Laser-induced Fluorescence Excitation Spectroscopy Research Articles

Experimental Determination of Vibrational Potential Energy Surfaces and Molecular Structures in Electronic Excited States

For more than three decades far-infrared and Raman spectroscopies, along with appropriate quantum mechanical computations, have been effectively used to determine the potential energy functions which govern the conformationally important large-amplitude vibrations of nonrigid molecules. More recently, we have utilized laser-induced fluorescence (LIF) excitation spectroscopy and ultraviolet absorption spectroscopy to analyze the vibronic energy levels of electronic excited states in order to determine the potential energy surfaces and molecular conformations in these states. Transitions from the ground vibrational state in an S0 electronic state can typically be observed only to several excited vibronic levels. Hence, the LIF of the jet-cooled molecules generally provides data on only a few excited state levels. Ultraviolet absorption spectra recorded at ambient temperatures, however, often provide data on many additional excited vibronic levels. However, these can only be correctly interpreted if the elec...

Van der Waals Complexes of Tropolone with Carbon Dioxide

van der Waals complexes of tropolone (TRN) with carbon dioxide have been synthesized by expanding mixtures of TRN and CO2 in He in a supersonic free-jet expansion, and have been examined by laser-induced fluorescence excitation spectroscopy and by ab initio structural calculations. At higher partial pressures of CO2 large clusters of TRN(CO2)n are formed in which TRN excited to S1 remains fluorescent. At sufficiently low partial pressures of CO2, well-resolved blue-shifted features in the spectra due to TRN(CO2) and TRN(CO2)2 may be identified. Their microscopic solvent shifts and the effects of solvation on the proton tunneling doublets and vibrational frequencies of the chromophore have been measured. The experimental results and calculations all suggest that the CO 2 binds to TRN in the 1:1 complex in a fashion which is similar to common hydrogen-bonding addends. The CO2 lies in the plane of the TRN ring and interacts with the keto oxygen and the hydroxyl hydrogen in such a way that the intramolecular hydrogen bond of TRN is partially disrupted, lengthening the R O- -- H distance and opening the H-O-C angle. Consistent with this interaction, the proton tunneling rate is reduced so that the tunneling doublets can no longer be resolved, and the frequency of the out-of-plane wagging vibration of the two oxygen atoms in the chromophore increases. The microscopic solvatochromic shifts of these and other hydrogen-bonded complexes are well-correlated with the calculated binding energies of the addends to the chromophore and with their proton affinities. The latter correlation suggests that CO2 acts as a weak proton acceptor in a hydrogen-bonding-like interaction with TRN.

Hydrated Clusters of 2-Phenylethyl Alcohol and 2-Phenylethylamine: Structure, Bonding, and Rotation of the S1 ← S0 Electronic Transition Moment

Hydrated clusters of 2-phenylethyl alcohol (PEAL) and 2-phenylethylamine (PEA) have been studied in a jet-cooled environment, using laser-induced fluorescence excitation and mass-selected resonant two-photon ionization (R2PI) spectroscopy of the S1 ← S0 electronic transitions. Spectral features have been observed for clusters M(H2O)n, n = 1−4, and their stoichiometry assigned on the basis of the ion fragmentation patterns. Ionization of hydrated PEA(H2O)n clusters leads to the observation of PEA(H2O)n-1+ and CH2NH2(H2O)n+ ions. Partially resolved rotational band contours of several n = 1, 2 clusters have been analyzed with the aid of ab initio molecular orbital calculations, conducted at the MP2/6-31G*//HF/6-31G* level for the ground state, and CIS/6-31G* for the first electronically excited singlet state. The analysis reveals the supramolecular structure: the host molecular conformation within these clusters and the binding sites of the water molecules. In n = 1 clusters of 2-phenylethylamine, the primary binding site involves hydrogen bonding to the nitrogen atom in the amine group. Cyclic hydrogen-bonded structures are observed for n = 2 clusters. In 2-phenylethyl alcohol, two different 1:1 clusters have been assigned in which the water molecule binds alternatively as a proton acceptor and proton donor. Further interactions between water molecules and the host, e.g., Hwater···π and Owater···HC, lead to additional stabilization of certain complexes. The assignments are aided greatly by the extraordinary sensitivity of the S1 ← S0 transition moment alignment to both side chain conformation and long-range intermolecular interactions.

