Dispersed Fluorescence Spectroscopy Research Articles

Detection and characterization of scandium imide, ScNH

A supersonic molecular beam of scandium imide, ScNH, and deuterated imide, ScND, were produced using a laser ablation/reaction source and interrogated by laser induced fluorescence and dispersed fluorescence spectroscopy. The field free fine and magnetic hyperfine interactions and Stark effect in the (0,0) à 2Πr–X̃2Σ+ transition of ScNH (origin=15 023.033 cm−1) were analyzed. Ground state vibrational spacing for ScNH and ScND was measured. The determined permanent electric dipole moments are μ(à 2Π)=4.08(7)D and μ(X̃ 2Σ+)=2.28(15)D. Plausible molecular orbital descriptions are given for the à 2Πr and X̃ 2Σ+ states based upon the interpretation of the spectroscopic parameters and a comparison with other isovalent molecules is made.

Structure and vibrations of catechol(methanol)1 in the S and S1 state

The structure as well as the inter- and intramolecular vibrations of the catechol (methanol)1 cluster are investigated both experimentally and theoretically. By using resonant two-photon ionization (R2PI) and dispersed fluorescence (DF) spectroscopy, the vibrational transitions of the S0 and S1 state are obtained. In order to find the corresponding vibrations of the S0 and S1 state, DF spectra are recorded by pumping the electronic origin and the most intense vibrations of the R2PI spectrum. According to ab initio calculations performed at the Hartree–Fock level [6-31G(d,p) basis], including MP2, BSSE, and ZPE (zero point energy) corrections, the most stable structure turns out to be translinear. The calculated vibrational frequencies are in close agreement to the experimental values. Since the catechol(methanol)1 cluster has no symmetry, all intermolecular fundamental vibrations of the S0 and S1 state spectra can be assigned. A large number of combination bands and overtone vibrations are observed in the low frequency region (<200 cm−1) of the S0 and S1 state spectra. From the DF spectrum obtained by pumping the low frequency ρ1 vibration, it can be concluded that the catechol(methanol)1 cluster undergoes a geometry change in the S1 state. The OH…O bond turns out to be nonplanar with respect to the aromatic plane. This confirms the results reported for catechol and the catechol(H2O)1 cluster.

Algebraic approach to vibrationally highly excited states of SO 2. Vibrational wavefunctions from spectroscopy

A total of 311 vibrational level energies of SO 2 in the electronic ground state obtained from dispersed fluorescence spectroscopy are fitted to the algebraic Hamiltonian expansion and Dunham-type expansion. A comparison of the results of the numerical fits up to 20407 cm −1 shows that the algebraic approach developed by Iachello, Levine and co-workers is more appropriate than the Dunham-type expansion for describing vibrational level energies in a wide energy range covering the vibrationally highly excited region. It is demonstrated that the vibrational wavefunctions can be constructed directly from the algebraic fit to the experimental data, and that the characteristic bifurcation of the vibrational wavefunctions is identified in the excitation of the symmetric stretching mode. This bifurcation is interpreted as a transition from normal-mode to local-mode behavior on the basis of finding a local-mode doublet representing the vibrational energy flow between the two SO bonds.

Laser Spectroscopy of Dysprosium Monofluoride: Ligand Field Assignments of States Belonging to the 4f96s2, 4f106s, and 4f96s6pSuperconfigurations

The techniques of selectively detected fluorescence excitation and dispersed fluorescence spectroscopy have been used to characterize two [19.3]8.5 and [20.3]8.5 electronic states of DyF in the region of ∼2.5 eV and five electronic states withT0< 6500 cm−1. Superconfigurational assignments of the observed low-lying states have been made on the basis of vibrational frequencies. TheX(1)7.5 ground state, with ΔG1/2≈ 605 cm−1, is assigned to the Dy+4f96s2superconfiguration, whereas four states, (1)8.5, (2)7.5, (3)7.5, and (4)7.5, with ΔG1/2≈ 508–544 cm−1are assigned to the Dy+4f106ssuperconfiguration. The lowest 4f106s(Ω = 8.5) state is found to lie 2431 cm−1above the 4f96s2X(1)7.5 ground state, thus establishing the linkage between the 4f96s2and 4f106ssuperconfigurations. The upper states in the region of ∼2.5 eV ([19.3]8.5 and [20.3]8.5) are assigned to the Dy+4f96s6psuperconfiguration on the basis of computed ligand field monopoleB00(nl,nl) orbital destabilization energies. The observed electronic states are discussed in relation to predictions of the ligand field theoretical model.

