Excited Cl Research Articles

Crossed beam experiment on the validity of Born-Oppenheimer approximation in Cl(2P)+D2→DCl+D reaction

The reaction of chlorine atom Cl(2P) (Cl(2P3/2) and Cl∗(2P1/2)) with D2 was investigated at collision energy from 4.5 kcal/mol to 6.5 kcal/mol with a high-resolution crossed molecular beam apparatus using the technique of D-atom Rydberg tagging detection. The contribution from the spin-orbit excited reaction Cl∗(2P1/2)+D2, which is prohibited by Born-Oppenheimer (BO) approximation, was observed. Collision-energy dependence of differential cross sections (DCSs) near the backward scattering direction was measured. The BO-forbidden reaction Cl∗+D2 was found to be dominant at lower collision energy. As collision energy increases, reactivity of BO-allowed reaction Cl+D2 increases much faster than that of BO-forbidden reaction and becomes dominant at higher collision energy. Our experiment indicates that the additional energy of spin-orbit excitation in Cl∗ facilitates BO-forbidden reaction to pass through the barrier at lower collision energy, while BO approximation is still valid at collision energy near and above the reaction barrier. This tendency of reactivity of Cl/Cl∗+D2 is similar to the isotopic reaction of Cl/Cl∗+H2.

Mixed transitions in the UV photodissociation of propargyl chloride revealed by slice imaging and multireference ab initio calculations.

Resonance-enhanced multiphoton ionization (REMPI) and DC slice imaging were used to detect photoproducts Cl (2P3/2), spin-orbit excited Cl* (2P1/2), and C3H3 in the photodissociation of propargyl chloride at 212 and 236 nm. Cl and Cl* translational energy distributions peak at high recoil energy, suggesting impulsive energy release in the C-Cl coordinate. Near 236 nm, photofragment angular distributions show rapidly changing anisotropy across the main peak for both Cl and Cl*, indicating excitations arising from in-plane or out-of-plane π systems. At 212 nm, the distribution is broader and isotropic for both Cl products. Ionization of C3H3 at 212 nm via 1 + 1 REMPI provides additional insight into these processes. A portion of the C3H3+ distribution is momentum-matched to the Cl, but in addition, there is a component assigned to cyclic C3H3+ formed from dissociative ionization of propargyl chloride. Multireference ab initio calculations show that excitations to three triplet states contribute at the long wavelength region, while predissociation from the first excited singlet state plays a role at the shorter wavelength.

High sensitivity ultra-broad-band absorption spectroscopy of inductively coupled chlorine plasma

We propose a method to measure the densities of vibrationally excited Cl2(v) molecules in levels up to v = 3 in pure chlorine inductively coupled plasmas (ICPs). The absorption continuum of Cl2 in the 250–450 nm spectral range is deconvoluted into the individual components originating from the different vibrational levels of the ground state, using a set of ab initio absorption cross sections. It is shown that gas heating at constant pressure is the major depletion mechanism of the Cl2 feedstock in the plasma. In these line-integrated absorption measurements, the absorption by the hot (and therefore rarefied) Cl2 gas in the reactor centre is masked by the cooler (and therefore denser) Cl2 near the walls. These radial gradients in temperature and density make it difficult to assess the degree of vibrational excitation in the centre of the reactor. The observed line-averaged vibrational distributions, when analyzed taking into account the radial temperature gradient, suggest that vibrational and translational degrees of freedom in the plasma are close to local equilibrium. This can be explained by efficient vibrational-translational (VT) relaxation between Cl2 and Cl atoms. Besides the Cl2(v) absorption band, a weak continuum absorption is observed at shorter wavelengths, and is attributed to photodetachment of Cl− negative ions. Thus, line-integrated densities of negative ions in chlorine plasmas can be directly measured using broad-band absorption spectroscopy.

Near-threshold photodissociation dynamics of CHCl3

Energy- and angle-resolved photofragment distributions for ground-state Cl ((2)P3/2) and spin-orbit excited Cl* ((2)P1/2) have been recorded using the velocity map imaging technique after photodissociation of chloroform at wavelengths of 193 and ∼235 nm. Translational energy distributions are rather broad and peak between 0.6 and 1.0 eV. The spin-orbit branching ratios [Cl*]/[Cl] are 1 and 0.3 at 193 and 235 nm, respectively, indicating the involvement of two or more excited state surfaces. Considering the anisotropy parameters and branching ratios collectively, we conclude that the reaction at 193 nm takes place predominantly on the (1)Q1 surface, while the (3)Q1 surface gains importance at lower dissociation energies around 235 nm.

