Laser-induced Fluorescence Excitation Spectroscopy Research Articles

Spectroscopic Study of the B 1Π State of NaH.

In this study, the B1Π excited state of NaH has been experimentally studied for the first time. Pulsed laser-induced fluorescence excitation spectroscopy was used to investigate the B1Π electronic state of NaH. A total of 48 ro-vibronic transitions were observed, including within the B–X (0–0) and B–X (0–1) transition bands. Only one B-state vibrational level was identified, and a series of PQR lines, with eight e-parity and eight f-parity sublevels (v′ = 0, J′ = 1–8), were assigned. The level assignment was supported by a comparison of the experimental line positions with the ab initio calculations, the dispersed laser-induced fluorescence spectrum of the NaH B1Π → X1∑+ emission, and the V-type optical–optical double resonance spectra. The Dunham-type coefficients, the mean internuclear distance, the harmonic vibrational frequency ω, and the dissociation energies D0 and De of the B1Π state were determined.

High-temperature continuous molecular beam source for aggressive elements: An example of zinc.

Expansion of Zn2 or ZnRg (Rg = rare gas atom) in a molecular supersonic beam constitutes a considerable technical challenge due to the high zinc melting point and high zinc reactivity with stainless steel at high temperatures. In order to overcome these difficulties and meet the requirements for spectroscopy of van der Waals molecules containing zinc, a high-temperature source-module of the supersonic molecular beam for aggressive elements was designed. The module was tested in the laser-induced fluorescence excitation spectroscopy experiment using the b30u +43P1←X10g +(41S0) bound ← bound transitions in Zn2. The new source-module can be used for other aggressive elements for which a laser-vaporization technique has been used to date.

Effect of Facial Encumbrance on Excimer Formation and Charge Resonance Stabilization in Model Bichromophoric Assemblies

Excimer formation and charge resonance stabilization in covalently linked bichromophoric systems with flexible spacers are important processes relevant to biochemistry and functional materials. Requiring a π-stacked cofacial arrangement of a pair of aromatic molecules at a van der Waals contact, the underlying geometrical reorganization that accompanies these events continues to be debated. Here we use a variety of methods including two-color resonant two-photon ionization spectroscopy (2CR2PI), ion yield measurements, hole-burning spectroscopy (HB), and laser-induced fluorescence (LIF) excitation and emission spectroscopy to compare the gas-phase spectroscopy and dynamics of the van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9′-dimethylfluorene (F1). The goal of this work is to probe the influence of methyl substitution on the dynamics of excimer formation and charge resonance (CR) stabilization. The fluorene dimer, (F)2, displays lifetime broadened electronic spectra and the dominance of...

Spectroscopy of 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) attached to rare gas samples: Clusters vs. bulk matrices. I. Absorption spectroscopy

The interaction between 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) and rare gas or para-hydrogen samples is studied by means of laser-induced fluorescence excitation spectroscopy. The comparison between spectra of PTCDA embedded in a neon matrix and spectra attached to large neon clusters shows that these large organic molecules reside on the surface of the clusters when doped by the pick-up technique. PTCDA molecules can adopt different conformations when attached to argon, neon, and para-hydrogen clusters which implies that the surface of such clusters has a well-defined structure without liquid or fluxional properties. Moreover, a precise analysis of the doping process of these clusters reveals that the mobility of large molecules on the cluster surface is quenched, preventing agglomeration and complex formation.

Experimental Determination of the Vibrational Constants of FeS(X5Δ) by Dispersed Fluorescence Spectroscopy

Based on previous laser-induced fluorescence excitation spectroscopy work, the vibrational constants of neutral FeS in the X5Δ electronic state were obtained by directly mapping the ground-state vibrational levels up to v=3 using conventional laser-induced dispersed fluorescence spectroscopy. The vibrational frequency of FeS(X5Δ) (518±5 cm−1) agrees well with that reported in a recent PES measurement (520±30 cm−1) [J. Phys. Chem. A 107, 2821 (2003)] which is the only one prior experimental vibrational frequency value for the 5Δ state of FeS. Careful comparisons of our experimental results and those documented in the literature (mainly from theoretical predictions) suggest that the ground state of FeS is 5Δ state.