Structure and vibrations of catechol and catechol⋅H2O(D2O) in the S and S1 state

The inter- and intramolecular vibrations in the S0 and S1 state of catechol, d2-catechol, catechol(H2O)1, and d2-catechol (D2O)1 have been investigated experimentally by resonant two photon ionization (R2PI), spectral hole burning (SHB), and dispersed fluorescence spectroscopy (DF). The experimental frequencies are compared to the vibrational frequencies obtained from ab initio normal mode calculations using the 6-31G(d,p) basis set. In order to get a complete interpretation of the S0 state spectra of d2-catechol the strong coupling of the two OD torsional motions has been taken into account. A two-dimensional calculation of the torsional eigenvalues based on an ab initio potential [6-31G(d,p) basis] obtained from single point calculations is presented. Due to these calculations all vibrations in the S0 state can be assigned. Furthermore a new assignment of the vibrations in the S1 state of d2-catechol is given. In the case of catechol (H2O)1 [d2-catechol(D2O)1] different structural isomers are discussed. Using HF ab initio calculations (including MP2, BSSE, and ZPE corrections) a trans-linear hydrogen bonding arrangement turns out to be more stable by an amount of 840 cm−1 compared to a cyclic structure which is also a minimum of the PES. Normal mode calculations have been carried out for both structures and anharmonic corrections are calculated for the τ and β2 mode of the trans-linear arrangement. The prediction of the ab initio calculations is supported by the vibrational transitions observed in the spectra of the S0 and S1 state, which can be assigned on the basis of the vibrations calculated for the trans-linear structure. The most important feature of the R2PI spectrum of catechol(H2O)1 [d2-catechol(D2O)1] is the occurrence of intermolecular vibrations of very low frequencies (14, 37 cm−1). These vibrations and the low frequency torsional modes in the spectra of the S1 state of the catechol monomer strongly support the assumption that catechol is nonplanar in the S1 state with respect to the OH groups. Due to this nonplanarity a double minimum potential for the intermolecular ρ1 mode of catechol(H2O)1 is postulated. Using this assumption the low frequency vibrations of the R2PI spectra as well as the vibrations observed in the spectra of the S0 state can be assigned.

Laser Spectroscopy of Cerium Monofluoride: Ligand Field Assignments of Some 4f5d6p← 4f5d6sTransitions

The techniques of selectively detected fluorescence excitation, dispersed fluorescence, and magnetic rotation spectroscopy have been applied to the CeF molecule and used to characterize one [17.6]4.5 electronic state in the ∼2 eV region and three electronic states withT0< 2000 cm−1:X(1)3.5 and (2)3.5 with Ω = 3.5 and (1)4.5 with Ω = 4.5. Two of these states, the [17.6]4.5 andX(1)3.5 states, had been observed previously [R. M. Clements and R. F. Barrow,J. Mol. Spectrosc.107,119 (1984); Y. Azuma, W. J. Childs, and K. L. Menningen,J. Mol. Spectrosc.145,413 (1991)]. Based on the pattern of Ω-values and the value of ΔG1/2≈ 550 cm−1, the three low-lying states are assigned to the Ce+(4f5d(3H)6s)F−superconfiguration. The upper state is assigned to the Ce+(4f5d(3H)6p)F−superconfiguration on the basis of computed ligand field monopoleB00(nl,nl) orbital destabilization energies and one-electron transition propensity rules.

Neutral photodissociation of CO into excited fragments investigated by dispersed fluorescence spectroscopy

Dispersed fluorescence of excited atomic fragments has been recorded after neutral photodissociation of CO as a function of the exciting-photon energy using photon-induced fluorescence spectroscopy. In the fluorescence wavelength range between 90 and 200 nm several lines have been detected which were not previously observed after neutral photodissociation of CO. The recorded relative partial emission cross sections show complicated structures due to resonance effects. The strength of the observed structures are strikingly different in the partial cross section functions.

