Spectroscopic characterization of structural isomers of naphthalene: (E)- and (Z)-phenylvinylacetylene

Near-pure samples of (E)-phenylvinylacetylene ((E)-PVA) and (Z)-phenylvinylacetylene ((Z)-PVA) were synthesized, and their ultraviolet spectroscopy was studied under jet-cooled conditions. The fluorescence excitation and UV-UV holeburning (UVHB) spectra of both isomers were recorded. The S0-S1 origin of (E)-PVA occurs at 33,578 cm(-1), whereas that for (Z)-PVA occurs at 33,838 cm(-1), 260 cm(-1) above that for (E)-PVA. The present study focuses primary attention on the vibronic spectroscopy of (E)-PVA. Single vibronic level fluorescence spectra of many prominent bands in the first 1200 cm(-1) of the S0-S1 excitation spectrum of (E)-PVA were recorded, including several hot bands involving low-frequency out-of-plane vibrations. Much of the ground-state vibronic structure observed in these spectra was assigned by comparison with styrene and trans-beta-methylstyrene, assisted by calculations at the DFT B3LYP/6-311++G(d,p) level of theory. Both S0 and S1 states of (E)-PVA are shown to be planar, with intensity appearing only in even overtones of out-of-plane vibrations. Due to its longer conjugated side chain compared with that of its parent styrene, (E)-PVA supports extensive Duschinsky mixing among the four lowest-frequency out-of-plane modes (nu45-nu48), increasing the complexity of this mixing relative to that of styrene. Identification of the v' = 0-3 levels of nu48, the lowest frequency torsion, provided a means of determining the 1D torsional potential for hindered rotation about the C(ph)-C(vinyl) bond. Vibronic transitions due to (Z)-PVA were first identified as small vibronic bands that did not appear in the UVHB spectrum recorded with the hole-burn laser fixed on the S0-S1 origin of (E)-PVA. The LIF and UVHB spectra of a synthesized sample of (Z)-PVA confirmed this assignment.     

Laser induced fluorescence spectroscopy of a mixed dimer between 2-pyridone and 7-azaindole

We report here the laser induced fluorescence excitation (FE) and dispersed fluorescence (DF) spectra of a 1:1 mixed dimer between 7-azaindole (7AI) and 2-pyridone (2PY) measured in a supersonic free jet expansion of helium. Density functional theoretical calculation at the B3LYP/6-311++G** level has been performed for predictions of the dimer geometry and normal mode vibrational frequencies in the ground electronic state. A planar doubly hydrogen-bonded structure has been predicted to be the most preferred geometry of the dimer. In the FE spectrum, sharp vibronic bands are observed only for excitation of the 2PY moiety. A large number of low-frequency vibronic bands show up in both the FE and DF spectra, and those bands have been assigned to in-plane hydrogen bond vibrations of the dimer. Spectral analyses reveal Duschinsky-type mixing among those modes in the excited state. No distinct vibronic band structure in the FE spectrum was observed corresponding to excitations of the 7AI moiety, and the observation has been explained in terms of nonradiative electronic relaxation routes involving the 2PY moiety.     

Cavity ring-down spectroscopy and theoretical calculations of the S1(1B3u)<--S0(1Ag) transition of jet-cooled perylene

As part of our long-term program to test the diffuse interstellar band-polycyclic aromatic hydrocarbon hypothesis, we have investigated the S(1)<--S(0) electronic transition of neutral perylene (C(20)H(12)) in a combined experimental and theoretical study. Jet-cooled perylene was prepared with a pulsed discharge slit nozzle and detected by cavity ring-down spectroscopy. A number of vibronic features were observed in the 24 000-24 900 cm(-1) spectral range. Density functional and ab initio calculations were performed to determine the geometries, harmonic vibrational frequencies, and normal coordinates of both the S(0) and S(1) electronic states. A rotational temperature of 52+/-5 K was derived from a rotational contour analysis of the vibronic band associated with the 0-0 transition. A Franck-Condon treatment was carried out to calculate the vibronic spectrum of the S(1)<--S(0) transition. A good agreement was found between the calculated and the experimental spectra. A vibrational assignment is proposed and six normal modes are identified. The contribution of neutral compact polycyclic aromatic hydrocarbons to the diffuse interstellar bands is briefly discussed.     

