The collision-induced non-adiabatic transitions from the f0g+ state of the iodine ion-pair second tier

Energy Transfer in the 2 u (1 D 2) Ion-Pair State of I2 by Inelastic Collisions with Noble Gas Atoms

We investigated the energy transfer in the 2 u (1 D 2) ion-pair state of I2 by collision with noble gas atoms, Ar, Kr, and Xe, using an optical-optical double resonance/fluorescence detection technique. By analyzing the temporal profiles of the emission from the laser-excited 2 u (1 D 2) state at various noble gas pressures, the quenching rate constants were determined to be (4.55 ± 0.42) × 10-10, (4.23 ± 0.11) × 10-10, and (6.83 ± 0.16) × 10-10 cm3 molecule-1 s-1 for quenching by Ar, Kr, and Xe, respectively. The 2 g (1 D 2) ion-pair state, lying in the vicinity of the 2 u (1 D 2) state, was identified as a destination state by collision with Ar and Kr. Collision with Xe provided a new reactive pathway forming the excimer XeI(B). The rate constants were determined to be = (9.61 ± 0.63) × 10-11 cm3 molecule-1 s-1 and = (4.87 ± 0.34) × 10-11 cm3 molecule-1 s-1 for the formation of the 2 g (1 D 2) state by collision with Ar and Kr, respectively, and = (6.55 ± 0.19) × 10-11 cm3 molecule-1 s-1 for the formation of XeI(B). The collisional cross sections calculated from the quenching rate constants were considerably larger than the molecular size, owing to the harpoon mechanism.

Collision induced state-to-state energy transfer dynamics between the 2u ((1)D2) and 2g ((1)D2) ion-pair states of I2

We report the first observation of collision induced state-to-state energy transfer from the 2u ((1)D2) (v2u = 3-7) ion-pair state of I2 using a perturbation facilitated optical-optical double resonance technique through the c (1)Πg∼ B (3)Π(0) hyperfine mixed double-faced valence state as the intermediate state. The excitation of the 2u ((1)D2) state yielded the weak UV fluorescence from the wide range of vibrational levels in the nearby 2g ((1)D2) state. The vibrational distribution in the 2g ((1)D2) state derived by the Franck-Condon simulation of the UV fluorescence showed that the population in the 2u ((1)D2) state transfers mostly to the 2g ((1)D2) vibronic levels which are located energetically above the laser-prepared level. The radiative lifetimes and the self-quenching rate constants were determined to be 21.3 ± 0.1 and 44.6 ± 0.8 ns, and (1.30 ± 0.01) × 10(-9) and (2.26 ± 0.17) × 10(-9) cm(3) molecule(-1) s(-1) for the 2u ((1)D2) (v2u = 3) and 2g ((1)D2) (v2g = 5) states, respectively. The rate constant for the 2u ((1)D2) - 2g ((1)D2) collision induced state-to-state energy transfer was also evaluated to be (1.89 ± 0.01), (3.07 ± 0.07), and (3.77 ± 0.05) × 10(-10) cm(3) molecule(-1) s(-1) for the v2u = 3, 5, and 7 levels, respectively. The very large self-quenching cross sections for the ion-pair states of I2 could be explained by the harpoon mechanism.

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.

Dynamics and mechanism of the E-->D, D', beta, gamma, and delta 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.

Theoretical and experimental studies of collision-induced electronic energy transfer from v=0-3 of the E(0g +) ion-pair state of Br2: collisions with He and Ar

Collisions of Br(2), prepared in the E(0(g)+) ion-pair (IP) electronic state, with He or Ar result in electronic energy transfer to the D, D', and beta IP states. These events have been examined in experimental and theoretical investigations. Experimentally, analysis of the wavelength resolved emission spectra reveals the distribution of population in the vibrational levels of the final electronic states and the relative efficiencies of He and Ar collisions in promoting a specific electronic energy transfer channel. Theoretically, semiempirical rare gas-Br(2) potential energy surfaces and diabatic couplings are used in quantum scattering calculations of the state-to-state rate constants for electronic energy transfer and distributions of population in the final electronic state vibrational levels. Agreement between theory and experiment is excellent. Comparison of the results with those obtained for similar processes in the IP excited I(2) molecule points to the general importance of Franck-Condon effects in determining vibrational populations, although this effect is more important for He collisions than for Ar collisions.

Collisional energy transfer in the intermediate states used for optical-optical double resonance excitation of ion-pair states in I2

The optical-optical double resonance spectra of I(2) and I(2)-Xe mixtures at room temperature reported in the literature using a fixed-wavelength, broad band pump laser have now been recorded using a tuneable, narrow band source. We show that during the time of the overlapped laser pulses ( approximately 10 ns) and with 10-20 Torr of Xe there is widespread collisional energy transfer in the intermediate state and that this phenomenon offers an alternative explanation for the broad bands in the excitation spectrum, assigned to XeI(2) complexes by the authors of the earlier study (M. E. Akopyan, I. Y. Novikova, S. A. Poretsky and A. M. Pravilov, Chem. Phys., 2005, 310, 287). Dispersed emission bands, previously attributed to direct fluorescence from the ion-pair state(s) of the complexes, are re-assigned to emission from ion-pair states of the parent I(2) that are populated by collisional energy transfer out of the initially excited state.

Rovibrational resonance effects in collision-induced electronic energy transfer: I2(E,v=0-2)+CF4

Collisions of I2 in the E(0(g)+) electronic state with CF4 molecules induce electronic energy transfer to the nearby D, beta, and D' ion-pair states. Simulations of dispersed fluorescence spectra reveal collision-induced electronic energy transfer rate constants and final vibrational state distributions within each final electronic state. In comparison with earlier reports on I2(upsilon(E)=0-2) collisions with He or Ar atoms, we find markedly different dynamics when I2, excited to the same rovibronic states, collides with CF4. Final vibrational state distributions agree with the associated Franck-Condon factors with the initially prepared state to a greater degree than those found with He or Ar collision partners and suggest that internal degrees of freedom in the CF4 molecule represent a substantial means for accepting the accompanying loss of I2 vibronic energy. Comparison of the E-->D transfer of I2 excited to the J=23 and J=55 levels of the upsilon(E)=0 state reveals the onset of specific, nonstatistical dynamics as the available energy is increased above the threshold for excitation of the low frequency nu2 bending mode of CF4.

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.

Franck-Condon effects in collision-induced electronic energy transfer: I2(E; v = 1,2) + He, Ar

Collisions of I2 in the E electronic state with rare gas atoms result in electronic energy transfer to the D, beta, and D' ion-pair electronic states. Rate constants for each of these channels have been measured when I2 is initially prepared in the J = 55, nu = 1 and 2 levels in the E state. The rate constants and effective hard sphere collision cross sections confirm the trends observed when nu = 0 in the E state is initially prepared: He collisions favor population of the D state, while Ar collisions favor population of the beta state. Final state vibrational level distributions are determined by spectral simulation and are found to be qualitatively consistent with the trends in the Franck-Condon factors. The experimental distributions are also compared to the recent quantum scattering calculations of Tscherbul and Buchachenko.