Pseudo Jahn-Teller Interactions Research Articles

Jahn-Teller and pseudo-Jahn-Teller effects on the vibronic structure of the photoionized spectrum of cyanopropyne.

Nonadiabatic quantum dynamics are carried out to illustrate the photoionized spectrum of the cyanopropyne (CH3-C≡C-C≡N) as reported in recent experimental measurements [Lamarre et al., J. Mol. Spectrosc. 315, 206 (2015)]. A detailed electronic structure calculation is performed to analyze the topographical details of the first five ionized states, of which three are degenerate states (X̃2E, B̃2E, and C̃2E) and two are non-degenerate states (Ã2A1 and D̃2A1). The degenerate E states of the C3V symmetry molecule are prone to Jahn-Teller (JT) instability, and in addition, symmetry allowed A1 - E vibronic coupling, i.e., pseudo-Jahn-Teller (PJT), effects are expected to have a significant impact in the detailed vibronic structure of these electronic states. The JT splittings of X̃2E and B̃2E degenerate states are small, whereas it is quite large at three high frequencies in the C̃2E electronic states. The large energy separation of X̃2E from the other states and the non-zero PJT coupling of the B̃2E state with the close-lying Ã2A1 state indicate the uncoupled nature of the X̃, Ã, and B̃ vibronic bands of C4H3N. The intersection minima of B̃ and C̃ states with the D̃ state nearly coincide with the energetic minimum of D̃ state. Therefore, the PJT couplings among these states will lead to a strong vibronic interaction to shape the respective band structure. To completely understand the JT and PJT interactions in the photoionized spectrum of C4H3N, the vibronic coupling model Hamiltonian was constructed to perform nuclear dynamics studies for these electronic states. The vibrational progressions in each vibronic band are identified and compared with the available experimental data in the literature. The impacts of JT and PJT effects in the first five ionized states of cyanopropyne are investigated and discussed in detail.

Unified one-electron Hamiltonian formalism of spin-orbit Jahn-Teller and pseudo-Jahn-Teller problems in tetrahedral and octahedral symmetries.

Heavy element compounds with high symmetries often feature both spin-orbit coupling and vibronic coupling. This is especially true for systems with tetrahedral and octahedral symmetries, whose electronic states may be threefold degenerate and experience complicated Jahn-Teller and pseudo-Jahn-Teller interactions. To accurately describe these interactions, high quality spin-orbit vibronic Hamiltonian operators are needed. In this study, we present a unified one-electron Hamiltonian formalism for spin-orbit vibronic interactions for systems in all tetrahedral and octahedral symmetries. The formalism covers all spin-orbit Jahn-Teller and pseudo-Jahn-Teller problems in the symmetries with arbitrary types and arbitrary numbers of vibrational modes and generates Hamiltonian expansion formulas of arbitrarily high order.

Pseudo–Jahn-Teller interaction among electronic resonant states of H3

We study the electronic resonant states of H3 with energies above the potential energy surface of the H3+ ground state. These resonant states are important for the dissociative recombination of H3+ at higher collision energies, and previous studies have indicated that these resonant states exhibit a triple intersection. We introduce a complex generalization of the pseudo-Jahn-Teller model to describe these resonant states. The potential energies and the autoionization widths of the resonant states are computed with electron scattering calculations using the complex Kohn variational method, and the complex model parameters are extracted by a least-square fit to the results. This treatment results in a non-Hermitian pseudo-Jahn-Teller Hamiltonian describing the system. The non-adiabatic coupling and geometric phase are further calculated and used to characterize the enriched topology of the complex adiabatic potential energy surfaces.

Manifestation of descent symmetry phenomena in tetrahedral structure of M42+ (M = P, As, Sb) analogues

Manifestation of descent symmetry phenomena induced by pseudo Jahn–Teller interactions is reported for the P4 dicationic tetrahedral structure and its As4 and Sb4 analogues. The symmetry descent phenomena in the dicationic tetrahedral structure for the series is caused by the pseudo Jahn–Teller effect (PJTE) where the unstable (high Td symmetry) configuration distorts to the equilibrium geometry with a lower, C2 symmetry. State averaging six low-lying electronic states via CASSCF(8,8)/cc-pVTZ–(PP) computations determined the adiabatic potential energy surfaces along the distorting normal coordinate. The (E(I) + A1 + E(II)) ⊗ e for the M42+ (M = P, As, Sb) series has been formulated accordingly. Subsequently, the coupling constants were estimated by fitting energies obtained from the PJTE equations. Moreover, to understand how removing or adding electrons affects the PJTE in the M42+ series, electronic configurations were analysed for M4(0,2+,4+) analogues in which the M4(0,4+) are stable in their tetrahedral structure.