Hydrogen-bond vibrations in the S(1)<-->S(0) spectra of a nucleobase pair analog: a mixed dimer between 2-pyridone and formamide

The vibronically resolved electronic spectra for S(1)<-->S(0) transitions of a mixed dimer between 2-pyridone (2PY) and formamide have been measured in a supersonic free jet expansion using laser-induced fluorescence spectroscopy. Quantum chemistry method at different levels of theory has been used to optimize the geometries of the dimer for the S(0) and S(1) electronic states and also to calculate the normal vibrational modes. Assignments for the vibronic bands observed in the dispersed fluorescence spectrum of the 0(0) (0) band have been suggested with the aid of the ground state frequencies calculated by density functional theoretical method. Spectral analysis reveals that electronic excitation causes extensive mixing of the low-frequency intermolecular vibrational modes of the dimer with some of the intramolecular modes of the 2PY moiety. This spectral behavior is consistent with the complete active space self-consistent field theoretical prediction that with respect to a number of geometrical parameters the dimer geometry in S(1) is significantly distorted from the geometry of the S(0) state.     

Laser-induced fluorescence spectroscopy of NC3S

In a discharged supersonic jet of acetonitrile and carbon disulfide, we have for the first time observed an electronic transition of the NC(3)S radical using laser-induced fluorescence (LIF) spectroscopy. A progression originating from the C-S stretching mode of the upper electronic state appears in the excitation spectrum. Each band of the progression has a polyad structure due to anharmonic resonances with even overtones of bending modes. Rotationally resolved spectra have been observed by high-resolution laser scans, and the electronic transition is assigned to A 2Pii-X 2Pii. For the vibronic origin band, the position and the effective rotational constant of the upper level have been determined to be 21 553.874(1) and 0.046 689(4) cm(-1), respectively. The dispersed fluorescence spectrum from the zero vibrational level of A 2Pi3/2 has also been observed; its vibrational structure is similar to that of the LIF excitation spectrum, showing a prominent C-S stretching progression with polyad structures. The vibrational frequencies of the C-S stretching mode in the ground and excited electronic states are determined to be 550 and 520 cm(-1), respectively. Fluorescence decay profiles have been measured for several vibronic levels of the A state.     

Experimental and theoretical investigation of the dispersed fluorescence spectroscopy of HC4S

A high-resolution single vibronic level emission study from the A (2)Pi(32) state of the HC(4)S radical is reported. Ground state density functional theory frequencies have been used to assign ground state vibronic levels involving three stretching modes nu(2), nu(3), and nu(5) in the region of 0-3250 cm(-1), while the frequency of nu(4) remains speculative. Tentative assignments are given for the complicated structures arising from Renner-Teller and spin-orbit interactions within the bending energy levels. From analysis of the dispersed emission spectra, Fermi resonances involving pairs of bands have been identified in the A (2)Pi(32)<--X (2)Pi(32) laser induced fluorescence spectrum.     

Potential-energy surface, van der Waals energy spectrum, and vibronic transitions in s-tetrazine-argon complex

The van der Waals vibrational states and the structure of the vibronic spectrum of s-tetrazine-argon complex have been studied by the ab initio methods. The potential-energy surface of the ground S(0) electronic state of the complex has been constructed by fitting the analytical many-body expansion to a large set of the interaction energy values computed using the second-order M?ller-Plesset perturbation theory combined with the standard aug-cc-pVDZ basis set. The equilibrium structure of the complex found is that with argon located above the tetrazine ring at a distance of 3.394 A. The calculated dissociation energy of 354 cm(-1) is compatible with the experiment. The van der Waals energy spectrum calculated from the potential-energy surface is explained analyzing a correlation with a simpler energy spectrum of benzene-argon. A new assignment of the S(0)-S(1) vibronic spectrum is proposed on the basis of the rigorous selection rules, vibrational energy levels in S(0) and S(1) states and vibronic transition intensities calculated from the electronic transition dipole moment surfaces.     

