B3Π State Research Articles

An Analysis of the E(0g+) Ion-pair State of Bromine through Stepwise Excitation Technique

Abstract The pulses of the fundamental and its frequency doubled outputs from a tunable dye laser are used to promote a stepwise exciation of the E(0g+)–X1Σg+ transition through the B3Π(0u+) state in molecular bromine. The double resonance is detected by the associated UV emission which originates from the E(0g+) state radiating back to the B3Π(0u+) state. The E(0g+) state is analyzed in detail on the basis of the selection rules of two transition components and the E–B fluorescence spectrum. The molecular parameters are derived as Te=49778.5 cm−1, ωe=150.8 cm−1, ωexe=0.41 cm−1, and re=3.197 Å for the 79Br2 isotope species.

Time Resolved Visible and UV Absorption Spectroscopy of Bromine Clusters in Argon Matrices

AbstractMatrix deposition was observed by a laser interference technique at deposition rates of 0.1 – 0.2 mmol/h at T = 10 K up to a matrix thickness of 1.1 · 10−4 m. Absorption spectra of Br2/Ar matrices (1 : 1000 – 1 : 300) examined quantitatively at λ = 300 nm and 415 nm showed an increase of molar bromine absorptivity with increasing Br2‐concentration. The increase of absorptivity may be explained by formation of Br2‐dimers during matrix deposition. Br2/Ar matrices were exposed to laser pulses at λ = 490 nm and showed a transient increase of absorption in the near UV (λ < 370 nm), a spectral region mainly due to Br2‐dimers. This increase presumably depends on laser excitation of Br2‐molecules situated in dimers into the B3π(O) state followed by deactivation into the ground state X1Σ. At least three exponentials are necessary to fit the transient signals with the lifetimes t1 = 3.5 μsec, t2 = 64 μsec, t3 = in the order of some msec.

The identification of the b3Π state of copper monofluoride

The identification of the b3Π state of copper monofluoride

NO fluorescence during vacuum-ultraviolet photodissociation of CFCl2NO

Gas-phase photodissociation of CFCl2NO at 147 nm results in the formation of electronically excited NO in the B2Π state, giving rise to the well known NO β-emission bands. Collisional effects consequent upon the addition of helium lead to the appearance of fluorescence from the NO(A2∑+) state with emission from the ν′= 2 and 3 levels dominating the spectrum. The results are compared with data obtained previously for other nitrosocompounds.

Laser Spectroscopy of Iodine Monofluoride Produced in Situ in a Molecular Beam Experiment

AbstractNascent IF molecules in various vibrational‐rotational manifolds of the electronic ground state have been prepared in a molecular beam experiment. High‐resolution laser spectroscopy has been applied to determine accurate band head positions and band origins of the B→X transition. Dunham coefficients have been calculated for the X1Σ+ and B3Π(O+) states. The hyperfine structure of the rotational lines has been observed.

Interpretation of the spontaneous predissociation of Cl2[B3Π(0+u)

The spontaneous predissociation of the B3Π(0+u) state of chlorine is discussed. Calculation of predissociation rate constants has been used to investigate the form of the repulsive potential which provides the dissociation channel. The calculations showed that the experimentally observed vibrational dependence of the predissociation rate cannot be accounted for by predissociation via the 1Π(1u) state. This is in contrast to the similar predissociations of the I2 and Br2B states, which are both predissociated by 1Π(1u) states.Further calculations demonstrate that the A3Π(1u) state provides the dissociative channel. Calculations were made utilizing an A-state potential-energy curve derived from spectroscopic data. Good quantitative agreement between the experimental and calculated predissociation rate constants was obtained.

