Dye Laser Spectroscopy Research Articles

From clusters to semiconductor nanostructures.

In the last two decades there has been tremendous interest in various aspects of nanoscience and nanotechnology. Now the subject has matured into an important field in the general area of science and technology that has been identified as one of the thrust areas. Nanoscience and nanotechnology is really an interdisciplinary and/or multidisciplinary subject. This review describes work on transition metal molecular level clusters. The clusters were fabricated by making very dilute solutions and also by doping them in lattices of alkali halides and alkali cyanides. Size dependence of the bandgap was shown for the first time by using site-selective dye laser spectroscopy, time resolved spectroscopy, optical absorption and molecular orbital calculations. A number of semiconductor nanostructures such as quantum wires, quantum wells, and coupled quantum wells, and many-body effects in nanostructures such as self-assembled quantum dots, exciton-phonon interactions, four wave mixing, polymer composites, metallic nanoparticles and metal oxides are discussed.

Experimental evidence of Eu 3+ pairs in K 2EuF 5

Low temperature, dye-laser spectroscopy of K 2EuF 5 reveals the presence of additional structures in the 7F 0 → 5D 0,2 excitation bands. These structures are associated with the formation of Eu 3+ pairs even in stoichiometric material. The kinetics of the emission decay are observed to be different when excitation occurs either at the centre or into the additional structures, suggesting faster energy transfer inside the pairs following excitation in the satellite structures compared to the analogous processes occurring for radiation absorbed in the centre of principal line.

Studies of problems connected with wide scan dye laser spectroscopy

The nonlinearity of a galvo-controlled laser frequency scanning Brewster plate has been studied. A theoretical treatment for the deviation from linearity due to rotation of the Brewster plate in the laser resonator is presented. The calculations are based on the difference between the optical length at any selected incident angle and that at Brewster angle. A basic computer program has been developed to calculate the following parameters: the optical length, deviation from linearity, driving current, free spectral range, and frequency change as a function of the incident angle. Furthermore, calculations for increasing the tuning range of laser resonator frequencies using a beam compensated displaced glass wedge and its losses as a function of wavelength are discussed.

Sub-Doppler spectroscopy of thioformaldehyde: Excited state perturbations and evidence for rotation-induced vibrational mixing in the ground state

High-resolution intracavity dye laser spectroscopy has been used to obtain sub-Doppler spectra of transitions to 350 rotational levels in the 410 band of the à 1A2–X̃ 1A1 electronic transition of thioformaldehyde. Ground state combination differences from the sub-Doppler spectra, combined with microwave and infrared data, have been used to improve the ground state rotational and centrifugal distortion constants of H2CS. The upper state shows a remarkable number of perturbations. The largest of these are caused by nearby triplet levels, with matrix elements of 0.05–0.15 cm−1. A particularly clear singlet–triplet avoided crossing in Ka′ = 7 has been shown to be caused by interaction with the F1 component of the 3162 vibrational level of the ã 3A2 state. At least 53% of the S1 levels show evidence of very small perturbations by high rovibronic levels of the ground state. The number of such perturbations is small at low J, but increases rapidly beyond J=5 such that 40%–80% of the observed S1 levels of any given J are perturbed by ground state levels. Model calculations show that the density and J dependence of the number of perturbed levels can be explained if there is extensive rotation-induced mixing of the vibrational levels in the ground state.

Vibrational predissociation of 9,10-dichloroanthracene—Mixed and homo rare gas atom clusters

Clusters of the form DCA(R1R2), where DCA=dichloroanthracene and R1, R2=Ar, Kr, and Xe were synthesized in a supersonic molecular beam. The mixed clusters were efficiently formed by the coexpansion of DCA in a mixture of two different rare gases with the heavier rare gas being in the minority. The clusters’ vibrational predissociation dynamics was probed using a nanosecond excimer pumped dye laser spectroscopy and energy resolved emission diagnostics. DCA was chosen for this study because of its high emission quantum yield and relatively few spectral interferences at high vibrational energy. The emission quantum yield of DCA–rare gas atom complexes was found to be unity at the electronic origin. The emission quantum yield is greatly reduced upon the increase of the vibrational energy being 0.052 at the 1390 cm−1 vibrational level. It was more than three times higher in the DCA–rare gas atom clusters at the 1390 cm−1 vibration, in comparison with that of the bare molecule, due to vibrational predissociation. The vibrational predissociation products have been identified using energy resolved emission. At 1390 cm−1, excess vibrational energy two argon atoms or one xenon atom could dissociate. For DCA(Kr)n, it is not clear whether one or two krypton atoms had dissociated. In DCA(XeAr) or DCA(XeKr) excited to the 1390 cm−1 vibration, either one of the two rare gas atoms could dissociate, but not both of them. The results indicate that predominantly the weakest bound rare gas atom dissociates, although its vibrational modes seems less effectively coupled to the excited skeleton modes. The dissociation rates were determined by the relative emission intensity before and after the dissociation, which could be spectrally identified. The time scales for vibrational predissociation of all the various DCA clusters were found to be about 1 ns, independent of the rare gas identity. The results are interpreted by assuming the excitation of a vibrationally mixed 1390 cm−1 state which undergoes a secondary intramolecular vibrational energy redistribution (IVR) within the DCA chromophore to a combination mode which contains a low lying promoting vibrational character. This secondary IVR is the ‘‘bottleneck’’ precursor process whose time scale is intramolecular, being independent of the rare gas atom attached to the DCA. After this secondary IVR, the vibrational energy flows on a much shorter time scale to and between the rare gas atom–DCA vibrational modes, and the weaker bound atom statistically dissociates.