S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene in supersonic jets and rare gas matrices

The study of the S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene (BghiP, C22H12) in supersonic jets and solid rare gas matrices is reported. In the jet-cooled spectrum, the origin band position is located at 25,027.1+/-0.2 cm-1, the assignment being supported by the analysis of vibrational shifts and rotational band contours. Except for the origin band, which is weak, all bands are attributed to the fundamental excitation of nontotally symmetric b1 vibrational modes of S1. The intensity pattern is interpreted as a consequence of the weak oscillator strength of the electronic transition combined with intensity-borrowing through vibronic interaction between the S1(1A1) and S2(1B1) states. The spectra of the S1(1A1)<--S0(1A1) and S2(1B1)<--S0(1A1) transitions have also been measured for BghiP in solid neon and argon matrices. The comparison of the redshifts determined for either transition reveals that the polarizability of BghiP is larger in its S2 than in its S1 state. Bandwidths of 2.7 cm-1 measured in supersonic jets, which provide conditions relevant for astrophysics, are similar to those of most diffuse interstellar bands. The electronic transitions of BghiP are found to lie outside the ranges covered by present databases. From the comparison between experimental spectra and theoretical computations, it is concluded that the accuracy of empirical and ab initio approaches in predicting electronic energies is still not sufficient to identify astrophysically interesting candidates for spectroscopic laboratory studies.     

Large amplitude out-of-plane vibrations of 1,3-benzodioxole in the S0 and S1 states: an analysis of fluorescence and excitation spectra by ab initio calculations

We report the analytical expressions of the two-dimensional potential energy surfaces (PES) spanned by the puckering and flapping vibrations in the S0 and S1 states of 1,3-benzodioxole (BDO). Both PES are obtained from S0 and S1 energies computed on a grid of 2500 molecular geometries at the CASPT2 level. Both the S0 and S1 PES are anharmonic, and the planar geometry corresponds to a barrier that separates two minima at nonplanar geometries along the puckering/flapping deformations. Eigenvalues and eigenvectors of the mixed puckering/flapping modes are calculated by the Meyer flexible model. Improved vibronic levels, in better agreement with the observed spectra, are obtained by suitably optimized CASPT2 surfaces. To assign the lower-energy (0-500 cm(-1)) portion of emission and absorption spectra, we evaluate the band intensities by estimating the Franck-Condon factors between the puckering/flapping eigenvectors of the S0 and S1 states. From these calculations, we obtain a satisfactory assignment of the ground state IR spectra and of the fluorescence excitation spectrum. Both assignments are supported by the analysis of the vibrational structures of several single vibronic level (SVL) fluorescence spectra. The successful interpretation of these spectra shows that the S0 and S1 PES that we derive for BDO are substantially correct. The barrier heights in the two states are similar: 125.7 and 190.4 cm(-1) in S0 and in S1, respectively. In S0, the barrier is associated essentially with the puckering motion. In S1, it involves to a considerable extent also the flapping coordinate, whose vibrational frequency is much lower in S1 than in S0. This fact introduces a substantial Duschinsky effect in the S0-S1 transitions of BDO.     

Fluorescence and ultraviolet absorption spectra, and the structure and vibrations of 1,2,3,4-tetrahydronaphthalene in its S1(pi,pi*) state

The ultraviolet absorption spectra in the static vapor phase and the laser induced fluorescence spectra (both fluorescence excitation and single vibronic level fluorescence spectra) of jet-cooled 1,2,3,4-tetrahydronaphthalene have been used along with theoretical calculations to assign many of the vibronic levels in the S1(pi,pi*) state. These have been compared to the corresponding vibrational levels for the S0 ground state. Analysis of the upper states of the ring-twisting vibration nu(31) and three other low-frequency modes has allowed us to construct an energy map of the lowest vibrational quantum states for both S0 and S1. The molecule is highly twisted in both electronic states with high barriers to planarity, which are calculated to be 4811 cm(-1) for S0 and 5100 cm(-1) for S1. However, the experimental data show that the barrier should be lower in the S1 state.     

