Renner-Teller Interaction Research Articles

Laser induced fluorescence spectroscopy of the transition of jet cooled 14NO3 and 15NO3 II: the dispersed fluorescence spectrum from the 3rd E level of the ν4 mode (approximately the 2ν4 (e'), l = 0, level).

14NO3 and 15NO3 isotopomers were generated in a supersonic free jet expansion, and laser induced fluorescence (LIF) of the electronic transition was observed. Dispersed LIF spectra from the vibronic level at ∼770 cm-1 above the vibrationless level have been measured for each isotopomer. One remarkable characteristic of the dispersed fluorescence (DF) spectrum is that the ν2 fundamental is clearly observed, even though the transition 201 is forbidden in the electronic transition. Another is that two sets of vibrational structure are observed in the DF spectrum: one of the two sets is the structure with the origin at the excitation energy, and the other is that with the origin at the ν2 fundamental. These observations suggest that the fluorescent level has contribution from the ν2 mode. In part I (M. Fukushima, J. Mol. Spectrosc., 2022, 387, 111646), it has been reported that the ν4 progressions are one of the typical characteristics of the vibrational structure of the DF spectrum from the vibrationless level; the progressions have intensity gradually decreasing with increasing ν4 quantum number (named the "regular" distribution), and have members forbidden for the Δl = 0 selection rule. The intensity distribution of the ν4 progression with the origin at the ν2 fundamental is similar to that from the vibrationless level, while that with the origin at the excitation energy is drastically different from that from the vibrationless level. The Jahn-Teller (J-T) and Renner-Teller (R-T) interactions in the B̃2E' state enable us to interpret the intensity distributions of the ν4 progressions, in which relatively weak interactions are enough to reproduce the observed distributions. The spectral intensity distribution analyses adopting the two vibronic couplings suggest that the fluorescent level at ∼770 cm-1 above the vibrationless level is the 3rd E eigenstate of the ν4 mode. The major component of the ν4 3rd E state is the |Λ = ±1; v4 = 2, l4 = 0〉 level of the B̃2E' state, which is a vibrationally and a vibronically E' level, and it is therefore concluded that the major components of the fluorescent level are both of the ν2 = 1 level and ν4 = 2 level .

A vibrational action spectroscopic study of the Renner-Teller- and spin-orbit-affected cyanoacetylene radical cation HC3N.

The linear radical cation of cyanoacetylene, HC3N+ (2Π), is not only of astrophysical interest, being the, so far undetected, cationic counterpart of the abundant cyanoaceteylene, but also of fundamental spectroscopic interest due to its strong spin-orbit and Renner-Teller interactions. Here, we present the first broadband vibrational action spectroscopic investigation of this ion through the infrared pre-dissociation (IRPD) method using a Ne tag. Experiments have been performed using the FELion cryogenic ion-trap instrument in combination with the FELIX free-electron lasers and a Laservision optical parametric oscillator/optical parametric amplifier system. The vibronic splitting patterns of the three interacting bending modes (ν5, ν6, ν7), ranging from 180to1600 cm-1, could be fully resolved revealing several bands that were previously unobserved. The associated Renner-Teller and intermode coupling constants have been determined by fitting an effective Hamiltonian to the experimental data, and the obtained spectroscopic constants are in reasonable agreement with previous photoelectron spectroscopy (PES) studies and ab initio calculations on the HC3N+ ion. The influence of the attached Ne atom on the infrared spectrum has been investigated by ab initio calculations at the RCCSD(T)-F12a level of theory, which strongly indicates that the discrepancies between the IRPD and PES data are a result of the effects of the Ne attachment.

Rotational Modulation of Ã2A″-State Photodissociation of HCO via Renner-Teller Nonadiabatic Transitions.

By examining the product-state distribution of a prototypical nonadiabatic predissociation system, HCO(Ã2A″-X̃2A'), we demonstrate that the dissociation dynamics is strongly modulated by parent rotational quantum numbers. The predissociation of the nominal (νC-H = 0, νbend, νC-O = 1) vibronic levels of the Ã2A″ state surprisingly gives rise to both vibrational ground and excited states of the CO product, despite the assumed spectator nature of the CO moiety. This anomaly is attributed to the dependence of the lifetime of the vibronic resonance facilitated by the Renner-Teller interaction on the parent rotational angular momentum quantum numbers coupled with transient intensity borrowing from nearby vibronic resonances with νC-O = 0. This unique phenomenon is a purely quantum mechanical behavior that has no classical analogue.

