Laser Spectroscopic Investigations Research Articles

Nonlinear Luminescence Quenching in Eu2O3

We report a laser spectroscopic investigation of saturation effects in the spectra and dynamics of Eu2O3. The saturation effects occur at high laser intensity and appear as dips in the center of the fluorescence excitation lines and as a shortening of the 5D0 excited‐state decay curve. The saturation effects are observed in nanoparticles, micrometer‐sized particles, and a fused crystal of monoclinic‐phase Eu2O3 and in micrometer‐sized particles of cubic‐phase Eu2O3. We attribute the mechanism of the nonlinear luminescence quenching to upconversion by energy transfer.

Laser spectroscopic investigation of isotope shifts in Nd II lines

The isotope shift of 11 optical transitions in Nd II in the spectral range 420–450 nm have been recorded (in all cases but one for all pairs of even Nd isotopes). For all observed transitions the values of the field shifts, the specific mass shifts and Δ∣ ψ(0)∣ 2 have been evaluated. Using our new data, combined with data reported in earlier papers, term isotope shifts for all pairs of even Nd isotopes have been determined for 27 energy levels and for the configurations: 4 f 46 s, 4 f 45 d, 4 f 35 d6 s and 4 f 46 p as well. It is shown that configuration assignment of a level on the basis of term shift values only is in some cases not satisfactory (as e.g. of the level at 22,696 cm −1) and obviously additional information is required.

Laser Spectroscopic Investigation of Salicylic Acids Hydrogen Bonded with Water in Supersonic Jets: Microsolvation Effects for Excited State Proton Dislocation

Geometric structures and excited-state proton dislocation of size-selected salicylic acid clusters (salicylic acid and 5-methoxysalicylic acid) with water were studied by using laser spectroscopic techniques. Fluorescence excitation, dispersed fluorescence, and infrared (IR) spectra of those clusters in supersonic jets were examined for both the electronic ground (S0) and first excited (S1) states. The geometric structures of the clusters were determined on the basis of the IR spectra of the OH stretch region with the help of quantum chemical calculations. The hydroxyl group of the water moiety in the clusters forms a ring involving the carboxylic group of the salicylic acid moiety. The IR spectra in S0 show that the intramolecular hydrogen bond in the salicylic acid moiety is still held upon cluster formation, but the phenolic OH stretch band intensity is remarkably weaken in the clusters. The IR spectra in the S1 state and dispersed fluorescence spectra indicated that the intramolecular excited state proton dislocation is hardly affected by the microsolvation with water, in contrast with the strong suppression of the dislocation in the self-solvation.

Thirteen pump-probe resonances of the sodiumD1line

We present the results of a pump-probe laser spectroscopic investigation of the Doppler-broadened sodium $D1$ resonance line. We find 13 resonances in the resulting spectra. These observations are well described by the numerical predictions of a four-level atomic model of the hyperfine structure of the sodium $D1$ line. We also find that many, but not all, of these features can be understood in terms of processes originating in a two-level or three-level subset of the full four-level model. The processes we observed include forward near-degenerate four-wave mixing and saturation in a two-level system, difference-frequency crossing and nondegenerate four-wave mixing in a three-level V system, electromagnetically induced transparency and optical pumping in a three-level lambda system, cross-transition resonance in a four-level double-lambda system, and conventional optical pumping. Most of these processes lead to sub-Doppler or even subnatural linewidths. The dependence of these resonances on the pump intensity and pump detuning from atomic resonance are also studied.

Laserspectroscopic Investigation of the Hyperfine Structure of Ta II-lines

The hyperfine structure of 39 lines of the singly charged tantalum ion was investigated by means of dopplerlimited laser spectroscopy. From the spectra we have deduced the magnetic hyperfine interaction constants A and the electric quadrupole interaction constants B of 22 levels with even parity and 17 levels with odd parity.

