Dispersed Fluorescence Research Articles

Spectroscopic and Theoretical Investigations of ThS and ThS+

Gas-phase ThS has been produced via the reaction of laser ablated Th with H2S. Rotationally resolved electronic spectra were recorded by laser-induced fluorescence (LIF) over the range 17500-24000 cm(-1). Resonance-enhanced multiphoton ionization was used in conjunction with a time-of-flight mass spectrometer to confirm the assignments of nine LIF bands to ThS. Using excitation of a ThS band centered at 22118 cm(-1), a dispersed fluorescence spectrum revealed a vibrational progression of the X(1)Σ(+) ground electronic state and the term energies of two low-lying excited states ((3)Δ1 and (3)Δ2). Two-color photoionization spectroscopy was used to study ThS(+). An accurate ionization energy for ThS was obtained (54425(3) cm(-1)); ThS(+) vibronic term energies up to v = 7 in the X(2)Σ(+) ground state and v = 3 in the (2)Δ3/2 first excited state were recorded. High-level electronic structure calculations, with inclusion of the spin-orbit interactions yielded predictions that were in good agreement with the experimental data for ThS and ThS(+). The spectroscopic properties of ThS/ThS(+) are compared with those of the valence isoelectronic pairs ThO/ThO(+), HfO/HfO(+), and HfS/HfS(+).

Three new electronic transitions of CoS probed by LIF and DF spectroscopy

The laser-induced fluorescence excitation spectrum of jet-cooled CoS molecules has been recorded in the energy range of 20 100–22 600cm−1. Twenty-one bands have been assigned as three transition progressions: [21.58] 4Φ9/2(υ′=0–2) – A4Φ9/2(υ″=0–3), [21.65] 4Δ7/2(υ′=0–1) – A4Φ9/2(υ″=0–2), and [21.73] 4Φ9/2(υ′=0–2) – A4Φ9/2(υ″=0–4). Spectroscopic constants for the three newly identified electronically excited states and the A state have been determined for the first time. The dispersed fluorescence spectra related to most of these bands have been investigated, vibrationally excited levels of the ground and the A state have been observed. In addition, the lifetimes for almost all the observed bands have also been measured.

Excitonic splitting and vibronic coupling in 1,2-diphenoxyethane: Conformation-specific effects in the weak coupling limit

Vibrationally and rotationally resolved electronic spectra of 1,2-diphenoxyethane (C6H5-O-CH2-CH2-O-C6H5, DPOE) are reported for the isolated molecule under jet-cooled conditions. The spectra demonstrate that the two excited surfaces are within a few cm(-1) of one another over significant regions of the torsional potential energy surfaces that modulate the position and orientation of the two aromatic rings with respect to one another. Two-color resonant two-photon ionization (2C-R2PI) and laser-induced fluorescence excitation spectra were recorded in the near-ultraviolet in the region of the close-lying S0-S1 and S0-S2 states (36,400-36,750 cm(-1)). In previous work, double resonance spectroscopy in the ultraviolet and alkyl CH stretch regions of the infrared was used to identify and assign transitions to two conformational isomers differing primarily in the central C-C dihedral angle, a tgt conformation with C2 symmetry and a ttt conformation with C2h symmetry [E. G. Buchanan, E. L. Sibert, and T. S. Zwier, J. Phys. Chem. A 117, 2800 (2013)]. Comparison of 2C-R2PI spectra recorded in the m∕z 214 (all (12)C) and m∕z 215 (one (13)C) mass channels demonstrate the close proximity of the S1 and S2 excited states for both conformations, with an upper bound of 4 cm(-1) between them. High resolution spectra of the origin band of the tgt conformer reveal it to consist of two transitions at 36,422.91 and 36,423.93 cm(-1), with transition dipole moments perpendicular to one another. These are assigned to the S0-S1 and S0-S2 origin transitions with excited states of A and B symmetry, respectively, and an excitonic splitting of only 1.02 cm(-1). The excited state rotational constants and transition dipole coupling model directions prove that the electronic excitation is delocalized over the two rings. The ttt conformer has only one dipole-allowed electronic transition (Ag→Bu) giving rise to a pure b-type band at 36,508.77 cm(-1). Here, the asymmetry induced by a single (13)C atom in one of the rings is sufficient to localize the electronic excitation in one or the other ring. Dispersed fluorescence (DFL) spectra are used to provide assignments for all vibronic structure in the first 200 cm(-1)of both conformers. In the tgt conformer, both "a" and "b" symmetry fundamentals are observed, consistent with extensive vibronic coupling between the two dipole-allowed, nearly degenerate excited states. In the ttt conformer, the lowest frequency vibronic transition located 46 cm(-1) above the Bu origin is assigned to a bu fundamental (labeled R[overline]) built off the dipole-forbidden Ag state origin. The DFL spectrum of the Ag(R[overline](1)) level contains strong transitions to v(")(R[overline]) = 0, 1, and 2, seemingly at odds with vibronic coupling models. Studies of the DFL spectrum of this band as a function of distance from the nozzle reveal that much of the intensity in v(") = 1 arises from collisions of DPOE while in the excited state Ag(vb' = 1) level with He, producing Bu(R[overline] = 1) levels with large collision cross section. The remaining intensity in the fundamental at large x∕D is ascribed to emission from the (13)C isotopomer, for which this emission is dipole-allowed.

