Collisions Of I2 Research Articles

Photoemission resulting from collisions of I2 with 5–100 eV electrons

We have determined absolute apparent excitation cross sections for photoemission in the 300–1050 nm spectral region resulting from collisions of I2 with electrons of 4.8–100 eV translational energy. Experiments were done in single collision conditions and emission is observed from I2, I2 +, I and I+. All the emission features appear to turn on at the energy thresholds for formation of the respective excited state species. At higher electron beam energy the spectra are dominated by emissions from the strong I2 + A–X band, as well as the intense emission from several I 5s 25p 46s states. An attempt to determine the initial I2 + A populations generated from electron impact is discussed. These populations are determined from simulations of the I2 + A–X emission band using both experimentally and theoretically determined potential energy surfaces available in the literature. Direct, vertical formation of I2 + A from I2 X is only supported in the case of the ab initio potential energy surface.

A guided-ion beam study of the collisions and reactions of I(+) and I2 (+) with I2.

Growing interest in developing and testing iodine Hall effect thrusters requires measurements of the cross sections of reactions that generate low energy plasma following discharge. Limited experimental and theoretical work necessitates a decisive experiment to elucidate the charge exchange and collision-induced dissociation channels. To this end, we have used guided-ion beam techniques to measure cross sections for both I(+) + I2 and I2 (+)+I2 collisions. We present total collision cross sections as well as collision-induced dissociation cross sections for center-of-mass collision energies ranging from 0.5 to 200 eV for molecular iodine cations. Similarly, we present total collision cross section and charge-exchange cross sections for atomic iodine cations for center-of-mass collision energies ranging from 0.67 to 167 eV. Time-of-flight measurements of the collision products allow determination of velocity distributions, which show evidence of complex formation of I3 (+) from the I(+) + I2 reaction at collision energies below 6 eV.

Electron transfer and bond-forming reactions following collisions of I2+ with CO and CS2

Collisions between I2+ and CO have been investigated using time-of-flight mass spectrometry at a range of centre-of-mass collision energies between 0.5 and 3.0 eV. Following I2++CO collisions, we detect I++CO+ from a single-electron transfer reaction and IO++C+ from bond-forming reactivity. Reaction-window calculations, based on Landau–Zener theory, have been used to rationalise the electron transfer reactivity and computational chemistry has been used to explore the [I–CO]2+ potential energy surface to account for the observation of IO+. In addition, collisions between I2+ and CS2 have been investigated over a range of centre-of-mass collision energies between 0.8 and 6.0 eV. Both single- and double-electron transfer reactions are observed in the I2+/CS2 collision system, an observation again rationalised by reaction-window theory. The monocations IS+ and IC+ are also detected following collisions of I2+ with CS2, and these ions are clearly products from a bond-forming reaction. We present a simple model based on the structure of the [I–CS2]2+ collision complex to rationalise the significantly larger yield of IS+ than IC+ in this bond-forming process.

On the dissociation of I2 by O2(a 1Δ): Pathways involving the excited species I2(A′ Π32u,A Π31u), I2(X 1∑,υ), and O2(a 1Δ,υ)

Kinetic studies were carried out to explore the role of the excited species I2(A′ Π32u,A Π31u), I2(X 1∑,υ), and O2(a 1Δ,υ) in the dissociation of I2 by singlet oxygen. A flow tube apparatus that utilized a chemical singlet oxygen generator was used to measure the I2 dissociation rate in O2(a 1Δ)/I2 mixtures. Vibrationally excited I2(X) is thought to be a significant intermediate in the dissociation process. Excitation probabilities (γυ) for population of the υth I2(X) vibrational level in the reaction I2(X)+I(P21/2)→I2(X,υ>10)+I(P23/2) were estimated based on a comparison of calculated populations with experimentally determined values. Satisfactory agreement with the experimental data [Barnault et al., J. Phys. IV 1, C7–647 (1991)] was achieved for total excitation probabilities partitioned in two ranges, such that Γ25≤υ≤47=∑υ=2547γυ≈0.1 and Γ15≤υ≤24=∑υ=1524γυ≈0.9. A multipathway I2 dissociation model was developed in which the intermediates are I2(A′ Π32u,A Π31u) and I2(X,υ). It was shown that the iodine dissociation process passes predominantly through the I2(A′ Π32u,A Π31u) intermediate. These states are populated by collisions of I2 with vibrationally excited O2(a 1Δ,υ) at the initiation and the chain stages, when the mole fraction of I2 is small (ηI2<1%). For higher I2 concentrations (ηI2≥1%) the excited states are populated in the chain stage by collisions of I2(X,15≤υ≤24) with O2(a 1Δ).

Formation of I2(B3Π) in the presence of O2(a1Δ)

The mechanism by which I2(B3Π0) is excited in the chemical oxygen-iodine laser was studied by means of emission spectroscopy. Using the intensity of the O2(b1Σ,υ′=0)→O2(X3Σ,υ″=0) band as a reference, I2(B3Π0) relative number densities were assessed by measuring the I2(B3Π0,υ′)→I2(X1Σ,υ″) emission intensities. Vibrationally excited singlet oxygen molecules O2(a1Δ,υ′=1) were detected using infrared emission spectroscopy. The measured relative density of O2(a1Δ,υ′=1) for the conditions of a typical oxygen-iodine laser medium amounted to ∼15% of the total O2(a1Δ) content. Mechanisms for I2(B3Π0) formation were proposed for both the I2 dissociation zone and the region downstream of the dissociation zone. Both pumping mechanisms involved electronically excited molecular iodine I2(A′3Π2u, A3Π1u) as an intermediate. It is proposed that in the dissociation zone the molecular iodine A′3Π2u and A3Π1u states are populated in collisions with vibrationally excited singlet oxygen molecules O2(a1Δ,υ′), whereas in the downstream region of the dissociation zone those intermediate states are populated by the atomic iodine recombination process. I2(B3Π0) is subsequently formed in collisions of I2(A′3Π2u, A3Π1u) with singlet oxygen. We also demonstrated that I2(B3Π0) does not participate measurably in the I2 dissociation process, and that energy transfer from O2(b1Σ) does not excite I2(B3Π0) to a significant degree.

