High-resolution Laser-induced Fluorescence Research Articles

Hyperfine structure and isotope shifts of high-lying levels of Zr I investigated by laser-induced fluorescence spectroscopy

High-resolution laser-induced fluorescence spectroscopy has been applied to investigate the hyperfine structure constants and isotope shifts of high-lying even-parity levels of zirconium in the energy range 34 000–37 000 cm-1. By analysis of the measured spectra, the hyperfine-structure constants of seven levels and isotope shifts of eight transitions are determined.

The Torsion–Inversion–Bending Energy Levels in theS1(n, π*) Electronic State of Acetaldehyde: A High-Resolution Study of the Bands #7 to #20 in the Jet-Cooled Fluorescence Excitation Spectrum

The band assignments and analyses of the jet-cooled high-resolution laser-induced fluorescence excitation spectrum of acetaldehyde that results from theS1(n, π*) electronic state have been extended to +600 cm−1from the 000system origin. The new assignments start at Band #7 and finish at Band #21. Bands #8 and #9, originally assigned to 1420, have now been assigned to 1530. The assignments of the lower energy bands remain unaltered. The origins of the bands that involve the torsional modes ν15(v= 1 to 4) in combination with the wagging mode ν14(v= 1 and 2) and the ν10(v= 1) were determined by analyses with a rigid rotational Hamiltonian. These origins were fitted to a set of levels that were derived from a torsion–wagging–bending Hamiltonian that employed flexible large amplitude coordinates. The resulting potential surface was found to have barriers to torsion and inversion of 712.5 and 638.6 cm−1, respectively, with minima in the potential hypersurface at θ = 59.9° and α = 33.5° for the torsion and wagging coordinates.

On the Geometry of the CuCl(2) Molecule.

High-resolution laser-induced fluorescence spectroscopy coupled with isotopic substitution shows that the molecule CuCl2 has a linear 2Πg(3/2) ground state with a Cu−Cl interatomic distance of 203.52 pm. Values for the harmonic wavenumbers ω1, ω2, and ω3 are reported, together with a number of rotational and anharmonicity constants.

General analytical form for the long-range potential of the (ns+npJ)0u+states of alkali dimers applied toLi26

The pooling of high-resolution laser-induced fluorescence measurements with photoassociation binding energies in $^{6}\mathrm{Li}_{2}$ has provided a strict test of models proposed for extrapolation of long-range molecular potential functions. An analytical expression is given for the long-range part of the ${0}_{\mathrm{u}}^{+}$ molecular states originating from M(ns${\mathrm{}}^{2}$S)+M(np${\mathrm{}}^{2}$P) alkali-metal atoms, replacing earlier expressions which assumed negligible spin-orbit coupling. The success of this model is illustrated by the excellent agreement between the experimentally determined dissociation energy ${\mathrm{D}}_{\mathrm{e}}$=9352.032(8) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and the value 9352.032(12) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ obtained by extrapolation of E(R) values of a Rydberg-Klein-Rees potential for the A${\mathrm{}}^{1}$${\mathrm{\ensuremath{\Sigma}}}_{\mathrm{u}}^{+}$ state of $^{6}\mathrm{Li}_{2}$. The lifetime of the $^{2}\mathrm{P}_{1\mathrm{/}2}$ atomic level is 27.13(2) ns.

High-Resolution Laser-Induced Fluorescence Spectra of 7-Azaindole−Water Complexes and 7-Azaindole Dimer

The geometric structures of 7-azaindole−water complexes, (7-AzI)-(H2O)n (n = 1−3), and 7-AzI dimer were investigated by laser-induced fluorescence (LIF) spectroscopy with high resolution (∼0.01 cm-1). For the 7-AzI−(H2O)n complexes (n = 1−3), the LIF spectra show partially resolved rotational structure, which has been analyzed in combination with theoretical calculations. This analysis yields the rotational constants and characterizes the structures. In 7-AzI−(H2O)1, the H2O molecule is located in the molecular plane, forming a six-membered ring with two hydrogen bonds. Analyses for 7-AzI−(H2O)2 and −(H2O)3 show, furthermore, that both the second and the third H2O are located in the molecular plane of 7-AzI, forming a network ring of hydrogen bonds. For the 7-AzI dimer, the LIF spectrum shows an unresolved rotational envelope. The rotational contour has been analyzed, and we discuss the dimer's likely structure.