Observation of a new Ω = 1 excited state in LaH

We report the observation of an Ω=1 electronic state with T0=20041.96(1) and T1=21359.1(5)cm−1. This state was identified by analyzing the laser-induced fluorescence spectra of jet-cooled LaH molecule. The spectra showed two new bands having P, R and strong Q branch structures with band origin at 19972.00(2) originated from v=1 and 20041.96(1)cm−1 from v=0 of the X1Σ+ ground state. The molecular constants of the excited state were determined. The dispersed fluorescence study also lead to the observation of Ω=2 spin-orbit component of the low-energy b3Π state, which remain unobserved in earlier studies.

Franck–Condon Fingerprinting of Vibration-Tunneling Spectra

We introduce Franck-Condon fingerprinting as a method for assigning complex vibration-tunneling spectra. The B̃ state of thiophosgene (SCCl2) serves as our prototype. Despite several attempts, assignment of its excitation spectrum has proved difficult because of near-degenerate vibrational frequencies, Fermi resonance between the C-Cl stretching mode and the Cl-C-Cl bending mode, and large tunneling splittings due to the out-of-plane umbrella mode. Hence, the spectrum has never been fitted to an effective Hamiltonian. Our assignment approach replaces precise frequency information with intensity information, eliminating the need for double resonance spectroscopy or combination differences, neither of which have yielded a full assignment thus far. The dispersed fluorescence spectrum of each unknown vibration-tunneling state images its character onto known vibrational progressions in the ground state. By using this Franck-Condon fingerprint, we were able to determine the predominant character of several vibration-tunneling states and assign them; in other cases, the fingerprinting revealed that the states are strongly mixed and cannot be characterized with a simple normal mode assignment. The assigned transitions from vibration-tunneling wave functions that were not too strongly mixed could be fitted within measurement uncertainty by an effective vibration-tunneling Hamiltonian. A fit of all observed vibration-tunneling states will require a full resonance-tunneling Hamiltonian.

Vibrations of porphycene in the S and S1 electronic states: Single vibronic level dispersed fluorescence study in a supersonic jet

Supersonic jet-isolated porphycene has been studied using the techniques of laser-induced fluorescence excitation, single vibronic level fluorescence, and spectral hole burning, combined with quantum mechanical calculations of geometry and vibrational structure of the ground and lowest electronically excited singlet states. Porphycene is a model for coherent double hydrogen tunneling in a symmetrical double well potential, as evidenced by tunneling splittings observed in electronic absorption and emission. The results led to reliable assignment of low frequency modes in S0 and S1 electronic states. The values of tunneling splitting were determined for ground state vibrational levels. In the case of tautomerization-promoting 2A(g) mode, tunneling splitting values significantly increase with the vibrational quantum number. Mode coupling was demonstrated by different values of tunneling splitting obtained for coexcitation of two or more vibrations. Finally, alternation of relative intensity patterns for the components of 2A(g) tunneling doublet observed for excitation and emission into different vibrational levels suggests that the energy order of levels corresponding to (+) and (-) combinations of nuclear wave functions is different for even and odd vibrational quantum numbers.

