Excitation Line Shape Research Articles

Theory of dispersive optical phonons in resonant inelastic x-ray scattering experiments

The community currently lacks a complete understanding of how resonant inelastic x-ray scattering (RIXS) experiments probe the electron-phonon interaction in solids. For example, most theoretical models of this process have focused on dispersionless Einstein phonons. Using a recently developed momentum average variational approximation for computing RIXS spectra of band insulators, we examine the influence of both electron and phonon dispersion in the intermediate state of the scattering process. We find that the inclusion of either, and their mutual interplay, introduces significant momentum variations in the RIXS intensity, even for momentum-independent electron-phonon coupling. The phonon dispersion also induces nontrivial changes in the excitation line shapes, which can have a quantitative impact on the data analysis. These results highlight the considerable challenges of interpreting RIXS data in actual materials.

Lattice dynamics of the excitonic insulator Ta2Ni(Se1−xSx)5

Recently, we employed electronic polarization-resolved Raman spectroscopy to reveal the strongly correlated excitonic insulator (EI) nature of Ta2NiSe5, Volkov et al. [arXiv:2007.07344], and also showed that for Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$ alloys the critical excitonic fluctuations diminish with sulfur concentration x exposing a cooperating lattice instability that takes over for large x, Volkov et al. [arXiv:2104.07032]. Here we focus on the lattice dynamics of this EI family. We identify all Raman-active optical phonons of fully symmetric and ac-quadrupole-like symmetries and study their evolution with temperature and sulfur concentration. We demonstrate the change of selection rules at temperatures below the orthorhombic-to-monoclinic transition at Tc(x) that is related to the EI phase. We find that Tc(x) decrease monotonically from 328 K for Ta2NiSe5 to 120 K for Ta2NiS5 and that the magnitude of lattice distortion also decreases with the sulfur concentration x. For x < 0.7, the two lowest-frequency B2g phonon modes show strongly asymmetric lineshapes at high temperatures due to Fano interference with the broad excitonic continuum present in a semimetallic state. Within the framework of extended Fano model, we develop a quantitative description of the interacting exciton-phonon excitation lineshape, enabling us to derive the intrinsic phonon parameters and determine the exciton-phonon interaction strength, that affects the transition temperature Tc(x). We also observe signatures of the acoustic mode scattered assisted by the structural domain walls formed below Tc. Based on our results, we additionally present a consistent interpretation of the origin of oscillations observed in time-resolved pump-probe experiments.

Interaction signatures and non-Gaussian photon states from a strongly driven atomic ensemble coupled to a nanophotonic waveguide

We study theoretically a laser-driven one-dimensional chain of atoms interfaced with the guided optical modes of a nanophotonic waveguide. The period of the chain and the orientation of the laser field can be chosen such that emission occurs predominantly into a single guided mode. We find that the fluorescence excitation line shape changes as the number of atoms is increased, eventually undergoing a splitting that provides evidence for the waveguide-mediated all-to-all interactions. Remarkably, in the regime of strong driving the light emitted into the waveguide is non-classical, with a significant negativity of the associated Wigner function. We show that both the emission properties and the non-Gaussian character of the light are robust against voids in the atom chain, enabling the experimental study of these effects with present-day technology. Our results offer a route towards novel types of fiber-coupled quantum light sources and an interesting perspective for probing the physics of interacting atomic ensembles through light.

Excited atoms-wall collisions and their manifestations in the fluorescence excitation line shapes

Since the invention of the extremely thin cell which sustains prolonged action of hot and dense alkali vapours, a number of spectroscopic problems have been solved with its aid. As the width of the cell is comparable to, or smaller than, the wavelength of the actual atomic transition, The Doppler width of the fluorescence light is reduced due to the cage effect. Stimulated by recent measurements of the fluorescent excitation line shapes in the extremely thin cell we studied the dependence of these line shapes on the cell thickness.