Rovibrational resonance effects in collision-induced electronic energy transfer: I2(E,v=–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.

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.

Phase and Energy Relaxation of Vibrational Motion and Its Manifestation in Femtosecond Pump−Probe Experiments on I2 in Rare Gas Environments

We present a classical description of femtosecond time-resolved experiments on collision-induced vibrational relaxation processes. For I2 embedded in Ar and He gases, a hot distribution of molecular vibrational states is prepared by a pump laser pulse. Collisions of I2 with gas atoms lead to dephasing and energy relaxation in the vibrational degree-of-freedom. In employing energy-phase coordinates, we are able to separate these two effects. Decay rates as obtained from the Fourier analysis of pump−probe signals are compared to those derived from the coordinate autocorrelation-function. It is shown that the vibrational energy relaxation influences the decay of the oscillatory structure in transient signals, leading to a nontrivial density dependence of the dephasing rate as observed in the experiments of Liu et al. [J. Phys. Chem. 1996, 100, 18650].

Fractal behavior in classical collisional energy transfer

A plot of final vibrational action vs initial vibrational phase for a two degree of freedom model for He + I2 collisions is examined and shown to exhibit a chattering region. The chattering region is shown to exhibit a very intricate structure, including an infinite number of regions of regularity which we term icicles. Structure is evident on all scales of examination and a fractal dimension close to 2 is obtained for some parts of the chattering region. The survival probability associated with the complexes shows an initial fast decay, due to the icicles, but can be roughly characterized over a longer and wider time range by a more slowly decaying exponential.

Angular momentum limitation in vibrationally inelastic collisions of I2(B 3Π) with He and Xe

We present measurements of thermally averaged level-to-level rate constants for vibrationally and rotationally inelastic collisions of I2(B 3Π) with Xe and He:I*2(vx=13, ji=41 or 91) → I*2(vf=12, jf). Comparison of these results with our previous work on the same systems for rotationally inelastic collisions with no vibrational inelasticity reveals a similarity in the collision dynamics for the vibrationally inelastic and vibrationally elastic processes. (AIP)

Pulsed molecular beam study of state-to-state vibrational excitation in He+I2 collisions: Energy dependence of the v=0→1 cross section

We report measurements of the kinetic energy dependence of the v=0→1 vibrational excitation cross section for He+12 collisions. These results were obtained with a new crossed, pulsed molecular beam apparatus having a variable molecular beam interaction angle and laser-induced fluorescence detection. The cross section shows a sharp onset at threshold, and rises nearly linearly with increasing energy to the highest experimental value of kinetic energy, about 13 times the vibrational quantum. At the higher energies, v=0→2 excitation is also observed. These data constitute the first measurements of the energy dependence of a microscopic state-to-state cross section for vibrational transitions in collisions of neutral species.

Dynamical constraints on the transfer of angular momentum in rotationally inelastic collisions of I2(B 3Π) with He and Xe

We have measured thermally averaged level-to-level rate constants for rotationally inelastic collisions of I2(B 3Π) with Xe and He using the method of laser induced fluorescence. We present the values of 149 rate constants with average errors less than 10% for initial rotational levels ranging from 41 to 113 in the v′ = 13 vibrational level. For I*2–He, we report the first experimental evidence for a limitation of maximum angular momentum transfer on level-to-level rate constants and conclude that dynamical, rather than energetic constraints govern the collisional process. We find that the I*2–Xe rate constants follow a power law in both energy and angular momentum transfer, a behavior observed in most previous experimental work on rotationally inelastic collisions.

Laser fluorescence studies of molecular iodine

We have studied quantitatively the circular polarization of rotationally resolved features in the laser-induced fluorescence from I2 excited by the 5145 A argon laser line, and its variation with various experimental parameters such as light intensity, iodine and foreign gas pressure. Optical pumping of the ν″ = 0, J″ = 13 level of 1Σ g + has been unambiguously demonstrated and the variation of C with gas pressure has been analysed to yield cross sections for elastic and inelastic collisions which relax the oriented ground state. Experiments on the dipolar array of oriented 3Π0u + molecules at very high pressures show that the circular polarization ratio does not change under conditions of many gas kinetic collisions per lifetime. This indicates that a selection rule ΔM = 0 operates on elastic collisions of I2* with other atoms and molecules, which appears to be rigorously adhered to.

Detection of Iodine Atoms by an Atomic Fluorescence Technique: Application to Study of Diffusion and Wall Recombination

An atomic fluorescence method has been used to detect iodine atoms in concentrations lower than 1010/cm3. The heart of the apparatus is a “solar-blind” photomultiplier having practically no sensitivity to radiation of wavelengths longer than 2500 Å, but with good response in the region of the two strong iodine atomic resonance lines near 1800 Å. In combination with a small photolysis flashlamp, the system is used to study recombination of ground-state (2P3/2) iodine atoms in pure I2 vapor. At pressures below 0.2 torr the removal of atoms is seen to be predominantly a first-order diffusion-controlled process, characterized by a wall sticking probability of 0.040–0.065 and a binary diffusion cross section of 100 Å2. The reaction I+I2→I3 is found to be of minor significance in this study; a rough estimate indicates that fewer than one in 104 I–I2 collisions lead to the formation of I3.