The electric dipole moment and hyperfine structure of [formula omitted]: high resolution laser-induced fluorescence spectroscopy of the [formula omitted] bands

The dipole moment of NO( B 2Π) was determined for the first time by high resolution laser-induced fluorescence molecular beam spectroscopy. Specific values obtained are as follows: μ( ν′ = 3) = 0.293 3 D, μ( ν′ = 4) = 0.276 9 D and μ( ν′ = 5) = 0.261 7 D. An RKR analysis allowed the first two moments of the dipole moment function to be derived. Quantum beats were observed for the f-parity Λ-doublets and the partial resolution of the hyperfine structure allowed the hyperfine energy level ordering to be determined. Accurate molecular constants were also obtained for B( ν′ = 4 and 5) levels.

Laser-induced fluorescence spectroscopy of ethyl and methyl p-aminobenzoates weakly bound complexes with polar solvents (CH2F2, CH3F and CHF3)

Abstract Weakly bound complexes derived from the combination of ethyl and methyl p-aminobenzoates (ABEE, ABME) with various polar halomethanes (CH2F2, CHF3, CH3F) have been studied by low- and high-resolution nanosecond laser-induced fluorescence spectroscopy. The complexes were prepared in a jet pulsed supersonic expansion. Both blue- and red-shifted bands have been observed for ABME/ABEE-(CH2F2, CHF3) complexes and interpreted as resulting from dipole-dipole and H-bonding interactions. Partially resolved rovibronic bands provided by high-resolution pulsed laser-induced fluorescence, in addition to spectral shifts, have been used to determine relative positions of host and solvent components and also as the final criteria to establish the proposed geometry of the complexes. ABME/ABEE-(CH3F) complexes depict a different spectral pattern, showing a comparatively large red-shift structured band. This effect has been assigned to the selective deep of one of the potential energy wells produced by dipole-dipole stabilisation, effect also found in other H-bond weakly bound complexes. A discussion of the geometries of the complexes in relation to the anisotropy of the H-bonding and dipole-dipole intermolecular forces is presented.

High-resolution laser-induced fluorescence spectrum of the B 4Σu−-X 4Σg− transition of C 2+

We have usedlaser-induced fluorescence on a fast ion beam to study the B 4 Σ u −-X 4 Σ g − system of the homonuclear molecular ion C 2 +. We have observed rotational lines of the (1,1) band by using a Doppler tuning technique and the high resolution obtained has permitted a detailed study of spin splitting. Molecular constants and interaction parameters were evaluated including revised results for the (1, 0) band.

Photochemistry in jet-cooled aniline derivatives

Abstract High-resolution laser-induced fluorescence excitation spectra of four para-substituted pyrrolidino- and dimethylamino-benzoic acid nitriles and esters have been compared under jet-cooled conditions and the fluorescence decay profiles of jet-cooled dimethylaminobenzoic acid nitrile (DMABN) and methyl ester (DMABME) have been determined using synchrotron radiation. The resonant two-photon ionization mass spectra (REMPI-TOF-MS) of DMABME revealed the presence of monomeric and dimeric DMABME and of van der Waals complexes with water. The loss of a well-resolved B-type rotational structure in the LIF excitation spectra for DMABME as compared to DMABN is indicative of state mixing between the initially populated 1 L b and the energetically close-lying 1 L a excited state and is paralleled by the onset of continua in the excitation spectra and associated ‘anomalous’ red-shifted emission and non-exponentiality in the fluorescence decays. However, the REMPI-TOF-MS spectra of jet-cooled DMABME show that complexes with water make a major contribution to the underlying continuum and the associated red-shifted fluorescence and serve to emphasise the essential role of the ‘solvent’. A further contribution comes from DMABME dimers. The results regarding the enhancement of red fluorescence of DMABME with respect to DMABN are discussed in terms of an adiabatic photochemical reaction, i.e. formation of a rotamer on the excited state hypersurface and nearly full electron transfer from the donor to the acceptor moeity, the so-called twisted intramolecular charge transfer (TICT) state. Microsolvation and state mixing in DMABME facilitate this process.

