S.A. Poretsky

Collision-induced nonadiabatic transitions in the second-tier ion-pair states of iodine molecule: Experimental and theoretical study of the I2(f0g+) collisions with rare gas atoms

Nonadiabatic transitions induced by collisions with He, Ar, Kr, and Xe atoms in the I(2) molecule excited to the f0(g)(+) second-tier ion-pair state are investigated by means of the optical-optical double resonance spectroscopy. Fluorescence spectra reveal that the transition to the F0(u)(+) state is a dominant nonradiative decay channel for f state in He, Ar, and Kr, whereas the reactive quenching is more efficient for collisions with Xe atom. Total rate constants and vibrational product state distributions for the f-->F electronic energy transfer are determined and analyzed in terms of energy gaps and Franck-Condon factors for the combining vibronic levels at initial vibrational excitations v(f)=8, 10, 14, and 17. Quantum scattering calculations are performed for collisions with He and Ar atoms, implementing a combination of the diatomics-in-molecule and long-range perturbation theories to evaluate diabatic PESs and coupling matrix elements. Calculated rate constants and vibrational product state distributions agree well with the measured ones, especially in case of Ar. Qualitative comparison is made with the previous results for the second-tier f0(g)(+)-->F0(u)(+) transition in collisions with I(2)(X) molecule and the first-tier E0(g)(+)-->D0(u)(+) transition induced by collisions with the rare gas atoms.

Hyperfine coupling of the iodine E0+g, vE = 19 and γ1u, vγ = 18 ion-pair states

A considerable difference of iodine emission spectra after optical–optical double resonance excitation of the E0+g, vE = 19, JE ≈ 55 and JE ≈ 85 rovibronic levels is observed. The latter include strong transitions from the γ1u ion-pair state to valence ones. This feature is interpreted as a result of hyperfine coupling of near-resonant E0+g, vE = 19, JE = 81 and γ1u, vγ = 18, Jγ = 80 states. The hyperfine matrix element and dipole moment functions of transitions from the γ state are estimated. The hyperfine g/u mixing of the iodine ion-pair states is observed for the first time.

Non-adiabatic transitions from I2(E0g+ and D0u+) states induced by collisions with M = I2(X0g+) and H2O

The stepwise two-step two-color and three-step three-color laser excitation schemes are used for selective population of rovibronic levels of the first-tier ion-pair E0(g)(+) and D0(u)(+) states of molecular iodine and studies of non-adiabatic transitions to the D and E states induced by collisions with M = I(2)(X) and H(2)O. Collection and analysis of the luminescence after excitation of the v(E) = 8, 13 and v(D) = 13, 18 vibronic levels of the E and D states in the pure iodine vapor and the gas-phase mixtures with H(2)O provide rate constants for the non-adiabatic transitions to the D and E state induced by collisions with these molecules. Vibrational distributions for the [formula: see text] collision-induced non-adiabatic transitions (CINATs) are obtained. Rather strong λ(lum)(max) ≈ 3400 Å luminescence band is observed in the I(2) + H(2)O mixtures, whereas its intensity is ~100 times less in pure iodine vapor. Radiative lifetimes and quenching rate constants of the I(2)(E,v(E) = 8, 13 and D,v(D) = 13, 18) vibronic state are also determined. Rate constants of the [formula: see text], v(E) = 8-54, CINATs are measured again and compared with those obtained earlier. New data confirm resonance characters of the CINATs found in our laboratory about 10 years ago. Possible reasons of differences between rate constant values obtained in this and earlier works are discussed. It is shown, in particular, that differences in rate constants of non-resonant CINATs are due to admixture of water vapor in iodine.

Spectroscopic constants and potential energy curve of the iodine weakly bound 1u state correlating with the I(2P1/2) + I(2P1/2) dissociation limit

The stepwise three-step three-color laser population of the I2(β1g, νβ, Jβ) rovibronic states via the , νB, JB rovibronic states and rovibronic levels of the 1u(bb) and (bb) states mixed by hyperfine interaction is used for determination of rovibronic level energies of the weakly bound I2(1u(bb)) state. Dunham coefficients of the state, Yi0 (i = 0–3), Yi1 (i = 0–2), Y02 and Y12 for the = 1–5, 8, 10, 15 and ≈ 9–87 ranges, the dissociation energy of the state, De, and equilibrium I–I distance, Re, as well as the potential energy curve are determined. There are aperiodicities in the excitation spectrum corresponding to the β, νβ = 23, Jβ ← 1u(bb), ν1u = 4, 5, J1u progressions in the I2 + Rg = He, Ar mixture, namely, a great number of lines which do not coincide with the R or P line progressions. Their positions conflict with the ΔJ-even selection rule. Furthermore, they do not correspond to the ΔJ-odd progression.

Ion-pair states in the complexes of iodine molecule with rare gases

The luminescence excitation spectra and the luminescence spectra of the I2 + Rg (Rg = He, Ar, Xe; pRg = 2–20 Torr) mixtures measured at room temperature by the method of double optical resonance in the spectral range corresponding to the population of the I2(f0g+, vf = 8.9) levels and in its vicinity are analyzed in this work. The experimental data and their interpretation, according to which these spectra can be explained by the energy transfer in the intermediate I2(B0u+) and final I2(f0g+) states of the free iodine molecule rather than by the optical population, luminescence, and predissociation of the ion-pair RgI2(IP) complexes, are discussed. It is shown that these data can be explained only with account taken of the optical population of the RgI2(IP) complexes.