Extending resonant inelastic X-ray scattering to the extreme ultraviolet

In resonant inelastic X-ray scattering (RIXS), core hole resonance modes are used to enhance coupling between photons and low energy electronic degrees of freedom. Resonating with shallow core holes accessed in the extreme ultraviolet (EUV) can provide greatly improved energy resolution at standard resolving power, but has been found to often yield qualitatively different spectra than similar measurements performed with higher energy X-rays. This paper uses experimental data and multiplet-based numerical simulations for the M-edges of Co-, Ni- and Cu-based Mott insulators to review the properties that distinguish EUV RIXS from more commonly performed higher energy measurements. Key factors such as the origin of the strong EUV elastic line and advantages of EUV spectral functions over soft X-ray RIXS for identifying intrinsic excitation line shapes are discussed.

Time-synchronized continuous wave laser-induced fluorescence on an oscillatory xenon discharge

A novel approach to time-synchronizing laser-induced fluorescence measurements to an oscillating current in a 60 Hz xenon discharge lamp using a continuous wave laser is presented. A sample-hold circuit is implemented to separate out signals at different phases along a current cycle, and is followed by a lock-in amplifier to pull out the resulting time-synchronized fluorescence trace from the large background signal. The time evolution of lower state population is derived from the changes in intensity of the fluorescence excitation line shape resulting from laser-induced fluorescence measurements of the 6s(')[1/2](1)(0)-6p(')[3/2](2) xenon atomic transition at λ = 834.68 nm. Results show that the lower state population oscillates at twice the frequency of the discharge current, 120 Hz.

Analytical model of transit time broadening for two-photon excitation in a three-level ladder and its experimental validation

We revisit transit time broadening for one of the typical experiment designs in molecular spectroscopy, that of a collimated supersonic beam of particles crossing a focused Gaussian laser beam. In particular, we consider a Doppler-free arrangement of a collimated supersonic beam of Na${}_{2}$ molecules crossing two counterpropagating laser beams that excite a two-photon transition in a three-level ladder scheme. We propose an analytical two-level model with a virtual intermediate level to show that the excitation line shape is described by a Voigt profile and provide the validity range of this model with respect to significant experimental parameters. The model also shows that line broadening due to the curvature of laser field wave fronts on the particle beam path is exactly compensated by increased transit time of particles farther away from the beam axis, such that the broadening is determined solely by the size of the laser beam waist. The analytical model is validated by comparing it with numerical simulations of density-matrix equations of motion using a split propagation technique and with experimental results.

Photon emission statistics and photon tracking in single-molecule spectroscopy of molecular aggregates: Dimers and trimers

Based on the generating function formalism, we investigate broadband photon statistics of emission for single dimers and trimers driven by a continuous monochromatic laser field. In particular, we study the first and second moments of the emission statistics, which are the fluorescence excitation line shape and Mandel's Q parameter. Numerical results for this line shape and the Q parameter versus laser frequency in the limit of long measurement times are obtained. We show that in the limit of small Rabi frequencies and laser frequencies close to resonance with one of the one-exciton states, the results for the line shape and Q parameter reduce to those of a two-level monomer. For laser frequencies halfway the transition frequency of a two-exciton state, the photon bunching effect associated with two-photon absorption processes is observed. This super-Poissonian peak is characterized in terms of the ratio between the two-photon absorption line shape and the underlying two-level monomer line shapes. Upon increasing the Rabi frequency, the Q parameter shows a transition from super- to sub- to super-Poissonian statistics. Results of broadband photon statistics are also discussed in the context of a transition (frequency) resolved photon detection scheme, photon tracking, which provides a greater insight in the different physical processes that occur in the multi-level systems.