Models for Intrinsic Non-RRKM Dynamics. Decomposition of the SN2 Intermediate Cl––CH3Br

Chemical dynamics simulations, based on both an analytic potential energy surface (PES) and direct dynamics, were used to investigate the intrinsic non-RRKM dynamics of the Cl−–CH3Br ion-dipole complex, an important intermediate in the Cl− + CH3Br SN2 nucleophilic substitution reaction. This intermediate may dissociate to Cl− + CH3Br or isomerize to the ClCH3–Br− ion-dipole complex. The decomposition of microcanonical ensembles of the Cl−–CH3Br intermediate were simulated, and the ensuing populations vs. time of the excited intermediate and Cl− + CH3Br and ClCH3–Br− products were fit with multi-exponential functions. The intrinsic non-RRKM dynamics is more pronounced for the simulations with the analytic PES than by direct dynamics, with the populations for the former and latter primarily represented by tri- and bi-exponential functions, respectively. For the analytic PES and direct dynamics simulations, the intrinsic non-RRKM dynamics is more important for the isomerization pathway to form ClCH3–Br− than for dissociation to Cl− + CH3Br. Since the decomposition probability of Cl−–CH3Br is non-exponential, the Cl−–CH3Br unimolecular rate constant depends on pressure, with both high and low pressure limits. The high pressure limit is the RRKM rate constant and for the simulations with the analytic PES the rate constant decreased by a factor of 3.0, 5.6, and 4.3 in going from the high to low pressure limit for total energies of 40, 60, and 80 kcal/mol. For the direct dynamics simulations these respective factors are 2.4, 1.4, and 1.2. A separable phase space model with intermolecular and intramolecular complexes describes some of the simulation results, but overall models advanced for intrinsic non-RRKM dynamics give incomplete representations of the intermediate and product populations vs. time determined from the simulations.

The vibrationally mediated photodissociation of Cl2

The photodissociation of vibrationally excited Cl(2)(v = 1) has been investigated experimentally using the velocity mapped ion imaging technique. The experimental measurements presented here are compared with the results of time-dependent wavepacket calculations performed on a set of ab initio potential energy curves. The high level calculations allow prediction of all the dynamical information regarding the dissociation, including electronic polarization effects. Using a combination of theory and experiment it was found that there was negligible cooling of the vibrational degree of freedom of the parent molecule in the molecular beam. The results presented are compared with those following the photodissociation of Cl(2)(v = 0). Although the same electronic states are found to be important for Cl(2)(v = 1) as for Cl(2)(v = 0), significant differences were found regarding many of the observables. The overall level of agreement between theory and experiment was found to be reasonable and confirms previous assignments of the photodissociation mechanism.

Laser-induced fluorescence study of spin–orbit quenching of Cl(2P1/2) by H2, D2, and HD gases

Employing ultraviolet laser flash photolysis in combination with pulsed vacuum ultraviolet laser induced-fluorescence spectroscopy room-temperature rate constants kQ,H2=(5.8±0.6)×10−11cm3s−1, kQ,D2=(5.6±0.6)×10−12cm3s−1, and kQ,HD=(2.7±0.3)×10−11cm3s−1 for electronically non-adiabatic quenching of spin–orbit excited Cl(2P1/2) atoms in collisions with H2, D2, and HD gases were determined, which allow for comparison with results of recent quantum-scattering calculations [14,15].

