A new, definitive analysis of a very old spectrum: The highly perturbed A2Πu–X 2Πg band system of the chlorine cation (${\rm Cl}_2^ +$ Cl 2+)

Abstract

The laser-induced fluorescence spectrum of jet-cooled chlorine cation has been recorded in the 500-312 nm region with high sensitivity and rigorous vibrational and spin-orbit cooling. More than 80 bands of the highly vibrationally perturbed A (2)Π(u)-X (2)Π(g) electronic transition have been detected and shown to originate from the Ω = 3/2 spin-orbit component of v = 0 of the ground state. The spectrum extends some 3700 cm(-1) to the red of any previously published report and the 0-0 band has been identified for the first time. The bands have regular rotational structure but exhibit irregular vibrational intervals and isotope splittings. Our ab initio studies show that the perturbations are due to a ΔΩ = 0 spin-orbit interaction between the A(2)Π(3/2u) and B(2)Δ(3/2u) states which have an avoided crossing at ~2.5 Å, which corresponds to v ≈ 4 of the A state. The nonadiabatic coupled equations have been solved for this two-state interaction after constructing the diabatic potentials including only the diagonal (ΔΛ = 0) spin-orbit coupling, yielding low-lying vibrational energy levels, isotope splittings, and rotational constants in good agreement with experiment. The calculations show that many of the observed bands are actually transitions to predominantly B state vibrational levels, which borrow oscillator strength from the A-X transition through spin-orbit mixing. Starting from the coupled equations solutions, we have fitted the experimental data using an effective Hamiltonian matrix that includes the vibrational energy levels of the A and B states and a single electronic spin-orbit coupling term H(SO)(AB) which has a value of 240 cm(-1). Transitions up to v' = 32 in both states have been satisfactorily fitted for all three chlorine isotopologues, providing a quantitative description of the perturbations. Transitions to higher levels are complicated by interactions with other electronic states.

The X+ 2Πg, A+ 2Πu, B+ 2Δu, and $\mathrm{a}^+\ ^4\Sigma _{\mathrm{u}}^-$a+Σu−4 electronic states of ${\rm Cl}_2^+$ Cl 2+ studied by high-resolution photoelectron spectroscopy

“…However, it is only very recently that a full understanding of the A + 2 u, 3/2 → X + 2 g, 3/2 band system, including a complete treatment of the interaction between the A + 2 u, 3/2 and the B + 2 u, 3/2 states, could be reached, based on a new high-resolution measurement of the laser-inducedfluorescence spectrum of Cl + 2 and ab initio calculations. 3 The low temperature of the Cl + 2 sample used in this study, however, prevented the observation of transitions involving the A + 2 u, 1/2 state, the assignment of which remains incomplete.…”

“…8, 17, and 18). These studies indicated that a broad range of vibrational levels of the X + and A + states could be studied (σ g ) 2 (π u ) 4 (π * g ) 3 X + 2 g (σ g ) 2 (π u ) 3 (π * g ) 4 A + 2 u (σ g ) 1 (π u ) 4 (π * g ) 4 2 + g (σ g ) 2 (π u ) 4 (π * g ) 2 (σ * u ) 1 4 − u , 2 u , 2 − u , 2 + u at high resolution by pulsed-field-ionization zero-kineticenergy (PFI-ZEKE) photoelectron spectroscopy and that information on the mechanisms leading to ion-pair formation in the photoionization of Cl 2 might be obtained from such studies. PFI-ZEKE photoelectron spectra of the low vibrational levels (v + ≤ 6) of the X + state with partial rotational resolution were reported by Li et al 9 who also determined precise values of the first adiabatic ionization energy of Cl 2 and of the threshold for Cl + ( 1 D 2 ) + Cl − ( 1 S 0 ) ion-pair formation.…”

“…Experimental information on the rovibrational and spin-orbit energy level structure of several of these low-lying states has been obtained by photoelectron spectroscopy [4][5][6][7][8][9] and by optical spectroscopy. 2,3,[10][11][12][13][14][15][16] At low resolution, the He I photoelectron spectrum reveals a very simple structure with three main bands centered at ≈11.8, 14.3, and 16 eV, which correspond to three S = 1/2 cationic states that are formed from the (σ g ) 2 (π u ) 4 (π * g ) 4 X 1 + g ground state of Cl 2 by (π * g ) −1 , (π u ) −1 , and (σ g ) −1 single-photon ionization. 5 The spin-orbit and vibrational structure of the bands is observable at higher resolution and the similarity of the vibrational and spin-orbit intervals in the X + and A + states leads to a clustering of the photoelectron bands.…”

“…6 of Ref. 3 give an excellent overview of the electronic structure of Cl + 2 at low energies. Table I lists the configurations that need to be considered in the region up to 3 eV of internal energy, which is at the focus of the present article, and the corresponding electronic states.…”

“…Ab initio calculations of the potential energy curves of the low-lying doublet and quartet states of Cl + 2 have been performed [1][2][3] and the potential energy functions depicted in Fig. 4 of Ref.…”