Vibronic Levels Research Articles

Spectroscopic observation of higher vibrational levels of C2 through visible band systems

Higher vibrational levels of the C2 molecule than those observed so far were investigated for the X(1)Σ(g)(+), A(1)Π(u), a(3)Π(u), c(3)Σ(u)(+), and d(3)Π(g) states through the Phillips, Swan, and d(3)Π(g)-c(3)Σ(u)(+) band systems under a jet-cooled condition. The term values and the molecular constants for 21 new vibronic levels were determined from rotationally resolved excitation spectra. The determined term values and rotational constants were compared to those derived from high-level ab initio potential curves. Perturbations identified in low J levels of the d(3)Π(g) (v = 8) state are most likely to be caused by the 1(5)Π(g) (v = 3) state.

Jet-Cooled Fluorescence Spectroscopy of a Natural Product: Anethole

The jet-cooled fluorescence spectroscopy of the natural product molecule anethole ((E)-1-methoxy-4-(1-propenyl)benzene) has been studied. Single vibronic level fluorescence spectroscopy was used to verify the existence of two rotamers, syn and anti, with electronic origins at 32,889 and 32,958 cm(-1), respectively. The excitation and emission spectra show characteristics similar to those of styrene and styrene derivatives, including Cs symmetry and low amplitude motions of the propenyl (vinyl) group. As in styrene, the low amplitude modes show substantial Duschinsky mixing. Interestingly, the methoxy group shows very little activity in the spectroscopy of anethole but is found to influence the activity of the propenyl group. This activity is easily observed in the Franck-Condon activity of the propenyl-bending mode. Differences are explained using simple molecular orbital arguments. The observed torsional structure was modeled and compared to ab initio calculations, allowing us to determine a barrier to internal rotation of 623 cm(-1) for the propenyl group, in agreement with similar molecules. Calculated potential energy surfaces (using density functional theory) were used to construct a more complete representation of the torsion surface of anethole, incorporating the torsions of both substituents. Several anomalous features of the excitation spectra were assigned to van der Waals clusters of anethole with water. The assignments and analyses presented here are also consistent with density function calculations.

Formation of , v = 0–3) metastable species in decaying nitrogen streamer

Evolution of , v = 0−3) metastable species produced by triggered single filamentary streamer discharge was investigated utilizing emission spectrometry and laser-induced fluorescence techniques. Triggered streamers were produced repeatedly in point-to-plane DBD electrode geometry in pure nitrogen at 200 Torr and metastable species were monitored in the centre of the discharge gap. Populations of all four monitored vibronic levels increase during streamer channel decay phase, reaching local maximum in a microsecond timescale and decreasing afterwards. Maximum population of an individual , v) vibronic level occurs with a certain delay after streamer onset τd(v), which decreases with increasing vibrational number. The longest delay τd = 5 ± 0.5 µs was fixed for the lowest metastable level v = 0, whereas τd values of 4.4 ± 0.5, 1.75 ± 0.25 and 0.85 ± 0.25 µs were fixed for v = 1, 2 and 3, respectively. Coupled evolution of the two lowest , v = 0, 1) levels was monitored independently by applying an indirect method based on emission intensity of the N2 Herman infrared system. Very good agreement between emission and fluorescence based diagnostics was achieved for later post-discharge times t > τd. A simple 0-D model of , v) post-discharge kinetics based on vibrational relaxation and electronic quenching of individual vibronic levels reproduces experimental observations fairly well. According to the model, observed maxima likely occur due to the collisional cascade that transfers metastable species from higher even/odd vibrational levels towards v = 0/v = 1 terminal levels through the Δv = 2 vibrational relaxation mechanism.

An experimental and theoretical study of the electronic spectrum of HPS, a second row HNO analog

The à (1)A" - X (1)A' electronic spectra of jet-cooled HPS and DPS have been observed for the first time, using a pulsed discharge jet source. Laser induced fluorescence spectra were obtained in the 850-650 nm region. Although the 0(0)(0) band was not observed, strong 3(0)(n) and 2(0)(1)3(0)(n) progressions and 3(1) hot bands could be assigned in the HPS LIF spectrum. Single vibronic level emission spectra were also recorded, resulting in the determination of all three HPS ground state vibrational frequencies. High level ab initio calculations were used to help confirm the vibronic assignments by calculation of transition energies, anharmonic vibrational frequencies, and anharmonic Franck-Condon factors. Ab initio potential energy surfaces gave an equilibrium structure for the X (1)A' state of r"(PH) = 1.4334 Å, r"(PS) = 1.9373 Å, θ" = 101.77° and for the à (1)A" state of r'(PH) = 1.4290 Å, r'(PS) = 2.0635 Å, and θ' = 91.74°. The rotational contours observed are consistent with these structures, confirming that the bond angle of HPS decreases on electronic excitation. Although the bond angles of HNO and HNS open in the excited state, in accord with the Walsh predictions for 12 valence electron HAB molecules, HPO, HAsO and now HPS all show the opposite behavior.