Laser-induced fluorescence excitation spectroscopy of N(2)(+) produced by VUV photoionization of N(2) and N(2)O.

Synchrotron radiation emitted from the UVSOR storage ring is monochromated by a grazing-incidence monochromator and introduced coaxially with the second harmonic of a mode-locked Ti:sapphire laser. Sample gases, N(2) and N(2)O, are photoionized into vibronically ground N(2)(+) with the fundamental light of the undulator radiation at 18.0 and 18.6 eV, respectively. Laser-induced fluorescence (LIF) excitation spectra of N(2)(+) from N(2) and N(2)O are measured in the laser wavelength region of the (B (2)Sigma(u)(+), v' = 0) <-- (X (2)Sigma(g)(+), v" = 0) transition at 389-392 nm. The LIF excitation spectra of N(2)(+) exhibit two maxima due to the P and R branches in which rotational bands are heavily overlapped. The rotational temperature is determined by simulating an LIF excitation spectrum by using the theoretical intensity distribution of rotation bands convoluted with the spectral width of the laser.

Pump-Probe Spectroscopy of N2 and N2O by Using a Laser-Synchrotron Radiation Combination Technique.

Gas-phase N2 or N2O is photoionized with the fundamental light of an undulator radiationinto N2+ (X2Σg+, v″ =0) which is then probed by laser induced fluorescence excitation spectroscopy in thelaser wavelength regionof the (B2Σu+, v″=0) ← (X2Σg+, v″=0) transition at 389-392 nm. Thefluorescence excitation spectra of N2+exhibit two maxima due to the P and R branches in which rotational lines are heavily overlapped. By fixing thelaser wavelength at the maximum position of the P branch, partial cross sections forproduction of N2+ (X2Σg+, v″=0) are measured as a function of the undulator photon energy. The cross sectioncurve for N2+ from N2 or N2O shows peaks originating from transitions to autoionizing Rydberg states converging to N2+ (A2IIu, v=0 or 1) or to N2O+ (C2Σ+, v1=v2=v3=0), respectively.

A computer-controlled apparatus for laser-induced fluorescence spectroscopy in a supersonic jet

An experimental apparatus for laser-induced fluorescence excitation spectroscopy (FES) and for dispersed fluorescence studies in a supersonic jet has been constructed and optimized for the collection of spectra from weakly fluorescing samples. A detailed description of the hardware and software is presented here.

Enantiodifferentiation in jet-cooled van der Waals complexes of chiral molecules

Laser-induced fluorescence excitation spectroscopy in a supersonic expansion is used to evidence intermolecular chiral discrimination at the microscopic level. Measurements have been performed in (R)-(+)- and (S)-(−)-α-methyl-2-naphthalenemethanol (a naphthalene compound made chiral by the asymmetric carbon in the 2 position) in the presence of alcoholic solvents (methanol, 1-propanol, and (S)-(+)-chloro-1-propanol). While identical spectral shifts are observed for 1:1 complexes of (R)-(+)- and (S)-(−)-α-methyl-2-naphthalenemethanol with non-chiral solvents, complexation of each enantiomer with the chiral (S)-(+)-2-chloro-1-propanol molecule gives rise to different solvent shifts and allows differentiation between the two forms in the racemic mixture.

