ALBERT

Notes: The lifetimes of the low-lying vibrational levels of the X2Π state of the recently identified dication HS2+ [Miller et al., Int. J. Mass Spectrom. Ion Proc. 100, 505 (1990)] are considered. The stability of this state is attributable to a barrier formed from the avoided crossing of 2Π states asymptotically characterized as H++S+ and H+S2+. As a result of this barrier, the nonrelativistic X2Π potential energy curve supports several quasibound vibrational levels that are long lived with respect to tunneling. However, this is not the principal decay mechanism. We show that the lifetimes of the low-lying vibrational levels, v=0−4, are controlled entirely by the spin–orbit induced perturbation, 14Σ−∼X2Π, and the corresponding allowed crossing of the X2Π potential energy curve by the dissociative 14Σ− potential energy curve which correlates with the ground state asymptote H++S+(4S).

Notes: In this work the fine structure splitting of the X 3Σ− state of NH together with the spin-forbidden dipole-allowed radiative transitions(b 1Σ+,a 1Δ)→X 3Σ− in that system are considered. In addition the spin-allowed A 3Π→X 3Σ− and c 1Π→(b 1Σ+,a 1Δ) transitions which provide valuable optical probes of the NH radical are studied. Symbolic matrix methods permit the use of large configuration state function (CSF) spaces (170–280×103 CSFs) in characterizing these effects. The fine structure splitting and spin-forbidden decay are described within the context of the Breit–Pauli approximation. In the determination of the fine structure splitting both HˆSO, the full microscopic spin–orbit and spin-other-orbit operator and HˆSS, the dipolar spin–spin operator, are considered through second order in pertubation theory. Thecompletely ab initio determination of λ0≡[E(X 3∑−1, v=0)−E(X 3∑−0+ , v=0)]/2, presented here gives λ0=0.903 (0.9198) cm−1 in good accord with the experimental value given parenthetically. The predicted radiative lifetimes for the v=n level of the A 3Π state τn (A 3Π)(corresponding to A 3Π, v=n→X 3Σ−) are τ0(A 3Π)=392[418±8, 453±10]ns and τ1(A 3Π)=438[420±35, 488±10] ns in good agreement with the experimental values given parenthetically. The predicted radiative rate for the v=0 level of the c 1Π state is somewhat slower than the total decay rate measured experimentally suggesting predissociation of even the lowest rotational levels. The radiative lifetime for the v=0 level of the a 1Δ state, τ0(a 1Δ) corresponding to the spin-forbidden dipole-allowed transition a 1Δ2→X 3∑−1 was found to be τ0(a 1Δ)=2.18(〉1.9)s which compares favorably with the lower bound determined from matrix isolation experiments given parenthetically.For the b 1Σ+→X 3Σ−(0,0) transition the ratio of the parallel to the perpendicular transition moment was found to be μ(parallel)/μ⊥=−0.30 (−0.35±0.05) which again compares favorably with the experimental result given parenthetically. This result is qualitatively different from that in the isovalent systems NF, NCl, and NBr for which −μ(parallel)||μ⊥ 〉1. A partial explanation for this result in terms of molecular dipole moments is offered. The predicted radiative lifetime for the b 1Σ+, v=0→X 3Σ− transition τ0(b1 Σ+)=100 ms, which is in excellent agreement with the independent theoretical determination τ0(b 1Σ+)=97 ms of Marian and Klotz, is significantly longer than the most recent gas phase measurement τ0(b 1Σ+)=53(+17−13)ms.