Translational energy dependence of the Cl + CH4(v b = 0, 1) reactions: a joint crossed-beam and quasiclassical trajectory study

The Cl + CH4(v b = 0, 1) reactions were studied over a wide range of collision energies, from threshold up to 20 kcal mol−1, using joint experimental and theoretical methods. Experiments were performed under crossed-beam conditions using a time-sliced velocity imaging detection method. Both the pair-correlated integral and differential cross sections were measured. Theoretically, quasiclassical trajectory calculations were performed on a highly accurate ab initio potential energy surface. The computed results show very good agreement with experimental findings. The underlying reaction mechanisms are as below: the formation of HCl(v′ = 0) products is mainly governed by direct scatterings, whereas the HCl(v′ = 1) channel is mediated by a time-delayed mechanism, likely invoking reactive resonances. In addition, the spin-orbit excited Cl*(2 P 1/2) reactivity was experimentally characterized. The results compare favourably with a recently reported reduced dimensionality quantum dynamics calculation, and with the previous reports on the isotopically analogous reactions. Possible involvement of resonances in this spin-orbit nonadiabatic process is suggested and awaits further investigations.

The photodissociation dynamics of tetrachloroethylene

We present a direct current slice imaging study of tetrachloroethylene (C(2)Cl(4)) photodissociation, probing the resulting ground state Cl ((2)P(3/2)) and spin-orbit excited state Cl* ((2)P(1/2)) products. We report photofragment images, total translational energy distributions and the product branching ratio of Cl*/Cl following dissociation at 235 and 202 nm, obtained using a two-color reduced-Doppler dissociation/probe. Near 235 nm, the Cl translational energy distribution shows a peak at the limit of the available energy, indicating a direct dissociation through a σ*(C-Cl) ← π (C=C) transition, which is superimposed on a broader underlying distribution. The ground state Cl image and associated translational energy distribution at 202 nm is broad and peaked at lower energy, suggesting either internal conversion to the ground state or a lower excited state prior to dissociation. The Cl* images are similarly broad at both wavelengths. The branching ratio is presented as a function of recoil energy, but after integration shows a near-statistical average of Cl:Cl* as 70:30 at both wavelengths. All the images are largely isotropic, with anisotropy parameters (β) of 0.05 ± 0.03.

Excited Cl(2P1/2) Atoms: Yield from the Photodissociation of SOCl2 and Collisional Deactivation by NO2, CCl3H, C2H4, C3H6, and SOCl2

Rate constants for the collisional deactivation of spin-orbitally excited Cl* ( identical withCl((2)P(1/2))) atoms by some selected gases at T = 298 K have been determined using time-resolved laser magnetic resonance (LMR) techniques. Cl* atoms were produced by photodissociation of SOCl(2) at 248 nm, and the relative quantum yield of Cl* atoms is determined to be 0.52 +/- 0.03. This yield is much larger than the yield at 235 nm (0.35 +/- 0.06). The rate constants for the relaxation of Cl* (x10(-11) cm(3)/s, +/- 2sigma) by NO(2)(1.5 +/- 0.4), C(2)H(4) (18 +/- 5), CCl(3)H (1.8 +/- 0.4), CH(3)-CH=CH(2) (16 +/- 4), and SOCl(2) (0.62 +/- 0.2) are reported for the first time. All of them are pressure-independent, and in all cases the dominant channel is physical quenching. It was established that the spin-orbital excitation of chlorine atoms decreases the probability of chemical reactions in collisions with propylene molecules. The rate constants of the reactions of ground state Cl((2)P(3/2)) atoms with C(2)H(4), CH(3)-CH=CH(2), and SOCl(2) at T = 298 K were found to be (5.0 +/- 1) x 10(-30) cm(6)/s (P(Ar) = 8-15 Torr), (5.2 +/- 1) x 10(-11) cm(3)/s (P(Ar) = 9-12 Torr), and (6 +/- 4) x 10(-14) cm(3)/s, respectively; the first one is termolecular, the last two are bimolecular, and the buffer gas is Ar.

Deactivation Study of Electronically Excited Cl(2p1/2) Atoms by Socl2 , Ccl3h, C2h4 , No2 Molecules with Laser Magnetic Resonance (Lmr) Technique

The method of time-resolved laser magnetic resonance (LMR) has been employed to detect spin-orbital excited chlorine Cl*(≡Cl(2P1/2)) atoms at room temperature. The rate constants for the deactivation of Cl* 11 3 10 cm /molecule , 2 s by SOCl2(0.62 0.2), CCl3H (18 5), C2H4(1.5 0.4) and NO2 (1.5 0.4) are reported. The unknown in literature rate constant for the thermo-neutral reaction of ground state Cl(2P3/2) atoms with SOCl2 were measured. The relative quantum yield of Cl* in photodissociation of SOCl2 is determined to be 0.52 ± 0.03.

