Vibronic Band System Research Articles

Severely perturbed vibrational structure in the 266–310 nm electronic transition of C3

The laser induced fluorescence (LIF) spectra of C3 are measured under jet-cooled conditions in the wide UV region of 266–310 nm, and 173 vibronic bands are identified. Among them, 77 and 68 vibronic bands were found to exhibit Σ–Σ type and Π–Σ type rotational structures, respectively. From the rotational analysis, the band-origin wave numbers and the rotational constants of the upper Σ and Π vibronic levels and K-type doubling parameters for the Π vibronic levels are determined. The rotational constants for the Σ and Π vibronic levels exhibit, respectively, similar distributions with almost the same mean values, B̄′=0.395(14) and 0.398(17) cm−1, indicating that the Σ and Π vibronic levels have the same electronic origin and that the average C–C bond distance of the upper electronic state in the observed energy range is about 1.331(25) Å, which is longer than the electronic ground X̃ 1Σg+ state by 0.054(25) Å. The convolution and Fourier-transform analyses are performed separately for the spectra composed only of Σ–Σ type and Π–Σ type vibronic bands to derive estimates of the vibrational fundamentals in the upper electronic state. The low ν1 fundamental of 940(60) cm−1 derived from these analyses and the seemingly regular ν1 progression in the convoluted spectra indicate that the bright character is carried primarily by the transitions to the vibrational levels in the Δu1 electronically excited state and is distributed into a large number of the originally dark transitions to the vibrational levels in the Πg1 electronically excited state, resulting in the exceedingly complex vibronic band system. © 2000 American Institute of Physics.

Electronic Absorption Spectra of Neutral Pentacene (C22H14) and Its Positive and Negative Ions in Ne, Ar, and Kr Matrices

The absorption spectra of neutral pentacene (C22H14) and its radical cation (C22H14+) and anion (C22H14-) isolated in solid Ne, Ar, and Kr have been measured from the ultraviolet to the near-infrared. The associated vibronic band systems and their spectroscopic assignments are discussed together with the physical and chemical conditions governing molecular anion production in matrices doped with alkali metal (Na or K). TD-DFT calculations were carried out to assist in the assignments for the measured spectral features of the pentacene ions.

Measurement of relaxation times of NH in atmospheric pressure flames using picosecond pump-probe degenerate four-wave mixing

Pump-probe degenerate four-wave mixing (DFWM) experiments with picosecond pulses were performed to study the collision induced temporal decay of the transient signal intensity generated in the A 3Π− X 3∑ 0,0 vibronic band system of NH radicals at 336 nm in atmospheric pressure flames. NH radicals were produced in the reaction zone of a premixed ammonia/oxygen/nitrogen flame stabilized on a slot burner. NH radicals were excited in various rotational transitions in the R 1- and Q 1-branches using radiation from a tunable frequency doubled distributed feedback dye laser with pulse widths of 34 ps and an average linewidth of 2 cm −1. In the R-branch the decay rates slightly decrease with increasing rotational quantum number, and are larger for orthogonal polarization of the pump beam electric fields compared to the case where all fields are parallel polarized. Within the signal-to-noise ratio the temporal decay of the DFWM signal intensity from simultaneous excitations in the strongly overlapped Q-branch is monoexponential with a time constant of (350 ± 40) ps for all electric fields polarized parallel to each other.

The near-infrared bands of NO2 observed by high-resolution Fourier-transform spectroscopy

Fourier-transform absorption spectra of NO2 at 298 K were recorded between 9000–15 000 cm−1 at Doppler-limited resolution (0.012 cm−1), in the region near the à 2B2–X̃ 2A1 conical intersection. The strong rovibronic bands are rather isolated and organized into polyads which can be assigned by the number of bending quanta of the à 2B2 state, corresponding to the transitions with the largest Franck–Condon factors. Around 9735 cm−1, where the origin of the à 2B2–X̃ 2A1 vibronic band system is located and up to about 11 200 cm−1, hot bands dominate the spectrum. The complexity of the spectrum increases with energy due to an increasing density of à 2B2 and X̃ 2A1 vibrational levels. The rotational structure is dense and irregular as already observed for higher bands.

