Experimental Excitation Spectra Research Articles

Two-dimensional laser-induced fluorescence thermometry in a medium- to high-enthalpy CO2 flow

Ground-based experimental facilities are needed to qualify materials subjected to aerothermodynamic loads during an entry into the Martian atmosphere. In the present work, the focus is on quantifying the temperature field in a medium- to high-enthalpy CO_{2} flow (from 4.5 to {10.5}, hbox {MJ}/hbox {kg}) of a plasma wind tunnel using one- and two-dimensional two-photon absorption laser-induced fluorescence (CO-TALIF) thermometry at a CO number density in the range of 1–2 times 10^{17} cm^{-3}. However, the measurement method developed here can also be used to study satellite propulsion systems. Due to absorption by air, a two-photon excitation scheme of the CO molecule at 115 nm (VUV) was used. Experimental and simulated excitation spectra are compared for temperature determination. For spectrum simulation in the B^1varSigma ^+leftarrow X^1varSigma ^+ system, an existing program was extended and a polynomial regression correlation was developed for signals with low signal-to-noise ratios. After the first one-dimensional temperature measurements using CO-TALIF in the free-stream to quantify the radial temperature profile for three conditions in the medium- to high- enthalpy range, respectively, two-dimensional measurements in a steady state test chamber at room temperature were carried out. For the two-dimensional investigations, a light sheet optic was designed to generate a light sheet free of divergence in width and thickness. The results of the one- and two-dimensional temperature measurements performed at medium- to high-enthalpy conditions provided plausible results concerning both the values and the course in radial and axial direction, the latter being, to the authors’ knowledge, the first two-dimensional temperature measurements under such conditions using CO-TALIF.

Silver nanoclusters prepared in water-in-oil emulsions

Stable silver clusters can be prepared by a simple electroless reduction reaction taking place in water-in-oil emulsions. An emulsion containing AgNO3 in the water droplets was mixed with a similar emulsion containing aqueous NaBH4 droplets. The droplet diameter, based on Rayleigh scattering, was 41 nm and the mean number of Ag+ ions in each droplet varied from 2.0 to 21.7 as the concentration increased from 90 μM to 1 mM AgNO3. The low number of Ag+ ions in each droplet inhibits the growth of large nanoparticles and these emulsions do not show the large plasmon band observed for Ag nanoparticles obtained by the analogous reaction in bulk solution at the same Ag+ concentrations. Atomic force microscopy provides evidence of small Ag nanoclusters and a much lower number of larger nanoparticles. Electrospray mass spectrometry suggests that the clusters are mainly Ag4 species coordinated to water and BH4 −. The Ag nanocluster-containing emulsions are fluorescent and show an emission band with a peak wavelength of 427 nm and a Stokes shift of 81 nm from the first peak at 346 nm in the excitation spectrum. The intensity of fluorescence decreased as the [Ag(I)] increased and our most fluorescent samples were prepared from 90 μM AgNO3 because at higher concentrations more Ag nanoparticles are formed. DFT calculations on Agn clusters indicated that Ag4 species favour a planar rhombic geometry even in the presence of coordinating water molecules or BH4 −. However calculations of vertical excitation energies for Ag4 species do not match the experimental excitation spectra and this suggests the fluorescence arises from bright AgNCs of different nuclearity present at lower abundance in the mixture of species produced by the emulsion reaction. Calculated excitation energies for Ag6 give the best fit to the available data.

Theoretical studies on thermodynamic stabilities and luminescent mechanisms of defects and lanthanide ions in BaZrSi3O9

Theoretical studies on thermodynamic stabilities and luminescent properties for the defects and lanthanide ions in the BaZrSi3O9 are performed in order to better understand luminescence mechanisms of undoped and lanthanide-doped BaZrSi3O9. The defect formation energies derived from density functional theory (DFT) calculations with the hybrid PBE0 functional are devoted to analyze thermodynamic stabilities of intrinsic (i.e., the vacancies and antisite defects) and Ti-related defects in the host. The experimental excitation spectra of Ce3+- and Eu2+-doped BaZrSi3O9 phosphors are assigned according to 4f → 5d transition energies obtained from ab-initio calculations at the complete-active-space self-consistent field/complete-active-space second-order perturbation theory/restricted-active-space state-interaction spin-orbit (CASSCF/CASPT2/RASSI−SO) level. Moreover, the mechanisms of the charge transfer (CT), thermoluminescence (TL) and long-lasting luminescence (LLL) in the undoped and lanthanide-doped BaZrSi3O9 are reasonably interpreted on the basis of the thermodynamic transition energy levels of the defects in the host.