Electronic spectroscopy of the HCCN radical

The Ã(3)A"-X̃(3)Σ(-) electronic transition of the HCCN∕DCCN radical was observed by laser-induced fluorescence spectroscopy. Rotationally resolved excitation spectra were observed for HCCN and DCCN, and effective molecular constants of the upper state were determined. The observed intensities of the rotational lines were anomalous, probably due to a level-dependent non-radiative decay process in the excited state. Fluorescence depletion spectroscopy was applied in order to observe non-fluorescent vibronic levels. A dispersed fluorescence spectrum was also observed to determine the vibrational level structure in the ground electronic state. The observed vibrational structures in the fluorescence depletion and dispersed fluorescence spectra were tentatively assigned based on the results of ab initio calculations.

Comparable Strength of OH–O versus OH−π Hydrogen Bonds in Hydrogen-Bonded 2,3-Benzofuran Clusters with Water and Methanol

Two different types of hydrogen bond, which are classified into a familiar OH-O and a relatively weak OH-π one, have been compared in the 1:1 hydrogen-bonded 2,3-benzofuran clusters with water and methanol molecules. By applying fluorescence-detected infrared spectroscopy and dispersed fluorescence spectroscopy, two isomers having different types of hydrogen bonds are distinguished. From the calculated stabilization energy as well as the frequency shift of the OH stretching vibration in each cluster, these two isomers are almost equally stable, although that of OH-π type is usually thought to be relatively weak. It is suggested that the origin of the weak OH-O hydrogen bond is derived from the lower availability for a hydrogen bond acceptor on the oxygen atom of a heteroaromatic ring, which is attributed to the larger furan aromaticity.

Electronic Relaxation after Resonant Laser Excitation of Cr in Superfluid Helium Nanodroplets

Chromium (Cr) atoms embedded into helium nanodroplets (HeN) are ejected from the droplets upon photoexcitation. During ejection they undergo electronic relaxation resulting in bare Cr atoms in various excited states. In a study of the relaxation process we present absorption spectra observed via laser induced fluorescence and beam depletion as well as dispersed fluorescence spectra and time-resolved fluorescence measurements. Broad and shifted absorption structures were found for the strong z7P° ← a7S3 and y7P° ← a7S3 excitations from the ground state. Emission lines are, in contrast, very narrow, which indicates that fluorescence is obtained from bare excited Cr atoms after ejection. Upon excitation into the y7P2,3,4° states we observed fluorescence from y7P2°, z5P1,2,3°, and z7P2,3,4°, indicating that these states are populated by electronic relaxation during the ejection processes. Relative population ratios are obtained from the intensities of individual spectral lines. Excitation into the z7P2,3,4° states resulted in fluorescence only from z7P2°. Estimates of the time duration of the ejection process are obtained from time-resolved measurements.

Towards a first-principles model of Fermi resonance in the alkyl CH stretch region: Application to 1,2-diphenylethane and 2,2,2-paracyclophane

The spectroscopy of two flexible hydrocarbons, 1,2-diphenylethane (DPE) and 2,2,2-paracyclophane (TCP) is presented, and a predictive theoretical model for describing the alkyl CH stretch region of these hydrocarbons is developed. Ultraviolet hole-burning spectroscopy identified two isomers of DPE and a single conformation of TCP present in the supersonic jet expansion. Through the analysis of the ground state low-frequency vibronic spectroscopy obtained by dispersed fluorescence, conformational assignments were made for both DPE and TCP. The two isomers of DPE were found to retain the low energy structures of butane, being present in both the gauche and anti structures. TCP forms a C(2) symmetric structure, differing from the predicted lower energy C(3) conformation by the symmetry of the ethano bridges (-CH(2)CH(2)-) linking the phenyl substituents. Resonant ion-dip infrared spectroscopy is used to record single-conformation IR spectra of the two conformers of DPE and the single conformer of TCP in the alkyl CH stretch region and in the mid-IR that covers the CH bend fundamentals. A local mode Hamiltonian that incorporates cubic stretch-bend coupling is developed. Its parameters are obtained from density functional theory methods. Full dimensional calculations are compared to those that use reduced dimensional Hamiltonians in which anharmonic CH stretches and scissor modes are Fermi coupled. Excellent agreement is found. Scale factors of select terms in the reduced dimensional Hamiltonian are determined by fitting the theoretical Hamiltonian to the anti-DPE spectrum. The scaled Hamiltonian is then used to predict successfully structures for the remaining lower symmetry experimentally determined spectra in the alkyl CH stretch region.

