Open shell systems curve

Conventional QDPT [12-21] has been developed to be applied to open-shell systems and excited states. Although much effort has been made to develop the QDPT, it is not widely used among chemists. The major reason is that the QDPT often provides incorrect potential curves, since the perturbation series frequently diverges owing to the existence of intruder states [22]. 

Hydroxyl radical (OH) is a key reactive intermediate in combustion and atmospheric chemistry, and it also serves as a prototypic open-shell diatomic system for investigating photodissociation involving multiple potential energy curves and nonadiabatic interactions. Previous theoretical and experimental studies have focused on electronic structures and spectroscopy of OH, especially the A2T,+-X2n band system and the predissociation of rovibrational levels of the M2S+ state,84-93 while there was no experimental work on the photodissociation dynamics to characterize the atomic products. The M2S+ state [asymptotically correlating with the excited-state products 0(1 D) + H(2S)] crosses with three repulsive states [4>J, 2E-, and 4n, correlating with the ground-state fragments 0(3Pj) + H(2S)[ in... 

More complicated anisotropies of the potential are, for example, encountered in the association of two linear dipoles. Adiabatic channel potential curves for this case have been calculated and expressed analytically in Ref. 16. More systematic studies, also comparing SACM and trajectory results, were reported in Ref. [36], One may as well consider open-shell effects for example, the association of two open-shell HO radicals in their lowest rotational state was treated in Ref. 37. Figure 12 shows the lowest rovi-bronic adiabatic channel potential curves for this system. The ultimate goal... 

This work provides accurate potential energy curves as well as coupling matrix elements for the B2+/H and B4+/ He systems. From the molecular point of view, it appears important to involve all levels correlated to the entry channels in the collision dynamics and, in particular, to take into account rotational effects, which might be quite important. The results concerning the double electron capture process in the (B4+ + He) collision point out the limitations ofthe potential approach model, especially to account for open shell levels, for which more elaborate calculations are necessary. 

The interaction-energy curves for alkali metal-rare gas pairs are also of interest experimentally in scattering and radiation problems, and theoretically because of the expected reliability of the HF energy for this class of half-open-closed-shell systems. Calculations on LiHe and NaHe (X22+, A2U, if 22+) and their X1 + ions have been reported by Krauss et al.285 from R = 3 to 10 bohr. Both STO and GTO expansion bases were used, with comparable results except for the ASH state of NaHe. The variation of dipole and quadrupole moments with R was investigated. The X2S+ curve is... 

Fig. 8. Electronic spectra of the radical anions of hydrocarbons 15—20 and results of open-shell PPP-like calculations. Two arrows indicate the positions of the experimentally found long-wavelength maxima of 15 (Balk, P. et al.. Ref. 8)). The band at 27.3 AK designated by a question mark was attributed to an impurity or to the protonated anion (Hoijtink, G. et al., Ref. 48)). Origin of data systems 15 and 16 (absorption curves), Brandes, K. K., Gerdes, J. J. Phys. Chem. 71, 508 (1967) systems 17—20 (absorption curves), Hoijtink, G. J. and coworkers. Refs. 48,48,49) calculations, Zahradnik, R., Cdrsky, P., Ref. 30)...

Laser induced fluorescence (LIF) techniques in supersonic free jets can also yield useful information on the potential energy curves of open shell atomic systems. This type of studies has provided high quality data on tbe ground and excited states of NaAr type of molecules. The LIF technique was also successfully applied for probing the potential surfaces of XeF. The B<-X fluorescence excitation spectrum of XeF in a supersonic free jet is sufficiently simplified that rotational analysis and accurate vibrational spacing are readily obtained, overcoming the complexity of gas phase emission spectroscopy, mainly due in this case to isotopic richness of natural Xe. 

In both cases, the closed shell and open shell curves intersect at 1.4 A. The geometries are actually different for the two curves and the system needs some more energy to jump from one curve to another. 

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