Laser Spectroscopy of Gadolinium Monofluoride: Ligand Field Assignments of States in the 0–3 eV Range

The techniques of selectively-detected fluorescence excitation and dispersed fluorescence spectroscopy have been used to characterize the (0,2) band of the [18.3]8Σ−–X8Σ−transition and the (1,0), (0,0), and (0,1) bands of the [15.8]8Σ−–X8Σ−transition of156GdF. The effective spin–spin and spin–rotation constants (in cm−1) for the groundX8Σ−state (v″ = 2) are λ″ = −0.181 38, γ″ = 9.79 × 10−4and for the [18.3]8Σ−state (v′ = 0) are λ′ = −3.279, γ′ = −9.47 × 10−3, which are consistent with 4f7[8S]6s2and 4f7[8S]5d6sconfigurational assignments, respectively. Five (0,0) bands of an extensive ΔΛ = +1 system in the near infrared region were analyzed and assigned as a10Φ(4f7[8S]5d2[3F])–a10Δ(4f7[8S]5d6s) transition. Five (0,0) subbands of a ΔΛ = 0 transition in the visible region were analyzed and assigned as a10Δ(4f7[8S]5d2[3F])–a10Δ(4f7[8S]5d6s) transition. Using a semiempirical ligand field model, all observed states were assigned to one of the three lowest energy superconfigurations of GdF: 4f76s2, 4f75d6s, and 4f75d2.

An accurate quartic force field for formaldehyde

An accurate quartic force field for formaldehyde is obtained by refining the ab initio quartic force field of Martin, Lee, and Taylor [J. Mol. Spectrosc. 160, 105 (1993)]. The refinement was achieved by iteratively fitting a subset of the coefficients of a Taylor-series expansion of the potential-energy surface to 138 of the observed transition frequencies, many of which were obtained by Bouwens et al. [J. Chem. Phys. 104, 460 (1994)] using dispersed fluorescence spectroscopy. We fit the vibrational energies (≤7600 cm−1) for 138 states with an absolute mean deviation of 1.5 cm−1.

Intermolecular vibrations of the phenol dimer revealed by spectral hole burning and dispersed fluorescence spectroscopy

Inter- and intramolecular vibrations of the Phenol dimer have been examined using spectral hole burning and dispersed fluorescence spectroscopy. The dispersed fluorescence spectra have been recorded via excitation of the electronic donor origin and all intermolecular vibrational transitions. The Franck–Condon intensity pattern allowed a straightforward assignment of the ground state vibrational frequencies to the excited state frequencies, which were examined by hole-burning spectroscopy. The existence of another conformer that possibly absorbs in the region of interest was ruled out by hole-burning spectroscopy. For obtaining the hole-burning spectra, the electronic origin of the donor chromophore was analyzed, while the hole-burning laser was scanned over the region of interest. As both acceptor and donor part belong to one molecule the hole-burning signal could be analyzed via fluorescence from one of the chromophores.

The electronic spectroscopy of jet-cooled m-difluorobenzene

The S1(1B2)–S0(1A1) electronic transition of m-difluorobenzene cooled in a supersonic free jet has been investigated in detail by use of laser induced fluorescence excitation and dispersed fluorescence spectroscopy. By analysis of over 50 vibronic transitions we have assigned 16 new S1 vibrational frequencies, and have confirmed seven previous assignments. This advances the total number of known S1 fundamental vibrational frequencies to 23 out of the possible 30. The assignments of two S0 frequencies have also been somewhat modified. Two different types of symmetry-forbidden transitions were identified: transitions involving one-quantum changes in b2 modes were found; and transitions involving combinations of b1 vibrations were observed to be unusually active in the vibronic spectrum. The observation of these forbidden transitions in both the fluorescence excitation and dispersed fluorescence spectra are explored in terms of first- and second-order vibronic coupling.

Dispersed fluorescence spectroscopy of excited rovibrational states in S formaldehyde

Dispersed fluorescence (DF) spectroscopy is used to explore the rovibrational structure of highly excited S0 formaldehyde (H2CO). A narrowband laser excites formaldehyde molecules to a single S1 rovibronic quantum state, and the resulting fluorescence is dispersed with a monochromator. DF spectra of ten vibrational levels with excitation in ν2, the carbon–oxygen stretch, and ν4, the out-of-plane bend, have been recorded, and the effective A, B, and C rotational constants are extracted. Five of the effective A rotational constants and seven of the effective B and C rotational constants are new to the literature. The dependence of these effective rotational constants on vibrational state are both calculated and discussed with regard to both the present and previous experiments. Particular attention is given to the manner in which that the effective A rotational constant depends on increasing excitation in ν4 due to the strong A-axis ν4/ν6 Coriolis interaction. For states where v2 is less than two, quantitatively accurate values for the nonlinear dependence of the A rotational constant on quanta in ν2 and ν4 is achieved by a simultaneous consideration of the strong A-axis ν4/ν6 Coriolis interaction and the 11↔42, 11↔62, and 51↔2161 Fermi interactions.