Intrinsic fluorescence properties of rhodamine cations in gas-phase: triplet lifetimes and dispersed fluorescence spectra

We have investigated the gas phase triplet state lifetimes and dispersed fluorescence spectra of several types of rhodamine cations confined in a quadrupole ion trap and thermalized to 85 K. The measured triplet lifetimes of rhodamine cations Rh6G(+), Rh575(+), RhB(+), and Rh101(+) are found to be on the order of seconds, several orders of magnitude longer than those typically observed for the same dyes in optical condensed phase measurements. In addition dispersed fluorescence emission spectra in the gas phase at 85 K have been measured. The experimental gas phase results as well as solution measurements are compared to density functional calculations and the previous literature. Possible explanations for the discrepancy of gas and solution phase triplet lifetimes are discussed.

Enhanced production of anionic and excited neutral fragments of gaseous HCCl3 near the Cl 2p1/2,3/2 ionization threshold

Following photoexcitation of Cl 2p electrons to various resonances, the fragmentation dynamics of anionic and excited neutral fragments of gaseous HCCl3 were investigated on combining measurements of photon-induced anionic dissociation, X-ray absorption and UV/visible dispersed fluorescence. The Cl 2p core-to-Rydberg excitations near the ionization threshold produce a notable enhancement of excited neutral fragments (C* and CH*), which is explained in terms of the contribution from a shake-modified resonant Auger decay and post-collision interaction. The anion yields significantly reinforced just above Cl 2p ionization threshold of gaseous HCCl3 are predominantly mediated by photoelectron recapture through post-collision interaction. The present work demonstrates that the detection of excited neutral fragments and anions offer specific insight into the fragmentation dynamics of high-lying molecular-ion states of core-excited molecules.

Towards a Spectroscopic and Theoretical Identification of the Isolated Building Blocks of the Benzene–Acetylene Cocrystal

Isomer- and mass-selective UV and IR-UV double resonance spectra of the BA3, B2A, and B2A2 clusters of benzene (B) and acetylene (A) are presented. Cluster structures are assigned by comparison with the UV and IR spectra of benzene, the benzene dimer, as well as the BA, BA2, and B2A clusters. The intermolecular vibrations of BA are identified by dispersed fluorescence spectroscopy. Assignment of the cluster structures is supported by quantum chemical calculations of IR spectra with spin-component scaled second-order Møller-Plesset (SCS-MP2) theory. Initial propositions for various structures of the BA3 and B2A2 aggregates are generated with model potentials based on density functional theory combined with the symmetry-adapted perturbation theory (DFT-SAPT) approach. Shape and relative cluster stabilities are then confirmed with SCS-MP2. T-shaped geometries are the dominant structural motifs. Higher-energy isomers are also observed. The detected cluster structures are correlated with possible cluster formation pathways and their role as crystallization seeds is discussed.