The B3Π(0+) → X1Σ+ emission spectrum of 79BrF and 81BrF in the range 6250–8700 Å: Rotational analysis of bands in the v′ = 0, 1, 2, 3, 4 progressions, and merged molecular constants for the X1Σ+ and B3Π(0+) states

Sixteen emission bands of the B3Π(0+) → X1Σ+ systems of 79BrF and 81BrF have been analyzed rotationally. Measured line frequencies are fitted by least-squares to determine the rotational constants and origin for each band. The least-squares results for 0 ≤ v′ ≤ 4 and 4 ≤ v″ ≤ 12 are merged with constants from previously reported B ← X absorption and X-state microwave data. RKR potentials for the two states and Franck-Condon factors and r-centroids for the BrF (B-X) system are reported.

Theoretical treatment of the X 1Σ+, A 1Σ+, and B 1Π states of LiH

A b initio self-consistent-field plus configuration-interaction calculations are reported for the X 1Σ+, A 1Σ+, and B1Π states of LiH using a 22σ12π7δ function Slater basis set. The resulting dissociation energies, with the experimental values in parentheses, are De (X 1Σ+)=19 972 (20 288) cm−1, De(A 1Σ+)=9042 (8682) cm−1, and De (B 1Π)=284 (288) cm−1. This is the first ab initio treatment to quantitatively account for the binding in the B 1Π state. Calculated dipole moments and electronic transition moments for the X 1Σ+–A 1Σ+, X 1Σ+–B 1Π, and A 1Σ+–B 1Π band systems are in excellent agreement with existing theoretical and experimental data. Radiative transition probabilities and lifetimes, including both the bound→bound and bound→free contributions, are computed for all vibrational levels of the A 1Σ+ and B 1Π states. Comparison with previous results using experimentally based potentials provides insight into the sensitivity of the radiative lifetimes to the detailed nature of the uppermost region of the potentials. Our calculated lifetimes for the lower vibrational levels of the A 1Σ+ state are within the experimental uncertainty. Our calculated lifetimes for the three vibrational levels of the B 1Π state are in excellent agreement with those of Zemke and Stwalley (values in parentheses), increasing with v′ from 11.3 (11.3) nsec at v′=0, to 17.0 (17.0) nsec at v′=1, and then to 23.5 (24.0) nsec at v′=2.

Les perturbations dans l'état B2Π de NS : interactions avec les états B′2Σ+, b4Σ− et a4Π

The emission bands of the B2Π–X2Π transition of 14N32S and 15N32S have been reanalysed in the spectral range 2500–5650 Å. The B2Π state, observed up to ν = 12, shows strong perturbations caused by interactions with the b4Σ− and a4Π valence states and with the B′2Σ+ state. The study of the two molecules allows the vibrational numbering and gives approximate constants of the different states from the positions of the observed perturbations and the isotopic displacements.

Revised molecular constants, RKR potential, and long-range analysis for the B3Π(0 u+) state of Cl 2, and rotationally dependent Franck-Condon factors for Cl 2 ( BX1Σ g+)

Literature data for the line frequencies of the B 3Π(0 u +) ← X 1Σ g + transition of Cl 2 are fitted directly by least squares to obtain new molecular constants. The constants from individual bands are merged to obtain single-valued estimates of the rotational constants for each vibrational level of the B state. The results are combined with recent data from the B → X system in emission to obtain new RKR turning points for the B and X states, and Franck-Condon factors for the B- X system. The new constants are also used to provide revised long-range parameters for Cl 2( B) which differ from those of earlier work. In particular, the coefficient C 5 of the leading term in the inverse-power long-range potential is now found to be C 5 = 1.16(2) × 10 5 A ̊ 5 cm −1 . Theoretical results for the variation of centrifugal distortion parameters for levels near dissociation are tested for D v and H v , and an extrapolation based on this behavior is used to facilitate determination of reliable B v and G( v) values for the highest observed B-state levels.

