Laser Excitation Spectra Research Articles

Correlations between laboratory line lists for FeH, CrH, and NiH and M-star spectra collected with ESPaDOnS and SPIRou

Context. Molecular bands of metal oxides and hydrides dominate the optical and near-infrared spectra of M dwarfs. High-resolution spectra of these bands have immense potential for determining many properties of these stars, such as effective temperature, surface gravity, elemental abundances, radial velocity, or surface magnetic fields. Techniques are being developed to do this but remain limited by the current availability and accuracy of molecular data and spectral line lists. Aims. This paper reports metal monohydride line lists selected from near-infrared and visible laboratory data to show that specific bands in several electronic transitions can be used to identify CrH, NiH, and FeH in M stars and to determine radial velocities from Doppler shifts. The possibility of measuring magnetic fields is also investigated for FeH and CrH. Methods. We used systematic cross-correlation analysis between unpolarised spectra from a selection of M stars and state-specific laboratory line lists. These lists were generated from a combination of existing data and new laboratory laser-excitation spectra recorded at Doppler-limited resolution, in zero-field conditions or in magnetic fields up to 0.6 tesla. Results. We show that transitions at visible wavelengths in FeH and NiH, usually neglected in the analysis of the spectra of M-type stars, do in fact contribute to the spectra, and we demonstrate the influence of magnetic sensitivity on selected transitions in CrH and FeH. Conclusions. Although the new line lists focus on transitions recorded at temperatures significantly lower than those of stellar objects, they remain pertinent because they cover some band-head regions of high spectral density. FeH bands can provide a useful supplement to atomic lines for the analysis of high-resolution optical and near-infrared spectra of M dwarfs. We demonstrate the influence of a magnetic field on CrH signatures around 862 nm.

Cavity-Enhanced Fluorescence in Colliding Droplets of Rhodamine 6G Aqueous Solutions.

A liquid droplet can act as a microscale high-Q optical cavity via a whispering gallery mode, where light resonates with enhanced intensity. A collision of two droplets temporarily exhibits a unique morphology, which provides a more effective optical cavity than a single droplet. We investigated the mechanisms of cavity-enhanced fluorescence in colliding droplets of aqueous rhodamine 6G. Laser-excited spectra and fluorescence generation times were acquired. The fluorescence spectra had two peaks: one attributed to amplified spontaneous emission (ASE) and the other to lasing. The lasing generation time had a longer delay relative to that of ASE, which indicated that it required a longer propagation distance for the positive feedback. Overall, this provides a basis for the development of a highly efficient dye laser using colliding droplets.

Laser Spectroscopy of Aromatic Molecules with Optical Cycling Centers: Strontium(I) Phenoxides.

We report the production and spectroscopic characterization of strontium(I) phenoxide (SrOC6H5 or SrOPh) and variants featuring electron-withdrawing groups designed to suppress vibrational excitation during spontaneous emission from the electronically excited state. Optical cycling closure of these species, which is the decoupling of the vibrational state changes from spontaneous optical decay, is found by dispersed laser-induced fluorescence spectroscopy to be high, in accordance with theoretical predictions. A high-resolution, rotationally resolved laser excitation spectrum is recorded for SrOPh, allowing the estimation of spectroscopic constants and identification of candidate optical cycling transitions for future work. The results confirm the promise of strontium phenoxides for laser cooling and quantum state detection at the single-molecule level.

Laser-induced fluorescence spectroscopy of the B 5Π-1 – X 5Π-1 system of the chromium monosulfide molecule

Laser induced fluorescence and dispersed fluorescence spectroscopy was used to investigate the B 5Π-1 – X 5Π-1 system of the chromium monosulfide molecule. The rotational constant and Λ-doubling parameter of B 5Π-1 (v′ = 7), as a newly observed upper vibrational level, were obtained from the rotationally resolved laser excitation spectrum. For the ground X 5Π-1 state, the vibrational frequency and vibrational anharmonicity constant were experimentally determined for the first time from the dispersed fluorescence spectrum. In addition, the lifetimes of the upper B 5Π-1 (v′ = 0 − 7) levels were measured by fitting the time-dependent fluorescence decay.