Density functional theory study of vibronic structure of the first absorption Qx band in free-base porphin

Harmonic vibrational frequencies and vibronic intensities in the first S(0)-->S(1) (pipi( *)) absorption band of free-base porphin (H(2) P) are investigated by hybrid density functional theory (DFT) with the standard B3LYP functional. The S(0)-S(1) transition probability is calculated using time-dependent DFT with account of Franck-Condon (FC) and Herzberg-Teller (HT) contributions to the electric-dipole transition moments including displacements along all 108 vibrational modes. Two weak wide bands observed in the gas phase absorption spectra of the H(2) P molecule at 626 and 576 nm are interpreted as the 0-0 band of the X(1) A(g)-->1B(3u) transition and the 0-1 band with largest contributions from the nu(10)(a(g))=1610 cm(-1) and nu(19)(b(1g))=1600 cm(-1) modes, respectively, in agreement with previous tentative assignments. Both bands are induced by the HT mechanism, while the FC contributions are negligible. A number of fine structure bands, including combination of two vibrational quanta, are obtained and compared with available spectra from supersonic jet and Shpolskij matrices. Both absorption and fluorescence spectra are interpreted on ground of the linear coupling model and a good fulfillment of the mirror-symmetry rule.     

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.     

Laser-induced fluorescence and pure rotational spectroscopy of the CH2CHS (vinylthio) radical

Laser-induced fluorescence (LIF) excitation spectra of the B-X (2)A(") electronic transition of the CH(2)CHS radical, which is the sulfur analog of the vinoxy (CH(2)CHO) radical, were observed under room temperature and jet-cooled conditions. The LIF excitation spectra show very poor vibronic structures, since the fluorescence quantum yields of the upper vibronic levels are too small to detect fluorescence, except for the vibrationless level in the B state. A dispersed fluorescence spectrum of jet-cooled CH(2)CHS from the vibrationless level of the B state was also observed, and vibrational frequencies in the X state were determined. Precise rotational and spin-rotation constants in the ground vibronic level of the radical were determined from pure rotational spectroscopy using a Fourier-transform microwave (FTMW) spectrometer and a FTMW-millimeter wave double-resonance technique [Y. Sumiyoshi et al., J. Chem. Phys. 123, 054324 (2005)]. The rotationally resolved LIF excitation spectrum for the vibronic origin band of the jet-cooled CH(2)CHS radical was analyzed using the ground state molecular constants determined from pure rotational spectroscopy. Determined molecular constants for the upper and lower electronic states agree well with results of ab initio calculations.     

Potential-energy surface, dynamics of van der Waals motions, and vibronic transitions in p-difluorobenzene-argon complex

The dynamics of van der Waals vibrational motions and vibronic spectrum of the complex of argon with p-difluorobenzene (ArDFB) are investigated using the ab initio method. The electronic ground-state potential-energy surface of the complex is calculated at the second-order M?ller-Plesset level of theory using a well-balanced basis set aug-cc-pVDZ and its reduced version without tight polarization functions. The dissociation energy of 351 cm(-1) and the binding energy of 402 cm(-1) determined at the Ar distance of 3.521 Angstroms from the DFB ring well agree with the experimental data available. The character of calculated vibrational levels is analyzed and the effect of a strong coupling between the stretching and bending motions is investigated. A new class of hybrid states created by this coupling is found. To investigate the vibronic S(1)-S(0) spectrum, the surfaces of the electronic transition dipole moment are calculated using the ab initio method. From these surfaces, the vibronic transition intensities are determined and employed to assign the Franck-Condon- and Herzberg-Teller-induced transitions.     