Beyond Born-Oppenheimer constructed diabatic potential energy surfaces for F + H2 reaction.

First principles based beyond Born-Oppenheimer theory has been implemented on the F + H2 system for constructing multistate global diabatic Potential Energy Surfaces (PESs) through the incorporation of Nonadiabatic Coupling Terms (NACTs) explicitly. The spin-orbit (SO) coupling effect on the collision process of the F + H2 reaction has been included as a perturbation to the non-relativistic electronic Hamiltonian. Adiabatic PESs and NACTs for the lowest three electronic states (12A', 22A', and 12A″) are determined in hyperspherical coordinates as functions of hyperangles for a grid of fixed values of the hyperradius. Jahn-Teller (JT) type conical intersections between the two A' states translate along C2v and linear geometries in F + H2. In addition, A' and A″ states undergo Renner-Teller (RT) interaction at collinear configurations of this system. Both JT and RT couplings are validated by integrating NACTs along properly chosen contours. Subsequently, we have solved adiabatic-to-diabatic transformation (ADT) equations to evaluate the ADT angles for constructing the diabatic potential matrix of F + H2, including the SO coupling terms. The newly calculated diabatic PESs are found to be smooth, single-valued, continuous, and symmetric and can be invoked for performing accurate scattering calculations on the F + H2 system.

Experimental and theoretical threshold photoelectron spectra of methylene.

The threshold photoelectron spectrum of methylene (CH2), produced by consecutive H atom abstractions on methane, has been recorded using synchrotron radiation. The experimental spectrum spans the region of the X + 2Π u ← X 3 B 1 ionizing transition. It is modeled starting from ab initio bending potentials and using the bending approach introduced by Coudert et al. [J. Chem. Phys. 148, 054302 (2018)] accounting for the quasilinearity of CH2 and the strong Renner-Teller interaction in . This first calculation yields a theoretical threshold photoelectron spectrum which is in moderate agreement with the experimental one. A more accurate approach treating the three vibrational modes is developed for computing the threshold photoelectron spectrum of triatomic molecules. This new treatment is tested modeling the already measured threshold photoelectron spectrum of the X + 2Π u ← X 1 A 1 ionizing transition of the water molecule. The threshold photoelectron spectrum of CH2 computed with the new approach compares more favorably with the experimental spectrum and yields an adiabatic ionization potential of 10.386(6) eV.

Theoretical investigation of intersystem crossing in the cyanonitrene molecule, 1NCN → 3NCN.

The NCN diradical is an important intermediate of prompt nitric oxide formation in flames. The mechanism of intersystem crossing (ISC) in the NCN molecule formed via pyrolysis or photolysis of NCN3 is of relevance to the interpretation of experiments that utilize NCN3 as a precursor for laboratory studies of NCN kinetics. This mechanism has been investigated by means of multi-reference configuration interaction calculations. From the potential energy surfaces for NCN3 dissociation, it was inferred that both thermal and photo-chemical decomposition initially lead to NCN in its lowest singlet state, ã1Δg, with a possible contribution from the b̃1Σg+ state at low photolysis wavelengths. Direct formation of the triplet ground state X̃ 3Σg- is also feasible for the photolytic pathway. An analysis of surface crossings between ã or b̃ and the triplet ground state X̃ 3Σg- in the absence and presence of a helium atom revealed an ISC channel NCN1(ã)→3NCN(X̃) via a strongly bent structure. However, its barrier of 38 kcal mol-1 relative to the singlet minimum turned out to be much too high to explain the fast ISC observed in experiments. A rigid-bender model including Renner-Teller interactions was used to examine the occurrence of mixed-multiplicity rovibrational states-so-called gateway states-that could enhance collision-induced ISC. The results of this study indicate that a gateway mechanism is probably not operative in the case of the ã/X̃ pair of states in NCN.

Dispersed fluorescence spectroscopy of the SiCN Ã2Δ - X̃2Π system: Observation of some vibrational levels with chaotic characteristics.