Structure and vibrations of phenol⋅CH3OH (CD3OD) in the electronic ground and excited state, revealed by spectral hole burning and dispersed fluorescence spectroscopy

The intermolecular vibrations of phenol(CH3OH)1 and its deuterated isotopomer d-phenol(CD3OD)1 were examined by comparing the vibrational frequencies of the electronic ground and excited state with the results of ab initio normal mode calculations at the Hartree–Fock level, using the 4-31G* and 6-31G** basis sets. Full energy minimization showed a translinear structure similar to phenol(H2O)1 or to the water dimer. Dispersed fluorescence spectra have been recorded via excitation of the electronic cluster origin and several intermolecular vibrational transitions. The Franck–Condon intensity pattern allowed an assignment of the ground state vibrational frequencies to the excited state frequencies, which were examined by resonance enhanced multiphoton ionization and hole burning spectroscopy. The existence of another conformer that possibly absorbs in the region of interest was ruled out by hole burning spectroscopy of the phenol(CH3OH)1 cluster. A full assignment of all intermolecular vibrations of this hydrogen bonded cluster in the S0 state could be given for the first time on the basis of ab initio calculations and a combination of different spectroscopical methods.

Dispersed fluorescence spectroscopy of jet-cooled DCO

We obtain dispersed fluorescence (DF) spectra of jet-cooled DCO from three excited-state levels in the B̃ 2A′-X̃ 2A′ system. These spectra yield term energies and relative fluorescence intensities for 112 levels (mostly quasibound resonances) of DCO (X̃) over an energy range of 0–19000 cm −1. We make zeroth-order, normal-mode vibrational assignments to 99 levels; however, their term energies are not well described by the customary (Dunham) expression due to extensive vibrational mixing and the weak CD bond. The use of an improved spectroscopic Hamiltonian that includes Fermi resonance and Darling-Dennison coupling terms is discussed.

Vibrational energy transfer from the 61 level of S1 (1B2u) benzene in a supersonic expansion. I. Monatomic collision partners

Vibrational energy transfer has been monitored from the 61 level of 1B2u(S1) benzene seeded in rare gas supersonic free jet expansions at X/D=5, where the temperature is calculated to be ∼10 K. The monatomic collision partners helium, neon, argon, and krypton form the subject of this study. Consequently, transfer is limited to one mechanism, transfer of vibrational energy in benzene to translational energy of the collision pair. The vibrational energy transfer is followed using time resolved, dispersed fluorescence spectroscopy. While there are five possible destination levels, only three are found to be important. These are transfer to the 162 level and transfer to the spectrally unresolved 111 and 161 levels. Negligible transfer is observed to both of the remaining two accessible levels, 00 and 41. It is found that the branching ratio for the two destination channels is insensitive to the identity of the collision partner. The branching ratios are reproduced by calculations based on SSH-T theory. The calculations suggest that the insensitivity of the branching ratio to the collision partner is fortuitous: while the combined 111/161 channel retains approximately the same ratio to 162 for all collision partners, the relative importance of the 111 and 161 levels themselves is collision partner dependent. Evidence is presented suggesting that there is significant rotational excitation accompanying the vibrational energy transfer in the case of heavy collision partners. This study, by establishing the behavior of vibration to translation transfer, forms the basis for further studies of the role of vibration to rotation and, subsequently, vibration to vibration mechanisms in vibrational energy transfer in benzene at low temperatures.

