Laser Excitation Spectra Research Articles

Optical-optical double-resonance spectroscopic study of four ion-pair states of ClF and identification of the ClF(A 3Π1) valence state

Four of the six ion-pair states of ClF that correlate to Cl+(3PJ) and F−(1S0), the E(0+,3P2), f(0+,3P0), β(1,3P2), and G(1,3P1) states, have been identified by sequential, two-photon excitation via the ClF(B 3Π0+) valence state. The Ω=1 states, β and G, were studied by selection of rotational levels of the B 3Π0+ state that are perturbed by ClF(A 3Π1). Spectroscopic data from laser excitation and fluorescence spectra permit the assignment of vibrational energies and rotational constants to 30 levels. These four ion-pair states exhibit extensive homogeneous and heterogeneous interactions, and neither the vibrational energy nor the rotational constants are regular with increasing vibrational quantum number. The vibrational and rotational constants of the A 3Π1 state were identified from the low resolution ClF[β(1)−A 3Π1] emission spectra, and the dissociation limits of the A 3Π1 and B 3Π0+ states are compared. The dissociation energy of ClF(X) is confirmed to be 21 110 cm−1. Some qualitative information also was obtained about the D′(2,3P2) and A′ 3Π2 states of ClF.

First Spectroscopic Observation of the CdC2H5Radical

We report the first spectroscopic observation of the monoethyl cadmium free radical, CdC2H5, using laser-induced fluorescence spectroscopy. This molecule was prepared in a supersonic jet by reaction of cadmium atoms with an ethyl precursor in a recently developed dual-channel electrical discharge nozzle. The laser excitation spectrum of CdC2H5consists of a single, unresolved, band attributed to the ÖX̃ origin transition. However, the dispersed fluorescence spectrum obtained by laser-pumping the origin transition does show some vibrational structure. A short progression in the Cd–C stretching mode has been observed giving a frequency of 280 cm−1for this mode. Comparison with the related molecule ZnC2H5shows that the increase in reduced mass is not sufficient to account for the much lower metal–carbon stretching frequency in CdC2H5and the evidence therefore points to a substantially weaker metal–carbon bond in the latter. The absence of vibrational structure in the excitation spectrum of CdC2H5is attributed to the depopulation of excited vibronic levels by predissociation.

Origin of porous silicon photoluminescence: Evidence for a surface bound oxyhydride-like emitter

Time-dependent excitation spectroscopy coupled with quantum chemical calculations is used to demonstrate that the photoluminescence (PL) resulting from the ultraviolet optical pumping of an etched porous silicon (PS) surface results from a silicon oxyhydride-like fluorophor bound to the PS surface. The time-dependent PL, in both aqueous (${\mathrm{H}\mathrm{F}/\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{H}\mathrm{F}/\mathrm{C}\mathrm{H}}_{3}{\mathrm{O}\mathrm{H}/\mathrm{H}}_{2}\mathrm{O}$) and nonaqueous [MeCN/HF (anhydrous)] etching media, has been monitored both in situ, during the etching cycle and before the PS sample is removed from the etching solution, and ex situ, after removal of the PS sample from the etching solution. The early appearance in time of the PS luminescence is consistent with the formation of a surface bound emitter created on a time scale $(l~10\mathrm{s})$ much shorter than that needed for pore formation. Laser excitation spectra (PLE) over the wavelength range extending from 193 to 400 nm produce an almost identical time-dependent PL emission feature between 550 and 700 nm. Influenced strongly by the chemical composition of the etch solution, an intermediate ``green'' emitter can be excited with select laser pumping wavelengths and observed to transform to the final ``orange-red'' luminescent product. In conjunction with experiments whose focus has been to compare the time-dependent PL after ArF (193 nm) and ${\mathrm{N}}_{2}$ (337 nm) laser excitation (PLE), the data suggest the pumping of an excited-state manifold for a molecule-like species followed by rapid relaxation via nonradiative transitions down the manifold and the subsequent emission of radiation at much longer wavelength. Detailed quantum chemical modeling supports this interpretation and suggests a correlation to changes in the bonding associated with electronic transitions that involve silanone-like ground electronic singlet states and their low-lying triplet excitons. Especially important are those changes involving SiO related bonds. A substantial shift in the excited-state manifold, relative to the ground state, correlates with the character of the observed PL spectra as the excitation to a manifold of states greatly shifted from the ground electronic state produces a considerable redshift of the PL spectrum $(\ensuremath{\sim}600--800\mathrm{nm})$ compared to the known peak wavelength of the PLE (excitation) spectrum at 350 nm. The combination of quantum chemical modeling and time-dependent spectroscopic studies also suggests that the multiexponential PL decay commonly observed as a function of increasing wavelength (550--750 nm) after excitation at 355 nm results primarily from nonradiative cascade. The optical detection of magnetic resonance (ODMR) spectrum obtained for PS and associated with a triplet exciton is assigned to an oxyhydride-like emitter possessing silicon-oxygen and silicon-hydroxide fluorophors similar to the much more complex annealed siloxene. Calculated infrared spectra are correlated with experimentally observed features and are consistent with a surface-based oxyhydride-like emitting fluorophor. A recent analysis that associates the linewidth of the triplet ODMR spectrum with an inhomogeneous distribution of quantum confined crystallites is shown to be in error. We demonstrate that the correct extension of the arguments used in this analysis provides clear evidence for the existence of a common radiative center associated with a molecule-like species bound to the surface of the PS framework. The results obtained in this study are thus not consistent with quantum confinement and suggest a surface bound emitter as the source of the PS photoluminescence.