The laser induced fluorescence (LIF) spectrum of the à 2Δ - X̃ 2Π transition was obtained for SiCN generated by laser ablation under supersonic free jet expansion. The vibrational structures of the dispersed fluorescence (DF) spectra from single vibronic levels (SVL's) were analyzed with consideration of the Renner-Teller (R-T) interaction. Analysis of the pure bending (ν2) structure by a perturbation approach including R-T, anharmonicity, spin-orbit (SO), and Herzberg-Teller (H-T) interactions indicated considerably different spin splitting for the μ and κ levels of the X̃ 2Π state of SiCN, in contrast to identical spin splitting for general species derived from the perturbation approach, where μ and κ specify the lower and upper levels, respectively, separated by R-T. Further analysis of the vibrational structure including R-T, anharmonicity, SO, H-T, Fermi, and Sears interactions was carried out via a direct diagonalization procedure, where Sears resonance is a second-order interaction combined from SO and H-T interactions with Δ K = ± 1, ΔΣ = ∓1, and Δ P = 0, and where P is a quantum number, P = K + Σ. The later numerical analysis reproduced the observed structure, not only the pure ν2 structure but also the combination structure of the ν2 and the Si-CN stretching (ν3) modes. As an example, the analysis demonstrates Sears resonance between vibronic levels, (0110) κ Σ(+) and (0200)μΠ12, with Δ K = ± 1 and Δ P = 0. On the basis of coefficients of their eigen vectors derived from the numerical analysis, it is interpreted as an almost one-to-one mixing between the two levels. The mixing coefficients of the two vibronic levels agree with those obtained from computational studies. The numerical analysis also indicates that some of the vibronic levels show chaotic characteristics in view of the two-dimensional harmonic oscillator (2D-HO) basis which is used as the basis function in the present numerical analysis; i.e., the eigen vectors for some of the observed levels have several components of the basis, and we have not been able to give precise vibronic assignments for the levels, but just vibronically labeled, referring the largest component in their vectors. (To emphasize this situation, we do not use the word "assignment," but prefer to use "label" as the meaning of just "label," but not "assign," throughout this paper.) The latter shows that the vibronic labels of the levels are meaningless, and the P quantum number and the order of their eigen states in the P matrix block derived in the numerical analysis only characterize the vibronic levels. Comparing the constants obtained for all of the interactions with those of species showing Sears resonance and studied previously, it is found that none of them are strong, but are moderate. It is thus concluded that the chaotic appearance is not derived by any strong interaction, but is induced by complex and accidental proximities of the vibronic levels caused by the moderate interactions.

Ab initio constructed diabatic surfaces of NO2 and the photodetachment spectra of its anion.

A thorough investigation has been performed for electronic structure, topological effect, and nuclear dynamics of NO2 molecule, where the adiabatic potential energy surfaces (PESs), conical intersections between the ground (X(2)A1) and the first excited state (A(2)B2), and the corresponding non-adiabatic coupling terms between those states are recalculated [Chem. Phys. 416, 11 (2013)] to achieve enough accuracy in dynamics. We employ beyond Born-Oppenheimer theory for these two state sub-Hilbert space to carry out adiabatic to diabatic transformation (ADT) to obtain the ADT angles and thereby, to construct single-valued, smooth, and continuous diabatic PESs. The analytic expressions for the adiabatic PESs and ADT angles are provided to represent a two-state three-mode diabatic Hamiltonian of NO2 for performing nuclear dynamics to calculate the photo-electron spectra of its anion. It appears that not only Jahn-Teller type coupling but also Renner-Teller interaction contributes significantly on the overall spectrum. The coupling between the electronic states (X(2)A1 and A(2)B2) of NO2 is essentially through the asymmetric stretching mode, where the functional form of such interaction is distinctly symmetric and non-linear.

Vibronic and Rydberg series assignments in the vacuum ultraviolet absorption spectrum of nitrous oxide

We report a comprehensive photoabsorption study of nitrous oxide (N2O) in the vacuum ultraviolet (45,000–95,000cm−1) region using synchrotron radiation. The observed spectrum comprises of a few valence transitions and low lying Rydberg series converging to the two spin–orbit components (2П1/2,3/2) of the ground state of N2O+. Spectral analysis is aided by extensive quantum chemical calculations of vertical excited states, oscillator strengths and potential energy curves using the time dependent density functional theory. Vibronic bands observed in the first absorption system (45,000–60,000cm−1) are assigned to hot band progressions in υ2′ originating from v″=1 or 2. New insights into the assignment of the well-formed progression of bands in the X1Σ+→C1П system (60,000–72,000cm−1) are afforded by consideration of the Renner–Teller interaction. A set of molecular vibrational parameters (ω2=467cm−1, x22=−2.9, ε=−0.24) for the C1П state are derived from a fitting of the experimental data. The 3pπ1Σ+ state at ~77,600cm−1 shows a large quantum defect (0.96) which is explained as arising due to mixed valence–Rydberg character. In the 85,000–95,000cm−1 region, a number of absorption features are observed with greater clarity than in earlier photoabsorption studies and assigned to Rydberg series of type nlλ (n=3,4; l=s,p,d; λ=σ,π,δ) and accompanying vibronic bands. This work has resulted in clarification of several discrepancies in earlier Rydberg series assignments. Additionally, the 3pπ 3Σ− Rydberg state at 85,788cm−1, the valence transition 7σ→3π (1П) at 87,433cm−1 and the 3dλ Rydberg series in the 91,700–92,600cm−1 region are assigned for the first time.