Calculated energy loss of swift light ions in platinum and gold: importance of the target electronic excitation spectrum

Understanding and predicting the energy loss of swift ions in metals is important for many applications of charged particle beams, such as analysis and modification of materials, and recently for modelling metal nanoparticle radiosensitisation in ion beam cancer therapy. We have calculated the stopping power of the transition metals Pt and Au for protons and alpha particles in a wide energy range, using the dielectric formalism, which realistically accounts for the excitation spectrum of each metal through the Mermin Energy Loss Function - Generalised Oscillator Strength methodology. For each combination of projectile, energy and target, we have considered: (i) the equilibrium charge state of the projectile through the target, (ii) the energy-loss due to electron capture and loss processes, and (iii) the energy loss resulting from the polarisation of the projectile’s electronic cloud due to the self-induced electric field. Our calculated stopping powers show a fairly good agreement with the available experimental data for platinum and gold, particularly the most recent ones around the stopping power maximum, which validates the methodology we have used to be further extended to other transition metals. For the materials studied (platinum and gold), two commonly used and different sources of the experimental excitation spectrum yield comparable calculated stopping powers and mean excitation energies, the latter being closer to the most recent data provided in a recent ICRU Report than to previous compilations. Despite the small differences in the sources of excitation spectra of these metals, they lead to practically the same stopping power results as far as they reproduce the main excitation features of the material and fulfil physically motivated sum rules.

Morphological, structural, and photophysical properties of luminescent coordination compounds based on Eu3+ and a zwitterionic ligand: A theoretical and experimental investigation

The development of functional materials with photoluminescent properties based on coordination compounds and trivalent lanthanide ions has attracted interest from the scientific community. In this regard, two materials based on a combination of zwitterion ligand (3-(1-methylimidazolium-1-yl) butane-1-sufonate) (Z) and Eu3+ ions (EuZCrystal and EuZGel) were prepared by a simple and efficient method, and the chemical, morphological, and photophysical properties were evaluated. Nuclear Magnetic Resonance (NMR) analysis reveals a broadening and shift from EuZCrystal and EuZGel spectra relative to the Z ligand, especially for the S–O bond, which indicated that the sulfone group is the coordinating site responsible for the bond between the Z molecule and Eu3+. The emission spectra reveal the line profiles of the Eu3+ ion in the coordination compounds for EuZCrystal and EuZGel, which agree with the formation of the new luminescent material. Although the ligand → Eu3+ energy transfer rates are at least 100 times higher than the rates calculated for the Eu3+ → Eu3+ energy transfer, the experimental excitation spectrum suggests that there is not enough electron population to populate the 5D0 emitter state and its consequent decay.

Joining a host-guest platform and a light-emission motif: Pyrazinamide-calixarene hybrids

Calixarenes are macrocyclic polyphenols widely used as hosts in supramolecular chemistry approaches. Recently, aminopyrazine has emerged as a high-efficiency blue light emitting moiety. Inspired by this knowledge, here we prepared two pyrazinamide-calixarene hybrids, in order to join in a single molecular platform both host-guest and light-emission properties. Both compounds were decorated with pyrazinamide at the upper rim, on two phenyl rings alternated into the four-units of the calixarene macrocycle. Their difference was the t‑butyl substitution on the other two alternate phenyl rings. The t‑butyl substituted derivative crystallized in two crystal forms, a non-solvated and a DMF mono-solvate phase, both with just one crystallographically independent hybrid molecule, while the compound without t‑butyl moieties crystallized with three hybrid conformers and four DMF molecules. The molecular substitution pattern has also impacted on the crown conformation, being related to more inclined pyrazinamide-phenyl average planes in the absence of the t‑butyl substituents. The coplanarity pattern between pyrazinamide and its bonded phenyl ring and the conformational profile of this substituent changed among the three conformers of derivative without the t‑butyl group and between those assembling the two crystals forms of the other compound, which was also found in our DFT calculations. Under an excitation wavelength of 388 nm, a broad band emission between 400 nm and 550 nm was found for the three crystal forms, with maxima at ca. 500 nm, with low quantum yields of up to 0.4%. TD-DFT absorption spectra were overlaid to the experimental excitation spectra, while the low efficiency was rationalized both as an effect of the DMF guest and the tail-to-tail π…π stacking interactions between coplanar pyrazinamide-phenyl moieties of the centrosymmetric related molecules.