Spectroscopy of Metastable Species in a Free-Jet Expansion: The β ←ATransition in Br2

TheA1u(3Π) state of Br2is observed in free-jet expansions of UV-photolyzed BBr3/Ar mixtures and electrically discharged Br2/Ar mixtures. Laser excitation spectra are recorded for the β 1g(3P2) ←Atransition, providing the first high-resolution data forv= 0 in theAstate. Thirteenv′–0″ bands spanningv′ = 23–33 are rotationally analyzed, yielding a total of 475 assignments. The new results are combined with existing spectroscopic data forv′ = 0–10 to yield an improved description of the β state and the following constants (cm−1) forv= 0 of theAstate:T″0(79Br2) = 13984.22(5),T″0(79Br81Br) = 13983.80(5),T″0(81Br2) = 13983.32(5),B″0(79Br2) = 0.05815(2). The new information for theAstate is analyzed together with existing data for levelsv= 7–24 and 30–34 to achieve compact expressions for the vibronic energy and rotational constant in terms of the standard polynomials in (v+ 12) at lowv, merging smoothly into near-dissociation expansions (NDEs) at highv. The resulting “mixed representations” have the usual properties of the polynomials at lowvand yield the estimates (cm−1, for79Br2):Te= 13910,De= 2147, ωe= 150,Be= 0.05844, andRe= 2.704 Å. The NDEs have superior extrapolating ability above the highest observed level to dissociation, which occurs atvD= 41.7. Spectroscopic constants and RKR potentials are tabulated for the first ∼40vlevels of both states, and Franck–Condon factors are given for transitions from the lowv″ levels.

Optical–Optical Double Resonance Spectroscopy of Jet-Cooled TiO: New3Φ and3Π States near 4 eV

Laser excitation spectra of TiO have been recorded in the 30500–34500 cm−1energy region using the technique of optical–optical double resonance. Twenty3Φ and 243Π excited vibrational levels have been observed in experiments that use theA3Φ andB3Π electronic states as intermediates. The new levels can be assigned to four electronic states, here labeledG3Φ,H3Φ,I3Π, andJ3Π. All the excited states are found to be perturbed to some degree; estimates of the upper state constants have been obtained from least-squares fitting of the data. Laser-induced fluorescence to a new low-lying electronic state of TiO has been observed, following excitation to theI3Π andJ3Π states; this state lies 12284 ± 5 cm−1above theX3Δ1,J= 1 ground rotational level of the molecule and is believed to be the hitherto unobserved δ2D3Σ−state.

Breast cancer diagnosis using N2 laser excited autofluorescence spectroscopy.

This article reports results of an in vitro study involving 63 patients for the evaluation of the diagnostic potential of N2 laser excited autofluorescence spectroscopy of human breast tissues. The N2 laser-excited spectra were recorded from benign (fibroadenomas, 35 patients), cancerous (ductal carcinomas, 28 patients), and normal (the uninvolved areas of the resected cancerous specimens). A stepwise multivariate linear regression (MVLR) analysis was developed to analyze the diagnostic content of the breast tissue fluorescence spectra. Significant changes were observed in the autofluorescence from normal, benign, and cancerous breast tissues, particularly in the spectrally integrated fluorescence intensity. The ratio of mean spectrally integrated intensity from cancerous tissues to that from benign tumor and normal tissues were 3.2 and 2.8, respectively. A discrimination parameter based on spectrally integrated intensity alone provided a sensitivity and specificity of up to 99.6% over the sample size investigated for discrimination of cancerous breast tissues from benign/normal. Our results suggest that a straightforward measurement of the total integrated fluorescence intensity can provide excellent discrimination between cancerous and benign/ normal breast tissues.