Conical intersections and nonadiabatic coupling terms in 1,3,5-C6H3F3 +: A six state beyond Born-Oppenheimer treatment.

In order to circumvent numerical inaccuracy originating from the singularity of nonadiabatic coupling terms (NACTs), we need to perform kinetically coupled adiabatic to potentially coupled diabatic transformation of the nuclear Schrödinger Equation. Such a transformation is difficult to achieve for higher dimensional sub-Hilbert spaces due to inherent complicacy of adiabatic to diabatic transformation (ADT) equations. Nevertheless, detailed expressions of ADT equations are formulated for six coupled electronic states for the first time and their validity is extensively examined for a well-known radical cation, namely, 1,3,5-C6H3F3 + (TFBZ+). While implementing this formulation, we compute ab initio adiabatic potential energy surfaces (PESs) and NACTs within the low-lying six electronic states (X̃2E'', Ã2A2 '', B̃2E', and C̃2A2 '), where several types of nonadiabatic interactions, like Jahn-Teller conical intersections (CI), accidental CIs, accidental seams (series of degenerate points), and pseudo Jahn-Teller interactions can be observed over the Franck-Condon region of nuclear configuration space. Those interactions are depicted by exploring degenerate components of C-C asymmetric stretching, C-C symmetric stretching, and C-C-C scissoring motion (Q9x, Q9y, Q10x, Q10y, Q12x, and Q12y) to compute complete active space self-consistent field level adiabatic PESs and NACTs as implemented in the MOLPRO quantum chemistry package. Subsequently, we perform the ADT using our newly devised fifteen (15) ADT equations to locate the position of CIs, verify the quantization of NACTs, and to construct highly accurate diabatic PESs.

Jahn-Teller and pseudo-Jahn-Teller effects in trifluoromethane radical cation

Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) effects in the ground, first and second excited electronic states of the trifluoromethane radical cation are theoretically examined here. Extensive ab initio computation of electronic potential energy surfaces and their coupling surfaces are performed. Full quantum dynamics are obtained by both time-independent and time-dependent quantum mechanical methods. This system belongs to (E+A)⊗e JT-PJT family. Our results compare well with the experimental data. JT interactions are fairly strong in the second excited B˜2E electronic state and the PJT interaction between A˜2A2 - B˜2E electronic states is stronger which cause an increase of the spectral line density of the vibronic spectrum.

Novel Effect Induced by Pseudo-Jahn-Teller Interactions: Broken Cylindrical Symmetry in Linear Molecules.

It is shown that in linear molecules the pseudo-Jahn-Teller (PJT) interaction of a Σ or Π term with a Δ term induces a bending instability that is angular dependent, reducing the symmetry of the adiabatic potential energy surface from expected D∞h to D4h and C∞v to C2v or C4v. This spontaneously broken cylindrical symmetry (BCS) emerges from the solution of the vibronic coupling equations of the PJT effect (PJTE) problems (Σ+Δ)⊗w, (Π+Δ)⊗w, (Π+Σ+Δ)⊗w, and (Δ+Δ)⊗w, where w includes linear, quadratic, and fourth order vibronic coupling terms, and it is confirmed by ab initio calculations for a series of triatomic molecules with ground or excited Δ terms. The BCS is due to the angular symmetry of the electronic wave functions of the Δ term, ∼cos 2φ, and ∼sin 2φ, split by the fourth order vibronic coupling, which in overlap with the other symmetry wave functions of the Σ or Π term provides for the periodical symmetry of the added covalency that facilitates the bending. The mechanism of this PJT-induced BCS effect is discussed in detail; the numerical values of the vibronic coupling parameters for the molecules under consideration were estimated by means of combining separate ab initio calculations of some of them with a procedure fitting the analytical expressions to ab initio calculated energy profiles. It is also shown that the bending of linear molecules in Δ states, similar to Π states, is exclusively a PJT (not Renner-Teller) effect. The BCS revealed in this paper illustrates again the predicting power of the PJTE.