Theoretical Investigations on CaO Ions: Vibronic States and Photoelectron Spectroscopy

The low-lying electronic states, X(2)Π and A(2)Σ(+) of CaO(+) and X(2)Σ(+) and A(2)Π of CaO(-), have been determined at the MRCI+Q level of theory with the aug-cc-pV5Z(O) and cc-pCV5Z(Ca) basis sets. The two states of CaO(+) are close within <0.1 eV and coupled via spin-orbit effect. The X(2)Σ(+) and A(2)Π states of CaO(-) are energetically separated by <1 eV such that the first excited state is close to the electronic ground state of neutral CaO and unstable with respect to electron detachment. Using the potential energy curves and the spin-orbit coupling terms, the vibronic energy levels of these ions have been determined. The ionization energy and the electron affinity of CaO are calculated at 6.79 and 0.79 eV, respectively. The photoelectron spectra of CaO(-) and CaO have also been simulated.

Fluorescence Lifetimes of the Ã1Πu State of C3

The fluorescence lifetimes of 115 vibrational levels of the Ã1Π(u) state of C3 have been measured under supersonic molecular beam conditions. Of these, ninety-one are Π(u) vibronic levels, for which the lifetimes lie in the range 190-700 ns. The lifetimes of those Π(u) levels where only the bending vibration is excited lie in the range 190-235 ns. There is very little variation with bending quantum number, and the lifetimes of the two orbital components of the 1Π(u) state are essentially the same. When ν1 and ν3 are excited, the lifetimes become longer and/or reach a maximum for levels with v1 + v3 ~ 4. Excitation of the bending vibration in addition to the stretching vibrations shortens the lifetime slightly. Several of the levels show double-exponential decays. Another 23 levels, of Σ(u)+ vibronic symmetry, mostly have lifetimes that are longer than 300 ns. Interaction with nearby "dark" electronic states, such as B1Σ(u)-, B'1Δ(u), C1Π(g), and b3Π(g), is proposed to account for the observed lifetime lengthening. A particularly clear instance of such an interaction is the long lifetime (914 ns) of a perturbing Σ(u)+ level at 30,181 cm(-1), which is confirmed as belonging to the perturbing B'1Δ(u) state. A single level of Δ(u) symmetry at 29,170 cm(-1), which perturbs one of the Π(u) levels, is shown to belong to the à state.

S1 and S2 States of Linear and Zigzag Cata-Condensed Hydrocarbons

We investigated the S1 and S2 states of linear and zigzag cata-condensed hydrocarbons on the basis of the results of jet spectroscopy and theoretical calculations. The S1 states of anthracene and tetracene are represented by the HOMO → LUMO configuration (Φ(A)), whereas those of phenanthrene and chrysene are represented by HOMO-1 → LUMO and HOMO → LUMO+1 configurations (Φ(B)). We found that the fluorescence lifetime varied with different vibronic levels in the S1 states of linear cata-condensed hydrocarbons due to the mode-selective internal conversion to the S0 state. This selectivity is likely to be seen in the S1 Φ(A) state of the D(2h) molecule.

Zeeman spectroscopy of NiH: Landé factors of three Ω = 3/2 excited electronic states

We report molecular Landé factors for three Ω′=3/2 vibronic levels of NiH: E[17.8], D[17.6], and I[17.2], lying 17000–18000cm−1 above the ground electronic state. The molecular Landé factors of these three states exhibit unusual variations with J and with parity. Also, molecular Landé factors of the D[17.6] excited electronic state are unexpectedly sensitive to Ni isotope substitution at low J. These observations provide evidence for extensive mixing among electronic states, deviation from Hund’s case (a) coupling, and the existence of a local perturbing state. We also report polarization-dependent discrepancies between experimental and theoretical spectral intensities [1] for transitions involving the I[17.2] excited electronic state.

Jahn–Teller Effect in VCl4

A theoretical study is made of the Jahn-Teller and other properties of vanadium tetrachloride. Relativistic effective core potentials and corresponding valence spin-orbit operators are used with Gaussian atomic orbitals to compute self-consistent-field and spin-orbit configuration-interaction wave functions. Energy-surface parameters, electronic excitation energies, vibronic energy levels, and g factors are computed. Electron correlation is shown to have a substantial effect on the Jahn-Teller properties. As have others, we find the Jahn-Teller effect in VCl4 to be of the dynamic form.