Two-dimensional laser induced fluorescence spectroscopy of van der Waals complexes: fluorobenzene-Ar(n) (n = 1,2)

The technique of two-dimensional laser induced fluorescence (2D-LIF) spectroscopy has been used to observe the van der Waals complexes fluorobenzene-Ar and fluorobenzene-Ar(2) in the region of their S(1)-S(0) electronic origins. The 2D-LIF spectral images reveal a number of features assigned to the van der Waals vibrations in S(0) and S(1). An advantage of 2D-LIF spectroscopy is that the LIF spectrum associated with a particular species may be extracted from an image. This is illustrated for fluorobenzene-Ar. The S(1) van der Waals modes observed in this spectrum are consistent with previous observations using mass resolved resonance enhanced multiphoton ionisation techniques. For S(0), the two bending modes previously observed using a Raman technique were observed along with three new levels. These agree exceptionally well with ab initio calculations. The Fermi resonance between the stretch and bend overtone has been analysed in both the S(0) and S(1) states, revealing that the coupling is stronger in S(0) than in S(1). For fluorobenzene-Ar(2) the 2D-LIF spectral image reveals the S(0) symmetric stretch van der Waals vibration to be 35.0 cm(-1), closely matching the value predicted based on the fluorobenzene-Ar van der Waals stretch frequency. Rotational band contour analysis has been performed on the fluorobenzene-Ar 0(0)(0) transition to yield a set of S(1) rotational constants A' = 0.05871 ± 0.00014 cm(-1), B' = 0.03803 ± 0.00010 cm(-1), and C' = 0.03103 ± 0.00003 cm(-1). The rotational constants imply that in the S(1) 0(0) level the Ar is on average 3.488 ? from the fluorobenzene centre of mass and displaced from it towards the centre of the ring at an angle of ~6° to the normal. The rotational contour for fluorobenzene-Ar(2) was predicted using rotational constants calculated on the basis of the fluorobenzene-Ar geometry and compared with the experimental contour. The comparison is poor which, while due in part to expected saturation effects, suggests the presence of another band lying beneath the contour.     

Jet-cooled phosphorescence excitation spectrum of the T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one

The T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one (2CP) was investigated by using phosphorescence excitation (PE) spectroscopy in a free-jet expansion. The origin band, near 385 nm, is the most intense feature in the T1(n,pi*) <-- S0 PE spectrum. A short progression in the ring-bending mode (nu'(30)) is also observed. The effective vibrational temperature in the jet is estimated at 50 K. The spectral simplification arising from jet cooling helps confirm assignments made previously in the room-temperature cavity ringdown (CRD) absorption spectrum, which is congested by vibrational hot bands. In addition to the origin and nu'(30) assignments, the jet-cooled PE spectrum also confirms the 28(0)(1) (C=O out-of-plane wag), 29(0)(1) (C=C twist), and 19(0)(1) (C=O in-plane wag) band assignments that were made in the T1(n,pi*) <-- S0 room-temperature CRD spectrum. The temporal decay of the T1 state of 2CP was investigated as a function of vibronic excitation. Phosphorescence from the v' = 0 level persists the entire time the molecules traverse the emission detection zone. Thus the phosphorescence lifetime of the v' = 0 level is significantly longer than the 2 micros transit time through the viewing zone. Higher vibrational levels in the T1 state have shorter phosphorescence lifetimes, on the order of 2 micros or less. The concomitant reduction in emission quantum yield causes the higher vibronic bands (above 200 cm(-1)) in the PE spectrum to be weak. It is proposed that intersystem crossing to highly vibrationally excited levels of the ground state is responsible for the faster decay and diminished quantum yield. The jet cooling affords partial rotational resolution in the T1(n,pi*) <-- S0 spectrum of 2CP. The rotational structure of the origin band was simulated by using inertial constants available from a previously reported density functional (DFT) calculation of the T1(n,pi*) state, along with spin constants obtained via a fitting procedure. Intensity parameters were also systematically varied. The optimized intensity factors support a model that identifies the S2(pi,pi*) <-- S0 transition in 2CP as the sole source of oscillator strength for the T1(n,pi*) <-- S0 transition.     