NO and CF2 fluorescence during vacuum ultraviolet photodissociation of CF2ClNO

Photolysis of CF2ClNO at 147 and 123 nm results in electronically excited fragments, fluorescence from which is observed. At 147 nm NO (B2Π) is produced, giving rise to the well known NO β-bands, contrasting with previous studies on CF3NO, in which NO (A2∑+) dominated. Addition of non-reactive gases (e.g., He) results in quenching of fluorescence from the higher vibronic levels of the NO (B2Π) state, while at the same time fluorescence from NO(A2∑+) becomes increasingly important. Photolysis at 123 nm give a very different emission spectrum, the most important contributor being the (1B1) state of CF2. Ionization also occurs at 123 nm (10.03 eV) with a quantum yield of ca. 0.08. These results are compared and contrasted with data obtained previously for CF3NO.

Laser-induced fluorescence studies: the B–X transition of Cl2. Part 5.—Radiative lifetime of the B state

A redetermination of the radiative lifetime of the long-lived B3Π(Ou+)state of 35Cl35Cl is described. Pulses of narrow-band radiation from a scanning dye laser (> 1 mJ pulse–1) were used to excite the v′= 5, J′= 16 state of Cl2(B). Decay of fluorescence in real time was used to determine τR for the v′= 5 state: τR=(305 ± 15)µs (1σ). Reasons for error in previous work are discussed. The long radiative lifetime of Cl2(B) is consistent with estimates based on absorption coefficient measurements in the ultraviolet continuum of Cl2.

Laser-induced fluorescence studies: the B–X transition of Cl2. Part 6.—Rotationally-dependent predissociation

The collision-free lifetimes of ro-vibrational levels (v′,J′) of excited Cl2B3Π(0u+) are strongly dependent upon rotational and vibrational quantum numbers for the predissociated levels v′ > 12: τ–100=τ–1R+kv′J′(J′+ 1). Those states with zero rotation (J′= 0) are not predissociated and have lifetimes equal to the radiative lifetime, τR≃ 305 µs. The rotationally-dependent predissociation is assigned to a heterogeneous interaction (|ΔΩ|= 1) of the bound B3Π(0u+) state and one or more 1Π(1u) states. Homogeneous predissociation (|ΔΩ|= 0), which is forbidden by selection rules, is not present in the B state of Cl2.The lifetime of the J′= 0 state shows a strong dependence on pressure of Cl2, which is interpreted in terms of rapid collisional pre-dissociation and rotational energy transfer into states J′ 1 which have negligible fluorescence quantum yields. The rate constant for overall transfer out of the v′= 13 state was found to be: kCl2=(2.8 ± 0.1)× 10–10 cm3 molecule–1 s–1.

Lifetime of the B3Π state of ZrO

The radiative lifetime of the J′ = 77, v′ = 0 level of the B3Π state of ZrO is 32.5 ± 2 nsec, as measured by the decay of resonant fluorescent radiation of vapor in a furnace. Transition rates and oscillator strengths have been calculated for the β-system (B3Π - X13Δ).

Angular distributions, energy disposal, and branching studied by photoelectron–photoion coincidence spectroscopy: O2+, NO+, ICl+, IBr+, and I2+ fragmentation

It is shown that some levels of the A2Πu states of I2+, IBr+, and ICl+ are stable toward dissociation, and ought to decay by emission. The B2Σg+ state of I2+ is fully dissociated to ground state I++I, while the B2Σg+ states of IBr+ and ICl (probably) yield excited decay products. The c3Π and B1Π states of NO+ dissociate to ground state N++O fragments, while the c4Σg− and 2Πu states of O2+ near 24 eV dissociate to excited levels of O++O. The origin of anomalous time-of-flight peak shapes is discussed, and several examples are attributed to anisotropic fragment angular distributions.