Double-resonance spectroscopy of the high Rydberg states of HCO. III. Multiple pathways in the vibrational autoionization of the bending overtone

Ultraviolet first-photon absorption selects individual rotational levels in the Renner–Teller split (020) vibronic band system in the 3pπ 2Π Rydberg state of HCO. These gateway states serve as originating levels for vertical second-photon transitions to vibrationally autoionizing Rydberg series converging to individual rotational levels associated with the (0200) and (0220) states of HCO+. Linewidths of assigned series convey information on autoionization dynamics. Resonances throughout the (020) autoionization spectrum match sharp profiles seen earlier for series converging to HCO+(010). Linewidths for autoionization via relaxation both of the bending fundamental and its overtone are measurably narrower than resonances built on CO stretch, (001), which indicates that mode-selectivity inhibiting bending autoionization relative to stretch extends to the bending overtone. Features in the (020) spectrum that fall below the threshold energy for decay by autoionization to the (010) continuum appear with diminished intensity and yet narrower linewidth. This provides evidence that vibrational autoionization in HCO conforms with a Δv=1 propensity rule that favors relaxation of (020) states to the (010) continuum over the two-quantum decay to produce HCO+(000). Analysis of detailed line shapes and intensities associated with individual vibrational angular momentum components of the relaxing-core overtone point to the way in which vibrational and orbital-electronic symmetries factor in the bound molecule and cation plus free electron, and fix the order in vibrational coordinates of necessary coupling terms.

Quantum beat spectroscopy of OH radicals in atmospheric pressure flames

Using time-resolved polarization spectroscopy we have observed quantum beats of coherently excited states in the ( A 2Σ +− X 2П ) vibronic (0, 0) band system of OH in atmospheric-pressure flames. With a spectral bandwidth of 1 cm −1 of the tunable coherent light source employed, line splittings were deduced from the temporal beat structure. Depending on the rotational quantum number of the probed transition and the collisional environment the relaxation times of anisotropic orientational distributions in which the quantum beat oscillations were observed ranged between 120 ps ( J″ = 5) and 368 ps ( J″ = 10).

Electronic absorption spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. II. The phenanthrene cation (C14H10+) and its 1-methyl derivative.

The ultraviolet, visible, and near infrared absorption spectra of phenanthrene (C14H10), 1-methylphenanthrene [(CH3)C14H9], and their radical ions [C14H10+; (CH3)C14H9+], formed by vacuum-ultraviolet irradiation, were measured in neon matrices at 4.2 K. The associated vibronic band systems and their spectroscopic assignments are discussed. The oscillator strengths were calculated for the phenanthrene ion and found lower than the theoretical predictions. This study presents the first spectroscopic data for phenanthrene and its methyl derivative trapped in a neon matrix where the perturbation of the isolated species by its environment is minimum; a condition crucial to astrophysical applications.

Spectroscopic detection and characterization of the FS2 free radical

An extensive vibronic band system of the FS2 radical in the 700–485 nm region has been observed for the first time by laser-induced fluorescence (LIF) probing of the products of the reaction of F2 with various sulfur-containing compounds. Jet-cooled spectra were obtained by reacting fluorine with COS, H2S, or CS2 in the body of a continuous supersonic jet, just prior to expansion. Vibrational analysis of the jet spectra gives ν1′ = 768, ν2′ = 217, ν3′ = 495, ν1″ = 705, ν2″ = 293, ν3″ = 684, and T00=14 922 cm−1, all in good agreement with ab initio predictions for the FS2 free radical. A rotational analysis of the high-resolution spectrum of the 350 band shows that the transition is polarized out-of-plane, with spin splittings characteristic of a molecule with a single unpaired electron. The rotational constants and spin–rotation interaction constants were determined with good precision for the ground and excited states. By combining the rotational constants with our ab initio estimates of the S–F bond length, the following structural parameters were obtained: r(S–F)=1.651 Å, r(S–S)=1.865(5) Å, and θ(FSS)=109.1(1)° for the X̃ 2A″ state and r(S–F)=1.642 Å, r(S–S)=2.105(5) Å, and θ(FSS)=97.6(1)° for the à 2A′ state.