Photophysics of soret-excited tetrapyrroles in solution. II. Effects of perdeuteration, substituent nature and position, and macrocycle structure and conformation in zinc(II) porphyrins

The steady-state absorption, fluorescence, and excitation spectra and upper excited-state temporal fluorescence decay profiles of 11 tetrapyrroles in several fluid solvents are presented and analyzed to ascertain the factors that control their S2 population decay times. The S2 lifetimes, which vary by more than 2 orders of magnitude, are controlled exclusively by their rates of radiationless decay. The only important electronic relaxation path is S2-S1 internal conversion, the efficiency of which is near 1.0 in all compounds studied (except CdTPP where it is 0.69). The rate of S1 population rise equals the rate of S2 population decay in all cases. Among the compounds studied, only MgTPP exhibits S2-S1 decay behavior that corresponds to the weak coupling limit of radiationless transition theory; all zinc metalloporphyrins exhibit intermediate to strong coupling. Perdeuteration of ZnTPP produces no significant change in the rate of S2 decay or in the quantum yield of S2-S0 fluorescence, indicating that in-plane C-C and C-N vibrations are the accepting modes in S1 with the largest Franck-Condon factors. The initial vibrational energy content of the S2 states (0 < E(vib) < 3500 cm(-1) over the range of compounds) plays no significant role in determining their overall population decay rates in solution. The S2 population decay rates of these tetrapyrroles are controlled by two factors: the Franck-Condon factor, which is inversely proportional to the exponent of the S2-S1 electronic energy spacing and the S2-S1 coupling energy. The S2-S1 electronic energy spacing is determined in solution by the difference in the polarizabilities of the S2 and S1 states and can be controlled by varying the polarizability of the solvent. The S2-S1 coupling energy is influenced by the nature, location, and effect of the substituents, with beta-alkyl substitution and reduction of symmetry in the tetrapyrrole--for example by loss of planarity--increasing the interstate coupling energy.     

Twisted intramolecular charge transfer states: rotationally resolved fluorescence excitation spectra of 4,4'-dimethylaminobenzonitrile in a molecular beam

We report the observation at high resolution of seven vibronic bands that appear within approximately 200 cm(-1) of the electronic origin in the S(1)-S(0) fluorescence excitation spectrum of 4,4'-dimethylaminobenzonitrile (DMABN) in a molecular beam. Surprisingly, each band is found to be split into two or more components by a (coordinated) methyl group tunneling motion which significantly complicates the analysis. Despite this fact, high quality [(Observed-Calculated)< or =30 MHz] fits of each of the bands have been obtained, from which the rotational constants, inertial defects, torsion-rotation interaction constants, methyl group torsional barriers, and transition moment orientations of DMABN in both electronic states have been determined. The data show that DMABN is a slightly pyramidalized (approximately 1 degree) but otherwise (heavy-atom) planar molecule in its ground S(0) state, and that its electronically excited S(1) state has both a more pyramidalized (approximately 3 degrees) and twisted (approximately 25 degrees) dimethylamino group. Large reductions in the methyl group torsional barriers also show that the S(1)<--S(0) electronic transition is accompanied by significant charge transfer from the nitrogen atom to the pi* orbitals of the aromatic ring. Thereby established is the participation of all three vibrational coordinates in the dynamics leading to the "anomalous" emissive behavior of DMABN in the condensed phase.     

Laser induced fluorescence spectroscopy of NC3O

Laser induced fluorescence spectra of the NC(3)O radical in a supersonic jet have been observed. The radical was produced in a pulsed electric discharge of HC(3)N and O(2) diluted to 0.3% with Ar. A total of 17 vibronic bands with a radiative lifetime of approximately 30 ns have been observed in a region from 27 000 to 27 500 cm(-1). The observed vibronic bands are classified as (2)Pi(12)-(2)Pi(12), (2)Pi(32)-(2)Pi(32), and (2)Sigma-(2)Sigma types. The upper states of the (2)Sigma-(2)Sigma bands have large spin-rotation constants, which should be denoted as Sigma((+)) and Sigma((-)). From high-level ab initio calculations and rotational analyses, the observed transition was assigned to the B (2)Pi-X (2)A(") transition. Dispersed fluorescence spectra from the upper (2)Sigma and (2)Pi vibronic levels have also been observed, yielding fundamental vibrational frequencies for the nu(1), nu(2), nu(3), and nu(7) modes of the ground state.