OH laser-induced fluorescence velocimetry technique for steady, high-speed, reacting flows

A high-resolution OH laser-induced fluorescence velocity measurement technique was developed for use in steady, high-speed, reacting flows. A narrow-linewidth laser source was tuned through an isolated OH absorption line to measure the Doppler-shifted line center frequency relative to an iodine reference line. A counterpropagating beam approach was used to eliminate the collisional impact shift and minimize systematic errors. Results from pointwise measurements of velocity in a unique reacting underexpanded jet facility are compared to an axisymmetric Navier-Stokes calculation with finite-rate chemical kinetics as a test of the technique over a wide range of flow conditions. The measured and calculated velocities in the supersonic jet core agree on average to within 1.3%. The uncertainty in the velocity measurement in the jet core was on average +/- 6.0% for a single measurement and +/- 3.5% for the average value of three scans. Potential errors caused by absorption effects were not detected in these measurements. 50 refs.

Spectroscopy of pendular molecules in strong parallel electric and magnetic fields

We report high-resolution laser-induced fluorescence spectra of the A3Π1, [Formula: see text] ν″ = 0 system of ICl subjected to strong parallel electric and magnetic fields, scanned independently up to 50 kV cm−1 and 0.7 T. Either field induces coherent superposition or hybridization of the rotational states within the A3Π1 electronic state, which possesses both an electric and a magnetic dipole moment along the internuclear axis. Only the electric field induces such hybridization within the ground state [Formula: see text], since it is polar but diamagnetic. Comparisons with computed spectra illustrate several salient features. The hybridization creates directional pendular states in which the molecular axis is confined to librate over a limited angular range about the field direction. For a polar paramagnetic electronic state, such as the A state, parallel electric and magnetic fields conjoin in producing hybridization for half of the ensemble but they counteract for the other half. For the latter component, the electric and magnetic effects can be tuned to balance out, yet the concomitant field-induced hyperfine splitting remains, despite the resulting free rotation of the molecule. The dependence of the transition intensities on the combined fields provides an unequivocal means to determine the relative sign of the electric dipole moments in the upper and lower electronic states. We find that the sign is the same for the X and A states (chlorine more negative than iodine).

Fluorescence Line-Narrowing Spectroscopy as an Off-Line Identification Method for Narrow-Bore Column Liquid Chromatography

Storage of the effluent from a column liquid chromatograph (LC) on a thin-layer chromatography (TLC) plate was utilized to identify analytes with fluorimetric techniques in the absence of time constraints. As an example, the deposited LC trace of an impure 1-chloropyrene sample was studied in situ by conventional excitation/emission spectroscopy and by fluorescence line-narrowing (FLN) spectroscopy, a high-resolution laser-induced fluorescence technique requiring cryogenic conditions. Pyrene and several mono-, di-, and trichlorinated pyrenes, present in the 1-chloropyrene sample, were identified with the use of the fingerprint structure of the vibrationally resolved FLN spectra. Quantitation on the basis of the FLN signals was achieved by standard addition; compounds could be detected down to the low picogram level. Various TLC materials were used to study the influence of the matrix on the FLN spectra; although peak intensities and hole burning effects are matrix-dependent, the line positions are not influenced.