Dynamics and mechanism of the E→D, D′, β, γ, and δ nonadiabatic transitions induced in molecular iodine by collisions with CF4 and SF6 molecules

Nonadiabatic transitions among the first-tier ion-pair states of the iodine molecule in collisions with CF(4) and SF(6) partners are investigated by detecting the luminescence following the optical-optical double resonance excitation of the E0(g) (+)-state to the vibrational levels v(E)=8, 13, and 19. Total and partial rate constants, as well as vibrational product state distributions, are determined. It is found that electronic energy transfer in all channels is predominantly assisted by excitation of the dipole-allowed nu(3) and nu(4) modes of the partner. The measurements are accompanied by quantum scattering calculations that implement a close coupling treatment for the electronic and vibrational degrees of freedom and combine diatomics-in-molecule and long-range models for diabatic potential energy surfaces and coupling matrix elements. The analysis of experimental and theoretical data shows that the transitions without excitation of the partner are due to short-range couplings, whereas the vibrational excitation of the partner in the D0(u) (+) channel originates from the long-range coupling of two transition dipole moments: electronic of the iodine molecule and vibrational of the partner. Unexpectedly efficient excitations of the partner in the other ion-pair states, which are not coupled to the initial E0(g) (+)-state by the transition dipole, are interpreted within the postcollision mechanism. Qualitatively, this implies that during a single collision the long-range nonadiabatic transitions to D, nu(3) and D, nu(4) channels are followed by secondary short-range transitions without changing the state of the partner.

Heterogeneous and hyperfine interactions between valence states of molecular iodine correlating with the I(2P1/2) + I(2P1/2) dissociation limit

Detailed analysis of interactions between all 0g (+), 1u, and 0u (-) weakly bound states of iodine molecule correlating with the I((2)P1/2) + I((2)P1/2) (bb) dissociation limit has been performed. For this purpose, the 0u (-) (bb) state has been described using analysis of rotationally resolved excitation spectra of luminescence from the g0g (-) state populated in a three-step three-color perturbation facilitated excitation scheme via the 0u (-) state. Energies of 41 rovibrational levels, molecular constants, and potential energy curve have been determined. Energy gaps between closest rovibrational levels of the 0u (-) and 0g (+), 1u (bb) states are found to be large, ∼6 cm(-1). However, interaction of all three 0g (+), 1u, and 0u (-) (bb) states has been observed. It has been found that the 0u (-) and 1u electronic states are mixed by heterogeneous interactions, while their mixing with the 0g (+) one is due to hyperfine interactions predominantly. Admixture coefficients and electronic matrix elements of the coupling between the 0g (+) ∼1u, 0g (+)∼0u (-), and 0u (-) ∼1u states have been estimated.

Optical population of iodine molecule ion-pair states via MI2 vdW complexes, M = I2, Xe, of valence states correlating with the third, I(2P1/2) + I(2P1/2), dissociation limit

The first results of measurements and analysis of excitation spectra of the λlum = 3250 Å luminescence corresponding to I2(D0u+ → X0g+) transition as well as luminescence at λlum = 3400 Å, where I2(D′2g → A′2u and/or β1g → A1u) transitions occur, observed after three-step, λ1 + λf + λ1, λ1 = 5321–5508.2 Å, λf = 10644.0 Å, laser excitation of pure iodine vapour and I2 + Xe mixtures at room temperature via MI2 vdW complexes, M = I2, Xe, of the I2(0g+, 1u(bb)) valence states correlating with the third, I(2P1/2) + I(2P1/2) (I2(bb)), dissociation limit are presented. Luminescence spectra in the λlum = 2200–3500 Å spectral range are also analyzed. Strong luminescence from the I2(D) and, probably, I2(D′ and β) states is observed. We discuss three alternative mechanisms of optical population of the IP state. In our opinion, the mechanism including the MI2 complexes is the most probable.

Dipole moment functions for electronic transitions from ion-pair states of the second tier of molecular iodine

The emission spectra caused by the transitions from the ion-pair states and f0g+ and G1g of the I2 molecule are obtained by excitation of individual rovibronic levels of the molecule by the method of optical-optical double resonance. The emission spectra from the state F0u+ populated due to collisions I2(f) + I2(X) are also measured. By modeling the experimental emission spectra, the dipole moment functions for the electronic transitions fg+-B0u+, A0u+, and B″0u+; G1g-A0u+ and B″0u+; and F0u+-X0g+ and a′0g+ of the iodine molecule are reconstructed.

Optical population of iodine molecule ion-pair states via valence states correlating with the third, I(2P1/2) + I(2P1/2), dissociation limit and their MI2 vdW complexes, M = I2, Xe

The first results of measurements and analysis of excitation spectra of the I2(D0u+ → X0g+) and I2(D0u+ → X0g+ and/or β1g → A1u) luminescence, observed after three-step, λ1 + λf + λ1, λ1 = 5508–5530 Å, λf = 10644.0 Å, laser excitation of pure iodine vapour and I2 + Xe mixtures at room temperature via bound parts of the I2(0g+, 1u(bb)) valence states correlating with the third, I(2P1/2) + I(2P1/2), dissociation limit and their MI2 vdW complexes, M = I2, Xe, are presented. Luminescence spectra in the λlum = 2200–5000 Å spectral range are also analyzed. Strong luminescence from the I2(D, γ, D′, and/or β) states is observed, though the two latter may be populated in optical transitions in a free iodine molecule if hyperfine coupling of the I2(0g+ and 1u(bb)) state rovibronic levels occurs. We discuss possible mechanisms of optical population of the IP state.