Laser induced fluorescence spectra of the [formula omitted][formula omitted] – [formula omitted][formula omitted] transition of jet cooled [formula omitted]NO3 and [formula omitted]NO3 I: [formula omitted] progressions in the ground [formula omitted][formula omitted] state

14NO3 and 15NO3 were generated in a supersonic free jet, and their laser excitation spectra and dispersed laser induced fluorescence (LIF) spectra of the B∼2E′ – X∼2A2′ transitions from the vibrationless B∼2E′ state were observed. Because of the cooling of the gas in the supersonic jet, the observed spectra are with resolution an order of magnitude higher than those previously reported and allowed us to resolve, for example, the splitting between the a1′ and e′ components of the 2ν4 state. The spectra of 14NO3 and 15NO3 are similar but small isotope shift in the latter allowed us to separate the ν1 band from the 3ν4 (a1′) band which were overlapped in the former.The high resolution dispersed LIF spectra revealed panoramic views of the vibrational level structures in the X∼2A2′ ground electronic states of the two isotopomers. Although the accuracies of the measurement of energy levels are much less than those obtained from the direct high resolution infrared spectroscopy, the obtained patterns are more complete because totally symmetric (a1′) vibrations such as ν1,2ν2,2ν1,ν1 + 2ν2 etc. are also observed. Such clear and complete spectra have revealed remarkable ν4 progressions with detailed vibrational structures in the ground X∼2A2′ state. Near complete progressions of nν4 overtone bands, nν4 with n = 1 ∼ 5, ν1 + nν4 bands with n = 1 ∼ 4, some 2ν2 + nν4 and ν3 + nν4 have been assigned leaving few lines unassigned. There is no evidence of anomaly in the degenerate ν4 bending progression of the B∼2E′ – X∼2A2′ electronic transition of NO3.

Rotational and hyperfine analysis of the [formula omitted] (0,0) band of TaO

The D2Π1/2-X2Δ3/2 transition of TaO has been analyzed for the first time by recording a laser excitation spectrum of its (0,0) band, which lies near 696 nm. This band was first observed many years ago by Cheetham and Barrow (Trans. Faraday Soc. 63 (1967) 1835–1845), who did not undertake an analysis due to the dense rotational structure. The spectrum displays broad hyperfine structure arising from the 181Ta nucleus that is well resolved at the Doppler-limited resolution of the experiment. The data were fitted using an effective Hamiltonian expressed in a Hund’s case (c) basis, confirming the upper state symmetry as 2Π1/2 and yielding an accurate set of molecular constants describing its rotational and hyperfine structure.

Effect of thermochemical treatments on laser-induced luminescence spectra from strontium titanate: comparison with swift ion-beam irradiation experiments

Results recently reported on the effect of thermochemical treatments on the (He-Cd) laser-excited emission spectra of strontium titanate (STO) are re-analyzed here and compared with results obtained under ion-beam irradiation. Contributing bands centered at 2.4 eV and 2.8 eV, which appear under laser excitation, present intensities dependent upon previous thermal treatments in oxidizing (O2) or reducing atmosphere (H2). As a key result, the emission band centered at 2.8 eV is clearly enhanced in samples exposed to a reducing atmosphere. From a comparison with the ionoluminescence data, it is concluded that the laser-excited experiments can be rationalized within a framework developed from ion-beam excitation studies. In particular, the band at 2.8 eV, sometimes attributed to oxygen vacancies, behaves as expected for optical transitions from conduction-band (CB) states to the ground state level of the self-trapped exciton center. The band at 2.0 eV reported in ion-beam irradiated STO, and attributed to oxygen vacancies, is not observed in laser-excited crystals. As a consequence of our analysis, a consistent scheme of electronic energy levels and optical transitions can now be reliably offered for strontium titanate.Graphical abstract