Vibrational Energy Transfer in Highly Excited Bridged Azulene-Aryl Compounds: Direct Observation of Energy Flow through Aliphatic Chains and into the Solvent

Intra- and intermolecular vibrational energy flow in vibrationally highly excited bridged azulene-(CH2)n-aryl (n = 0,1,3; aryl = benzene or anthracene) compounds is observed using time-resolved pump−probe laser spectroscopy. Light absorption in the azulene S1-band, followed by fast internal conversion, leads to vibrational excitation at the azulene side of the molecules. Subsequent energy flow through the aliphatic chain to the aryl group at the other side of the molecules and vibrational energy transfer into a surrounding liquid solvent bath are measured either by probing the red edge of the azulene S3-absorption band at 300 nm and/or the anthracene S1-absorption band at 400 nm. The data are analyzed by representing the intramolecular energy flux as a diffusion process and using hot absorption spectra of the two chromophores of the compounds for measuring their energy contents. A fit to all of the experimental signals leads to an energy conductivity of a single C−C bond of κCC = (10 ± 1) cm-1 K-1 ps-1 (with energies measured in cm-1). Depending on the substituent and the length of the chain, this models yield intramolecular energy transfer times of 1.2−4 ps. Energy transfer to the solvent 1,1,2-trichloro-trifluoro-ethane, on the other hand, is characterized by an exponential loss profile with a cooling time constant of (21 ± 2) ps, independent of the substituent and the same as for bare azulene.

Binding energy and structure of the ground, first electronic and ion states of p-methoxyphenethylamine(H 2O) 1 isomers: a combined experimental and theoretical study

An extensive laser spectroscopic, structural and computational investigation on the neurotransmitter analogue p-methoxyphenethylamine (MPEA) chromophore weakly bound to one molecule of water, (MPEA(H 2O) 1) is reported. The complex was prepared by supersonic expansion of a gas mixture of seeded MPEA and water molecules in rare gas He. Two well-characterised experimental isomers have been observed, with geometries resulting from large contributions of the ordinary hydrogen bond and other intermolecular forces. Four key properties of the complex and related species are addressed in this paper: the characterisation of the isolated isomers observed spectra, the measurements of the binding energies of the ground, first electronic and ground ion states of the complex, the identification of each isomer spectrum with the computed geometries and the relationship of the complex stable isomers with the precursor conformers. A number of mass resolved laser spectroscopies have been used in order to get a complete set of experimental results, that complemented with ab initio density functional computations at four basis set, finally permitted us to settle and assign the mentioned properties of the two observed MPEA(H 2O) 1 isomers.

Crystal field studies in Eu 3+ doped Bi 12SiO 20 and Bi 12SiO 20:V 5+ single crystals

Site-selective laser spectroscopy investigations of Eu 3+ in Bi 12SiO 20 (BSO) and Bi 12SiO 20:V 5+ (BSO:V) single crystals have been carried out. The line narrowed fluorescence shows the existence of a single crystal field site for the rare earth in BSO whereas in BSO:V five different crystal field sites for Eu 3+ may exist, which depend on the vanadium concentration. The number of Stark components observed in 5D 0→ 7F J transitions corresponds to a complete absence of degeneracy in 7F J multiplets and indicates a symmetry C 2 v or lower for these optically active centres. Simulations of the sequence of Eu 3+ energy levels for each of the five observed sites have been worked out by using the single-particle crystal-field theory. The nature of the new active optical sites has also been investigated by comparing the phenomenological crystal-field parameters with those calculated by a semi-empirical model which takes into account the distortions of the oxygen bonds produced by the presence of V 5+.

Vibrational wave packets in the B 1Πu and D 1Σu+ states of Cs2: Determination of improved Cs2+(X) and Cs2(B) spectroscopic constants

Vibrational wave packets in the B 1Πu and D 1Σu+ excited states of Cs2 have been studied on the ∼100 fs time scale by pump–probe laser spectroscopy. The temporal behavior of the wave packets was monitored by photoionizing the electronically excited molecule with a time-delayed probe pulse and recording the time and energy-integrated photoelectron signal as a function of time delay between the pump and probe pulses. For the B 1Σu+ experiments, wave packets were produced by exciting the B 1Σu+←X 1Σg+ transition in the ∼740–790 nm region and subsequently detected by photoionizing the molecule at wavelengths between 565 nm and 600 nm. By simulating the experimentally observed transients with the density matrix formalism (and explicitly accounting for laser chirp and |Δv|>1 coherences), improved values for the equilibrium internuclear separation for the Cs2(B 1Πu) state and Te for the Cs2+(X) state were determined to be Re(B 1Πu)=4.93±0.03 Å and Te[Cs2+(X)]=29 930±100 cm−1, respectively. Similar experiments were conducted for the D 1Σu+ state. Wave packets composed of vibrational levels (v′≈40–50) perturbed by the bound 2 3Πou state were produced on the D 1Σu+ potential surface by driving the D 1Σu+←X 1Σg+ transition in the 575–610 nm spectral interval.

