Threshold Photoelectron Spectroscopy of Cl2 and Br2 up to 35 eV

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

“…The observation of transitions to all vibrational levels in the range v + = 24-41 in the PFI-ZEKE photoelectron spectrum is consistent with the threshold photoelectron spectrum 8 but stands in sharp contrast to the He I photoelectron spectrum (see Fig. 3(d)), in which such transitions are entirely missing.…”

“…The band origins determined in this way are listed in Tables II and III. To assign vibrational quantum numbers, the molecular constants derived for low vibrational levels, given in Table IV, were employed to obtain approximate band positions. Band positions determined from threshold photoelectron spectra 7,8 and LIF spectra 3 were used to bridge the gap between the v + = 0-6 and the v + = 24-41 regions. Whereas the molecular constants ω + e and ω e x + e are sufficient to describe the vibrational structure of the X + state at low v + values, anharmonic constants of higher order (ω e y + e and ω e z + e , see Eq.…”

“…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. 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.…”

“…The A + ← X photoelectron band further reveals a pronounced perturbation below 14.35 eV. 6 The intensity distribution of the threshold photoelectron spectrum of Cl 2 differs strongly from that observed in He I and Ne I photoelectron spectra 8 and enabled the observation of long vibrational progressions in the Franck-Condon gaps between the X + ← X and A + ← X band systems. In their study, Yencha et al 8 noted the resemblance of the overall contour of the threshold photoelectron spectrum and both the electron energy-loss spectrum of Cl 2 reported by Stubbs et al 17 and the photoionization and ion-pair production spectra reported by Berkowitz et al 18 They also suggested that the structures observed in all three spectra result from the excitation of the same autoionizing Rydberg states of Cl 2 , namely, those belonging to the J and K series which are thought to be nsσ g series converging on the A + 2 u state of Cl + 2 with an onset at n = 5 and an energy of 12.244 eV (see discussion in Refs.…”

“…The goals of our investigation of the PFI-ZEKE photoelectron spectrum of Cl 2 were (i) to obtain precise information on the high vibrational levels of the X + state of Cl + 2 observed in the threshold photoelectron spectrum of Yencha et al, 8 (ii) to complete the analysis of the A + ← X band system in the region below 14.35 eV, where it is affected by perturbations, by observing and assigning the vibrational structure of the A + 2 u, 1/2 spin-orbit component, (iii) to study the mechanisms that lead to the observation of high vibrational levels of the X + state and of Cl + + Cl − ion pairs in the photoionization of Cl 2 , and (iv) to try and observe quartet states of Cl + 2 for the first time.…”