Investigation of N(4S) kinetics during the transients of a strongly emissive pulsed ECR plasma using ns-TALIF

A nanosecond-two-photon absorption laser-induced fluorescence (ns-TALIF) technique was employed to investigate the transients of a strongly emissive, pulsed microwave discharge. We analysed the conditions that have to be fulfilled in order to use the TALIF intensities measured with the laser central frequency tuned to the absorption peak instead of the fully integrated laser excitation spectrum absorption for measuring N-atom densities. We demonstrated the validity of the method under our conditions and applied it to the straightforward monitoring of N-atom densities during the transients of a pulsed ECR plasma. In particular, we demonstrated the existence of an unexpected increase in N(4S) atom density of about 10% at the early stage of the transition from high- to low-power phase. Using a self-consistent, quasi-homogenous plasma model encompassing a detailed state-to-state kinetics, a thorough analysis of the N-atom and N2(B) kinetics was carried out and enabled this enhancement to be attributed to surface de-excitation of N(2D) and N(2P) atoms. The rise of the N(4S) atom density can serve as an indicator of the concentration of N(2D) and N(2P) atoms in the discharge.

Infrared spectroscopy of carbocations upon electron ionization of ethylene in helium nanodroplets

The electron impact ionization of helium droplets doped with ethylene molecules and clusters yields diverse CXHY + cations embedded in the droplets. The ionization primarily produces C2H2 +, C2H3 +, C2H4 +, and CH2 +, whereas larger carbocations are produced upon the reactions of the primary ions with ethylene molecules. The vibrational excitation of the cations leads to the release of bare cations and cations with a few helium atoms attached. The laser excitation spectra of the embedded cations show well resolved vibrational bands with a few wavenumber widths-an order of magnitude less than those previously obtained in solid matrices or molecular beams by tagging techniques. Comparison with the previous studies of free and tagged CH2 +, CH3 +, C2H2 +, C2H3 +, and C2H4 + cations shows that the helium matrix typically introduces a shift in the vibrational frequencies of less than about 20cm-1, enabling direct comparisons with the results of quantum chemical calculations for structure determination. This work demonstrates a facile technique for the production and spectroscopic study of diverse carbocations, which act as important intermediates in gas and condensed phases.

Indirect and direct excitation of Nd3+ ions in as-deposited and annealed Nd3+-doped ZnO films

Indirect (Ar+-laser) and direct (dye-laser) excitation of Nd3+ ions in unannealed (as-deposited) and annealed r-f sputtered ZnO:1.5mol%Nd3+ films resulted in emission in the 875–915 nm region. This is attributed to the 4F3/2 → 4I9/2 transitions of the Nd3+ ions. Direct excitation of the films in the 10–75 K temperature range enabled the identification of one Nd3+ centre in the ZnO:1.5Nd3+ pristine films and a different one in the ZnO:1.5mol%Nd3+ films annealed in air at 600 °C. A broad emission band characteristic of ZnO spans the 500–875 nm wavelength range of the Ar+-laser excited spectra. Films were characterized by grazing incidence X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy for structural, morphology and compositional properties.

Investigation of the mechanism and influence of laser wavelength and energy on laser opto-ultrasonic dual detection.