Coupling of Planar Waveguide Modes in All-Dielectric Multilayer Structures: Monitoring the Dependence of Local Electric Fields on the Coupling Strength

Except for a few cases to which sophisticated techniques are applicable, it is difficult to observe experimentally local electric fields or near fields associated with excitation of electromagnetic modes supported by building blocks of nanostructures. Although elucidating the behavior of the local electric fields or near fields is very useful to develop potential applications of the nanostructures, in many cases, their behaviors have been only speculated from simulations based on the electromagnetic theory. Here, we develop a simple technique that allows us to study experimentally the average behavior of the local electric fields inside two planar waveguide layers interacting with each other in all-dielectric multilayer structures. The study is made possible by doping the two waveguide layers with two different fluorophores. Fluorescence intensities detected at two different wavelengths allow us to study separately the behaviors of the local electric fields inside each waveguide layer. Angle-scan fluorescence excitation spectra are measured for two different series of multilayer samples by exciting the samples in an attenuated total reflection geometry. The line shapes observed are in good agreement with theoretical ones calculated based on the electromagnetic theory, confirming that the experimental fluorescence excitation spectra reflect well the average behaviors of the local electric fields. Applying the developed technique, we investigate systematically the changes in the local electric fields inside the waveguide layers caused by the change in the strength of coupling between the two waveguide modes. In one series of the samples, a gradual transition in the behavior of the local electric fields, from a Fano-resonant behavior to a double-peak behavior, is demonstrated with increasing the coupling strength. In another series of the samples, the coupling strength is widely changed and the formation of the coupled modes in a strong coupling regime is clearly observed. The observed changes in the local electric fields can be reproduced very well by simple analytical calculations based on a coupled oscillator model. The model is helpful to gain physical insights into the coupling of the waveguide modes, which cannot be obtained by the electromagnetic calculations.

A study of the effects of the polarity of the solvents acetone and cyclohexane on the luminescent properties of tryptophan

The luminescent properties of tryptophan in solvents less polar than water, such as acetone, and non-polar ones, such as cyclohexane, are experimentally studied and compared with theoretical calculations using time-dependent density functional theory (TD-DFT) methods. Since tryptophan may present different configurations and charge distributions, the most stable conformer is analyzed for both solvents, including its neutral and zwitterionic forms. To perform the simulation two clusters are proposed with the Zpt conformer in acetone: (Trp)1-(C3H6O)17 and (Trp)2-(C3H6O)34, and four clusters with the Nag+ conformer in cyclohexane: (Trp)1-(C6H12), (Trp)2-(C6H12), (Trp)3-(C6H12) and (Trp)4-(C6H12), in order to conveniently emulate the concentration in each solvent by reducing the distance between adjacent tryptophan molecules as the concentration increases, since there is no control over the volume parameter. In each case, the UV–vis absorption is computed and compared with the experimental excitation spectra; the results show a good agreement. This calculation allows a more detailed analysis of the experimental results based on the properties of the molecular orbitals involved in electronic transitions. In the present work, a strong effect of the solvent acetone on tryptophan is observed; for this solvent, a charge transfer from the solute to solvent happens. This behavior does not occur with water (polar solvent) or cyclohexane (non-polar solvent). Finally, experimental spectroscopic data of Trp in cyclohexane are explained through the hydrogen bonds between amino acid molecules present in the fluorescent states. In this case, the theoretical and experimental results are compared and also show good agreement.

Extreme ultraviolet photoemission of a tin-based photoresist

Tin is a suitable element for inclusion in extreme ultraviolet photoresists because of its relatively high-absorption cross section at 92 eV. The electrons emitted after photon absorption are expected to generate secondary electrons in the solid film. In this way, several pathways lead to reactive species that cause a solubility switch. Here, we report the photoelectron spectra of tin oxo cage photoresists over the photon energy range 60–150 eV, and the relative yields of photoelectrons from the valence band of the resist, from the Sn 4d orbitals, and of inelastically scattered electrons. The experimental excitation spectra differ considerably from those predicted by commonly used database cross section values, and from the combined computed subshell spectra: the maximum efficiency of ionization of Sn 4d both in the photoresists and in Sn metal occurs near the industrially relevant EUV wavelength of 13.5 nm.