Laser-induced excitation and dispersed fluorescence spectra of the ethoxy radical

Extensive laser excitation and dispersed fluorescence spectra of the ethoxy radical have been recorded in a supersonic jet expansion. Neon transitions have been used to calibrate both the wavelength of the excitation dye laser and the optical multichannel analyser system used to record the wavelength-resolved emission spectra. Both the excitation and dispersed spectra are characterized by prominent progressions involving the C - O stretch vibrational mode. Seven vibrational frequencies for the excited electronic state and eight for the ground state have been assigned. To the best of our knowledge, 10 of these 15 assigned frequencies are reported here for the first time. Vibrational and anharmonic constants for the C - O stretch mode have been determined via least-squares fits for the X state and the B state .

Recent developments in the theory of angular distribution and spin polarization of Auger electrons

The general theory of angular distribution and spin polarization of Auger electrons is reviewed. The angular distribution of open-shell atoms is investigated in more detail. Whereas the KLL Auger lines have to be isotropic for closed-shell atoms, this is no longer valid for open-shell systems. The angular distribution of the KLL spectra of sodium and laser-excited sodium is discussed. Large anisotropy parameters are predicted for the unresolved initial- and final-state multiplets of the laser-excited sodium KLL spectrum. An outlook is given for the angular distribution of Auger electrons emitted from diatomic molecules. The case of electron-impact ionization is discussed. The physical importance of a coherent excitation process is emphasized within a simple example. For the case of the rare gases a simpler experiment is discussed to measure the in-reaction plane components of the spin-polarization vector. Within the LS-coupling approximation theoretical predictions for the spin-polarization vector for Auger transitions on argon are given. A degree of spin polarization of up to 50% is predicted.

The near infrared electronic spectrum of tungsten methylidyne, WCH

The laser excitation spectrum of tungsten methylidyne, WCH, has been recorded in the 12 000–15 400 cm−1 region. A total of 14 vibronic bands of WCH and 16 bands of WCD have been observed in this region. Rotational analysis shows that the ground state is X̃3/2(2Δ), with the substitution structure r0(W≡C)=1.73665 Å and r0(C–H)=1.0765 Å. The excited vibronic levels have been assigned, on the basis of their WCH/WCD isotope shifts and their wavelength resolved fluorescence patterns, to three electronic states, Ã3/2(2Δ), B̃1/2(2Π), and C̃5/2(2Φ), at 12 090, 13 392, and 14 110 cm−1, respectively. The wavelength resolved fluorescence spectra have also established the low-lying vibrational levels of the ground state. The ground state bending fundamental lies at 660 cm−1, while the W–C stretching frequency is 1006 cm−1; the corresponding frequencies in WCD are 501 cm−1 and 953 cm−1, respectively. No evidence for the C–H stretching frequency has been found. Emission has also been observed to a low-lying electronic state, 813 cm−1 above the X̃3/2 state. The pattern of rotationally resolved emission to this state clearly indicates that it is a 4Σ1/2 state. Its bending frequency is 612 cm−1, and its W–C stretching frequency is 971 cm−1, indicating a slightly longer bond length than in the X̃3/2 state. High resolution cw laser spectra of the (0,0) bands of the two lowest excited electronic states [Ã3/2(2Δ) and B̃1/2(2Π)] reveal a small splitting of the four principal tungsten isotopes (182W, 183W, 184W, and 186W) which serves to confirm the presence of tungsten in the carrier. Hyperfine splitting associated with the 183W nucleus (I=1/2) has been observed for the (0,0) band of the Ã3/2−X̃3/2 system, allowing the electron configuration of the ground state to be elucidated.

Continuum resonance Raman scattering in 127I79 Br1

Abstract We have simulated high resolution continuum resonance Stokes- and anti-Stokes-Raman spectra of isotopically pure 127 I 79 Br using second order perturbation theory in the time independent and time dependent approach. By use of extensive fitting procedures of argon ion laser excited spectra potential function parameters for the repulsive branches of the excited states 1 Π 1u and 3 Π 0+u are determined. The high sensitivity of resonance Raman band shapes to small changes of excited state repulsive potential functions and to the strength of transition moments is shown. A symmetry principle of Stokes versus anti-Stokes Raman spectra is also shown and discussed as well as a reflection principle and scattering delay times for this diatomic molecule.