Laser opto-ultrasonic dual (LOUD) detection, which uses laser irradiation of samples to generate spectral and ultrasonic signals simultaneously, can perform multimodal detection of element composition and structural property. As such, it has been applied to the detection of additive manufacturing (AM) components. Further, optimized parameters lead to better detection results. To the best of our knowledge, however, there is no study on the effect of laser properties on LOUD detection. Therefore, we studied the mechanism and influence of laser wavelength and energy on LOUD detection. In this work, the intensity, signal-to-noise ratio (SNR), and stability evolution of the laser excitation spectrum and ultrasonic signals at different wavelengths and energies were analyzed. It was found in the plasma evolution that high electron number density means a large amount of ablated mass generated, which was favorable for laser ultrasonic excitation and can produce higher SNR and a more stable signal. However, it also led to more atoms of the ground-state, which resulted in the self-absorption effect and reduced spectrum intensity in the spectrum analysis. Therefore, with self-absorption correction, better stability, and higher signal intensity, an SNR of spectral and ultrasonic signals can be obtained using 355 nm laser excitation at optimal energy. As a result, in the quantitative analysis of Cu and Si elements by LOUD detection, the determination coefficients (R2) were higher than 0.995, and the average relative errors were less than 2.5%, the limit of detection could reach the order of 100 ppm. Further, the defect size of 0.55 mm in the wire +arc additive manufacturing sample was detected by LOUD detection, and the average relative error was 5.59% compared with the digital radiography results, which indicate that laser wavelength and laser energy affect the intensity and stability of spectral and ultrasonic signals in LOUD detection, which means selecting appropriate laser parameters is important to obtain a high precision detection.

High-resolution spectroscopy of the [formula omitted] and [formula omitted] bands in the [formula omitted] transition of [formula omitted

We present a high-resolution spectroscopic study of the A~1B2-X~1A1 transition of SiC2 by laser-induced fluorescence. The gas-phase SiC2 molecules are produced in a discharge plasma and cooled in a supersonic jet expansion. By employing a home-made single longitudinal mode optical parametric oscillator as the narrow bandwidth laser source, laser excitation spectra have been recorded with a transition frequency accuracy of ~0.004 cm-1 for the 000,201,301 and 311 bands. Rotational analyses of the high-resolution data have yielded accurate spectroscopic constants, including the centrifugal distortion constants, for the upper vibronic states studied here.

Sm2+-doped SiAlO sputter deposited coatings for self-absorption and scatter-free luminescent solar concentrator applications

Combinatorial reactive co-sputtering using Al, Si and Sm targets in an Ar + O2 atmosphere, resulted in Sm doped SiAlO thin films with a wide Sm concentration- and Si:Al composition gradient. By combining position dependent EDX spectra and laser excited emission spectra, ternary phase diagrams were constructed that directly show the relation between Sm emission intensity, index of refraction, thickness and composition. Using this approach, the Sm2+ and Sm3+ emission intensity ratio was controlled towards films with predominantly Sm2+ emission, which is most favorable for luminescent solar concentrator (LSC) applications. The optimum Sm2+ efficiency was reached when the Al content was about equal to the Sm content. When the Si:Al ratio decreases, the Sm2+ emission intensity strongly drops to almost zero. However, sputtering without Al resulted in no Sm2+ emission intensity at all. The excitation and emission properties of Sm2+ in the optimized thin films, especially the ratio between the 4f→4f and 5d→4f emission that is sensitively susceptible to the co-ordination polyhedron, closely resembles that of Sm2+ doped crystalline powders with the same composition. This strongly suggest that the Sm2+ ions in our amorphous films are coordinated in the same way. A homogeneous thin film on float glass clearly shows the light concentration effect of the red Sm2+ emission. Due to an unexplained low Sm2+ absorption of our films, even the optimized thin films do not luminesce brightly.

A uranium atlas, from 365 to 505 nm

A Fourier-transform spectrum of the emission from a commercial uranium hollow-cathode lamp 19,800–27,400 cm−1 is proposed, in ascii format, as a possible aid to calibration of laser excitation spectra.

The red bands of TaH: Identification of the ground and low-lying electronic states

Doppler-limited laser excitation spectra were recorded for 12 bands of the molecule tantalum hydride (TaH) in the red region, covering the wavelengths 605–665 nm. These excitation bands originate from three different lower electronic states. Dispersed fluorescence experiments revealed three additional low-lying electronic states of the molecule. The identities and energies of these six states are found to be in good agreement with the multiconfiguration self-consistent field calculations of Koseki et al. (2004). The ground state is X3Φ2, with the [0.1]0+ state lying only 76 ± 6 cm−1 above it. The upper states display energy shifts caused by numerous local perturbations with other dark states.