A four-component quadratic vibronic coupling approach to the Renner–Teller effect in linear triatomic molecules. The , manifold of BrCN+ and ClCN+

In this work, we present the four-component quadratic vibronic coupling model for the description of the Renner–Teller effect (RTE) in the presence of the spin–orbit coupling. The interaction of the two potential energy surfaces emerging from the cationic 2Π states of singly ionized linear triatomic molecules is described by the quadratic coupling constant c for the genuine RT repulsion and the second parameter, d, for a nonconstant spin–orbit coupling varying with the bond angle of the triatomic. The emergence of a linear RT constant in the presence of the spin–orbit operator was originally shown by Poluyanov and Domcke (2004 Chem. Phys. 301 111–27) and is based on the application of the Breit–Pauli Hamiltonian in combination with nonrelativistic wavefunctions. In contrast to this methodology, we generate the diabatic RT Hamiltonian in a 4-spinor basis where the symmetry transformation properties of the electronic and vibrational wavefunctions completely determine the RT matrix structure. Explicit access to highly correlated wavefunctions is not required in our approach. In addition, the four-component vibronic coupling model takes into account the full spatial orbital relaxation upon the inclusion of the spin–orbit coupling and is therefore well suited for heavy systems. The third parameter, p, accounting for a possible pseudo-Jahn–Teller interaction is not considered here, but it does not introduce a principal difficulty. As the initial systems for this study, we considered the BrCN+ and ClCN+ cations and determined the c and d parameters by a numerical fit to accurate adiabatic potential energy surfaces obtained by the relativistic Fock-space coupled-cluster method. New values for the computed linear RT parameter d amount to 14.7 ± 0.5 cm−1 for ClCN+ and 73.2 ± 0.7 cm−1 for BrCN+.

Electronic states and pseudo Jahn-Teller distortion of heavy metal-monobenzene complexes: M(C6H6) (M = Y, La, and Lu)

Monobenzene complexes of yttrium (Y), lanthanum (La), and lutetium (Lu), M(C(6)H(6)) (M = Y, La, and Lu), were prepared in a laser-vaporization supersonic molecular beam source and studied by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and ab initio calculations. The calculations included the second-order perturbation, the coupled cluster with single, double, and perturbative triple excitation, and the complete active space self-consistent field methods. Adiabatic ionization energies and metal-benzene stretching frequencies of these complexes were measured for the first time from the ZEKE spectra. Electronic states of the neutral and ion complexes and benzene ring deformation were determined by combining the spectroscopic measurements with the theoretical calculations. The ionization energies of M(C(6)H(6)) are 5.0908 (6), 4.5651 (6), and 5.5106 (6) eV, and the metal-ligand stretching frequencies of [M(C(6)H(6))](+) are 328, 295, and 270 cm(-1) for M = Y, La, and Lu, respectively. The ground states of M(C(6)H(6)) and [M(C(6)H(6))](+) are (2)A(1) and (1)A(1), respectively, and their molecular structures are in C(2v) point group with a bent benzene ring. The deformation of the benzene ring upon metal coordination is caused by the pseudo Jahn-Teller interaction of (1(2)E(2)+1(2)A(1)+2(2)E(2)) e(2) at C(6v) symmetry. In addition, the study shows that spectroscopic behaviors of Y(C(6)H(6)) and La(C(6)H(6)) are similar to each other, but different from that of Lu(C(6)H(6)).

The Jahn–Teller and pseudo-Jahn–Teller effects in the low-lying electronic states of 1,3,5-trifluorobenzene radical cation

Static and dynamic aspects of the multimode Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) interactions in the low-lying electronic states of 1,3,5-trifluorobenzene radical cation (TFBZ(+)) are investigated by an ab initio quantum dynamical approach. The electronic ground state (tilde X(2)E'') of TFBZ(+) is energetically well separated from its excited states. The first three excited electronic states (tilde A(2)A(2)", tilde B(2)E' and tilde C(2)A(2)') are however, energetically close. A sequence of low-energy conical intersections underlying the JT and PJT interactions among these electronic states is established. Nonadiabatic effects due to these intersections on the vibronic dynamics are examined in detail. The theoretical results are compared with the low resolution and also better resolved experimental data. The impact of increasing fluorination on the structure and dynamics of the excited states is discussed in relation to the parent benzene radical cation and its mono- and di-fluoro derivatives.