Publisher's Note: “Tunneling splittings in vibronic energy levels of CH3F+ (${\rm \tilde X}^2 {\rm E}$X̃2E) studied by high resolution photoelectron spectroscopy and ab initio calculation” [J. Chem. Phys. 139, 064302 (2013)

Publisher's Note: “Tunneling splittings in vibronic energy levels of CH3F+ (${\rm \tilde X}^2 {\rm E}$X̃2E) studied by high resolution photoelectron spectroscopy and <i>ab initio</i> calculation” [J. Chem. Phys. 139, 064302 (2013)

Rotationally resolved ${\tilde{B}} \leftarrow {\tilde{X}}$B̃←X̃ electronic spectra of the isopropoxy radical: A comparative study

The B̃-X̃ laser-induced-fluorescence spectrum of jet-cooled isopropoxy radical (i-C3H7O[middle dot]) has been recorded. Using an isolated state model the observed rotational and fine structure of the origin band has been well simulated to determine rotational constants for both the X̃ and B̃ states and the electron spin-rotation constants of the X̃ state. The line intensities are well simulated with a parallel transition type, requiring the same symmetry for the levels involved of each the X̃ and B̃ state, which confirms the previous suggestion that going from ethoxy (C2H5O[middle dot]) to isopropoxy, the energy ordering of the electron configurations with in- and out-of-plane half-filled p-orbitals of the oxygen atom is reversed and the ground vibronic symmetry changes from a" to a'. However, the observed spin-rotation coupling constants are not consistent with their predication from either semi-empirical theory or quantum chemical calculations. Additionally, the lack of observed transitions involving the out-of-plane transition moment component is not consistent with high level electronic structure calculations suggesting mixing of vibronic levels by strong spin-orbit coupling. A new twofold model has been developed that explicitly includes Coriolis and spin-orbit coupling between different vibronic levels. This model renders the discrepancy between theoretical and experimental spin-rotation constants moot. Moreover, it determines independently the contributions to the observed splitting between the lowest two levels, resulting from non-relativistic kinetic and Coulombic effects, and that due to the relativistic spin-orbit interaction. The experimental values show that these effects are comparable, but that the vibronic one is slightly more important. This result is at variance with state-of-the-art electronic structure calculations which otherwise do a remarkably good job of describing the ground state of isopropoxy.

Spectroscopic study on the [formula omitted] transition of the trans-2-methylvinoxy radical

Electronic excitation spectra of the B̃2A″-X̃2A″ transition of the trans-2-methylvinoxy radical were observed in the region of 29000–32000cm−1 by laser-induced fluorescence and fluorescence depletion spectroscopy. Fourteen vibronic bands observed in the depletion spectrum were unambiguously identified to be originating from the radical. Dispersed fluorescence spectra from three different vibronic levels were also obtained, and a total of 69 emission bands were assigned, yielding the ground state vibrational constants.

Spectral and thermodynamic properties for the exciplexes of N-alkyl carbazoles with dicyanobenzenes in THF

The exciplex emission spectra of N-ethylcarbazole with 1,2-dicyanobenzene (NEC/1,2-DCB), N-methylcarbazole with 1,2-dicyanobenzene (NMC/1,2-DCB), 1,3-dicyanobenzene (NMC/1,3-DCB), and 1,4-dicyanobenzene (NMC/1,4-DCB) are studied in tetrahydrofuran (THF) for the temperature range starting from 253K to 334K. Thermochromic shifts along with the spectral properties including change in peak intensities and the ratio of exciplex peak intensity to fluorophore peak intensity are studied. Effect of temperature on the energy of zero–zero transitions hνo′, Huang–Rhys factor (S), Gauss broadening of vibronic level (σ) and the dominant high-frequency vibration (hνν) are also part of investigation. Enthalpy of exciplex formation (ΔHEX∗) calculated by the model proposed by A. Weller and the Gibb’s energy of electron transfer (ΔGet∗) for all exciplex systems are also discussed in the present paper. All the exciplexes under study were observed to be dipolar in nature. The exciplex of the N-methylcarbazole/1,4-dicyanobenzene was found to be the most stable and the N-methylcarbazole/1,3-dicyanobenzene was the weakest exciplex system.

Tunneling splittings in vibronic energy levels of CH 3F+X̃2E studied by high resolution photoelectron spectroscopy and ab initio calculation

The energy levels of CH3F(+) (X(2)E), which show strong vibronic coupling effect (Jahn-Teller effect), have been measured up to 3500 cm(-1) above the ground vibrational state using one-photon zero-kinetic energy photoelectron spectroscopic method. Theoretical calculations have also been performed to calculate the spin-vibronic energy levels using a diabatic model and ab initio adiabatic potential energy surfaces (APESs) including the energy gradients and derivative couplings between the APESs. The calculations showed that the tunneling splittings of the vibrational energy levels occur due to the deep potential energy wells formed by the Jahn-Teller deformation. The calculated spin-vibronic energy levels are in good agreement with the experimental data. For example, the energy splitting for the first excited vibrational energy level is calculated as 111 cm(-1) that is confirmed by the experimental value. The experimental spectrum was assigned based on the fundamental vibrational modes calculated at the energy minimum. The fundamental vibrational modes related to the H-C-F bending, H-C-H bending, C-F stretching, and C-H stretching vibrations have been observed.