Thermodynamics stability, electronic structures and spectroscopic properties of defects and Ce3+ ions in Y2O3

Thermodynamic properties, electronic structures and spectroscopic properties of defects and Ce3+ in Y2O3 are studied by using the hybrid density functional theory associated with multi-reference configuration interaction ab-initio calculations. Thermodynamic transition energy levels of the easily generated oxygen vacancies in the host are analyzed according to HSE06-calculated formation energies, which may be conducive to interpretations of the persistent luminescence (PersL) of Y2O3-based phosphors. Besides, the locations of impurity states (caused by VO and Ce3+) in energy bands are obtained from derived density of states. Moreover, energies and oscillator strengths of 4f1 → 5d1−5 transitions of Ce3+ ions (at Y1 and Y2 sites) calculated from the CASSCF/CASPT2/RASSI−SO method agree reasonably well with experimental excitation spectra of Y2O3: Ce3+ phosphors, achieving the assignment of excitation spectra. The presented calculations can be applied to identify luminescent centers in Ce3+-doped phosphors and reveals possible native defects and their roles in the PersL of phosphors.

Two-photon cross-section calculations for krypton in the 190-220 nm range.

This paper presents multi-path, two-photon excitation cross-section calculations for krypton, using first-order perturbation theory. For evaluation of the two-photon-transition matrix element, this paper formulates the two-photon cross-section calculation as a matrix mechanics problem. From a finite basis of states, consisting of 4p, 5s, 6s, 7s, 5p, 6p, 4d, 5d, and 6d orbitals, electric dipole matrix elements are constructed, and a Green's function is expressed as a truncated, spectral expansion of solutions, satisfying the Schrödinger equation. Electric dipole matrix elements are evaluated via tabulated oscillator strengths, and where those are unavailable, quantum-defect theory is used. The relative magnitudes of two-photon cross-sections for eight krypton lines in the 190-220 nm range are compared to experimental excitation spectra with good agreement. This work provides fundamental physical understanding of the Kr atom, which adds to experimental observations of relative fluorescence intensity. This is valuable when comparing excitation schemes in different environments for krypton fluorescence experiments. We conclude that two-photon excitation at 212.556 nm is optimal for single-laser, krypton tagging velocimetry or krypton planar laser-induced fluorescence.

First-principles study of persistent luminescence mechanisms in CaB2O4:Ce3+

Abstract First-principles calculations based on the density functional theory (DFT) are carried out to explore the mechanisms of the persistent luminescence (PersL) in CaB2O4:Ce3+ phosphors. Firstly, the energies and Stokes shifts of 4f → 5d transitions of Ce3+ ions in the CaB2O4 are successfully obtained and agree well with the experimental results. Thus, the experimental excitation spectra of the CaB2O4:Ce3+ are assigned successfully. The formation energies and thermodynamic transition levels of candidate intrinsic vacancies (of Ca, O, B and BO), antisite defects (BCa) and Ce3+ ions in the host are calculated in order to understand their effects on the PersL of CaB2O4:Ce3+ phosphors. By combining the presented calculation results and Dorenbos empirical model, the charge transition levels for all the trivalent and divalent lanthanides in the host are derived, which can be adopted to predict the candidate emission and trap centers for the PersL in the lanthanide-doped CaB2O4.

Site Occupation and Spectral Assignment in Eu2+-Activated β-Ca3(PO4)2-Type Phosphors: Insights from First-Principles Calculations

Eu2+-activated β-Ca3(PO4)2-type phosphors have attracted significant experimental interest for applications in solid-state lighting because of the presence of multiple cation sites, which is highly desirable for site engineering of activator emission. However, the site occupation and associated spectral assignment of dopant Eu2+, and hence the mechanism behind the site-regulated emission tuning, still remain elusive. Herein, we perform a systematic first-principles study on Eu2+-doped Ca3(PO4)2, Ca10M(PO4)7 (M = Li, Na, K), and Ca3(PO4)2:Mg by combining density functional theory and multiconfigurational ab initio calculations. The results show that, among the isovalent EuCa substitutions in Ca3(PO4)2, the occurrence probability correlates positively with the size of the substituted site, which is, nevertheless, weakened by the incorporation of codopant Mg2+ ions. In the presence of aliovalent EuM substitutions as in Ca10M(PO4)7, the site-size-controlled preference is neutralized by the requirement for charge compensation, and the effect becomes more pronounced with an increase of the M+ ionic size. On this basis, the emission spectra of the phosphors are interpreted with respect to the substituted sites and the mechanism behind the composition dependence of the emission color is consistently elucidated, as is also verified by a comparison between the calculated 4f → 5d transition energies and experimental excitation spectra. Our results provide a new perspective on the site preference of Eu2+ in β-Ca3(PO4)2-type compounds and may also serve as a theoretical guideline on the structure-property relationship for the design of other Eu2+-activated phosphors.