Investigation of optical fibers for gas-phase, ultraviolet laser-induced-fluorescence (UV-LIF) spectroscopy

We investigate the feasibility of transmitting high-power, ultraviolet (UV) laser pulses through long optical fibers for laser-induced-fluorescence (LIF) spectroscopy of the hydroxyl radical (OH) and nitric oxide (NO) in reacting and non-reacting flows. The fundamental transmission characteristics of nanosecond (ns)-duration laser pulses are studied at wavelengths of 283 nm (OH excitation) and 226 nm (NO excitation) for state-of-the-art, commercial UV-grade fibers. It is verified experimentally that selected fibers are capable of transmitting sufficient UV pulse energy for single-laser-shot LIF measurements. The homogeneous output-beam profile resulting from propagation through a long multimode fiber is ideal for two-dimensional planar-LIF (PLIF) imaging. A fiber-coupled UV-LIF system employing a 6 m long launch fiber is developed for probing OH and NO. Single-laser-shot OH- and NO-PLIF images are obtained in a premixed flame and in a room-temperature NO-seeded N(2) jet, respectively. Effects on LIF excitation lineshapes resulting from delivering intense UV laser pulses through long fibers are also investigated. Proof-of-concept measurements demonstrated in the current work show significant promise for fiber-coupled UV-LIF spectroscopy in harsh diagnostic environments such as gas-turbine test beds.

Study of isospin violating ϕ excitation in e+e− → ωπ0

We study the reaction e+e− → ωπ0 in the vicinity of the ϕ mass region. The isospin-violating ϕ excitation is accounted for by two major mechanisms. One is electromagnetic transition and the other is strong isospin violations. For the latter, we consider contributions from the intermediate hadronic meson loops and ϕ–ρ0 mixing as the major mechanisms via the t- and s-channel transitions, respectively. By fitting the recent KLOE data, we succeed in constraining the model parameters and extracting the ϕ → ωπ0 branching ratio. It shows that the branching ratio is sensitive to the ϕ excitation line shape and background contributions. Some crucial insights into the correlation between isospin violation and Okubo–Zweig–Iizuka rule evading transitions are also learned.

A study of the quantum classical crossover in the spin dynamics of the 2DS = 5/2 antiferromagnet Rb2MnF4: neutron scattering, computer simulations and analytic theories

We report comprehensive inelastic neutron scattering measurements of the magnetic excitations in the 2Dspin-5/2 Heisenbergantiferromagnet Rb2MnF4 as a function of temperature from deep in the Néel ordered phase up to paramagnetic,0.13<kBT/4JS<1.4. Well-defined spin waves are found for wavevectors larger than the inverse correlation lengthξ−1 for temperatures up to near the Curie–Weiss temperature,ΘCW. For wavevectorssmaller than ξ−1, relaxational dynamics occurs. The observed renormalization of spin wave energies, andevolution of excitation lineshapes, with increasing temperature are quantitatively comparedwith finite-temperature spin wave theory and computer simulations for classical spins.Random phase approximation calculations provide a good description of the lowtemperature renormalization of spin waves. In contrast, lifetime broadening calculatedusing the first Born approximation shows, at best, modest agreement around the zoneboundary at low temperatures. Classical dynamics simulations using an appropriatequantum classical correspondence were found to provide a good description ofthe intermediate and high temperature regimes over all wavevector and energyscales, and the crossover from quantum to classical dynamics observed aroundΘCW/S, wherethe spin S = 5/2. A characterization of the data over the whole wavevector/energy/temperature parameter space is given.In this, T2 behaviour is found to dominate the wavevector and temperature dependence of thelinewidths over a large parameter range, and no evidence of hydrodynamic behaviouror dynamical scaling behaviour found within the accuracy of the datasets. Anefficient and easily implemented classical dynamics methodology is presented thatprovides a practical method for modelling other semiclassical quantum magnets.

Energy-dependent damping of excitations over an elongated Bose-Einstein condensate

We report the measurement of Beliaev damping of bulk excitations in cigar shaped Bose Einstein condensates of atomic vapor. By using post selection, excitation line shapes of the total population are compared with those of the undamped excitations. We find that the damping depends on the initial excitation energy of the decaying quasi particle, as well as on the excitation momentum. We model the condensate as an infinite cylinder and calculate the damping rates of the different radial modes. The derived damping rates are in good agreement with the experimentally measured ones. The damping rates strongly depend on the destructive interference between pathways for damping, due to the quantum many-body nature of both excitation and damping products.