Emission spectrum of mass-selected C 4−:C 2Πu → X 2Πg in a neon matrix

The laser excited emission spectrum of the C 2 Π u → X 2 Π g electronic transition of mass-selected C 4 − embedded in a 5 K neon matrix has been observed. This is an extension of the technique coupling mass spectrometry and matrix isolation and allows information on the vibrational frequencies of the species in the ground state to be obtained. In the case of C 4 −, three vibrational frequencies ( ν 1( σ g +) = 2047(20), ν 2( σ g +) = 936(20) and ν 4( π g) = 396(20) cm −1) have been inferred for the X 2 Π g state.

The measurement of tropospheric OH radicals by laser‐induced fluorescence spectroscopy during the POPCORN Field Campaign

A highly sensitive OH measurement instrument has been developed. It is based on laser‐induced fluorescence (LIF) detection of OH using the A²Σ+v′ = 0‐X²Π v″ = 0 transition at 308.15 nm at low pressure. The LIF instrument detects OH directly and with high specificity, a fact that was demonstrated by recording laser excitation spectra (Q1(3), Q21(3) and P1(1) lines) of ambient OH. For high time resolution (typ. 60–100 s), the laser wavelength was modulated on‐/off‐ resonance with the P1(1) line. Here, we report some of the OH measurements obtained by this technique during its first application in a tropospheric field campaign (“POPCORN”), which was conducted in August 1994 in a rural environment in the North‐East of Germany. These include diurnal OH concentration profiles with maximum OH concentrations up to 1.4×107cm−3at noon. Minimum OH concentrations were measured in the morning and evening down to the detection limit of (3–6) × 105cm−3(SNR=2, measurement time 1 min.). During the day, OH fluctuations were observed on a time scale of minutes and hours. These were highly correlated to the flux of the solar UV radiation which is responsible for the primary OH production by photolysis.

Far-Wing Excitation Spectra of the Hg 6s6p 3P0 – 6s7s 3S1 Inter-Excited-State Transitions Broadened by Ar and Ne

Abstract We have measured the laser-excitation spectra of the Hg–Ar and Hg–Ne collisional quasimolecules in the far wings of the Hg 6s6p 3P0 – 6s7s 3S1 inter-excited-state transitions broadened by Ar and Ne. The simulation of the spectrum, based on an uniform-semiclassical treatment for the free-free Franck-Condon factor, has been carried out for the Hg–Ar system in order to estimate the potential curves for the E31 and a 30− states of the quasimolecule.

Spectroscopy of Metastable Species in a Free-Jet Expansion: TheD′ ← A′ Transition in Br2

TheA′2u(3Π) state of Br2is observed in free-jet expansions of UV-photolyzed BBr3/Ar mixtures and of electrically discharged Br2/Ar mixtures. Laser excitation spectra are recorded for theD′2g(3P2) ←A′ transition, providing the first information aboutv= 0 in theA′ state. Thirty-onev′ − 0″ bands spanningv′ = 23–47 are rotationally analyzed. The new results are combined with existing emission spectroscopic data forv′ = 0–6 andv″ = 5–21 (a total of 2940 assignments) to provide spectroscopic parameters which should be valid forv″ = 0–21 andv′ = 0–50. The key spectroscopic constants (cm−1) for79Br2are ΔTe= 35 702.67, ω′e= 150.86, ωex′e= 0.3911, ω″e= 162.8, ωex″e= 1.84,B′e= 0.04243,B″e= 0.05931,R′e= 3.1731 Å, andR″e= 2.6838 Å. Fluorescence decay data are recorded for excitation ofv′ = 24 and analyzed to yield an estimate of 11 ns for the lifetime of theD′ state.

Fluorescence velocity measurements in the interior of a hydrogen arcjet nozzle

The first use of laser-induced fluorescence for spatially resolved plasma velocity measurements within an arcjet nozzle are described.The arcjet was operated on a propellant of pure hydrogen.The Balmer-α transition of atomic hydrogen was scanned by the excitation laser source while simultaneously detecting the spectrally integrated resonance fluorescence.Velocity was determined from the Doppler shift in the peak of the laser excitation spectrum relative to a stationary reference.The arcjet operating conditions were chosen to facilitate a comparison between these measurements and arcjet modeling results of Butler et al.