Unraveling a trifecta of weak non-covalent interactions: The dissociation energy of the anisole-ammonia 1:1 complex

The anisole-ammonia 1:1 complex is a challenge for both experiment and theory. Early studies supported a non-planar structure, involving a trifecta of weak non-covalent interactions: N-H/O, N-H/π, and C-H/N. The calculated structure and binding energy of the complex proved remarkably sensitive to the level of theory employed. Here, we report the first experimental measurement of the ground state dissociation energy of the complex, and derive an excited (S1) state dissociation energy that is in excellent agreement with the cutoff observed in the experimental excitation spectrum. Results are compared with previous predictions and new calculations based on benchmarked Density Functional Theory methods.

Monte Carlo simulation and experimental evaluation of the quantum efficiency of Eu3+-doped glass at different temperatures.

The quantum efficiency (QE) is a key parameter to evaluate the optical properties of fluorescent glass. However, it is very difficult to measure the QE at high temperatures by the integrating sphere test system. In this paper, we report a new method to calculate the QE of five kinds of Eu3+-doped glasses at different temperatures based on experimental absorption and excitation spectra of Eu3+-doped glasses. The simulated QE values agree well with the experimental values of QE. Furthermore, the influence of the shape, refractive index and temperature on the QE and the spatial light intensity distribution of the Eu3+-doped glass is studied based on the Monte Carlo method. This work presents a simple method to calculate the QE and the spatial light intensity distribution at different temperatures.

Low-Energy Electronic Excitations of N-Substituted Heteroacene Molecules: Matrix Isolation Spectroscopy in Concert with Quantum-Chemical Calculations.

N‐Heteropolycycles are attractive as materials in organic electronic devices. However, a detailed understanding of the low‐energy electronic excitation characteristics of these species is still lacking. In this work, the matrix isolation technique is applied to obtain high‐resolution absorbance spectra for a series of tetracene and core‐substituted N‐analogues. The experimental electronic excitation spectra obtained for matrix‐isolated molecules are then analysed with the help of quantum‐chemical calculations. Additional lower energy excitation bands in the spectrum of the core‐substituted N‐derivatives of tetracene could be explained in terms of intensity borrowing from dipole‐forbidden transitions due to Herzberg–Teller vibronic coupling. In the case of tetracene, evidence for the additional formation of London dimers (J aggregates) is found at higher tetracene concentrations in the matrix.

A red-emitting Mn4+ activated phosphor with controlled morphology and two-dimensional luminescence nanofiber film: Synthesis and application for high-performance warm white light-emitting diodes (WLEDs)

Mn4+ activated fluoride red phosphors, Na2GeF6:Mn4+, with controllable morphology and size, were successfully synthesized through simple coprecipitation method. Using the hollow Na2GeF6:Mn4+ particles, we successfully fabricated two-dimensional red-emitting nanofiber film via electrospinning technology. The phase purity, morphology and composition of the samples were confirmed via X-ray powder diffraction (XRD), scanning electron microscope (SEM) and energy-dispersive X-ray spectrometer (EDS), respectively. Different surfactants were introduced into the experimental process, which successfully controlled the sample morphology. The broad excitation band from 300 nm to 550 nm matches well with commercial blue light emitting diodes, and narrow band emission red light can be observed in the range of 575–675 nm because of spin-forbidden 2Eg - 4A2g energy level transition of Mn4+ ion. The effect of doping concentration on luminescence and the mechanism of concentration quenching were discussed. On the basis of the experimental excitation (PLE) and emission (PL) spectra, the crystal field strength of the Mn4+ occupation environment was estimated and the nephelauxetic effect was researched for the Mn4+ ions in the Na2GeF6 host. Temperature-dependent properties of Na2GeF6:Mn4+ particles and nanofiber film were systematically analysed. Moreover, the Na2GeF6:Mn4+ particles and nanofiber film can significantly change the performance of white light-emitting diodes (WLEDs), the nanofiber film is a more appropriate candidate than Na2GeF6:Mn4+ particle for warm WLED applications.