Rotational Analysis of the A4Π r− X4Σ − Band System of Molybdenum Nitride (MoN)

The high-resolution laser-induced fluorescence spectrum of a supersonic molecular beam sample of molybdenum nitride, MoN, was recorded in the 596- to 633-nm spectral range. The A 4Π r − X 4Σ − band system of the 92Mo, 94Mo, 96Mo, 98Mo, and 100Mo isotopic forms was analyzed to produce an improved set of spectroscopic parameters. The isotopic dependence of the parameters is discussed and a comparison with a previous rotational analysis is given.

Laser-induced fluorescence spectroscopy of ethyl and methyl p-aminobenzoate van der Waals complexes with non-polar solvents (CH4, C2H6and CF4)

Both low- and high-resolution nanosecond-pulsed laser-induced fluorescence spectra of methyl and ethyl p-aminobenzoate van der Waals complexes (MAB and EAB) have been characterised following jet-expansion investigations. A combined MNDO–Lennard-Jones computation method has been used to determine the potential-energy surface minima in the complexes between the hosts MAB and EAB and the solvents CH4, C2H6 and CF4. Whilst this approach yielded satisfactory results for bare hosts, the method was found to be unsatisfactory for computing the complex equilibrium geometries consistent with the experimental rotational contours of the observed complexes. However, the method is still a valid tool for predicting possible complex conformers and relative host–solvent angles. Substitution of the Lennard-Jones computation by a trial and error grid-search geometry, followed by simulation of the geometry rotational band structure and fitting to experimental rotational contours leads to a successful determination of the geometry of the complexes. With the exception of the MAB–CH4 complex, equilibrium geometries, quantitatively close to those observed, have been simulated for all the complexes studied, including some conformers. No simple general picture emerges from the present intermolecular force interpretation of the complexes that are described.

Measurements of Hyperfine Structure and Isotope Shift of High-Lying Levels of Gd I by Laser Induced Fluorescence Spectroscopy.

High resolution laser-induced fluorescence spectroscopy has been applied to investigate the isotope shift and hyperfine constant of high-lying odd-parity levels of the gadolinium atom. The isotope shifts were obtained for seven-transitions and hyperfine constants were determined for three levels of the configuration 4F75d6s7s9D0.

High-resolution electronic spectroscopy of the Ne·OH and Ne·OD Ã ← X̃ bands

The high-resolution laser-induced fluorescence spectra of the Ne·OH A˜(0,11,1)←X˜(0,00,0) and Ne·OD A˜(0,00,0)←X˜(0,00,0),A˜(0,00,1)←X˜(0,00,0),A˜(0,10,1)←X˜(0,00,0),A˜(0,11,1)←X˜(0,00,0) transitions were obtained. Analyses of these spectra determine precise bond lengths and reveal interesting spin–rotation and parity structure. In the A (0, 1K=0,1, 1) levels of Ne·OH and Ne·OD, the spin–rotation structure is described with the parameter γ, whose magnitude is larger than in the other observed levels of Ne·OH, Ne·OD, and Ar·OH, and Ar·OD. For the K = 1 level an additional term κ is introduced and is found to be an order of magnitude larger than γ. A model based on perturbation theory is proposed and successfully explains the peculiar spin–rotation structure in the A (0, 1K, 1) levels. In addition, a parity-splitting (P-type doubling) for the X˜(0,00,0) level of Ne·OD was determined to be a factor of 10 larger than that in Ar·OD. The parity splitting (K-type doubling) for the A (0, 11, 1) level of Ne·OH is also observed and measured.