Effect of dipole blockade on the laser excitation spectra of mesoscopic ensembles of cold Rydberg atoms

The results of theoretical calculations of the multiatom laser excitation spectra of Rydberg atoms under conditions of a dipole blockade are presented, and the effect of the laser radiation line width and the interaction energy of Rydberg atoms on the dipole blockade completeness degree is discussed. It is shown that in order to increase the fidelity of performing quantum gates based on a dipole blockade, it is necessary to use resonance dipole − dipole interaction and exciting laser radiation with a line width smaller than 10 kHz.

High-resolution spectroscopy of the a4Σ−3/2 − X12Π3/2 system of gold monosulphide in the near infrared

ABSTRACTWe have recorded the a4Σ−3/2 − X12Π3/2 (0,0), (1,0), and (2,0) bands of gaseous gold monosulphide (AuS) at sub-Doppler resolution in the near-infrared region. The molecules were made in a hollow cathode discharge source by the reaction of sputtered gold with carbonyl sulphide. The high resolution of the laser excitation spectrum enabled the determination of molecular constants describing the rotational and 197Au hyperfine structure in both states, as well as the spin–rotation interaction in the a4Σ−3/2 state. The natures of the two electronic states are discussed in the context of the observed hyperfine structure.

Plasmon–Exciton Coupling at Individual Porphyrin-Covered Silver Clusters

The feasibility to couple surface plasmons with molecular excitations is an intriguing feature of plasmonic–organic hybrid systems. To date, investigations of plasmonic–excitonic coupling have largely focused on ensembles of nanoparticles and on purely optical methods. Here we present a single-particle approach based on laser-spectroscopic photoemission electron microscopy. Localized surface plasmons give rise to photoemission resonances in laser excitation spectra of individual silver nanoclusters. As a most striking manifestation of plasmon–exciton coupling, upon deposition of a thin zinc tetraphenylporphyrin (ZnTPP) film, a second resonance near the ZnTPP Soret band appears in our spectra. In accordance with simulations, spectral repulsion of both resonances as well as intensity redistribution between both modes indicates their plasmonic–excitonic hybrid character.

High-resolution electronic spectra of yttrium oxide (YO): The D2Σ+-X2Σ+ transition.

The D2Σ+-X2Σ+ electronic absorption spectrum of the astrophysically relevant yttrium oxide (YO) molecule has been recorded for the first time in the 400-440 nm region using laser induced fluorescence. YO molecules are produced by corona discharge of oxygen between the tips of two yttrium needles in a supersonic jet expansion. An unambiguous spectroscopic identification of the D2Σ+-X2Σ+ transition becomes possible from a combined analysis of the moderate-resolution laser excitation spectrum and dispersed fluorescence spectrum. We have also performed multi-state complete active space second order perturbation theory calculations on the first six doublets of YO, and the results support our assignment of the D2Σ+ state. Accurate spectroscopic constants for D2Σ+ν' = 0 and 1 levels have been determined from a rotational analysis of the high resolution spectra that are recorded with a resolution of ∼0.018 cm-1. Severe perturbations are observed in the experimental spectra and are considered to originate from interactions with at least one nearby 2/4Π electronic state, e.g., the undetected C2Π state. We have also measured the radiative lifetimes of B2Σ+ν' = 0, and D2Σ+ν' = 0 and 1 states, based on which the B2Σ+-X2Σ+ (0, 0) and D2Σ+-X2Σ+ (0/1, 0) band oscillator strengths have been determined.

Investigation of the spectral–luminescent properties of the threonine molecule

We have investigated the laser-excited photoluminescence spectra of powder and aqueous solution threonine. The spectral ranges of the photoluminescence and its intensity maxima have been determined. We have measured the spectral dependence of the molar extinction coefficient of an aqueous solution of threonine. Using quantum-chemical methods, we have calculated the electron absorption spectrum, the dipole moment, and the distributions of charges on individual atoms of the threonine molecule. The calculated electron absorption spectrum has been compared with experiment.