Laser-induced fluorescence and dispersed fluorescence spectroscopy of jet-cooled 1-phenylpropargyl radical

The D(1)((2)A("))-D(0)((2)A(")) electronic transition of the resonance-stabilized 1-phenylpropargyl radical, produced in a jet-cooled discharge of 3-phenyl-1-propyne, has been investigated in detail by laser-induced fluorescence excitation and dispersed single vibronic level fluorescence (SVLF) spectroscopy.The transition is dominated by the origin band at 21,007 cm(-1), with weaker Franck-Condon activity observed in a(') fundamentals and even overtones and combinations of a(") symmetry. Ab initio and density functional theory calculations of the D(0) and D(1) geometries and frequencies were performed to support and guide the experimental assignments throughout. Analysis of SVLF spectra from 16 D(1) vibronic levels has led to the assignment of 15 fundamental frequencies in the excited state and 19 fundamental frequencies in the ground state; assignments for many more normal modes not probed directly by fluorescence spectroscopy are also suggested. Duschinsky mixing, in which the excited state normal modes are rotated with respect to the ground state modes, is prevalent throughout, in vibrations of both a(') and a(") symmetry.

High resolution laser induced fluorescence excitation spectroscopy of jet-cooled terrylene

High resolution laser induced fluorescence excitation spectra upon absorption in the A 1 B 3 u ← X 1 A g band of jet-cooled terrylene have been recorded. Precise energies of three vibronic transitions are deduced. Low lying vibrations are found in both electronic states. Rotational constants in ground and excited state are determined by band contour analysis. Terrylene is a medium-size polycyclic aromatic hydrocarbon (PAH) and a possible carrier of diffuse interstellar bands (DIB). The results of the jet experiments are discussed regarding the PAH–DIB hypothesis.

Laser-induced fluorescence and single vibronic level emission spectroscopy of chiral (R)-1-aminoindan and some of its clusters in a supersonic jet

Two low energy conformers of the chiral (R)-1-aminoindan molecule are identified in supersonic jet and their ground and excited states vibrational spectroscopy has been investigated by laser-induced fluorescence (LIF) excitation and single vibronic level (SVL) emission spectroscopy. Ab initio calculations confirm the existence of two lowest-energy structures, where the amino group is in equatorial position with its lone pair directed opposite to the aromatic electron cloud. Harmonic frequencies have been calculated for these two conformers at the DFT level with B3LYP functional. A low-frequency progression of 118 cm(-1) and 114 cm(-1), respectively, appears in the fluorescence excitation spectrum of the two conformers, with its ground state counterpart at approximately 147 cm(-1). It has been assigned to the puckering motion coupled with the ring flapping mode. The other calculated low-frequency mode corresponds to the puckering motion coupled with the ring twisting mode and its ground state frequency has been observed at 119 cm(-1) and 111 cm(-1) from SVL spectra. Both conformers form similar 1 : 1 water clusters, whose 0-0 transitions are shifted to the blue by 41 cm(-1) and 44 cm(-1), respectively, and whose SVL spectra are similar. Interestingly, one of the conformers seems to preferentially make complexes with (S)-methyllactate, while the other one shows selective complexation to (R)-methyllactate.

Identification of isomeric dimers of o-fluorobenzoic acid using laser-induced fluorescence spectroscopy

Three conformational isomers of o-fluorobenzoic acid dimer have been identified in a supersonic jet expansion by use of laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. Using a mixed dimer of o-fluorobenzoic acid with benzoic acid, we have provided further evidence that the three isomers in the previous case originate due to two distinct internal rotational isomeric forms of each of the dimer moiety. Relative stability and geometries of all the dimers are computed by use of DFT theoretical method. The resolved fluorescence spectra of the three isomeric homodimers have been tentatively assigned by correlating the observed frequencies with the normal mode frequencies of the isomers predicted by DFT calculation.

Laser-induced fluorescence excitation and dispersed fluorescence spectroscopy of the Ã(1B1)–X̃(1A1) transition of dichlorocarbene