Laser-Induced Fluorescence Excitation Spectroscopy of Highly Vibrationally Excited Ã2A1 NH 2 Radical

Eighty spin-rovibronic transitions in NH 2 à 2A 1(0, 10, 0) K a =1 ← X̃ 2 B 1(0, 0, 0) K a =0,2 and à 2 A 1(0, 12, 0) K a =0 ← X̃ 2 B 1(0, 2, 0) K a =1 have been observed in the range 28810-29220 cm −1. The spectral resolution (Δν ∼ 0.03 cm −1) of the present investigation has resolved many electron-spin doublets. The observed high-lying Π(0, 10, 0) and Σ(0, 12, 0) vibrational states of NH 2 à 2 A 1 were only subjected to minor perturbations, despite the molecule undergoing a large-amplitude bending vibration. The molecular constants, T = 29036.13(7) cm −1, B = 9.671(6) cm −1, A = 0.93(25) cm −1, and q = 2.25(1) cm −1, of NH 2 à 2 A 1 Π(0, 10, 0) have been obtained from a least-squares fit to the spectroscopicaIly determined spin-rovibronic term values. The spectroscopic constants of NH 2 à 2 A 1Σ(0, 12, 0), T = 32115.9(14) cm −1 and B = 10.33(18) cm −1, have also been determined from the present data.

Production of Halomethylenes in Free-Jet Expansions from a Hot Nozzle: Identification and Characterization of HCBr and DCBr by Laser-Induced Fluorescence Excitation Spectroscopy

The efficient production of chloromethylene and bromomethylene by hot-nozzle pyrolysis and subsequent supersonic free-jet expansion has been demonstrated. In the case of the bromocarbene, the LIF-excitation spectral measurements represent the first direct observations of this species. Visible spectra for both HCBr and DCBr have been obtained and are in accord with expectations for a species with nonlinear ground-state and quasilinear excited-state geometries. For the protonated species the excited-state bending frequency is near 840 cm -1 when υ 2 =5 and the C-Br stretching frequency is υ 3 =783 cm -1

Spectroscopy of the indium argon van der Waals complex: A high resolution study of the B 2Σ1/2←X2 2Π3/2 system

The InAr van der Waals complex has been characterized by high resolution laser induced fluorescence excitation spectroscopy. Six vibronic bands of the B 2Σ1/2←X2 2Π3/2 transition have been observed and five of these (v′,0), where v′=1–5, have been rotationally analyzed. Rydberg–Klein–Rees potential curves were constructed for the B 2Σ1/2 state using the rotational and vibrational constants determined from these spectra. Equilibrium bond lengths were determined for the B and X2 states and a dissociation energy was determined for the B state. The stronger bonding present in the B state is rationalized in terms of penetration of the argon atom into the diffuse 6s orbital of indium. Evidence is presented that the B state potential energy curve has a barrier at long range, due to Pauli repulsion, of ∼60 cm−1. An analysis of the hyperfine structure involving the 115In nucleus was made. It is concluded that the X2 state conforms to Hund’s coupling case aβ, whereas the B state conforms to case bβs. The extent of 6s–6p hybridization in the upper state was measured from hyperfine splittings and was used in conjunction with a simple electrostatic model to estimate the polarizability of the indium atom in the 6s 2S1/2 state. A value of 68(4) Å3 was obtained (1σ error).

Van der Waals complexes of xanthione and benzopyranthione with rare gases. S 2S 0 fluorescence excitation spectroscopy and microscopic solvation effects

The 1:1 and 2:1 van der Waals complexes of xanthione and benzopyranthione with He, Ne, Ar, Kr and Xe have been observed by S 2S 0 laser-induced fluorescence excitation spectroscopy in a supersonic jet. The adatom is located over the pyranthione ring in the 1:1 complex, and the 2:1 complex has a symmetrical sandwich structure. An analysis of the microscopic solvent shifts indicates that the thiones have substantially smaller electric dipole moments in the S 2 state than in the ground state, in qualitative agreement with previous semi-empirical calculations. The dipole moment of xanthione S 2 is estimated to be only 2.0 D, so that thione—adatom interactions are almost exclusively dispersive in the upper state.