The vibronic level structure of the cyclopentadienyl radical

The 351.1 nm photoelectron spectrum of the cyclopentadienide ion has been measured, which reveals the vibronic structure of the X (2)E(1) (") state of the cyclopentadienyl radical. Equation-of-motion ionization potential coupled-cluster (EOMIP-CCSD) calculations have been performed to construct a diabatic model potential of the X (2)E(1) (") state, which takes into account linear Jahn-Teller effects along the e(2) (') normal coordinates as well as bilinear Jahn-Teller effects along the e(2) (') and ring-breathing a(1) (') coordinates. A simulation based on this ab initio model potential reproduces the spectrum very well, identifying the vibronic levels with linear Jahn-Teller angular momentum quantum numbers of +/-1/2. The angular distributions of the photoelectrons for these vibronic levels are highly anisotropic with the photon energies used in the measurements. A few additional weak photoelectron peaks are observed when photoelectrons ejected parallel to the laser polarization are examined. These peaks correspond to the vibronic levels for out-of-plane modes in the ground X (2)E(1) (") state, which arise due to several pseudo-Jahn-Teller interactions with excited states of the radical and quadratic Jahn-Teller interaction in the X (2)E(1) (") state. A variant of the first derivative of the energy for the EOMIP-CCSD method has been utilized to evaluate the strength of these nonadiabatic couplings, which have subsequently been employed to construct the model potential of the X (2)E(1) (") state with respect to the out-of-plane normal coordinates. Simulations based on the model potential successfully reproduce the weak features that become conspicuous in the 0 degrees spectrum. The present study of the photoelectron spectrum complements a previous dispersed fluorescence spectroscopic study by Miller and co-workers [J. Chem. Phys. 114, 4855 (2001); 114, 4869 (2001)] to provide a detailed account of the vibronic structure of X (2)E(1) (") cyclopentadienyl. The electron affinity of the cyclopentadienyl radical is determined to be 1.808+/-0.006 eV. This electron affinity and the gas-phase acidity of cyclopentadiene have been combined in a negative ion thermochemical cycle to determine the C-H bond dissociation energy of cyclopentadiene; D(0)(C(5)H(6),C-H)=81.5+/-1.3 kcal mol(-1). The standard enthalpy of formation of the cyclopentadienyl radical has been determined to be Delta(f)H(298)(C(5)H(5))=63.2+/-1.4 kcal mol(-1).

Local symmetry change in BaF2:Mn2+ at ∼50K: Microscopic insight

The microscopic origin of the abrupt cubic-tetrahedral symmetry change associated with the local a2u vibrational mode observed by electron paramagnetic resonance in BaF2:Mn2+ at ∼50K is explored by means of density functional theory calculations. It is found that while the a2u vibrational frequencies calculated for MnF86− in CaF2 (168cm−1) and SrF2 (132cm−1) are real, in the case of BaF2:Mn2+, the adiabatic potential curve along this mode exhibits a double well with a small barrier of 50cm−1. Although the ground and first excited vibrational states are localized around the energy minima, the rest of the excited states resemble those of a harmonic oscillator centered at Q(a2u)=0. Moreover, only the inclusion of the anharmonic coupling between a2u and t1u modes allows one to understand the Td-Oh transition temperature. It is shown that both the unusually high Mn2+–F− distance in BaF2:Mn2+ and the pseudo-Jahn–Teller interaction of the t2g(xy;xz;yz) antibonding orbital with filled t1u orbitals favor the a2u instability. The calculated a2u force constant for different electronic states supports this conclusion.

Vibronic dynamics in the low-lying coupled electronic states of methyl cyanide radical cation

Static and dynamic aspects of the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) interactions between the ground and first excited electronic states of the methyl cyanide radical cation are theoretically investigated here. The latter involves construction of a theoretical model by ab initio computation of electronic potential energy surfaces and their coupling surfaces and simulation of the nuclear dynamics employing time-independent and time-dependent quantum mechanical methods. The present system represents yet another example belonging to the ( E + A) ⊗ e JT–PJT family, with common JT and PJT active degenerate (e) vibrational modes. The theoretical results are found to be in very good accord with the recent experimental data revealing that the JT interactions are particularly weak in the ground X ∼ 2 E electronic manifold of methyl cyanide radical cation, On the other hand, the PJT interactions of this ground electronic manifold with the first excited A ∼ 2 A 1 electronic state of the radical cation are stronger which cause an increase of the spectral line density. The effect of deuteration on the JT–PJT dynamics of the methyl cyanide radical cation is also discussed.