Tomographic rf Spectroscopy of a Trapped Fermi Gas at Unitarity

We present spatially resolved radio-frequency spectroscopy of a trapped Fermi gas with resonant interactions and observe a spectral gap at low temperatures. The spatial distribution of the spectral response of the trapped gas is obtained using in situ phase-contrast imaging and 3D image reconstruction. At the lowest temperature, the homogeneous rf spectrum shows an asymmetric excitation line shape with a peak at 0.48(4)epsilonF with respect to the free atomic line, where epsilonF is the local Fermi energy.

Radio-frequency transitions on weakly bound ultracold molecules

We show that radio-frequency spectroscopy on weakly bound molecules is a powerful and sensitive tool to probe molecular energy structure as well as atomic scattering properties. An analytic expression of the excitation line shape is derived, which in general contains a bound-free component and a bound-bound component. In particular, we show that the bound-free process strongly depends on the sign of the scattering length in the outgoing channel and acquires a Fano-type profile near a Feshbach resonance. The derived line shapes provide an excellent fit to both numerical calculations and recent experimental measurements based on $^{6}\mathrm{Li}_{2}$.

Thermalization of fast cesium5D3∕2atoms in collisions with ground-state cesium atoms

We have investigated collisions involving fast, excited Cs atoms produced by photodissociating Cs{sub 2} molecules with a pulsed dye laser. The velocities of the atoms in the 5D state formed by the process Cs{sub 2}(X {sup 1}{sigma}{sub g}{sup +})+({Dirac_h}/2{pi}){omega}{sub pump}{yields}Cs{sub 2}{sup *}{yields}Cs(5D)+Cs(6S) are much greater than typical thermal velocities associated with the cell temperature. Using a narrow-band cw probe laser to observe the increased Doppler broadening of the 5D{sub 3/2}{yields}5F{sub 5/2} excitation line shape, we are able to monitor the time evolution of the velocity distribution of these 5D atoms. We analyze the data using a model that predicts the time-dependent excitation line shape of the fast atoms. Because the photons used to dissociate the molecules have a well-defined energy, the velocity distribution of the excited atoms in the early time after they are produced can be fairly well determined. Over time, velocity-changing collisions with ground-state Cs atoms cause the velocity distribution of excited atoms to approach the thermal limit. An analysis based on the strong-collision model leads to a prediction that the observed line shape at intermediate times will be a linear combination of contributions from distinct 'fast' and 'thermalized' atomic populations. By fitting our data to this model,more » a rate coefficient for velocity-changing collisions of fast Cs(5D{sub 3/2}) atoms with ground-state Cs atoms has been determined. The result k{sub VCC}=(6.1{+-}1.2)x10{sup -10} cm{sup 3} s{sup -1} corresponds to an effective velocity-changing collision cross section of {sigma}{sub VCC}{sup Cs,eff}=(1.2{+-}0.2)x10{sup -14} cm{sup 2}.« less

Non-lorentzian single-molecule line shape: pseudolocal phonons and coherence transfer

The excitation line shape of a single terrylene molecule in a naphthalene crystal has been investigated. In addition to the conventional Lorentzian, it consists of a dispersive component in the core region and a sideband. This is due to a pseudolocal phonon caused by the substitution of a host molecule with the chromophore. When the pseudolocal phonon is excited, the resonance frequency of the chromophore slightly changes, resulting in the appearance of a second, quasiresonant transition. Coherence transfer between these two optical transitions causes the deviation from the purely Lorentzian line shape.