The A(1B1)–(1A1) transition of jet-cooled CCl2 has been investigated using both laser-induced fluorescence (LIF) excitation and dispersed fluorescence (DF) spectroscopy. DF spectra were taken from over 90 emitting vibronic states over the breadth of the LIF spectrum. 68 ground state vibrational levels were assigned in the C35Cl2 isotopomer and 47 in C35Cl37Cl. The ground state frequencies were fit to an anharmonic potential yielding: ω″1 = 736 cm−1, ω″2 = 338 cm−1, ν″3 = 760 cm−1, x″11 = −3.19 cm−1, x″22 = −0.29 cm−1, x″12 = −1.80 cm−1, x″13 = −5.70 cm−1 and x″23 = −3.80 cm−1. This is the first gas phase measurement of ν″3. Despite the close frequency match between ν″1 and 2ν″2 there was no evidence for Fermi resonance. The intensities of transitions in the DF spectra were analysed to identify the nature of the emitting state. Levels below T00 + 2500 cm−1 in the A state could be assigned simply as combinations and overtones of ν′1 and ν′2. Between T00 + 2500 and T00 + 5300 cm−1 the emission revealed an increasing mixing between levels within the same polyad, i.e., (m,n,0), (m + 1,n − 2,0), (m + 2,n − 4,0) etc, presumably due to Fermi resonance. This mixing becomes so extensive that simple assignment of the emitting states is not possible. Above T00 + 5300 cm−1 only K′ = 0 states could be identified, indicating that the Renner–Teller intersection between the and A states had been exceeded. This intersection is therefore 22 600 cm−1 above the ground state, in excellent agreement with a previous theoretical calculation of 23 000 cm−1.

Laser spectroscopy of a chiral drug in a supersonic beam: conformation and complexation of S-(+)- Naproxen

The S 0–S 1 electronic transition of jet-cooled Naproxen has been investigated by laser-induced fluorescence excitation and emission spectroscopy. Two electronic transitions separated by 102 cm −1 have been evidenced and attributed to the presence of two conformers. With the help of DFT calculations, the structure of these conformers has been shown to correspond to a rotation by 180° of the chiral substituent with respect to the aromatic plane. When associated with alcohols, both conformers of the chromophore form complexes which give rise to different microscopic solvent shifts of the S 0–S 1 transition. In the case of complexation with R- or S-2-butanol, the hetero- and homo-chiral pairs are characterised by different spectroscopic patterns, which allow a clear discrimination between them.

Conformational stability of allylbenzene: A combined study by dispersed fluorescence spectroscopy and quantum chemistry calculation

Two conformational isomers of allylbenzene are identified in a supersonic free jet expansion by use of laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. With the aid of the predictions of ab initio quantum chemistry calculations at the MP2 level for a series of extended basis sets [6-311+G(d,p), 6-311++G(d,p), and cc-pVTZ], the major species of the electronic spectrum is shown to be an eclipsed conformer in which the allyl group is oriented perpendicular to the plane of the benzene ring and a terminal hydrogen atom of the ethylene moiety is poised nearly above the aromatic π electrons. The minor species is identified as an internal rotational isomer that is obtained by rotating the ethylene group about the Cα–Cβ bond by 120° from the eclipsed configuration. This predicted order of conformational preference is reversed for calculations at relatively low levels of theory: MP2/6-31G(d,p), HF/6-311++G(d,p), HF/6-31G(d,p), and B3LYP/6-31G(d,p). The relative intensities of the vibronically induced nontotally symmetric and totally symmetric transitions are significantly different in the electronic spectra of the two conformers.

Electronic and vibrational spectroscopy of jet-cooled complexes of o-cyanophenol

Hydrogen-bonded complexes of o-cyanophenol ( o-CNP) with a series of protic and non-protic solvents have been investigated in supersonic jet by means of laser-induced fluorescence excitation and dispersed emission spectroscopy. All the complexes exhibit a large red shift of the S 0–S 1 transition extending from −674 cm −1 for acetonitrile to −1321 cm −1 for the dimer of methanol. This shift is shown to correlate with the proton affinity of the protic solvents and has been related to the formation of cyclic hydrogen bond networks. For a non-protic solvent such as diethylether, the presence of a strong H bond has been confirmed by the study of the OH stretch using IR–UV double resonance spectroscopy. The low energy intermolecular vibrations observed in the cyclic complexes of o-CNP with H 2O and CH 3OH were assigned by comparing the experimental frequencies observed in the S 0 and S 1 states with the results of DFT calculations. The complexes with TEA undergo proton transfer as evidenced from the observation of the fluorescence attributable to the deprotonated form of o-CNP.