The photochemical decomposition of gaseous OClO at 436 nm: Dependence of products distribution on total pressure

A systematic experimental study of the photochemical decomposition of 5–80 Torr gaseous OClO at 293 and 303 K has been performed. The photolysis was also investigated in the presence of the added inert gases He, Ar, N 2, O 2, CO 2 and SF 6 at pressures ranging from 10 to 600 Torr. The final products Cl 2O 4, Cl 2O 6, Cl 2 and O 2 are formed. The amount of Cl 2O 4 and Cl 2O 6 was determined by IR and UV spectrophotometry, respectively. Both products reach a limit of about 30% of the total yield. Only very small amounts of ClClO 2 have been detected. The quantum yields for Cl 2O 4 and Cl 2O 6 formation satisfy the condition Φ(Cl 2O 4) + Φ(Cl 2O 6) ≅ 1 throughout. In the absence of an inert gas Φ(Cl 2O 4) decreases from a value close to 1 to about 0.5 as OClO pressure is decreased, indicating the formation of Cl 2O 4 by the reaction ClO 3 + ClO → Cl 2O 4. For about 5 Torr OClO mean pressure and in the presence of an inert gas, Φ(Cl 2O 4) increases as total pressure is increased reaching a value close to 1 for 300 Torr of N 2 or 50 Torr of SF 6. At the higher inert gas pressures Φ(Cl 2O 4) slightly decreases when OClO is increased. This result is attributed to the interplay established between reactive and collisional deactivation process in the excited radical intermediate Cl 2O 5 * generated in the process ClO 3 * + OClO → Cl 2O 5 * → ClO 4 + ClO. The generally accepted reaction for the formation of Cl 2O 6, namely 2ClO 3 → Cl 2O 6, can be ruled out and a complicated mechanism is needed to account for the experimental observations.

Is spin-orbit state branching in the photodissociation of CCl 4 isotope specific?

We have reinvestigated the ultraviolet photodissociation of CCl 4 around 235 nm. The yield of spin-orbit state excited Cl*( 2P 1/2) atoms was found to be 0.26 ± 0.03, independently of the isotopic mass of the chlorine atom. No evidence of isotope specific Cl* yields was observed. Our results suggest that the previously reported isotope specificity in the photodissociation of CCl 4 does not exist.

Kinetic study of the collisional quenching of spin–orbitally excited atomic chlorine, Cl( 2P 1/2), by H 2O, D 2O, and H 2O 2

Rate constants for the collisional quenching of spin–orbitally excited Cl( 2P 1/2) atoms by H 2O, D 2O, and H 2O 2 were determined at ∼295 K using laser-flash photolysis and laser-induced fluorescence techniques in the vacuum ultraviolet energy region. The rate constants reported for the quenchers of H 2O, D 2O, and H 2O 2, are (3.9 ± 0.3) × 10 −11, (4.7 ± 0.3) × 10 −11, and (3.1 ± 0.3) × 10 −10 cm 3 molecules −1 s −1, where the uncertainties correspond to 1 σ, respectively.

Photodissociation of state selected BrCl+ cations: branching ratios and angular anisotropies of the Br+ product forming channels

High resolution velocity map ion imaging methods have been used to explore the photofragmentation dynamics of state-selected BrCl+(X2ΠΩ, v=0) cations following excitation in the wavelength range 370–420 nm. The parent ions are formed by 2 + 1 resonance enhanced multiphoton ionization (REMPI) via the [2Π1/2]5sσ, v=0 Rydberg level. Experimental measurables include the wavelength dependent branching ratios and recoil anisotropies of the various Br+(3 P J ) + Cl(2 P J ) product channels. Ground state Br+(3 P 2) fragments dominate the fragment ion yield throughout the photolysis wavelength range investigated, with spin–orbit excited Cl(2 P 1/2) atoms the preferred co-fragment whenever energetically allowed. All fragmentation channels show a predominantly parallel recoil anisotropy, consistent with an initial A2Π ← X2Π (ΔΩ=0) excitation in the parent cation, but the best-fit description of the fragment angular distributions requires use of higher terms in the Legendre expansion than is traditionally the case for a one-photon dissociation process. The additional angular modulation is rationalised by recognizing that the REMPI preparation laser induces unwanted (but unavoidable) parent ion fragmentation, leaving a spatially aligned distribution of ions for the intended photolysis study. Interpretation of the measured product branching ratios and recoil anisotropies is guided by complementary ab initio calculations of spin–orbit free potential energy curves for many of the lower lying electronic states of BrCl+ and subsequent semi-empirical inclusion of spin–orbit effects. The adiabatic potentials so derived exhibit a plethora of avoided crossings. Preliminary wavepacket propagations following excitation to the A2Π state show rapid flux redistribution to adiabatic potentials that correlate with the lower dissociation asymptotes, but are unable to reproduce the details of the experimentally determined product branching ratios.