High-resolution laser-induced fluorescence and microwave-ultraviolet double resonance spectroscopy on 1-cyanonaphthalene

The rotationally resolved fluorescence excitation spectrum of the 0 0 0 band in the S 1 ← S 0 transition of 1-cyanonaphthalene (CNN), at ≈ 318 nm, has been recorded using laser-induced fluorescence in a molecular beam apparatus. This band exhibits pure a-type character and consists of ≈ 600 lines at a rotational temperature of 2.5 K, each with a linewidth of 17 MHz. A microwave-ultraviolet double resonance experiment on the 0 0 0 band of CNN has been performed to verify the rotational assignments of the fluorescence excitation spectrum and to obtain more accurate rotational constants in both the ground and electronically excited states. The band origin of the 0 0 0 band is at 31411.114 ± 0.003 cm −1 and the rotational constants are (in MHz) A″ = 1478.65(2), B″ = 956.75(1), C″ = 580.989(7), A′ - A″ = −21.363(9), B′ - B″ = −13.305(5), and C′ - C″ = −8.167(2).

High-resolution laser-induced fluorescence study of a cage molecule, 1,4-diazabicyclo [2,2,2] octane, DABCO

Rotationally resolved laser-induced fluorescence spectra of the caged amine DABCO (1,4-diazabicyclo [2,2,2] octane) seeded in a supersonic He beam are recorded using a high-resolution pulsed dye laser system. Both one-photon and two-photon spectra of the S 1A ′ 1) ← S 0(A ′ 1) transition are measured. In the one-photon spectra we observe electric quadrupole transitions to the S 1 origin and to a ′ 1 vibrational levels and vibrationally induced electric dipole transitions to a″ 2 and e′ vibrational levels. For the first time parallel one-photon transitions with their characteristic rotational structure have been observed. Rotational constants are extracted from both the two-photon and one-photon spectra. The quantum mechanical model previously applied to explain the occurrence of the “memory effect” in fluorescence spectra emanating from the S 1(e′) levels is extended here for the S 1(a″ 2) levels. Despite the presence of long progressions in several vibrational modes for the S 1(A′ 1) ← S 0(A′ 1) transition, the difference in S 0 and S 1 rotational constants are remarkably small, suggesting that compensating geometry changes take place on excitation to S 1.

Optical spectra of spatially oriented molecules: ICl in a strong electric field

Strong electric fields can hybridize rotational states of polar molecules and thus create pendular states in which the molecules are confined to librate over a limited angular range about the field direction. In this way, substantial spatial orientation can be attained for the lowest rotational states of many linear, symmetric and asymmetric top molecules. A large fraction of a molecular ensemble can often be condensed into these low rotational states by cooling in a supersonic expansion. Pendular eigenstates differ qualitatively from those of a rotor or an oscillator and can be observed spectroscopically; this provides a means to characterize the extent of the orientation achieved. Here we present high-resolution laser-induced fluorescence spectra of ICI A 3Π1â†� X 1Σ0 measured as a function of the electric field strength up to 36 kV cm–1. These spectra are compared with calculated transition frequencies and probabilities between pendulum/pinwheel states. The field-induced mixing of J states changes the transition probabilities markedly and enriches the spectra with many transitions that would be forbidden in the absence of the field. The transition probabilities fluctuate as the angular lobes of the two pendular wavefunctions involved come in and out of phase when the field strength is varied. The observed fluctuations are particularly pronounced in the present case because the dipole moment changes sign between the two electronic states.

CN temperatures above laser ablated polyimide

Polyimide is readily ablated by UV excimer lasers pulses. There exists an ongoing effort to determine the temperature (T) of the surface during the ablation process. The present experiment evaluates the temperature by means of high resolution laser-induced fluorescence measurements near the bandhead of the CN radical. The rotational state occupation indicates T=1710±140 K for ArF (193 nm) ablation with 100 to 500 mJ/cm2. This temperature is near the extrapolation of earlier results at lower fluences (≤80 mJ/cm2). Furthermore, modeling predicts T values essentially equal to the present experimental result when the following are included: thermal diffusion in the solid, above surface UV absorption and increasing specific heat for temperatures above ambient. The present T-value reinforces the concept that thermal energy probably promotes the intermediate steps in photoablation.