High-Resolution Near-Infrared Spectroscopy of CH2+ and Its Deuterated Isotopologues

Aiming to provide approximate rotational constants for millimeter wave spectroscopists to identify the corresponding species in space, we have recorded the near-infrared spectra of the methylene cation CH2(+) and its deuterated isotopologues, CD2(+) and CHD+, using a high resolution and high sensitivity spectrometer. Detection of CH2(+) in space will shed light on interstellar chemistry as it is the intermediate between the abundant CH+ and yet to be observed CH3(+), which is important in the formation of larger organic molecules. CH2(+) and its deuterated isotopologues are also of special interest for theoretical studies because of their unique intramolecular dynamics, i.e., the Renner–Teller interaction and quasi-linearity. This paper will discuss several new bands of CH2(+), the Ã(0,5[11],0)0 ← X̃(0,0[0],0)1 and Ã(0,4[11],0)2 ← X̃(0,0[0],0)1 bands of CD2(+), which have been identified and analyzed, and the candidate lines for the Ã(0,4[10],0)1 ← X̃(0,0[1],0)0 band of CHD+, in comparison with the theoretical predictions by Bunker and colleagues.

Study of the $\tilde{\rm X}^2\Pi$X̃2Π state of the SiCN/SiNC Renner-Teller system

The potential energy surfaces of both components of the X̃(2)Π electronic ground state of the double Renner-Teller SiCN/SiNC system are calculated using explicitly correlated coupled cluster approach. The SiNC minimum is found to lie at 628 cm(-1) above the SiCN one. The isomerization transition state is found at 7583 cm(-1) on the (2)A' surface and at 7936 cm(-1) on the (2)A(") surface. The cyclic local minimum on surface (2)A' is also reproduced by our potential energy surface and is located at 3901 cm(-1). The calculated potentials are used to simulate rovibrational spectroscopy employing the recently developed EVEREST variational code. It is shown that Renner-Teller interaction (ε = 0.3043 for SiCN and ε = 0.3874 for SiNC) and spin-orbit coupling are both very important for a correct description of the spectroscopy of this system. Comparison with available experimental measurement is reported.

Conical intersections between [formula omitted] and [formula omitted] electronic states of [formula omitted

We explore both the general symmetry-allowed accidental (SAA) conical intersections (CIs) and Renner–Teller (RT) interactions prevailed between the ground (X2A1) and first excited (A2B2) electronic states of NO2 in the configuration space of normal mode coordinates. Global ab initio potential-energy surfaces and Non Adiabatic Coupling Term (NACT) between those states are reported. For each of the three pairs of normal mode (Q1,Q2,Q3), calculated NACT show singularity at different positions, which are used to calculate Adiabatic-to-Diabatic Transformation (ADT) angles and thereby, the diagonal elements of ADT matrix display (i) sign change for odd number of SAA CI (s) and (ii) no sign change for even number of SAA CIs as well as Renner–Teller interactions. Similar to Jahn–Teller CI, the existence of SAA CIs are, further, confirmed by Longuet-Higgins’ phase change.

Linear polyatomic molecules withΠground state: Sensitivity to variation of the fundamental constants

In polyatomic molecules with \Pi\ electronic ground state the ro-vibrational spectrum can be strongly modified by the Renner-Teller effect. The linear form of C3H molecule has particularly strong Renner-Teller interaction and a very low lying vibronic \Sigma+ level, which corresponds to the excited bending vibrational mode. This leads to the increased sensitivities of the microwave and submillimeter transition frequencies to the possible variation of the fine structure constant alpha and electron to proton mass ratio mu.