Temperature dependence of the line shapes of two-photon excited transitions in KMgF3:Eu2+

The two-photon excitation line shapes of f-f transitions of Eu2+ in KMgF3 have been investigated as a function of temperature. Results for several transition lines of the 6P J and 6D J multiplets are presented in this work. The two-photon excited transitions appear as zero-phonon lines with a Lorentzian profile whose peak energy is shifted from the zero temperature position. The observed temperature dependence of the zero-phonon line shapes is ascribed to the elastic scattering of the electronic transitions among vibrational states via anharmonic coupling to a phonon mode of the lattice heat-bath. In those components of 4f7 multiplets whose energies are not resonant with the 4f65d broad excited state, a direct proportionality to the phonon occupation number of the coupled mode is found for the line-shapes and peak-positions temperature dependences.

Theory of the effect of odd-photon destructive interference on optical shifts in resonantly enhanced multiphoton excitation and ionization

We present a theory for two- and three-photon excitation, optical shifting, and four-wave mixing when a first laser is tuned onto, or near, a two-photon resonance and a second much more intense laser is tuned near or on resonance between the two-photon resonance and a second excited state. When the second excited state has a dipole-allowed transition back to the ground state and the concentration is sufficiently high, a destructive interference is produced between three-photon coupling of the ground state and the second excited state and one-photon coupling between the same states by the internally generated four-wave mixing field. This interference leads to several striking effects. For instance, as the onset of the interference occurs, the optical shifts in the two-photon resonance excitation line shape become smaller in copropagating geometry so that the line shapes for multiphoton ionization enhanced by the two-photon resonance eventually become unaffected by the second laser. In the same range of concentrations the four-wave mixing field evolves to a concentration-independent intensity. With counterpropagating laser beams the line shape exhibits normal optical shifts like those observed for both copropagating and counterpropagating laser beams at very low concentrations. The theoretical work presented here extends our earlier works by more » including the effect of laser bandwidth and by removing the restriction of having the second laser be tuned far from three-photon resonance. In this way we have now included, as a special case, the effect of both laser bandwidth and interference on laser-induced transparency. Unlike other effects related to odd-photon destructive interference, the effect of a broad bandwidth is to bring about the predicted effects at much lower concentrations. Studies in rubidium show good agreement between theory and experiment for both ionization line shapes and four-wave mixing intensity as a function of concentration. {copyright} {ital 1998} {ital The American Physical Society} « less

Rare earth dopants as probes of localized states in chalcogenide glasses

The recently observed phenomenon of broad band excitation of rare earth dopants in chalcogenide glasses indicates an interaction between the rare earth ions and localized background defect states in the glass. Low temperature (5 K) photoluminescence and photoluminescence excitation spectroscopy of Er 2S 3-doped As 12Ge 33Se 55 samples show similarities in excitation lineshape, Stokes shift, and spectral position to the intrinsic luminescence of an undoped As 12Ge 33Se 55 sample. Fatiguing of the broad band excitation has been observed in Er-doped As 12Ge 33Se 55 and As 2S 3 samples. These results indicate that the broad excitation band involves interaction with the same native defects in the host glass which give rise to important optical, electrical, and magnetic resonance properties of the chalcogenides. A model for the energy transfer process from the host glass to the rare earths, mediated by intrinsic defect states in the glass is presented. Some features of existing models for the behavior of chalcogenide glasses will be evaluated in light of our results.

Temperature and doping dependence of the Bi-Sr-Ca-Cu-O electronic structure and fluctuation effects

Angle-resolved photoemission data from bulk ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ show changes in the excitation gap and line shape versus both doping and temperature. We employ two different quantitative analyses; one to search for a gap closing temperature ${T}^{\ensuremath{\star}},$ and the other to further characterize its relation to the superconducting transition temperature ${T}_{c}.$ We present observations of the sharp feature near crystal momentum $\mathbf{k}=(1,0)$ in superconducting spectra, and its temperature and doping dependence. This temperature dependence is analyzed together with the shift in the spectral weight's lowest binding energy (leading edge). Finally, we find evidence for persistence of this sharp peak at temperatures slightly above ${T}_{c}.$