Atomic lifetime measurements by beam–gas–dye laser spectroscopy

Beam–gas–dye laser spectroscopy as a precise, cascade-free and collision-free method for measuring atomic lifetimes and individual oscillator strengths is described. Its recent application to fine-structure levels of the KrI 5p configuration is reported. The experimental uncertainty is reduced by one order of magnitude, with respect to previous work, down to 0.3% (1σ). The discussion of these results in comparison with experimental and theoretical ones from the literature underlines the precision of the method and its potential to guide future theoretical developments.

Relations structure-proprietes dans les photoamorceurs de polymerisation—6. Interactions avec des photosensibilisateurs

Excited state processes of a large variety of photoinitiators of polymerization (based on morpholino, hydroxyalkyl, amino ketones) in the presence of photosensitizers belonging to the thioxanthone series have been investigated through time-resolved laser-pumped dye-laser spectroscopy. Energy and electron transfer processes account for the experimental results. The high efficiency of the interaction reaction parallels the high reactivity of the couple photosensitizer-photoinitiator.

A spectroscopic view of internal conversion in a small polyatomic molecule: Sub-Doppler intracavity dye laser spectroscopy of thioformaldehyde

Intracavity dye laser spectroscopy has been used to obtain sub-Doppler spectra of selected rotational lines in the à 1A2–X̃ 1A1 410 band of thioformaldehyde with very high resolution and sensitivity. Many of the spectra show extra lines due to perturbations involving high vibrational levels of the ground state. Most of the perturbations observed for K′a = 0 and 4 are found to correlate well with previous observations of anomalously long single rotational level fluorescence lifetimes and reduced quantum yields [J. Dunlop and D. J. Clouthier, J. Chem. Phys. 93, 6371 (1990)]. S1–S0 interaction matrix elements of 0.001–0.006 cm−1 are found for levels involved in simple two level perturbations. The large number of small random perturbations by levels of the ground state is indicative of the first stages of the onset of quantum chaos in a small molecule. Some larger perturbations in the rotational structure are also observed; these are caused by additional local interactions with levels of the nearby triplet state.

Two-channel thermal decomposition of CH 3

The two-channel thermal decomposition of CH3 was investigated by time-resolved measurements of both H atom and CH radical concentrations using ARAS and ring dye laser spectroscopy respectively. As a source for methyl radicals, mixtures of a few ppm ethane diluted in argon were shock heated to temperatures between 2100 K and 3800 K and pressures between 1.1 bar and 1.8 bar. From the initial time histories of the measured H atom and CH radical concentration profiles the rate coefficients of the reaction channels.

Radiative lifetimes of He I 33S1 and 33DJ by beam-gas dye-laser spectroscopy

The lifetimes of the 33S1 and 33DJ levels of the He atom were measured by means of the cascade-free beam-gas dye-laser method. The resulting lifetimes are 35.27+or-0.24 ns and 14.02+or-0.08 ns, respectively, where the uncertainties quoted correspond to one standard deviation. The results have been discussed in comparison with other experimental and theoretical work.

Excited state kinetics and energy transfer in Pr 3+ doped YAG

Tunable pulsed, dye-laser spectroscopy was used to investigate the fluorescence kinetics of the Pr 3+ ions in Y 3Al 5O 12 (YAG) crystals. The decay time of the 1D 2 state was examined after selective excitation of the 3P 0 and 1D 2 absorption lines. The presence of non-equivalent crystal field Pr 3+ sites, the non-exponential character of the excited 1D 2 state decay as well as the anti-Stokes emissions from the 3P 0 and 1S 0 levels following excitation of the 1D 2 state are reported. The investigation of the fluorescence decay profiles have led to the conclusions on the dominant role of an excited state absorption (ESA) in the creation of the high energy states population in strongly dilute 0.12% Pr 3+: YAG crystal.

Cavity ring down dye laser spectroscopy of jet-cooled metal clusters: Cu 2 and Cu 3

Direct optical absorption measurements of jet-cooled metal clusters formed in a pulsed molecular beam have been made using a sensitive new absorption technique employing pulsed laser sources. Metal clusters and other metal containing species have been observed in this initial study. Absorption measurements of the Cu 3 2A′ 1 2E′ system reveal strong intensities for vibronic bands seen only weakly in previous photoionization and LIF studies due to predissociation of the upper level. A previously unobserved weak band has been identified and tentatively assigned as an excitation out of the higher energy Jahn—Teller component of the cluster ground state. These results illustrate the potential of this approach for studying larger metal cluster species.