Why Are Some ML2Molecules (M = Ca, Sr, Ba; L = H, F, Cl, Br) Bent while Others are Linear? Implications of the Pseudo Jahn−Teller Effect

The unexpected bent geometries of some alkaline earth dihalides and dihydrides, ML(2) (M = Ca, Sr, Ba; L = H, F, Cl, Br) have been explained in the literature using various models that attribute the effect to different phenomena like covalency, metal core polarization, sd-hybridization, and electron pair repulsion. We employ (based on first principles) the pseudo Jahn-Teller effect, as the only source of instability of high-symmetry configurations in nondegenerate states, to analyze the origin of the geometry of these systems and show that this approach explains all of their main structural features, including the topology of the Laplacian of the electron density and the vibrational frequencies. The main contribution to the distortion of the linear configuration is due to the pseudo Jahn-Teller mixing by bending of the sigma(u) HOMO formed by the ligand orbitals with the unoccupied pig orbitals of the metal (with main d(xz) and d(yz) character), resulting in new covalency which stabilizes the bent configuration. We show that the model approaches to the problem, mentioned above, are either restricted particular cases of the pseudo Jahn-Teller interaction, or they yield very small contributions to the instability that do not explain the origin of the bending. All of our conclusions are supported by high-quality ab initio calculations.

On the Jahn–Teller and pseudo-Jahn–Teller effects in the photoelectron spectrum of cyclopropane

We present a theoretical account of the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) effects in the photoelectron spectrum of cyclopropane. The PJT interactions between the two JT split X ˜ 2 E ′ and A ˜ 2 E ″ electronic states of the cyclopropane radical cation are examined. Nuclear dynamical simulations on the resulting four coupled electronic states including fourteen relevant vibrational modes are carried out by a wave packet propagation approach employing the multiconfiguration time-dependent Hartree algorithm. The theoretical results are compared with recent experimental photoelectron spectroscopic data.

Multistate and multimode vibronic dynamics: The Jahn–Teller and pseudo-Jahn–Teller effects in the ethane radical cation

Multimode Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) coupling effects in the photoelectron spectrum of ethane are theoretically investigated. In this article, we focus on the vibronic structure of the second excited B ∼ 2 E u electronic manifold of the ethane radical cation which reveals an asymmetric band in the 14.5–16.5 eV ionization energy range in the experimental recordings. Ionization of an electron from the third occupied 1 e u molecular orbital of ethane produces the radical cation in the degenerate B ∼ 2 E u electronic manifold, which is prone to the JT instability when distorted along the degenerate e g vibrational modes. The theoretical formalism employed here is based on a model diabatic Hamiltonian and a quadratic vibronic coupling scheme with the parameters derived from ab initio electronic structure calculations. The photoelectron band is calculated by carrying out quantum dynamical simulations in the coupled manifold of electronic states. The B ∼ 2 E u electronic manifold of the radical cation is estimated to be ∼2.75 eV and ∼2.40 eV above its X ∼ 2 E g and the A ∼ 2 A 1 g electronic states, respectively. The symmetry selection rule suggests PJT coupling of these electronic states along the vibrational modes of e g/ e u symmetry. The quadratic JT spectrum simulated within the B ∼ 2 E u electronic manifold shows two maxima at ∼ 14.96 eV and ∼15.76 eV which are attributed to the two JT split adiabatic sheets of this electronic manifold. This is in good accord with their position observed at ∼15.0 eV and ∼15.8 eV, respectively, in the experimental recording. The diffuse structure of the overall band can be accounted to a large extent by considering the A ∼ 2 A 1 g - B ∼ 2 E u PJT interactions. The overall shape of the theoretical band agrees very well with the experimental results. Further refinement of the theoretical results may be accomplished by including X ∼ 2 E g - A ∼ 2 A 1 g - B ∼ 2 E u PJT interactions in the theoretical model. Importance of the latter vibronic interactions is also discussed in the text.

