Collision-induced non-adiabatic transitions between the ion-pair states of molecular iodine: A challenge for experiment and theory

Abstract

The ion-pair states of molecular iodine provide a unique system for studying the efficiency, selectivity, and mechanisms of collision-induced non-adiabatic transitions. Non-adiabatic transitions between the first-tier ion-pair states in collisions with molecular partners and rare gases are analyzed and discussed. The qualitative features of the rate constants and product state distributions under single collision conditions are summarized and interpreted in terms of appropriate theoretical approaches. Two mechanisms for the non-adiabatic transitions are clearly identified. The first, operative for collisions involving molecular partners possessing permanent or transition electrostatic moments, is highly selective. It connects the initially prepared level in the E 0 þ g electronic state with the near-resonant vibronic level of the D 0 þ u state with a minimum change of the total angular momentum. In an extreme quasi-resonant case when the gap between initial and final rovibronic level is less than 1 cm À1 , this mechanism has a giant cross section, 40 times that of a gas kinetic collision. An electrostatic model, which includes the coupling of the giant E-D transition dipole moment with a moment of the colliding partner and the semiclassical Born approximation, provides a plausible interpretation of this mechanism. A second mechanism is shown to govern collisions with rare gas atoms. It results in population of several ion-pair states and broad distributions over rovibronic levels. This mechanism is successfully interpreted by quantum scattering calculations based on the diatomics-in-molecule diabatic potential energy surfaces and coupling matrix elements. The calculations provide good agreement with experimental measurements and reveal different mechanisms for the population of different electronic states. Unexplained features of the non-adiabatic dynamics and directions of future work are outlined.

Non-adiabatic , D′, β, γ, δ transitions in the first ion-pair tier of molecular iodine induced by collisions with I2, N2 and CO2

“…Strong difference between CINAT characteristics for M = I 2 and Rg is the consequence of various characters of long-range interaction between collision partners [7,[10][11][12][13]. The interaction between the E-D transition electric dipole and the permanent quadrupole of M, $R À4 , is dominant in the M = I 2 (X) case.…”

“…M other IP statesÞ non-adiabatic transitions induced by collisions with molecular partners different from the I 2 (X) is very scarce. Data on kinetics of the transitions from the E state to the D one induced by collisions with N 2 and CF 4 are only available in the literature [7,[12][13][14]. Besides, very recently, Hutchison et al studied I 2 ðE !…”

“…The majority of experimental work deals with the nonadiabatic transitions from the E0 þ g state of the first tier iodine IP states and f0 þ g one of the second tier following the optical-optical double resonance (OODR) excitation of the selected v E , J E and v f , J f rovibronic levels, respectively and collisions with Rg atoms and ground state iodine molecules [7][8][9][10]. The following features have been found out for the…”

“…These distinct spectroscopic features suggest use of the halogen IP states as a testing ground for general understanding of the efficiency and mechanisms of CINATs in electronically excited molecules. Therefore, collision-induced mixing of the IP states of molecular chlorine and iodine has attracted a great deal of interest since the late 1980s (see [7][8][9][10] and references therein).…”

Theoretical investigation of the He–I2(E3Πg) ion-pair state: Ab initio intermolecular potential and vibrational levels

“…Such potential surfaces are also of primary interest in studying the dynamics of non-adiabatic transitions in collisions of the I 2 (E) with He atoms. 29,30 Due to the lack of ab initio PES, semiempirical models have been developed and employed in the quantum scattering calculations, although in these studies the importance of the correct description of the surface for the interpretation and modeling of the collision mechanisms is clearly indicated. [29][30][31][32] In the present work, we use multireference configuration interaction (MRCI) methods, employing relativistic effective core potentials for the I atoms and large basis sets to compute the interaction energies and to provide an accurate description of the E ion-pair excited electronic state of the HeI 2 cluster.…”