Ultrafast carrier dynamics in undoped microcrystalline silicon

We have studied ultrafast dynamics of photoexcited carriers in μc-Si:H by pump and probe laser spectroscopy. We have found that the dynamics of photoexcited carriers in μc-Si:H depend on the crystallinity of the material: in the samples with low crystalline fraction, the dynamics have a fast decay and resemble those in a-Si:H, while in the samples with high crystallinity the dynamics are slower and similar to those in c-Si. We have identified an intensity dependent bimolecular recombination in the samples with lower crystalline fraction (coefficient B=2×10 −10 cm 3 s −1 for deposition with silane dilution ratio ≈5% at a fixed power of 6 W), and no bimolecular recombination in the samples with high crystallinity.

Ultrafast Phenomena in Copper Films

The investigation of ultrafast processes in copper films by femtosecond pump-supercontinuum probe laser spectroscopy is presented. The energy dependence of electron-electron and electron-phonon scattering rates in wide spectral region of probing 1.6 < ℏωprobe < 3.2 eV is clearly observed. It is shown that the relaxation rates sharply slow down in the spectral area, related to the optical transitions into the vicinity of the Fermi level. The position of rate minimum gives the location of the Fermi level. The rate energy dependence around minimum gives unique information on the quasiparticle damping rate near the Fermi surface and thus on the Fermi liquid vs non-Fermi liquid behavior.

Laser‐spectroscopic investigation of OH‐radical concentrations in the exhaust plane of jet engines

Hydroxyl radical (OH) emissions are relevant for oxidation reactions in the post flame chemistry of exhaust gases emitted from jet engines. No direct measurements of OH concentrations are available to date due to the low abundance and the short lifetime of this radical species. The first application of a combined technique based on Raman scattering and laser‐induced fluorescence (LIF) spectrometry is presented here for measurements in the exhaust gases of a commercial jet engine operated in a test rig. From the measurements, upper limits for OH concentrations in the exit plane were determined in the range of 90 ppbv for take off and 80 ppbv for ap‐idle. The values are significantly below the predictions of model calculations based on HONO and HNO3 in‐flight concentration measurements presented recently. Possibilities for further increase of the detection sensitivity for OH are discussed.

Laser-Spectroscopic Investigation of Higher Excited Electronic States of the KAr Molecule

Higher excited electronic states of the van der Waals molecule39KAr have been investigated using a supersonic beam and a two-step excitation. Four hundred seventy Doppler-free lines have been observed in the second step leading to the electronic states 5d2Σ, 5d2Π, 5d2Δ, and 7s2Σ. The spectroscopic parameters of rotation, of spin–orbit splitting, of spin–rotation coupling, and of λ-type doubling have been determined for altogether 19 vibrational levels of these electronic states. The molecular spin–orbit splitting turns out to be −0.42 cm−1for 5d2Δ and −0.15 cm−1for 5d2Π, which is in fair agreement with the theoretical prediction for a Hund's coupling casea. The vibrational numbering was deduced from theoretical calculations of the interatomic potential. Six vibrational levels have been assigned to the 5d2Δ and the 5d2Σ state, respectively, allowing a determination of the Dunham coefficients in these two states. For the equilibrium parameters of the 5d2Δ state, we obtainRe= 3.22(15) Å,De= 900(80) cm−1.

Femtosecond study of photo-induced electron dynamics in AgI and core/shell structured AgI/Ag2S and AgBr/Ag2S colloidal nanoparticles

Direct measurements of the dynamics of photoinduced electrons in AgI, AgI/Ag2S, and AgBr/Ag2S semiconductor colloidal nanoparticles have been performed using femtosecond pump–probe laser spectroscopy. In AgI the transient absorption signal features a laser pulsewidth limited rise followed by a double exponential decay with time constants of 2.5 ps and &amp;gt;0.5 ns. The decay dynamics were found to be independent of pump power, indicating that the decays are not second order kinetic processes. The dynamics were also independent of the probe wavelengths, suggesting that the absorption spectrum of the photogenerated electrons is fully developed within ∼150 fs. The fast 2.5 ps decay is attributed to trapping and nonradiative electron–hole recombination mediated by a high density of trap states while the slow decay is attributed to reaction of deep trapped electrons with silver ions to form Ag atoms. For core/shell structured cocolloids of AgI/Ag2S, similar decay dynamics were observed. The only interesting difference is that the fast decay becomes faster with increasing concentration of Ag2S. This is due to direct excitation of Ag2S, which has a faster initial decay (800 fs) than in AgI. This is supported by the results of AgBr/Ag2S for which the same 800 fs decay was observed while no signal was observed for AgBr colloids alone, indicating clearly that the signal is from Ag2S. These findings on the early time dynamics of photo-induced excitons in AgI, AgI/Ag2S, and AgBr/Ag2S are important in better understanding the photographic process involving silver halides.