Photodissociation of state selected BrCl cations: Branching ratios and angular anisotropies of the Br product forming channels

High resolution velocity map ion imaging methods have been used to explore the photofragmentation dynamics of state-selected BrCl+(X2ΠΩ, v=0) cations following excitation in the wavelength range 370-420 nm. The parent ions are formed by 2 + 1 resonance enhanced multiphoton ionization (REMPI) via the [2Π1/2]5sσ, v=0 Rydberg level. Experimental measurables include the wavelength dependent branching ratios and recoil anisotropies of the various Br+(3PJ) + Cl(2PJ) product channels. Ground state Br+(3P2) fragments dominate the fragment ion yield throughout the photolysis wavelength range investigated, with spin-orbit excited Cl(2P1/2) atoms the preferred co-fragment whenever energetically allowed. All fragmentation channels show a predominantly parallel recoil anisotropy, consistent with an initial A2Π←X2Π (ΔΩ=0) excitation in the parent cation, but the best-fit description of the fragment angular distributions requires use of higher terms in the Legendre expansion than is traditionally the case for a one-photon dissociation process. The additional angular modulation is rationalised by recognizing that the REMPI preparation laser induces unwanted (but unavoidable) parent ion fragmentation, leaving a spatially aligned distribution of ions for the intended photolysis study. Interpretation of the measured product branching ratios and recoil anisotropies is guided by complementary ab initio calculations of spin-orbit free potential energy curves for many of the lower lying electronic states of BrCl+ and subsequent semi-empirical inclusion of spin-orbit effects. The adiabatic potentials so derived exhibit a plethora of avoided crossings. Preliminary wavepacket propagations following excitation to the A2Π state show rapid flux redistribution to adiabatic potentials that correlate with the lower dissociation asymptotes, but are unable to reproduce the details of the experimentally determined product branching ratios.

Kinetics of the Reactions of Cl*(2P1/2) and Cl(2P3/2) Atoms with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH at 295 K

The title reactions were studied using laser flash photolysis/laser-induced-fluorescence (FP-LIF) techniques. The two spin-orbit states, Cl*(2P(1/2)) and Cl(2P(3/2)), were detected using LIF at 135.2 and 134.7 nm, respectively. Measured reaction rate constants were as follows (units of cm3 molecule(-1) s(-1)): k(Cl(2P(3/2))+CH3OH) = (5.35 +/- 0.24) x 10(-11), k(Cl(2P(3/2))+C2H5OH) = (9.50 +/- 0.85) x 10(-11), k(Cl(2P(3/2))+n-C3H7OH) = (1.71 +/- 0.11) x 10(-10), and k(Cl(2P(3/2))+i-C3H7OH) = (9.11 +/- 0.60) x 10(-11). Measured rate constants for total removal of Cl*(2P(1/2)) in collisions with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH were (1.95 +/- 0.13) x 10(-10), (2.48 +/- 0.18) x 10(-10), (3.13 +/- 0.18) x 10(-10), and (2.84 +/- 0.16) x 10(-10), respectively; quoted errors are two-standard deviations. Although spin-orbit excited Cl*(2P(1/2)) atoms have 2.52 kcal/mol more energy than Cl(2P(3/2)), the rates of chemical reaction of Cl*(2P(1/2)) with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH are only 60-90% of the corresponding Cl(2P(3/2)) atom reactions. Under ambient conditions spin-orbit excited Cl* atoms are responsible for 0.5%, 0.5%, 0.4%, and 0.7% of the observed reactivity of thermalized Cl atoms toward CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH, respectively.