High-Resolution Laser Excitation Spectroscopy of theB̃2Σ+–X̃2Σ+System of Jet-Cooled SrOD

The high-resolution laser excitation spectrum of the 001–000 and 000–000 bands of theB2Σ+–X2Σ+transition of SrOD was recorded. The SrOD molecules were made by pulsed laser ablation of Sr metal followed by reaction with D2O. Sub-Doppler resolution and rotational cooling were achieved by expansion into vacuum and excitation with a cw dye laser perpendicular to the molecular beam. A molecular linewidth of 200 MHz and the lack of spectral congestion allowed aQbranch to be detected. Spin–orbit mixing between theA2Π andB2Σ+states caused the unusual appearance of a moderate intensityQbranch in a nominally parallel2Σ+–2Σ+transition. From the relative intensities of the rotational lines, the ratio of the perpendicular to the parallel transition dipole moment was found to be 0.2 to 0.3.

Laser Spectroscopy of YCl: Observation of the d3Φ–a3Δ Transition

The yttrium monochloride molecule (YCl) has been produced in an electric furnace in a DC discharge tube containing YCl3powder and argon gas. Triplet transitions of YCl have been observed and identified for the first time in the emission spectrum and studied using LIF (laser induced fluorescence) techniques. High resolution laser excitation spectra have been recorded in the region 12200–13100 cm−1, using a cw single-mode Ti:sapphire laser. The triplet transition centered at 12789 cm−1has been rotationally analyzed and assigned as thed3Φ–a3Δ transition; molecular constants have been determined. TheX1Σ+∼ a3Δ separation has been investigated by detection of theD1Π–a3Δ intercombination transition induced by the Ar+4579 Å laser line.

Rotational analysis of bands in the 460 nm system of nickel dichloride produced in a free-jet expansion: Determination of the structure and electronic ground state of nickel dichloride

By use of a free-jet expansion which incorporates a heated nozzle, we have recorded the laser excitation spectrum of the 460 nm band system of NiCl2 at rotational resolution. The rotational temperature in these recordings was about 12 K. Several bands have been recorded and analyzed for three isotopomers, 58Ni35Cl2, 60Ni35Cl2, and 58Ni35Cl37Cl in natural abundance. Spin components with Ω values of 0 and 1 have been identified in both the upper and lower states of the transition. Accurate values for all three vibrational intervals ν1, ν2, and ν3 have been determined for nickel dichloride in the upper state and for the bending wave number ν2 in the lower state. The results show that the molecule is linear in both states involved in the transition and that the lower (ground) state is 3Σ−g in character. Evidence is presented from the nickel isotope shifts to show that the transition is vibronically induced through the bending vibration and that the upper state is vibronically 3Πu in character; it probably derives from an electronic 3Δg state. The zero-point averaged bond lengths are determined for both states as r0′=0.209 435(13) nm and r0″=0.205 317(14) nm. The fine structure parameters for the X̃ 3Σ−g state are interpreted in terms of low lying 1Σ+g and 3Πg states, which are shown to lie a few thousand reciprocal centimeters above the ground state.

Fluorescence line narrowing in Mn4+doped gadolinium gallium garnet1

Abstract The laser excitation and luminescence spectra of the R-lines of the Mn4+ in gadolinium gallium garnet show that the large broadening which they exhibit is partially related to presence of charge-compensating Ca2+ ions, even for the case of manganese center without close compensation. Remaining part of the broadening is due to antiferromagnetic exchange interaction between Mn4+ and surrounding them six Gd3+ ions. The spectra remain broadened in magnetic fields below 6 T and the higher magnetic fields up to 20 T removes the broadening related to the spin-spin interaction. Luminescence and excitation spectra at 2.2 K show that the transitions occur between levels which magnetic numbers differ by 0 or ± 1. The estimated value of the spin-spin exchange constant for the ground state of Mn4+ is approximately equal to J ≈ 1.9 cm−1.

Excited state absorption in holmium doped Gd3Ga5O12garnet

Abstract The purpose of the present work is the direct measurement of the 5I7 → 5S2—5F4(Ho) excited state absorption in Gd3Gd5012 garnet with a two beam experiment. The result is compared with the cross-section deduced from the 5S2 – 5F4 → 5I7 emission spectrum, using Mc Cumber relations. The result is also compared to the laser excitation spectrum near 753 nm of the green emission recorded in a previous work [1].