Notes: The nonadiabatic charge transfer reaction H++NO→H+NO+ is considered. Regions of significant nonadiabatic effects are located for specific C∞v nuclear configurations which correspond to the confluence of three potential energy surfaces, the 1,2 2A' and 1 2A‘ potential energy surfaces. The 2 2A' and 1 2A‘ states correlate asymptotically with the H++NO(2Π) system states while the 1 2A' state correlates asymptotically with the H(2S)+NO+(1Σ+) system state. The three surface confluence consists of two seams corresponding to hydrogen approaching NO from either the nitrogen or oxygen end and is energetically accessible from the H++NO(2Π) asymptote for re(NO+)≤R(NO)≤re(NO). The region of the three surface confluence represents a C∞v symmetry allowed Σ–Π crossing and consequently evinces properties of both a conical intersection and a Renner–Teller surface touching. It is therefore necessary to treat nonadiabatic (derivative) couplings originating from both internal nuclear motion and overall nuclear rotation. The rotational coupling provides a mechanism for the direct (and indirect via the 2 2A' state) coupling of the 1 2A‘ state correlating with H++NO(2Π) to the 1 2A' state correlating with H+NO+(1Σ+). No such coupling is possible if only internal modes are considered. Derivative couplings attributable to internal nuclear motion are evaluated using analytic gradient techniques introduced previously [Chem. Phys. Lett. 113, 159 (1985)] while derivative couplings attributable to overall nuclear rotation, which can also be evaluated using gradient methods, are evaluated, more efficiently, in terms of matrix elements of the total electronic angular momentum operator. The nonadiabatic interactions in the vicinity of the three surface confluence are compared with the analogous interactions along paths leading to the HNO+ and NOH+ equilibrium structures. These are regions of general Cs nuclear configurations for which avoided crossings are observed. An analysis of the wave functions in terms of the molecular dipole moment vector is presented.

Notes: A recently developed method for treating spin-forbidden electronic transitions within the Breit–Pauli approximation is shown to provide a means for characterizing such processes when the transitions derive their intensity by coupling to states embedded in a continuum. The success of this approach is attributable principally to the need to specify only the configuration state function space, rather than its spectrum relative to Hˆ0, in order to obtain the first order perturbation contribution (Ψ1I) to the wave function. Here Hˆ0 is the nonrelativistic Born–Oppenheimer Hamiltonian and Ψ1I is the solution of (Hˆ0−E0I) Ψ1I=−HˆsoΨ0I where Hˆso is the full microscopic spin–orbit portion of the Breit–Pauli interaction. A method for improving the molecular orbital basis used to describe Ψ1I based on the iterative natural orbital (INO) procedure is introduced. The a 1Δ→X 3Σ− transition in CH− was considered. Using the INO procedure, it was found that the optimum orbital space for describing Ψ1I includes a molecular orbital with character intermediate between the compact valence orbital and the diffuse orbital obtained from two alternative MCSCF procedures. Equivalent INO orbitals were obtained from these two distinctly different starting points. Using the INO orbital set, a total radiative rate for the ground vibrational level of the a 1Δ state of 0.163 s−1 was obtained which gives a lifetime, τ=6.14(±1.2) s. This value is in excellent agreement with the experimental value τ=5.9(+0.8, −0.6) s reported by Okumura et al.

Notes: Lifetimes for excited ND A 3Πv'=1–3 and NH v'=2 in specified rotation/fine-structure levels were determined by excitation on isolated rotational lines in the A 3Π–X 3Σ−Δv =+1 sequence, using molecules prepared in a pulsed supersonic beam. The observed lifetimes for NH v'=2 levels were found to be significantly shorter than those expected for purely radiative decay, indicative of additional removal of excited-state population by predissociation. The observed fine-structure dependence of the removal rates is consistent with a mechanism in which the A 3Π state is predissociated by spin–orbit coupling to the repulsive 1 5Σ− state which correlates with the ground-state asymptote, N(4S)+H(2S). This mechanism is also expected to be responsible for the previously observed predissociation of high N' levels in NH v'=0 and 1. By contrast, no significant evidence for predissociation was found for the decay of excited ND v'=1–3 for the low J' levels investigated. These observations were confirmed with a combination of ab initio electronic structure, and coupled electronic state dynamics, calculations. Using an extended contracted Gaussian basis and large configuration state function expansions(160 000–380 000 terms) potential energy curves for, and spin–orbit induced coupling (using the full microscopic Breit–Pauli interaction) between, the A 3ΠΩ and 1 5Σ−Ω states were determined. These electronic structure data were used as the basis for the determination of the radiationless decay rates using a semiclassical coupled state model. These results were in turn combined with radiative decay rates for the A 3Π→X 3Σ− transition to determine the total decay rates which were found to be in excellent accord with the available experimental observations, thereby confirming the mechanism of the predissociation.