Dissociative Recombination of Small Polyatomic Molecules

Simplified analytical expressions for dissociative recombination driven by the Jahn-Teller and Renner-Teller interactions are discussed. These expressions allow straightforward predictions of isotope effects, and the results are discussed for both H3+ and HCO+ and their isotopomers. Although the predicted effects are generally in good agreement with experiment, discrepancies indicate areas worthy of more extensive investigation. Some general features of isotope effects in the dissociative recombination of other systems are also discussed.

Renner−Teller and Fermi Resonance Interactions for the v3 = 1 and v7 = 2 Vibronic Levels in the A2Πu and X2Πg Electronic States of HC4H+

The excitation of the v(3) = 1 (σ(g)(+) C-C stretch) and the v(7) = 2 (π(g)(2) C≡C-C bend) modes in the A(2)Π(u) electronic state of diacetylene cations results in Renner-Teller (R-T) and Fermi interactions. The 3(0)(1) and 7(0)(2) vibronic bands in the A(2)Π(u)-X(2)Π(g) transition of HC(4)H(+) have been measured with rotational resolution using cavity ringdown spectroscopy in a supersonic slit jet discharge. The analysis yields T(00) = 20520.828(4) cm(-1), B' = 0.14047(2) cm(-1), and A' = -17.95(1) cm(-1) for the v(3) = 1 and T(00) = 20573.659(4) cm(-1), B' = 0.14018(3) cm(-1), and A' = -11.55(1) cm(-1) for the v(7) = 2 level in the A(2)Π(u) electronic state. A vibronic analysis has been carried out taking into consideration the R-T, spin-orbit, and Fermi resonance interactions between the ν(3) and ν(7) modes. The levels are fitted to the eigenvalues of an appropriate Hamiltonian matrix. This yields the vibrational frequencies ω(3)′ = 811.8 cm(-1) and ω(7)′ = 403.2 cm(-1), Renner parameter ε(7)′ = 0.065, Fermi coefficients W(1)′ = 10.3 cm(-1) and W(2)′ = 5.1 cm(-1), and spin-orbit interaction constant A(SO)′ = -31.1 cm(-1). A corresponding R-T analysis has been carried out for the X(2)Π(g) ground state of HC(4)H(+) using data available in the literature [Callomon, J. H. Can. J. Phys. 1956, 34, 1046]. This gives ω(3)" = 956.2 cm(-1), ω(7)" = 435.4 cm(-1), ε(7)" = 0.028, W(1)" = 7.2 cm(-1), W(2)" = 10.9 cm(-1), and A(SO)" = -33.3 cm(-1).

Quantum dynamics of Renner–Teller and isotopic effects in NH(a1Δ) + D(2S) reactions

A quantum dynamics study for the NH(a(1)Δ) + D((2)S) reactions using coupled channel time dependent real wavepacket formalism is presented. Moreover, the Renner-Teller (RT) interactions between two electronic states of NHD (X[combining tilde](2)A'' and Ã(2)A') have been taken into account by means of semiempirical RT matrix elements. The introduction of RT effects opens the possibility of studying not only the adiabatic reactions [depletion (d) to N((2)D) + HD(X(1)Σ(+)) and exchange (e) to ND(a(1)Δ) + H((2)S)] but also nonadiabatic ones [quenching (q) to NH(X(3)Σ(-)) + D((2)S) and exchange-quenching (eq) to ND(X(3)Σ(-)) + H((2)S)]. Reaction probabilities, cross sections, isotopic effects, and rate constants are presented for all the before mentioned reactions. RT results are compared with Born-Oppenheimer, quasiclassical, and experimental data. Contrasting with previous NH + H results, we point out interesting RT and isotopic effects, which depend on the D and H masses and on the tunneling of the H atom. In fact, RT effects, near-threshold cross sections, and rate constants are smaller in NH + D than in NH + H, as expected from the masses of the attacking atoms. Our rate constants and quenching branching ratio agree well with previous quasiclassical and experimental data, validating the semiempirical RT coupling we employ. Some small differences between calculated and measured rate constants might be due to the theoretical approximations and to the large experimental error bars.