DC Slice Imaging of CH3Cl Photolysis at 193.3 nm

The DC slice imaging method is used to record energy- and angle-resolved distributions for both ground-state Cl (2P3/2) and spin−orbit excited Cl* (2P1/2) photofragments produced from the A-band photolysis of CH3Cl at 193.3 nm. The dissociation is found to be highly impulsive, with ∼90% of the available energy being released into translation. The angular distributions are predominantly perpendicular in the case of both the ground state and spin−orbit excited chlorine atom fragments, as characterized by recoil-energy averaged values of β = −0.46 and β* = −0.74 respectively, although we find considerable variation in both channels as a function of CH3 internal energy. In contradiction with previous measurements, we conclude that the transition to the 1Q1 state dominates much of the A-band profile, and this has significant implications for the description of the dissociation dynamics. Our findings are rationalized in terms of similar measurements performed with other methyl halides and their H atom substitut...

Recoil velocity-dependent spin–orbit state distribution of chlorine photofragments

Abstract We present the results of experimental and theoretical studies of the speed-dependent spin–orbit state distributions of chlorine photofragments produced in the photodissociation of thiophosgene (CSCl 2 ) at 235 nm. Three-dimensional imaging has been employed for observing chlorine photofragments in their ground (Cl) and excited (Cl * ) spin–orbit states. The kinetic energy distributions for Cl and Cl * fragments reflect excitation of several electronic states of the partner fragment CSCl. The spin–orbit branching ratio of P (Cl * )/[ P (Cl) + P (Cl * )] was found to depend on the kinetic recoil energy increasing from 0.1 for low kinetic energy to 0.8 for high kinetic energy. The theoretical interpretation is based on the computation of the CSCl 2 potential energy surfaces (PES) along the C–Cl bond. Two completely different methods of determination of the PES were applied for small and for large values of the C–Cl bond separation R . In case of small and intermediate R values time-dependent density-functional theory has been used. In case of large R values we used an asymptotic method of computation of the PES, which is a generalisation of the Heitler–London approach for many-electron systems. Basis molecular wavefunctions with definite values of the total spin S and the spatial and spin reflection symmetry σ v with respect to reflection of the total electronic wavefunction in the molecular plane were used. The developed theoretical approach was used for the assignment of the molecular states involved in the photodissociation and for the qualitative explanation of the non-statistical population of the spin–orbit states of the chlorine photofragments as function of the kinetic energy. The spin–orbit branching ratio of P (Cl * )/[ P (Cl) + P (Cl * )] predicted by the theory strongly depends on the quantum state of the CSCl fragment. It is large in case of the CSCl(X) + Cl and CSCl(A) + Cl channels and small in case of the CSCl(B) + Cl channel which explains the experimental results.

High resolution ion imaging study of BrCl photolysis in the wavelength range 330–570 nm

The photodissociation of jet-cooled BrCl molecules has been investigated at many different excitation wavelengths in the range 330–570 nm by velocity map imaging of the ground state Br and ground and spin–orbit excited Cl atom products. Image analysis confirms literature values for the energies of the ground, Br(2P3/2)+Cl(2P3/2), and first excited [Br(2P3/2)+Cl(2P1/2)] dissociation asymptotes, and provides measures of the electronic branching into these two active product channels, and the recoil anisotropy of each set of products, as a function of parent vibrational level (v″⩽2) and excitation wavelength. The availability of such experimental data allows mapping of the partial cross-sections for parallel (i.e., ΔΩ=0) and perpendicular (i.e., ΔΩ=±1) absorption as a function of excitation energy, and thus deconvolution of the room temperature parent absorption spectrum into contributions associated with excitation to the A 3Π(1), B 3Π(0+), and C 1Π(1) excited states of BrCl. This analysis of the continuous absorption spectrum of BrCl, when considered in conjuction with existing spectroscopic data for the ground electronic state and for the bound levels supported by the B state potential, allows determination of key regions of the potential energy curves for, and transition moments to, each of these three excited states. Further wave packet calculations, which reproduce the experimentally measured wavelength dependent product channel branching ratios and product recoil anisotropies very well, serve to validate the excited state potential energy functions so derived and allow estimation of the strength (∼80 cm−1) of the coupling between the bound (B) and dissociative (Y) diabatic states of 0+ symmetry.