Laser spectroscopic studies of terbium monofluoride: ligand field assignments of some f8[7F]sp-f8[7F]s2transitions

The techniques of selectively-detected fluorescence excitation and dispersed fluorescence spectroscopy have been used to characterize four (CΘ = 6, B6, A7, A′6) electronic states in the 17 000 < T 0 < 20 000 cm-1 region and three electronic states with T 0 < 2800 cm-1: X(1)6 with Θ = 6, and (1)5 and (2)5 with Θ = 5. Four of these states, the C6, B6, A7, and X6 states, had been observed previously (D. J. W. Lumley and R. F. Barrow, 1978, J. molec. Spectrosc. 69, 494), but the vibrational numbering for the A7 state is reduced by 1 here. Based on the pattern of Θ-values and that ΔG 1/2 ∼ 610 cm-1, the three low-lying states are shown to belong to the Tb+(f8[7F]s2)F- superconfiguration. The four upper states are assigned to the Tb+(f8[7F]sp)F- superconfiguration on the basis of computed monopole ligand field B 0 0(nl, nl) orbital stabilization energies and one-electron transition propensity rules.

A Characterization of the CH2ã1A1(1,2,0),(2,0,0),(0,5,0),(1,3,0) and b̃1B1(1,142,0),(0,180,0),(0,191,0) Vibronic Levels by Fourier-Transform Dispersed Fluorescence Spectroscopy

The CH2ã1A1, (1, 2, 0), (2, 0, 0), (0, 5, 0), (1, 3, 0) and b̃1B1 (1, 142, 0), (0, 180, 0), (0, 191, 0) levels are characterized by Fourier-transform dispersed fluorescence spectroscopy (FTDFS). Rotational transitions in the b̃ ← ã 1102140, 2180, and 2190 fluorescence excitation bands and those of the b̃ → ã 1012162, 1012172, 1022160, 2185, 2195, 1012183 and 1102193 bands in dispersed fluorescence spectra have been assigned. The rotational constants and vibrational term values for these vibrational levels have been obtained. Due to the limits brought by Renner-Teller coupling and Franck-Condon factors, this work, along with previous studies on lower vibrational levels, has characterized almost all ã vibrational levels detectable in the b̃↔ã transitions.

Internal rotation of the methyl group in o-, m- and p-fluorotoluene and their ions as studied by supersonic jet spectroscopies

The potential for the internal rotation of a CH 3 group was determined for o-, m- and p-fluorotoluene in their S 0 and S 1 states by fluorescence excitation and dispersed fluorescence spectroscopies applied to the jet-cooled molecules. The potentials were also determined for the cationic ions in their ground states by pulsed field ionization (PFI) spectroscopy. The internal rotation of the methyl group changes greatly upon the electronic excitation and ionization.

Fourier transform dispersed fluorescence spectroscopy: Observation of new vibrational levels in the 5000–8000 cm−1 region of ã 1A1 CH2

Dispersed fluorescence spectra from the CH2 b̃ 1B1 state to highly excited vibrational levels of the ã 1A1 state were recorded by a new technique, Fourier transform dispersed fluorescence spectroscopy. The spectra obtained clearly show the advantages of using a Fourier transform spectrometer for dispersing fluorescence, namely emission over a wide spectral range can be efficiently detected with high sensitivity and resolution. These advantages allow four new vibrational levels in the CH2 ã 1A1 state to be observed; the (2,0,0) and (0,5,0) vibrational overtones and the (1,2,0) and (1,3,0) combination bands. The vibrational term values for these levels are given, along with the harmonic frequencies and anharmonicities for the v1 and v2 modes. From the (0,5,0) term value an improved estimate of the barrier height to linearity in the CH2 ã 1A1 state is made.

Vibrational predissociation and intramolecular vibrational relaxation in dimethyl-s-tetrazine–argon complexes

The one- and two-argon van der Waals complexes of dimethyl tetrazine (DMT) have been investigated by means of laser-induced excitation and dispersed fluorescence spectroscopy. The band shifts D″0 − D′0 range from −23 to −31 cm−1 for DMT–Ar relative to DMT. The excited state dissociation energy of the DMT–Ar bond was estimated to be 270 ≤ D′0 ≤ 484 cm−1. In all levels with energy greater than the argon dissociation energy, predissociation is very fast. A dissociation rate constant kD = 3 × 109 s−1 has been determined for dissociation from the 6a1 level. This rate constant is an order of magnitude larger than that observed in tetrazine–argon. The dissociation rate constant of DMT–Ar2 from the 6a1 level is approximately 2 × 1010 s−1. Unlike aminotetrazine–Ar2, DMT–Ar2 shows no mode-selective variations in the branching ratios between the DMT and DMT–Ar photoproducts.