Renner–Teller interactions in the vibrational autoionization of polyatomic molecules

Vibrational autoionization induced by the Renner-Teller interaction in linear polyatomic molecules is considered in the context of the three-state electrostatic model developed by Gauyacq and Jungen [Mol. Phys. 41, 383 (1980)]. For small interactions, simple formulas are derived for the quantum defect matrix elements and the autoionization rates in terms of the more common Renner-Teller parameters derived from spectroscopic analyses of low-lying Rydberg states. These formulas should provide guidance for empirical fitting of quantum defect parameters to spectra of high Rydberg states. Consideration of typical values of the Renner-Teller parameters also allows the estimation of vibrational autoionization rates induced by these interactions. These estimates support the validity of the Deltav=-1 propensity rule for vibrational autoionization. Constraints on the vibrational autoionization rates for the symmetric stretching vibration are also discussed. In the following paper, electron capture by polyatomic molecular ions into vibrationally autoionizing Rydberg states is considered from the same perspective, and a simple formula is derived to allow the estimation of the effect of this process on dissociative recombination cross sections.

Renner–Teller interactions in the dissociative recombination of HCO+

The formalism developed in the preceding paper for vibrational autoionization via Renner-Teller active vibrations is adapted to treat dissociative recombination and applied to the reaction of HCO(+)+e(-). Existing spectroscopic data on the rovibrational structure of the HCO(+) (2)Sigma(+) ion and the HCO 3ppi (2)Pi Rydberg state are fitted by using the semirigid bender model to extract the parameters required to calculate the autoionization and electron capture widths. The results of this simple model are in good agreement with more detailed first principles calculations of the dissociative recombination cross section and confirm the earlier conclusion that coupling due to the Renner-Teller interaction is largely responsible for the observed dissociative recombination cross section at electron energies below approximately 0.1 eV.

Coriolis coupling effects on the initial-state-resolved dynamics of the N(D2)+H2→NH+H reaction

We present Coriolis coupling effects on the initial-state-resolved dynamics of the insertion reaction N((2)D)+H(2)(X (1)Sigma(g) (+))-->NH(X (3)Sigma(-) and a (1)Delta)+H((2)S), without and with nonadiabatic Renner-Teller (RT) interactions between the NH(2) X (2)B(1) and A (2)A(1) electronic states. We report coupled-channel (CC) Hamiltonian matrix elements, which take into account both Coriolis and RT couplings, use the real wave-packet and flux methods for calculating initial-state-resolved reaction probabilities, and contrast CC with centrifugal-sudden (CS) results. Without RT interactions, Coriolis effects are rather small up to J=40, and the CS approximation can be safely employed for calculating initial-state-resolved, integral cross sections. On the other hand, RT effects are associated with rather large Coriolis couplings, mainly near the linearity of NH(2), and the accuracy of the CS approximation thus breaks down at high collision energies, when the reaction starts on the excited A (2)A(1) surface. We also present the CC-RT distribution of the X (3)Sigma(-) and a (1)Delta electronic states of the NH products.

Laser induced fluorescence spectroscopy of the C3N radical

Electronic spectra of the C3N radical have been observed for the first time in the near ultraviolet wavelength region by laser induced fluorescence (LIF) spectroscopy. Seventeen vibronic bands of the B 2Pii-X 2Sigma+ electronic transition system of C3N were identified in LIF spectra of products in a discharge of HC3N. The origin of the B 2Pii state was determined to be 27,929.985(1) cm(-1) from rovibrational analyses. It was found that observations of two types of 2Sigma vibronic levels, which have 2Sigma+ and 2Sigma+/- symmetries originated from excitations of the nu4 trans-bending mode (omega4=369.1(20) cm(-1)) with a large Renner-Teller (RT) interaction (epsilon4=-0.1549(50)), and the nu5 cis-bending mode (omega5=163.24(84) cm(-1)) with a small Renner-Teller interaction (epsilon5=-0.0503(68)), respectively. Vibronic levels, with excitations of the C-C stretching (omega3=869.7 cm(-1)) mode, were also identified. The spin-orbit interaction constant was determined to be Aso=-36.7(50) cm(-1) from the RT analysis. In dispersed fluorescence spectra from B 2Pii, vibrational structures of the low-lying electronically excited A 2Pii state were clearly observed with a strong progression due to the nu3' mode, together with those of the X 2Sigma+ state with weak intensities. The origin of A 2Pii, T0=1844(3) cm(-1), and the vibrational frequencies, omega3'=883(3) cm(-1) and omega5'=121(3) cm(-1) for A 2Pii, and omega3"=1054(3) cm(-1), omega4"=405(3) cm(-1), and omega5"=131(3) cm(-1) for X 2Sigma+, were determined. Time profiles of fluorescence from B 2Pii have short (50-200 ns) and long (>1 micros) decay components with quantum beats, indicating that there is a competition between radiative decay and the nonradiative internal conversion to vibrationally highly excited A 2Pii and X 2Sigma+.