Stark effect investigations of resonance lines of neutral potassium, rubidium, europium and gallium

Laserspectroscopic investigations of the Stark splitting and shift behaviour of the hyerfine components of resonance lines of potassium, rubidium, europium and gallium have been performed in external electric fields up to field strengths of 400 kV/cm. For the transitions {fx229-1} we derived the scalar polarizability difference Δα 0 between upper and lower level of the transitions and the individual tensor polarizabilities α 2.

Fast wavelength switching of narrow-band excimer lasers

A novel system was developed, which allows one to switch the wavelength of a narrow-band excimer laser between two successive light pulses at a repetition rate of at least 250 Hz. This is realized by a periodically driven piezo actuator, which is attached to the diffraction grating of the narrow-band KrF excimer laser. The achieved position accuracy of the grating leads to a wavelength reproducibility of ±0.2 pm, which allows one to apply this system to laser spectroscopic investigations like LIF or LIPF of OH in flames. Using the fast wavelength switching system background reduced concentration and temperature fields in flames can be measured within one sequence. Some possible realized and planned applications like the measurement of gas temperature, the diagnostic of turbulent combustion processes, and the investigation of combustion processes under microgravity are discussed.

Compositional control of rovibrational wave packets in the E(1Σg+) “shelf” state of Li2 via quantum-state-resolved intermediate state selection

Compositional control in the preparation of rovibrational wave packets is demonstrated in the E(1Σg+) state of gas-phase Li2 molecules using ultrafast pump–probe laser spectroscopy combined with quantum-state-resolved intermediate state selection. The intermediate state, from which subsequent ultrafast excitation occurs, is a stationary rovibrational level in the A(1Σu+) state of Li2, produced by cw laser excitation from the ground X(1Σg+) state. The effect that the intermediate state has on the final composition of the wave packet is investigated by comparing the transients resulting from ultrafast pump–probe excitation of two different intermediate states (vA=14, JA=18 versus vA=13, JA=18). In these experiments the pump wavelength is compensated so that in each case the same E-state eigenstates (vE=13–18, JE=JA±1) make up the wave packet, but with different amplitudes. Theory predicts, and experiments confirm, that the relative amplitudes of the rovibrational eigenstates are strongly dependent upon the intermediate state and determine the spatial and temporal evolution of the wave packet. Evidence for this includes differences in the observed pump–probe transients and dramatically different amplitudes of the beat frequencies in the Fourier analysis of the time-domain transients. Theoretical three-dimensional wave packet simulations highlight how the composition of the wave packet is used to vary its spatial and temporal evolution.

Stark- and Zeeman-effect of resonance lines of Ba I

Laserspectroscopic investigations were performed on a collimated atomic beam of barium (natural isotope abundance). The metastable ground levels (3D1,2,3 and 1D2) of the investigated lines were populated by a discharge burning in barium vapour directly in front of the oven hole. We could investigate 14 spectral lines between 580 and 690 nm. The tensor polarizabilities of the upper and the lower level as well as the differences in the scalar polarizabilities could be determined. Spectra in magnetic fields up to 0.03 Tesla showed no deviation from the pattern resulting from a simple Russell-Sounders coupling.

Laser Spectroscopic Studies of theC1Σ+,v= 0 andv= 1 States of CO

Two laser spectroscopic investigations were performed on theC1Σ+–X1Σ+(1,0) and (0,0) bands of CO. In the first experiment, using 2 + 1 resonance-enhanced multiphoton ionization, information on highly excited rotational states up toJ≈ 50 was obtained. In the second experiment, application of a narrow-band VUV laser provided highly accurate absolute calibrations. Molecular constants for12C16O,13C16O,12C18O, and, for the first time,12C17O are derived.

Laser-spectroscopic investigations of the lithium resonance lines

High-resolution laser-atomic-beam spectroscopy allowed us to investigate the lithiumD lines in magnetic and electric fields and to determine hyperfine constants, isotope shifts, scalar and tensor polarizabilities and the absolute transition frequencies with high accuracy. The methods used are discussed and the results are summarized.