Nitrogen ion dynamics in low-pressure nitrogen plasma and plasma sheath

Laser-induced fluorescence (LIF) excitation and time-resolved LIF spectroscopy have been used to explore the interaction of a nitrogen plasma with a metal surface. The rotational temperature, the concentration of N+2, and the lifetime of N+2(B2Σ+u)υ=0 were measured as a function of distance from the metal surface. It was observed that the lifetime of the excited ion decreases sharply in the plasma sheath which is created in front of the metal surface. The concentration of N+2(X 2Σ+g), which was measured simultaneously with the lifetime of excited N+2, indicates a quasilinear decrease from the cathode to the grounded surface. Trot undergoes a gradual reduction from the bulk plasma up to the surface where its value is a good approximation of the surface temperature. An interpretation of these phenomena including a numerical simulation of the lifetime decrease is proposed here. The calculated values are in good agreement with experimental results. The dependence of lifetime decrease on the sheath potential as well as on other important parameters (cross section, profile of the electric field) is further discussed.

The electric dipole moment of the A2Π state of ScO

The electric dipole moment of the A 2Π state ( v = 1) of ScO is determined to be 4.13 ± 0.21 D in A 2 Π 3 2 and 4.25 ± 0.25 D (2σ uncertainties) in A 2 Π 1 2 by laser induced fluorescence excitation spectroscopy of the RQ 24 + RR 24 (1) and PP 1G + PQ 1G (1) ( G = 3 or 4 where G  I + S) lines in the presence of a large homogeneous electric field. The positions of low- J lines in the A 2Π - X 2Σ + (1,0) band are also reported.

Remeasurement of the [formula omitted] (0, 0) band in 12C 16O + by laser-induced fluorescence excitation spectroscopy

A remeasurement and subsequent analysis of the (0, 0) band of the Comet-Tail System of 12C 16O + is reported. The measured line positions were fitted via a nonlinear least-squares technique. The constants obtained from these measurements were then merged with those obtained by fitting the nine measured microwave frequencies within the v ″ = 0 vibrational level of the X ̃ 2 Σ + state. The method yields the best estimates of the molecular constants for the v = 0 level of both the A ̃ 2 Π i and X ̃ 2 Σ + states.

Isolated ultracold porphyrins in supersonic expansions. III. Free-base porphine

The vibrational level structure of the S0→S1x transition (the Qx band) and of the S0→S1y transition (the Qy band) of free-base porphine in supersonic expansions of He was interrogated by laser-induced fluorescence excitation spectroscopy. Electronic relaxation in the S1x manifold was explored by time-resolved spectroscopy, revealing a constant value of the decay lifetime τ=9.5±1.0 ns for excess vibrational energies in the range Ev=0–5000 cm−1. The line broadening (FWHM) Δ=1.0–1.5 cm−1 of the electronic origin and of low-lying vibrational excitations in the S1x manifold originates from inhomogeneous unresolved rotational structure, while the large linewidth Δ=11.9 cm−1 of the electronic origin of the S1y state is due to homogeneous electronic relaxation broadening in the statistical limit. The line shape of the electronic origin of S1y was found to be Lorentzian, providing a quantitative determination of the lifetime τ=5×10−13 s for interstate S1y–S1x electronic relaxation within a bound level structure of a large isolated molecule.

A photochemical study of rotational state dependence by laser excitation of formaldehyde (? 1A2). I. Coriolis and singlet–triplet perturbation

A study of rotational dependence of radiationless transition rate and radical yield in the photodecomposition of formaldehyde, H2CO(? 1S2), was made with laser-induced fluorescence excitation spectroscopy and chemiluminescence excitation spectroscopy [Lewis, Tang and Lee, J. Chem. Phys., 65, 2910 (1976)]. The effect of perturbations on some rovibronic levels excited by the 220410 and 230410 transitions is examined.