Energy transfer processes in yttrium oxide activated with europium

Energy transfer and non-radiative decay between excited states of europium in yttrium oxide are studied with dye-laser spectroscopy as a function of the europium concentration and the sample temperature. Combining the results with luminescence spectra measured under cathode-ray excitation, relative feeding rates for the excited states are obtained. These transfer, decay and feeding rates are used to interpret measurements on the sublinearity of the luminescence as a function of cathode-ray excitation density.

Selective excitation of Ti2+Mn2+ and Mn2+Ti2+Mn2+ spin clusters in MgCl2

Ti2+Mn2+ dimers and three types of Mn2+Ti2+Mn2+ trimers obtained by codoping MgCl2 with Ti2+ and Mn2+ were studied by site-selective dye-laser spectroscopy. Very high selectivity was achieved by using the 6A1g→4A1g,4Eg excitation on Mn2+, which does not coincide with any Ti2+ single-ion excitation. Sharp-line 1Eg(T2g)→3Ta1g luminescence from Ti2+ was observed in the near infrared (NIR). Electronic ground-state splittings resulting from Ti2+–Mn2+ exchange interactions were observed for the various spin clusters and, as a result, accurate exchange parameters could be obtained. The luminescence behavior of Ti2+Mn2+ pairs was investigated as a function of temperature and compared with the behavior of Ti2+ single ions in MgCl2. As in Ti2+:MgCl2 luminescence from Ti2+Mn2+ clusters was not only observed in the NIR but also in the visible, it corresponds to 3Tb1g→3Ta1g transitions.

Optical study of praseodymium doped yttrium aluminium garnet

The absorption and fluorescence spectra of YAG crystals doped with praseodymium at 1% level are investigated through dye laser spectroscopy. The spectra can be interpreted as evidence for at least three series of crystallographic sites for Pr3+, but only the transitions which involve the ground state are really complex, that is “multi-sites” like. It is suggested that an electronphonon interaction occurs through resonance with peaks in the density of phonon states and ground state Stark levels in the range 350–750 cm−1. This creates new “vibronic” states and the appearance of a large number of lines for transitions involving ground state especially if laser excitation of the fluorescence is used.

Rotational analysis of the A1Σ +- X1Σ + transition of SrS

The previously unobserved A 1Σ +- X 1Σ + transition of gas phase SrS was studied by dye laser spectroscopy. The 0-0, 0–1, 1-1, 2-1, 3-0, 3-2, 4-1, 5-1, and 5-2 bands were rotationally analyzed at high resolution. The A 1Σ + state is extensively perturbed. Spectroscopic constants of individual vibrational levels and equilibrium molecular constants were determined. Ground state ( X 1Σ +) constants derived from the analysis include: ω e = 388.2643(11) cm −1, B e = 0.1208034(33) cm −1, and r e = 2.43969 A ̊ . Excited state ( A 1Σ +) constants determined include: T e = 13932.7068(10) cm −1, ω e = 339.1454(20) cm −1, B e = 0.1139895(38) cm −1, and r e = 2.51155 A ̊ .

Dye laser excitation spectroscopy of the DSO radical in the A2A′(022) ← X2A″(000) band

The A 2 A′(022) ← X 2 A″(000) band of the DSO radical has been studied by Doppler-limited dye laser spectroscopy. The band, observed in the region of 16 946 to 17 001 cm −1, was analyzed to determine molecular constants for the A 2 A′(022) state. The band origin is 16 992.4496 (2.5 σ = 0.0018) cm −1. By combining the present results with the previous ones, vibrotational constants ( α 2's and α 3's) for the A 2 A′ state are deduced, and rotational constants for the A 2 A′(000) state are obtained by extrapolation.

Energy transfer processes in yttrium oxide activated with europium

Energy transfer between excited states of europium in yttrium oxide is studied with dye-laser spectroscopy. Combining the results with spectra under cathode-ray excitation, relative feeding rates for the excited states are obtained. Using these transfer and feeding rates the sublinearity of the luminescence as a function of excitation density can be explained.

Time-resolved laser-pumped dye laser spectroscopy: energy and electron transfer in ketones

The synergic effects which are generally invoked to account for the specific features of a system of two ketones used as polymerization photoinitiators are reconsidered. The increase in reactivity observed when mixing these two initiators is reinterpreted in terms of a simultaneous energy and electron transfer in the pair. The relative efficiencies of these processes depend on the energy gap between the triplet states involved, which is known to be influenced by the polarity of the medium.

Frequency-modulated laser absorption spectroscopy of the HF fourth overtone

With the recent application of frequency modulation techniques to tunable dye laser spectroscopy, the detection of small concentrations of weakly absorbing atomic and molecular species has been made possible. Here we report measurement of the fourth overtone of X1∑+ HF in a single pass through a 25.5-cm cell at 1-Torr pressure. Pressure-dependent line shape profiles were obtained with ±0.0002-cm−1 resolution for the R4 transition centered at 18225.2 cm−1. Analysis of the data yielded the broadening coefficient, the Einstein A coefficient, as well as the Doppler linewidth and a system sensitivity of ΔI/I0 ≅ 8 × 10−7.