The 2pσu–1sσgelectronic spectrum of D+2

Non-adiabatic dissociation energies are calculated for the N≤ 5 vibration–rotation levels of the ground electronic state of D+2, together with the seven vibration–rotation levels of the first excited electronic state, using a transformed Hamiltonian and an artificial-channels scattering method. Coupling of rotational and electronic angular momenta is included. Relativistic and radiative corrections are made to give dissociation energies, that of the 0,0 level of the ground electronic state agreeing with experiment. In particular, calculated electronic transition energies are in excellent agreement with the available experimental values. The complete calculated 2pσu–1sσg vibronic band system of D+2 is reported.

Electronic absorption spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. I. The naphthalene cation (C1H+8)

The ultraviolet, visible, and near infrared absorption spectra of naphthalene (C10H8 ) and its radical ion (C10H+8 ), formed by vacuum ultraviolet irradiation, were measured in argon and neon matrices at 4.2 K. The associated vibronic band systems and their spectroscopic assignments are discussed together with the physical and chemical conditions governing ion production in the solid phase. The absorption coefficients were calculated for the ion and found lower than previous values, presumably due to the low polarizability of the neon matrix. This study presents the first spectroscopic data for naphthalene trapped in a neon matrix, where the perturbation of the isolated species by its environment is minimum; a condition crucial to astrophysical applications.

Vibronic absorption spectra of phosphorus molecules (P4+ and P3) in solid argon

White phosphorus vapor (P 4 ) was passed through a short argon discharge and subjected to vacuum ultraviolet radiation during condensation with excess argon. Visible absorption spectra revealed two vibronic band systems. A photosensitive progression with 436-nm origin and 550±10-cm -1 vibrational interval maps the phtoelectron spectrum of P 4 + , which confirms this assignment

Laser excitation spectra Ã2Π3/2â†�X2Π3/2of the bromoacetylene and deuterobromoacetylene cations in the gas phase

Laser excitation spectra of the A2Π3/2â†�X2Π3/2 electronic transition of rotationally cooled bromo- and deuterobromo-acetylene cations in the gas phase have been obtained. The cations were produced by Penning ionisation in a liquid-nitrogen-cooled environment. The vibronic band systems have been analysed and the vibrational frequencies of the fundamentals for these cations in their A2Π3/2 states inferred.

Absorption and laser-excited emission spectra of matrix-isolated Pb 2

Absorption and laser-induced emission spectra are reported for the diatomic lead molecule in neon, argon, and krypton matrices. In emission, vibronic band systems are observed at about 20 000, 14 500, 13 500, and 12 000 cm −1. The sharpness of the bands indicates only a small degree of coupling between the states of the molecule and the phonon states of the host matrix. The A → X band system near 20 000 cm −1 reveals the most structure and the results show practically no vibrational relaxation of the Pb 2 molecule between absorption of the laser radiation and the luminescence process. Multiple matrix site effects are observed. There is also evidence to suggest that the ground state of Pb 2 is degenerate and that an asymmetric matrix environment removes the degeneracy.

The Franck-Condon principle and the structures of excited electronic states of molecules

An approximate method is described for quantitatively applying the Franck-Condon principle to the problem of determining the structure of an excited electronic state of a simple polyatomic molecule. The method is applicable to an electronic transition for which both electronic states have structures of the same symmetry. Necessary information includes the geometrical structure and the totally symmetrical frequencies of the ground state, as well as the vibrational analysis of a vibronic band system and the resulting information about the totally symmetrical frequencies of the excited state. The relative intensities of a few selected vibronic bands must be measured. One dimensional Condon overlap integrals for these bands are expressed in terms of parameters which depend on the change of the geometrical structure with the electronic transition. A comparison of the squares of the overlap integrals with the band intensities permits the evaluation of the geometrical parameters. In an initial calculation the normal coordinates of the excited state are assumed to be parallel to those known for the ground state. The approximate excited state structure calculated on this basis is used to determine more exact normal coordinates of the excited state. A final calculation based on these coordinates yields an improved excited state structure. Actually there are always several excited state structures consistent with band intensities. It is therefore necessary to have some additional information from the band envelopes. As examples, the structures of one excited state of ClO 2 and of two excited states of SO 2 are determined.