Gas phase dihydrogen bonding: clusters of borane-amines with phenol and aniline

In this article, we present the electronic and the vibrational spectroscopy of dihydrogen bonded clusters of phenol and aniline with borane-amines in supersonic jets. Laser-induced fluorescence excitation spectroscopy was used to characterize the formation of binary clusters. Fluorescence-detected infrared spectroscopy was used to probe the vibrations in the hydride (O–H and N–H) stretching region. The spectral shifts in the electronic as well as vibrational transitions upon cluster formation indicate that phenol and aniline are hydrogen bonded to borane-amines. Density functional theory-based calculations were carried out to determine the structures of the binary clusters. The formation of B–H⋯H–X (X=O, N) dihydrogen bonded clusters between borane-amines and phenol/aniline was deduced by comparing experimental and theoretical results. Comparison of the present results with other hydrogen bonded clusters of phenol and aniline indicates that dihydrogen bonds are comparable in strength to conventional σ-type hydrogen bonds.

Electronic spectroscopy of jet-cooled CFCl: Laser-induced fluorescence, dispersed fluorescence, lifetimes, and C–Cl dissociation barrier

The Ã(1A″)–X̃(1A′) transition of jet-cooled chlorofluorocarbene (CFCl) has been measured by laser-induced fluorescence (LIF) excitation and dispersed fluorescence spectroscopy. Over 170 vibronic transitions were measured in the LIF spectrum, consisting of cold bands and hot bands of carbenes containing both Cl35 and Cl37 isotopes. Dispersed fluorescence spectroscopy was used both to map the ground-state vibrational levels and to provide confirmation of the vibronic identity of the emitting level. A predictor–corrector method was used to progressively assign almost all of the vibronic transitions, resulting in the positive assignment and measurement of almost every bound vibrational state within the Ã-state manifold. The vibrational structure is modeled well by a Morse potential with frequencies ν1′=1229 cm−1, ω2′=399.2 cm−1, and ω3′=748.0 cm−1 for CF 35Cl and 1235 cm−1, 397.0 cm−1, and 744.5 cm−1 for the same three vibrations in CF 37Cl. The standard diagonal and cross-anharmonicity constants for a three-coordinate Morse potential were also measured for each isotopic species. Dispersed fluorescence spectroscopy provided a map of ground-state vibrational levels up to about 4000 cm−1. Franck–Condon factors were modeled well by a simple, one-dimensional harmonic potential, and these were also used to confirm assignment of many transitions. The fluorescence lifetime of the excited vibronic states decreased markedly from a consistent 650 ns for most states, to <20 ns for the highest lying observed state. In addition, the Franck–Condon analysis indicates that higher lying members of progressions were missing in the LIF spectrum. This strongly indicated the presence of a nonradiative pathway that opens for energies above T00+4073 cm−1. Analysis of the rotational structure of many transitions indicated that the molecule was not reaching the Renner–Teller intersection, where the à and X̃ states are degenerate. We attribute the nonradiative channel to cleavage of the C–Cl bond directly on the à state, in exact analogy with the observed process in CFBr. The height of the barrier, and the vibrational frequencies are all in reasonable agreement with recent ab initio values.

Benzyl alcohol-ammonia (1:1) cluster structure investigated by combined IR-UV double resonance spectroscopy in jet andab initio calculation

Laser-induced fluorescence excitation and IR-UV double resonance spectroscopy have been used to determine the hydrogen-bonded structure of benzyl alcohol-ammonia (1:1) cluster in a jet-cooled molecular beam. In addition,ab initio quantum chemical calculations have been performed at HF/6-31G and HF/6-31G(d,p) levels for different ground state equilibrium structures of the cluster to correlate the calculated OH and NH frequencies and their intensities with experimental results. The broad red-shifted OH-stretching mode in the IR-UV double resonance spectrum suggests strong hydrogen bonding between the hydroxyl hydrogen and the lone pair of the ammonia nitrogen. The position and intensity distribution of the calculated NH and OH modes for the minimum-energy gauche form at HF/6-31G level have better correlation with the experimental results compared to other calculated ground state equilibrium conformers. These results lead to the conclusion that the minimum energy gauche form of the cluster is populated in the jet-cooled condition.

Laser-induced fluorescence excitation spectroscopy of the magnesium oxide B^1Σ^+–A^1Π system

Excitation scans of the MgO B(1)?(+)-A(1)? transition were made in a seeded acetylene-air flame. The combustion of magnesium in oxygen and carbon dioxide is well known and is of significant interest for fire safety, solid propellant applications, and recently for use as an in situ resource propellant for planetary exploration of Mars. This spectroscopic study expands the available data on this electronic transition, particularly data at high rotational states that are lacking in the literature. Rotational parameters of the v = 0, 1, and 2 states of B(1)?(+) are derived.