Selective Enhancement and Quenching Effects of Sulphur Hexafluoride on the Recombinative Emission of Nitrogen and Oxygen Atoms

Abstract Addition of sulphur hexafluoride to the nitrogen afterglow was found to selectively quench the δ and γ bands of NO while enhancing markedly the β bands involving v' = 1 and 2 levels. These effects could be satisfactorily explained on the basis of vibrational relaxation in the a4π state brought about by collisions with SF6 and the interactions of this state with the other excited states which are involved in the emission of the above band systems. Certain peculiar spectral features were also observed at the crossing of the b4σ− with the B2π state.

Rotational analysis of four bands of the γ′(B3Π–X3Δ) system of TiO

The (0,0), (1,0), (0,1), and (1,1) bands of the γ′(B3Π–X3Δ) system of TiO have been rotationally analysed up to J = 55. Accurate molecular constants have been determined for the ν = 0and ν = 1 levels of the B3Π and X3Δ states. The three substates of the B3Π state all show significant Λ-type doubling, which has been analysed using a full matrix treatment. Our new results give corrected values for the ground state spin–orbit splittings; they also illustrate very clearly the problem of the correlation of the various centrifugal distortion parameters appearing in the Hamiltonian for a triplet electronic state.

Laser-induced fluorescence studies: B–X transition of Cl2. Part 2.—Transition from bound to unbound excited states

Laser-induced fluorescence has been used to determine the collision-free lifetimes of resolved ro-vibrational levels (v′, J′) of the excited B3Π(0+u) states of 35Cl35Cl and 35Cl37Cl. Predissociation affects all vibrational states above v′= 12, giving undetectably weak fluorescence and lifetimes estimated to be 3 ns.

Le spectre électronique de PS. Première partie : le systeme B2Π–X2Π

The electronic transition B2Π–X2Π of PS has been studied in the region 3450–5700 Å. One hundred fourteen sub-bands have been put in the vibrational classification. They arise from 13 levels (ν = 0 to 12) of the B2Π state and from 10 levels (ν = 0 to 9) of the X2Π state.The rotational analysis of 50 sub-bands allows the determination of the equilibrium constants of the two states and presents the perturbations in the levels ν > 9 of the B2Π state.

Quantum-resolved dynamics of excited states. Part 1.—Predissociation in the B3Π(0+) state of BrF

Pulses from a narrow-band tunable dye laser (5–10 ns in duration; 0.04 cm–1 band width) have been used to excite fluorescence from the vibrational levels v′= 7 and 8 of the excited B3Π(0+) states of 79BrF and 81BrF. Individual rotational levels were readily excited, and the corresponding decay lifetimes were measured.The lower rotational levels (J′⩽ 23) of v′= 7 were stable, with an extrapolated zero-pressure lifetime τ0=(22.4 ± 3.3)µs for 21 J′ 4. Onset of predissociation was found near J′= 28. Above this level, τ0 shortened systematically with increase in rotational energy, giving τ0=(0.37 ± 0.10)µs for the highest rotational level studied (J′= 34).All rotational levels of the v′= 8 state were predissociated, and τ0 values were found to shorten systematically from (1.74 ± 0.15)µs for J′= 1 to 3 down to (0.11 ± 0.06)µs for J′= 31. A linear dependence of 1/τ0 upon rotational energy [found previously for the natural predissociation in B3Π(0+u)] was not observed for either the levels v′= 8, or for v′= 7 in the B3Π(O+) state of 79BrF. An improved upper limit for the dissociation energy of 79BrF is reported.Non-exponential fluorescence decay curves were seen only for excitation of rotational levels of BrF (B, v′= 7) whose energy is either above, or somewhat below, the minimum predissocation energy. These decay curves are interpreted in terms of near collisional rates of upward and downward rotational energy (R–T) transfer of excited BrF(B) with helium gas at pressures between 50 and 400 mTorr. The results indicate that collisional rotational relaxation of 79BrF, B3Π(0+), occurs with a high probability of multi-quantum transitions (ΔJ⩽± 10). Vibrational relaxation is slow and has an appreciable effect only on the fluorescence decay curves following excitation in the 8–0 band at pressures in the 500 mTorr range.