The Jahn–Teller and pseudo‐Jahn–Teller effects in the anion photoelectron spectroscopy of B3 cluster

The photodetachment spectroscopy of B3- anion is theoretically studied with the aid of a quantum dynamical approach. The theoretical results are compared with the available experimental photoelectron spectra of B3-. Both B3- and B3 possess D(3h) symmetry at the equilibrium configuration of their electronic ground state. Distortion of B3 along its degenerate vibrational mode nu2 splits the degeneracy of its excited C2E' electronic manifold and exhibits (E [symbol: see text] e)-Jahn-Teller (JT) activity. The components of the JT split potential energy surface form conical intersections, and they can also undergo pseudo-Jahn-Teller (PJT) crossings with the X2A1' electronic ground state of B3 via the degenerate nu2 vibrational mode. The impact of the JT and PJT interactions on the nuclear dynamics of B3 in its X2A1'-C2E' electronic states is examined here by establishing a diabatic model Hamiltonian. The parameters of the electronic part of this Hamiltonian are calculated by performing electronic structure calculations and the nuclear dynamics on it is simulated by solving quantum eigenvalue equation. The theoretical results are in good accord with the experimental data.

Magnetic properties of complex d1 and d5 ions: crystal field model and Jahn–Teller effect

In this paper, the low lying levels of d1 and low-spin d5 metal complexes possessing ground 2T2 term are considered. Strong spin–orbital interactions is taken into account along with the axial (trigonal or tetragonal) component of the crystal field. In the framework of the semi-classic adiabatic approximation, vibronic Jahn–Teller and pseudo Jahn–Teller interactions acting within the ground manifold are also considered. The influence of the low-symmetry crystal fields, covalence reduction factor, spin–orbital and Jahn–Teller interactions on the anisotropy of g-factors, temperature independent paramagnetic contributions, and magnetic susceptibility is elucidated.

Problem of the magnetic anisotropy in orbitally degenerate exchange and mixed-valence clusters

This contribution summarizes the results obtained in the problem of orbital degeneracy of the metal ions in exchange coupled and mixed-valence (MV) clusters. The theory of the double exchange is generalized and the orbitally degenerate systems are considered. The orbitally dependent double exchange parameter is deduced for the singlet–triplet and triplet–triplet transition metal pairs in three high-symmetric topologies. A new effective Hamiltonian of the magnetic exchange between the ions with unquenched orbital angular momenta is discussed. The technique of the irreducible tensor operators is applied to the problem of the kinetic exchange in these kind of metal clusters. Strong magnetic anisotropy is shown to appear in the exchange and MV clusters consisting of orbitally degenerate ions. The influence of the vibronic pseudo Jahn–Teller interaction on the degree of the magnetic anisotropy is discussed.

Theoretical investigation of Jahn–Teller and pseudo-Jahn–Teller interactions in the ammonia cation

The spectroscopic and dynamic aspects of Jahn–Teller and pseudo-Jahn–Teller interactions in the ammonia cation are investigated within an ab initio based vibronic-coupling model approach. Multireference second-order perturbation theory (CASPT2) has been employed to obtain the potential energies of the ground state and the first excited state of NH3+ as a function of symmetry-coordinate displacements. Vibronic-coupling parameters determining the Franck–Condon, Jahn–Teller, and pseudo-Jahn–Teller activity of the normal modes have been obtained from the ab initio data. The vibronic structures of the X̃ 2A1 and à 2E photoelectron bands of ammonia have been calculated by numerical diagonalization of the vibronic Hamiltonian matrix. All six vibrational degrees of freedom are taken into account. The effects of Jahn–Teller and pseudo-Jahn–Teller interactions on the band shape of the à 2E photoelectron band are analyzed. The calculation of the time-dependent population probability of the à 2E state reveals a radiationless decay process on a time scale of 30 fs caused by a conical intersection of the X̃ and à potential-energy surfaces, which arises from the combined effect of the Jahn–Teller splitting of the à 2E state and the X̃–à pseudo-Jahn–Teller interaction. In the X̃ 2A1 band, the X̃–à pseudo-Jahn–Teller coupling results in the weak excitation of a single quantum of the degenerate bending mode. This theoretical result corroborates the earlier assignment of the vibronic structure of the X̃ 2A1 photoelectron band of NH3 by Edvardsson et al. [J. Phys. B 32, 2583 (1999)].