Vibronically selective intersystem crossing in van der Waals complexes of 9-cyanoanthracene with xenon

Supersonically expanded 9-cyanoanthracene (9CNA) and its van der Waals complexes with xenon have been examined by laser-induced fluorescence excitation spectroscopy. Previously unassigned vibronic features in the spectrum of the bare molecule have been assigned. Measurements of the relative intensities of corresponding vibronic features in the corrected spectra of 9CNA and its 1:1 complex with Xe are used to calculate the quantum yields of fluorescence arising from single vibronic levels at energies in the range 0<E v <800 cm −1 . Variations are attributed to mode selectivity in S 1–T 1 intersystem crossing.

Intermolecular interactions between tropolone and fluoromethanes

Van der Waals complexes of tropolone (TRN) with CF4, CFH3, CF2H2, and CF3H have been synthesized by expanding mixtures of TRN and the fluorinated methane (FM) in a supersonic free-jet and have been examined using laser induced fluorescence excitation spectroscopy. The sign and magnitude of the microscopic solvent shifts and the magnitude of the tunneling doublet splittings of the origin bands of each distinct complex have been determined from the LIFE spectra. These data, together with both empirical and ab initio calculations of the potential energy surfaces of the 1:1 complexes, have been used to assign the structures of the complexes and determine their approximate binding energies. Expansion of TRN with CF4 produces one identifiable 1:1 complex in which the solvent is primarily dispersively bound and lies above the TRN ring in a symmetric three-legged stool conformation. Expansion of TRN with CFH3 produces two 1:1 complexes, both primarily dispersively bound, in which the solvent molecule lies above the seven-membered ring of TRN in a three-legged stool conformation but which differ in the conformational orientation of the CFH3 species on the TRN surface. Expansion of TRN with CF2H2 produces one 1:1 complex in which the solvent molecule lies above the plane of the TRN ring, but is considerably displaced from its centre of mass and in which binding is primarily electrostatic rather than dispersive. All three partially fluorinated methane molecules produce 1:1 complexes with TRN in which the solvent is bound in the TRN plane by intermolecular hygrogen-bonding. Such structures partially disrupt the intramolecular hydrogen bond of the chromophore and consequently exhibit LIFE spectra characterized by intense, strongly blue-shifted origin bands in which the proton tunneling doublets are unresolvable because of a large decrease in the intramolecular proton tunneling rate. The existence of good correlations between the solute-solvent binding energy and the microscopic solvent shift and between the binding energy and the proton affinities of the solvent for the entire group of hydrogen-bonding solvents, including the partially fluorinated methanes, suggests that C–F ... H–O and F–C–H ... O = C interactions result in weak hydrogen bonds which are not qualitatively different from those of more traditional hydrogen-bonding species. PACS No.: 33.20L, 35.20B

Excimer formation in the mixed dimers of naphthalene and 1-methoxynaphthalene in a supersonic jet

Electronic spectra for the S1←S0 transition of jet-cooled mixed clusters of naphthalene and 1-methoxynaphthalene have been measured by laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. Under jet-cooled conditions, naphthalene forms two isomeric 1:1 mixed dimers with 1-methoxynaphthalene, which exhibit significant differences in excimer formation dynamics from the locally excited states. One of the isomeric dimers emits excimer fluorescence when excited to the electronic origin of the lowest excited state but the other shows a significant barrier to formation of the excimer. The energy gap between the S1 states of naphthalene and 1-methoxynaphthalene is ∽350 cm−1. The observation of excimer emission, particularly when excited to the S1 origin of one of the isomeric dimers, supports our earlier proposal that for naphthalene–methoxynaphthalene mixed dimers energy exchange or exciton resonance interactions are not significant for the stabilization of the mixed excimers. The charge-transfer (CT) interaction and overlap between π-molecular orbitals of two molecules at a suitable geometry are considered to be the primary factors for the stability of the excimers. At higher naphthalene vapour pressure we have observed a mixed trimer of 2:1 composition of naphthalene and 1-methoxynaphthalene. The trimer does not emit excimer fluorescence when excited with additional vibrational energy in S1 up to 650 cm−1. The observation indicates that the geometry of the trimer is very different from the stack configuration essential for formation of the excimer.