Nuclear electromagnetic dipole response with the self-consistent Green's function formalism

Microscopic calculations of the electromagnetic response of medium-mass nuclei are now feasible thanks to the availability of realistic nuclear interactions with accurate saturation and spectroscopic properties, and the development of large-scale computing methods for many-body physics. The purpose is to compute isovector dipole electromagnetic (E1) response and related quantities, i.e. integrated dipole cross section and polarizability, and compare with data from photoabsorption and Coulomb excitation experiments. The single-particle propagator is obtained by solving the Dyson equation, where the self-energy includes correlations non-perturbatively through the Algebraic Diagrammatic Construction (ADC) method. The particle-hole ($ph$) polarization propagator is treated in the Dressed Random Phase Approximation (DRPA), based on an effective correlated propagator that includes some $2p2h$ effects but keeps the same computation scaling as the standard Hartree-Fock propagator. The E1 responses for $^{14,16,22,24}$O, $^{36,40,48,52,54,70}$Ca and $^{68}$Ni have been computed: the presence of a soft dipole mode of excitation for neutron-rich nuclei is found, and there is a fair reproduction of the low-energy part of the experimental excitation spectrum. This is reflected in a good agreement with the empirical dipole polarizability values. For a realistic interaction with an accurate reproduction of masses and radii up to medium-mass nuclei, the Self-Consistent Green's Function method provides a good description of the E1 response, especially in the part of the excitation spectrum below the Giant Dipole Resonance. The dipole polarizability is largely independent from the strategy of mapping the dressed propagator to a simplified one that is computationally manageable

Synthesis, Luminescence and Crystal Field Calculations of Mn4+- Doped Li2TiO3 Deep Red Phosphor

Red emission of the Mn4+ ions incorporated in different materials is used in various applications, such as lighting, holographic recording, thermoluminescence dosimetry, thermometry, etc. Mn4+ emission can improve color-rendering-index of phosphor-converted white-light-emitting diodes, and it is a promising candidate to replace the well-established rare-earth Eu activator in red phosphors. Here, synthesis, structure, detailed spectroscopic and crystal field analysis of Mn4+ doped Li2TiO3 are presented. We have successfully synthesized Li2Ti0.995Mn0.005O3 powder by a solid state synthesis. All precursors were mixed with stoichiometric ratio Li : Ti = 2 : 1 and calcined at 800⁰C for 5 h to produce pure-phase. Structural analysis was done by X-ray diffraction. It confirmed monoclinic Li2TiO3 structure with space group C2/2, in which the octahedral sites are occupied by Ti4+ and Mn4+. Optical characterization included measurements of photoluminescence excitation and emission spectra, emission decay analyses and diffuse reflection measurements. Optical spectra showed emission and absorption bands originating from d–d electronic transitions (d3 configuration). Kubelka-Munk function of the measured diffuse reflectance spectrum showed characteristic absorption of Mn4+. Deep red emission at 680 nm under λexc= 490 nm is a result of the spin-forbidden 2Eg→4A2 and 4T2→ 4A2 electron transition of the tetravalent manganese ions. The energy levels of the Mn4+ ions in Li2TiO3 were calculated using the crystal field exchange-charge model. Three parameters are needed to describe completely each energy level: the Racah parameters B and C, which determine the positions of the free ion energy levels, and the crystal field strength denoted by Dq, which describes the splitting of the free ion terms. These parameters were calculated and showed good agreement with the experimental photoluminescence excitation and emission spectra. Figure 1

Microscopic description of triaxiality in Ru isotopes with covariant energy density functional theory

The triaxiality in nuclear low-lying states has attracted great interests for many years. Recently, the reduced transition probabilities for levels near the ground state in $^{110}$Ru have been measured and provided strong evidences for a triaxial shape of this nucleus. The aim of this work is to provide a microscopic study of low-lying states for the Ru isotopes with $A\sim100$ and to examine in detail the role of triaxiality, and the evolution of quadrupole shapes with the isospin and spin degrees of freedom. The low-lying excitation spectra and transition probabilities of even-even Ru isotopes are described at the beyond mean-field level by solving a five-dimensional collective Hamiltonian with parameters determined by constrained self-consistent mean-field calculations based on the relativistic energy density functional PC-PK1. The calculated energy surfaces, low-energy spectra, intraband and interband transition rates, as well as some characteristic collective observables, such as $E(4_{\rm g.s.}^+)/E(2^+_{\rm g.s.})$, $E(2^+_\gamma)/E(4^+_{\rm g.s.})$, $B(E2; 2^+_{\rm g.s.}\to 0^+_{\rm g.s.})$, and $\gamma$ band staggerings are in a good agreement with the available experimental data. The main features of the experimental low-lying excitation spectra and electric transition rates are well reproduced, and thus strongly support the onset of triaxiality in the low-lying excited states of the Ru isotopes around $^{110}$Ru.