Intermolecular vibrations of fluorobenzene-Ar up to 130 cm−1 in the ground electronic state

Sixteen intermolecular vibrational levels of the S(0) state of the fluorobenzene-Ar van der Waals complex have been observed using dispersed fluorescence. The levels range up to ~130 cm(-1) in vibrational energy. The vibrational energies have been modelled using a complete set of harmonic and quartic anharmonic constants and a cubic anharmonic coupling between the stretch and long axis bend overtone that becomes near ubiquitous at higher energies. The constants predict the observed band positions with a root mean square deviation of 0.04 cm(-1). The set of vibrational levels predicted by the constants, which includes unobserved bands, has been compared with the predictions of ab initio calculations, which include all vibrational levels up to 70-75 cm(-1). There are small differences in energy, particularly above 60 cm(-1), however, the main differences are in the assignments and are largely due to the limitations of assigning the ab initio wavefunctions to a simple stretch, bend, or combination when the states are mixed by the cubic anharmonic coupling. The availability of these experimental data presents an opportunity to extend ab initio calculations to higher vibrational energies to provide an assessment of the accuracy of the calculated potential surface away from the minimum. The intermolecular modes of the fluorobenzene-Ar(2) trimer complex have also been investigated by dispersed fluorescence. The dominant structure is a pair of bands with a ~35 cm(-1) displacement from the origin band. Based on the set of vibrational modes calculated from the fluorobenzene-Ar frequencies, they are assigned to a Fermi resonance between the symmetric stretch and symmetric short axis bend overtone. The analysis of this resonance provides a measurement of the coupling strength between the stretch and short axis bend overtone in the dimer, an interaction that is not directly observed. The coupling matrix elements determined for the fluorobenzene-Ar stretch-long axis bend overtone and stretch-short axis bend overtone couplings are remarkably similar (3.8 cm(-1) cf. 3.2 cm(-1)). Several weak features seen in the fluorobenzene-Ar(2) spectrum have also been assigned.

The gas-phase vibrational frequency of the ground state, X4Φ, of titanium monohydride, TiH, and monodeuteride, TiD

Low resolution dispersed fluorescence following laser excitation of the R(1.5) line of the (0,0) band of the B4Γ5/2–X4Φ3/2 transition has been used to obtain the first direct gas phase measurements of the vibrational frequencies of the ground X4Φ state of TiH and TiD. Observation of a very weak (0,1) band in the dispersed fluorescence spectra allowed determination of ground state vibrational spacings, ΔG1/2 of 1520(10) cm−1 for TiH and 1109(10) cm−1 for TiD.

Jet-cooled laser-induced fluorescence spectroscopy of B2Σ+–X2Σ+ system of scandium monoxide: Improved molecular constants at equilibrium

The investigation on B2Σ+–X2Σ+ system of ScO was extended to higher vibrational levels by laser-induced fluorescence (LIF) spectroscopy in a free-jet. We have observed rotationally resolved excitation spectra for (4,0), (3,0), (2,0), and (1,2) bands in addition to the previously observed (0,0) and (1,0) bands. The wavenumbers of these bands were fitted to a Hamiltonian matrix to determine the molecular constants for the vibrational levels up to ν′=4 of the B2Σ+ state and ν″=2 of the X2Σ+ state. In addition, the vibration constants of the ground states were determined from the dispersed fluorescence wavenumbers between B2Σ+ (ν′=0–4) and X2Σ+ (ν″=0–8) transitions. The equilibrium molecular constants, derived from the extensive set of molecular constants for individual vibrational levels, were used to construct RKR potential energy curves for both the electronic states. The Franck–Condon factors were also calculated for the B2Σ+–X2Σ+ transition.

The electronic spectrum of Si3 I: Triplet D3h system

We report the measurement of a jet-cooled electronic spectrum of the silicon trimer. Si(3) was produced in a pulsed discharge of silane in argon, and the excitation spectrum examined in the 18 000-20 800 cm(-1) region. A combination of resonant two-color two-photon ionization (R2C2PI) time-of-flight mass spectroscopy, laser-induced fluorescence/dispersed fluorescence, and equation-of-motion coupled-cluster calculations have been used to establish that the observed spectrum is dominated by the 1(3)A(1)" - ã (3)A(2)' transition of the D(3h) isomer. The spectrum has an origin transition at 18,600 ± 4 cm(-1) and a short progression in the symmetric stretch with a frequency of ∼445 cm(-1), in good agreement with a predicted vertical transition energy of 2.34 eV for excitation to the 1(3)A(1)" state, which has a calculated symmetric stretching frequency of 480 cm(-1). In addition, a ∼505 cm(-1) ground state vibrational frequency determined from sequence bands and dispersed fluorescence is in agreement with an earlier zero-electron kinetic energy study of the lowest D(3h) state and with theory. A weaker, overlapping band system with a ∼360 cm(-1) progression, observed in the same mass channel (m/z = 84) by R2C2PI but under different discharge conditions, is thought to be due to transitions from the (more complicated) singlet C(2v) ground state ((1)A(1)) state of Si(3). Evidence of emission to this latter state in the triplet dispersed fluorescence spectra suggests extensive mixing in the excited triplet and singlet manifolds. Prospects for further spectroscopic characterization of the singlet system and direct measurement of the energy separation between the lowest singlet and triplet states are discussed.

