A Theoretical Study on the Electronic States and O‐Loss Photodissociation of the NO₂⁺ Ion

Abstract

CASPT2 (multiconfiguration second-order perturbation theory) calculations were performed at the molecular geometry for 17 low-lying singlet and triplet states of the NO(2)(+) ion. The CASPT2 vertical relative energies (T(v)') were obtained and the characters of these ionic states (primary or shake-up ionization states) were determined. For the eight low-lying states, we performed CASPT2 geometry optimization calculations and obtained the CASPT2 adiabatic relative energies (T(0)). We conclude that the 1(1)A(1), 1(3)B(2), 1(3)A(2), 1(1)A(2), 1(1)B(2), 1(3)A(1), 2(3)B(1), and 3(3)B(2) states of NO(2)(+) correspond to the X(1)Sigma(g)(+), a(3)B(2), b(3)A(2), A(1)A(2), B(1)B(2), c(3)A(1), d(3)B(1), and (3b(2))(-1) (3)B(2) states (the eight ionic states below 20 eV observed in the photoelectron spectra of Brundle et al.1 and Baltzer et al.2), respectively. The 1(1)A(1), 1(3)B(2), 1(3)A(2), 1(1)A(2), 1(1)B(2), 1(3)A(1), and 3(3)B(2) states are primary ionization states, and the CASPT2 T(v)' and T(0) values of these states are close to the corresponding experimental values from refs. [1] and [2]. The 2(3)B(1) state is not a typical primary ionization state, and the CASPT2 T(v)' and T(0) values for 2(3)B(1) are in reasonable agreement with the experimental values for d(3)B(1) from refs. [1] and [2] (the CASPT2 T(0) value for 1(3)B(1) is more than 2.5 eV smaller than the experimental values). Based on our CASPT2 T(0) calculations, we comment on the assignments of the d(3)A(1), C(1)B(1), and D(1)B(2) states below 20 eV observed by Jarvis et al. and on the MRCI T(0) values of Hirst for the 1(3)B(1), 1(1)B(1), and (3)A(1) states. On the basis of the CASPT2 potential energy curve (PEC) and CASSCF singlet/triplet minimum-energy crossing point (MECP) calculations, we reach the following conclusions concerning O-loss photodissociation from the X(1)Sigma(g)(+), a(3)B(2), b(3)A(2), A(1)A(2), and B(1)B(2) states, which are in line with the experimental facts. The adiabatic dissociation process of the X(1)Sigma(g) (+) state to the second limit [NO(+)(X(1)Sigma(+))+O((1)D)] cannot occur due to a high energy barrier (>5.0 eV) along the PEC, and the nonadiabitic process of X(1)Sigma(g)(+) to the first limit [NO(+)(X(1)Sigma(+))+O((3)P)] via the triplet states is unlikely since the MECPs lie very high above X(1)Sigma(g)(+). For the a(3)B(2) and b(3)A(2) states, adiabatic dissociation processes to the first limit may occur. Both the A(1)A(2) and B(1)B(2) states can undergo processes of predissociation to the first limit by a repulsive 2(3)A'' state, since the MECPs lie low above A(1)A(2) and B(1)B(2) and the calculated spin-orbit couplings at the MECPs are not small.

Assignment on the X, A, B, C, and D states of the C6H5Br+ cation based on high‐level calculations

“…It is known that the complete active space self‐consistent field (CASSCF) 12 and multiconfiguration second‐order perturbation theory (CASPT2) 13, 14 methods are effective for theoretical studies of excited electronic states of molecules and molecular ions 1, 15–17. We have carried out large‐scale calculations for some low‐lying doublet electronic states of the C 6 H 5 Br + ion using the CASSCF and CASPT2 methods.…”

Spectral dependence of photoemission in multiphoton ionization of NO2 by femtosecond pulses in the 375–430 nm range

“…This process may be interpreted as an ionisation into vibrationally excited NO2 + (X 1 g,1, 2, 3) followed by quasi-resonant dissociation with no excess of energy to the ground state limit L1 (NO + (X 2 ), O( 3 P)). We note that such a reaction necessarily involves an intersystem crossing mechanism 21 . Large spin-orbit coupling values are indeed found in the CASPT2 calculations 21 in particular between the NO2 + (X 1 1) and (a 3 B2) ionic states.…”

“…We note that such a reaction necessarily involves an intersystem crossing mechanism 21 . Large spin-orbit coupling values are indeed found in the CASPT2 calculations 21 in particular between the NO2 + (X 1 1) and (a 3 B2) ionic states.…”

“…This selective photodissociation acts thus as a sorting between these two populations,. The dissociation of the A 1 A1 or B 1 B2 excited state to the L1 limit occurs through intersystem crossing mechanism 21 , which is expected to occur on a time scale shorter than the molecular rotation or bending, preserving thus the validity of the axial recoil approximation. The stability of the two 2,e values characterizing the MPDI and non-dissociative MPI in the entire region (I) supports this proposed reaction mechanism for the whole excitation energy range.…”

Theoretical study on HBO+ and HOB+ cations using multiconfiguration second‐order perturbation theory

“…It is known that the CASSCF (complete active space self‐consistent field)27 and CASPT2 (multiconfiguration second‐order perturbation theory)28, 29 methods are effective for theoretical studies of excited electronic states of molecules and molecular ions 30–33. Recently, we34 studied the HBeN − radical anion and its isomer HNBe − which, being isoelectronic to the HBO + and HOB + cations, by using CASSCF and CASPT2 methods and found that a bending local minimum M1 along the 1 2 A′′ potential energy surface and the A 2 Π state of HNBe − should be the transition state of the isomerization reactions for M1↔ M1.…”