-PI bands for S(3P0), S(3P1), and S(3P2) have been used to determine the branching ratios for these fine structure states, S(3P0):S(3P1):S(3P2)=1.00:1.54:3.55, produced by photodissociation of CS2 at 193.3 nm.

Observation of many-body Coulomb interaction effects on the photoluminescence spectra of InAs/GaAs quantum dots

InAs quantum dots (QDs) on GaAs (0 0 1) substrates were grown by Molecular Beam Epitaxy (MBE) using two growth temperatures. Photoluminescence (PL) pump power dependence measurements at low temperature were carried out for sample grown at higher temperature (520 °C). With increasing excitation density, the ground-state transition energy is found to decrease by 8 meV, while the excited-state transition energies exhibit resonance behaviour. The redshift of the ground-state emission was related to the band-gap renomalization (BGR) effect whereas the blueshift of the excited-state emissions was assigned to the compensation between filling of fine structure states and BGR effects. Using a quasi-resonant PL measurement, we have shown that the renormalization of the band-gap had to occur in the QD barrier.

Monitoring the Variable Interstellar Absorption toward HD 219188 withHubble Space TelescopeSTIS

We discuss the results of continued optical and UV spectroscopic monitoring of the variable intermediate-velocity (IV) absorption at v☉ = -38 km s-1 toward HD 219188. After reaching maxima in mid-2000, the column densities of both Na I and Ca II in that IV component declined by factors ≳2 by the end of 2006. Comparisons between echelle spectra from HST STIS (2001, 2003, 2004) and HST GHRS (1994-1995) indicate the following: (1) The absorption from the dominant species S II, O I, Si II, and Fe II is roughly constant for all four epochs—suggesting that the total N(H) (~6 × 1017 cm-2) and the (mild) depletions did not change significantly over a period of nearly 10 years. (2) The column densities of the trace species C I (both ground and excited fine-structure states) and of the excited state C II* all increased by factors of 2-5 between 1995 and 2001—implying increases in the hydrogen density nH (from about 20 cm-3 to about 45 cm-3) and in the electron density ne (by a factor ≳3) over that 6 yr period. (3) The column densities of C I and C II*—and the corresponding inferred nH and ne—then decreased slightly between 2001 and 2004. (4) The changes in C I and C II* are very similar to those seen for Na I and Ca II. The relatively low total N(H) and the modest nH suggest that the -38 km s-1 cloud toward HD 219188 is not a very dense knot or filament. Partial ionization of hydrogen appears to be responsible for the enhanced abundances of Na I, C I, Ca II, and C II*. In this case, the variations in those species reflect differences in density and ionization [and not N(H)] over scales of tens of AU.

Fine-structure quantum method of electron impact broadening calculations for XUV lines in hot plasmas

Many atomic physics codes are now available to build synthetic line spectra of high temperature plasmas. They usually provide energy levels, radiative/autoionisation probability rates, and electron impact collision strengths. However, more information could be extracted, in particular electron impact line widths that are important for dense plasmas. We present such an extension implemented on well-known UCL code package, SUPERSTRUCTURE/DISWAV/JAJOM. Here the Baranger formulation for the line widths, rewritten by Seaton and formulated in for fine-structure states was used. The new code gives the different broadening contributions: excitation, de-excitation and “elastic”. As examples we present the electron line-broadening of the Ne-like Cu XX X-ray lasing line, 1s 22s 22p 53s ( 3P 1)–1s2s 22p 53p ( 1S 0), and the Ne-like Al IV inner-shell absorption line, 1s 22s 22p 6 ( 1S 0)–1s2s 22p 63p ( 1P 1). In the Al IV case these results emphasize the non-negligible contribution of the “elastic” process.