Spectroscopy of Lithium Atoms and Molecules on Helium Nanodroplets

We report on the spectroscopic investigation of lithium atoms and lithium dimers in their triplet manifold on the surface of helium nanodroplets (HeN). We present the excitation spectrum of the 3p ← 2s and 3d ← 2s two-photon transitions for single Li atoms on HeN. The atoms are excited from the 2S(Σ) ground state into Δ, Π, and Σ pseudodiatomic molecular substates. Excitation spectra are recorded by resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) mass spectroscopy, which allows an investigation of the exciplex (Li*–Hem, m = 1–3) formation process in the Li–HeN system. Electronic states are shifted and broadened with respect to free atom states, which is explained within the pseudodiatomic model. The assignment is assisted by theoretical calculations, which are based on the Orsay–Trento density functional where the interaction between the helium droplet and the lithium atom is introduced by a pairwise additive approach. When a droplet is doped with more than one alkali atom, the fragility of the alkali–HeN systems leads preferably to the formation of high-spin molecules on the droplets. We use this property of helium nanodroplets for the preparation of Li dimers in their triplet ground state (13Σu+). The excitation spectrum of the 23Πg(ν′ = 0–11) ← 13Σu+(ν″ = 0) transition is presented. The interaction between the molecule and the droplet manifests in a broadening of the transitions with a characteristic asymmetric form. The broadening extends to the blue side of each vibronic level, which is caused by the simultaneous excitation of the molecule and vibrations of the droplet (phonons). The two isotopes of Li form 6Li2 and 7Li2 as well as isotope mixed 6Li7Li molecules on the droplet surface. By using REMPI-TOF mass spectroscopy, isotope-dependent effects could be studied.

Photodissociation Spectra of OCS+ via B2Σ+↙X2Π Transitions

In the wavelength range of 231–275 nm, we have studied the mass-resolved dissociation spectra of OCS+ via B2Σ+↙X2Π3/2(000) and B2Σ+↙X2Π1/2(000, 001) transitions by preparing OCS+ ions in the well-defined spin-orbit states. The spectroscopic constants of ?1(CS stretch)=828.9 (810.4) cm−1, ?2 (bend)=491.3 cm−1 and ?3(CO stretch)=1887.2 cm−1 for OCS+(B2Σ+) are deduced. The observed dependence of the ?2(bend) mode excitation of B2Σ+ on the spin-orbit splitting of X2Π(Ω=1/2, 3/2) in the B2Σ+↙X2Π transition can be attributed to the K coupling between the (000)2Π1/2 and (010)2Σ1/2+ vibronic levels of X2Π state, which makes the B2Σ+(010)↙X2Π1/2(000) transition possible.

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.

Solvent Effects on Vibronic Coupling in a Flexible Bichromophore: Electronic Localization and Energy Transfer induced by a Single Water Molecule.

Size and conformation-specific ultraviolet and infrared spectra are used to probe the effects of binding a single water molecule on the close-lying excited states present in a model flexible bichromophore, 1,2-diphenoxyethane (DPOE). The water molecule binds to DPOE asymmetrically, thereby localizing the two electronically excited states on one or the other ring, producing a S1/S2 splitting of 190 cm(-1). Electronic localization is reflected clearly in the OH stretch transitions in the excited states. Since the S2 origin is imbedded in vibronic levels of the S1 manifold, its OH stretch spectrum reflects the vibronic coupling between these levels, producing four OH stretch transitions that are a sum of contributions from S2-localized and S1-localized excited states. The single solvent water molecule thus plays multiple roles, localizing the electronic excitation in the bichromophore, inducing electronic energy transfer between the two rings, and reporting on the state mixing via its OH stretch absorptions.

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.

Remarks on phase choices for the electronic wavefunctions when carrying out vibronic or rovibronic energy level and intensity calculations for molecules exhibiting a Jahn–Teller effect

In this pedagogical paper, material already in the Jahn–Teller literature is recast in a way that emphasizes how vibronic wavefunctions for the XY3 Jahn–Teller problem can conveniently be calculated using electronic and vibrational basis set functions that do not transform into their negatives upon going once around the conical intersection and do transform as irreducible representations of the ordinary C3v point group. We give as an explicit example, a recalculation of the pseudorotational energy levels and wavefunctions near the bottom of a deep Jahn–Teller moat that does not make use of half-integral quantum numbers. A brief discussion, also recast somewhat from material in the literature, is given of how the basis functions used here are related to the more frequently discussed electronic and vibrational basis functions for the Jahn–Teller problem, which transform into their negatives upon going once around the conical intersection.