Anomalous fluorescence from the second excited singlet state upon excitation to the S4 state of pyrene in a supersonic jet

Abstract The electronic spectra of pyrene-h 10 and pyrene-d 10 were measured in supersonic jets to investigate dynamics in the higher electronic states. The dispersed fluorescence of pyrene-h 10 obtained by excitation into the S 2 state shows structured bands in the 380 nm region. The 380 nm fluorescence is assigned to the S 1 fluorescence, which indicates that fast internal conversion occurs even jet-condition. By excitation into the S 4 state, an anomalous S 2 fluorescence is observed in addition to the S 1 fluorescence. The intensity ratio between the S 2 fluorescence and the S 1 fluorescence (S 2 /S 1 ) was calculated. The S 2 /S 1 ratios under jet-cooled conditions is 0.2 which is larger than that in static gas reported by Baba [H. Baba, A. Nakajima, M. Aoi, K. Chihara, J. Chem. Phys. 55 (1971) 2433]. On the other hand, the S 2 /S 1 is 0.1 for pyrene-d 10 , which indicates that the anomalous S 2 fluorescence of pyrene-d 10 is weaker than that of pyrene-h 10 . This result supports that the anomalous S 2 fluorescence occurs through a mechanism involving reverse internal conversion from the S 1 to the S 2 state.

Spectroscopic investigations of ThF and ThF+

The electronic spectra of ThF and ThF(+) have been examined using laser induced fluorescence and resonant two-photon ionization techniques. The results from high-level ab initio calculations have been used to guide the assignment of these data. Spectra for ThF show that the molecule has an X (2)Δ(3/2) ground state. The upper spin-orbit component, X (2)Δ(5/2) was found at an energy of 2575(15) cm(-1). The low-lying states of ThF(+) were probed using dispersed fluorescence and pulsed field ionization-zero kinetic energy (PFI-ZEKE) photoelectron spectroscopy. Vibronic progressions belonging to four electronic states were identified. The lowest energy states were clearly (1)Σ(+) and (3)Δ(1). Although the energy ordering could not be rigorously determined, the evidence favors assignment of (1)Σ(+) as the ground state. The (3)Δ(1) state, of interest for investigation of the electron electric dipole moment, is just 315.0(5) cm(-1) above the ground state. The PFI-ZEKE measurements for ThF yielded an ionization energy of 51 581(3) cm(-1). Molecular constants show that the vibrational constant increases and the bond length shortens on ionization. This is consistent with removal of a non-bonding Th-centered 6d or 7s electron. Laser excitation of ThF(+) was used to probe electronically excited states in the range of 19,000-21,500 cm(-1).

Intramolecular charge transfer reaction in 3-amino-4-methyl benzoic acid: a study from condensed phase to jet-cooled molecular beam

The present work reports the spectral signatures of intramolecular charge transfer (ICT) reaction in 3-amino-4-methyl benzoic acid (AMBA). AMBA exhibits large red-shifted emission in various solvents with the emission maxima being highly sensitive to the solvent polarity. Solvatochromic measurements of AMBA on the basis of its steady state absorption and emission properties appears to confirm the operation of photoinduced ICT reaction in it with the ICT state being characterized by a higher excited state dipole moment than that of the ground state. Laser induced fluorescence excitation spectra and dispersed fluorescence spectra of AMBA and its solvated clusters with polar solvent methanol in the cooled jet yield quite distinct evidence for the occurrence of ICT reaction in polar solvent.