John R. Barker

Polycyclic aromatic hydrocarbons and the unidentified infrared emission bands - Auto exhaust along the Milky Way

The unidentified infrared emission features (UIR bands) are attributed to a collection of partially hydrogenated, positively charged polycyclic aromatic hydrocarbons (PAHs). This assignment is based on a spectroscopic analysis of the UIR bands. Comparison of the observed interstellar 6.2 and 7.7-micron bands with the laboratory measured Raman spectrum of a collection of carbon-based particulates (auto exhaust) shows a very good agreement, supporting this identification. The infrared emission is due to relaxation from highly vibrationally and electronically excited states. The excitation is probably caused by UV photon absorption. The infrared fluorescence of one particular, highly vibrationally excited PAH (chrysene) is modeled. In this analysis the species is treated as a molecule rather than bulk material and the non-thermodynamic equilibrium nature of the emission is fully taken into account. From a comparison of the observed ratio of the 3.3 to 11.3-micron UIR bands with the model calculations, the average number of carbon atoms per molecule is estimated to be about 20. The abundance of interstellar PAHs is calculated to be about 2 x 10 to the -7th with respect to hydrogen.

Energy‐dependent energy transfer: Deactivation of azulene (S0, Evib) by 17 collider gases

Collisional deactivation of highly vibrationally excited azulene in the electronic ground state was investigated using infrared fluorescence detection. Azulene (S0, E) was prepared with E≂17 500 cm−1 and E≂30 600 cm−1 by laser excitation at 600 and 337 nm, respectively. Advantage was taken of the fast internal conversion rate to S0 azulene from S1(600 nm) and S2(337 nm) electronic states. The collider gases investigated are He, Ne, Ar, Kr, Xe, H2, D2, N2, CO, O2, CO2, H2O, NH3, CH4, SF6, n‐C4H10, and unexcited azulene. The results are expressed in terms of 〈ΔE(E)〉, the average energy transferred per collision, which can depend on the vibrational excitation energy E of the azulene. Using previously obtained knowledge of the dependence of infrared fluorescence intensity on E [M. J. Rossi and J. R. Barker, Chem. Phys. Lett. 85, 21 (1982)], two methods were used to obtain 〈ΔE(E)〉 values from the fluorescence decay curves: (1) an approximate method that considered only the average energy, and (2) solution of t...

Vibrational relaxation of highly excited toluene

The collisional loss of vibrational energy from gas‐phase toluene, pumped by a pulsed KrF laser operating at 248 nm, has been observed by monitoring the time‐resolved infrared fluorescence from the C–H stretch modes near 3.3 μm. The fragmentation quantum yield of toluene pumped at 248 nm was determined experimentally to be ∼6%. Energy‐transfer data were obtained for 20 collider gases, including unexcited toluene, and analyzed by an improved inversion technique that converts the fluorescence intensity to the bulk average energy, from which is extracted 〈〈ΔE〉〉, the bulk average amount of energy transferred per collision. Comparisons are presented of these results with similar studies of benzene and azulene, and with the time‐resolved ultraviolet absorption study of toluene carried out by Hippler et al. [J. Chem. Phys. 78, 6709 (1983)]. The present results show 〈〈ΔE〉〉 to be nearly directly proportional to the vibrational energy of the excited toluene from 5000 to 25 000 cm−1. For many of the colliders at hig...

Energy‐dependent collisional deactivation of vibrationally excited azulene

Collisional energy transfer parameters for highly vibrationally excited azulene have been deduced from new infrared fluorescence (IRF) emission lifetime data with an improved calibration relating IRF intensity to vibrational energy [J. Shi, D. Bernfeld, and J. R. Barker, J. Chem. Phys. 88, 6211 (1988), preceding paper]. In addition, data from previous experiments [M. J. Rossi, J. R. Pladziewicz, and J. R. Barker, J. Chem. Phys. 78, 6695 (1983)] have been reanalyzed based on the improved calibration. Inversion of the IRF decay curves produced plots of energy decay, which were analyzed to determine 〈ΔE〉, the average energy transferred per collision. Master equation simulations reproduced both the original IRF decays and the deduced energy decays. A third (simple) method of 〈ΔE〉 determination agrees well with the other two. The results show 〈ΔE〉 to be nearly directly proportional to the vibrational energy of the excited azulene from ∼8000 to 33 000 cm−1. At high energies, there are indications that the 〈ΔE〉 ...

Anharmonicity and the interstellar polycyclic aromatic hydrocarbon infrared emission spectrum

The hypothesis that interstellar infrared emission originates from vibrationally excited polycyclic aromatic hydrocarbons (PAHs) requires that emission can arise from all vibrational levels that are energetically accessible. Due to anharmonicity, the emission from the upper vibrational levels is shifted to longer wavelengths from that of the v = 1-0 transition. It is shown that structure in the 3-micron region is readily and quantitatively explained by emission from upper vibrational levels of excited PAHs that contain a maximum of 20-30 carbon atoms. The asymmetrical broadening of the 11.3-micron emission band may also be due to anharmonicity. 21 references.

Infrared multiphoton decomposition: A comparison of approximate models and exact solutions of the energy‐grained master equation

The approximate ’’thermal’’ model and ’’continuum’’ model have been compared to exact calculations, and neither gives satisfactory results. In particular, the thermal model is based on an inaccurate and unphysical approximation to the correct molecular density of states, as well as a restrictive expression for the absorption cross sections. As an alternative to the approximate models, the exact stochastic method gives exact results and can be implemented using small computers or programmable pocket calculators. The results obtained from this method are exact and precision depends only upon the number of ’’trajectories’’ calculated. It was discovered that the yield versus fluence results from exact numerical integration of the master equation approximately conform to a cumulative log–normal distribution function in decomposition time or fluence. Thus, to a good degree of approximation, computed yield versus fluence results can be expressed simply in terms of a mean and a dispersion parameter. This suggests...

Monte Carlo calculations on unimolecular reactions, energy transfer, and IR-multiphoton decomposition☆

Abstract Monte Carlo techniques are described for stochastic calculations involving energy transfer, unimolecular reactions, and IR-multiphoton decomposition; all of these phenomena can be simulated using the same computer code. The details of the model are presented along with the following example calculations: (1) weak collision energy-transfer following photoactivation, (2) chemical activation unimolecular reactions with multiple reaction pathways, (3) incubation times in shock-tube experiments on systems with multiple reaction pathways, and (4) IR-multiphoton absorption and decomposition in collisional environments. The model uses a modular approach that permits application to numerous experimental systems.

Infrared emission studies of the vibrational deactivation of benzene derivatives

Abstract This paper reviews the time-resolved infrared fluorescence technique and its application to studies of the deactivation of highly vibrationally excited benzene-d0, benzene-d6, toluene-d0, and toluene-d8. The energy transfer parameters obtained are summarized in terms of both «ΔE» and d, the average total energy transferred per collision and the average energy transferred in deactivating collisions, respectively. The mechanisms of energy transfer have also been investigated to determine the roles of vibration-to-translation/rotation and vibration-to-vibration energy transfer and the possible influence of quantum effects. The review concludes with a brief summary of a current view of large molecule energy transfer and suggestions for future work.

Infrared multiphoton generation of radicals: A new technique for obtaining absolute rate constants. Application to reactions of CF3

The infrared multiphoton dissociation of CF3I is used as a convenient source of CF3 radicals under very low pressure conditions. Measurement of thermal absolute rate constants at 298 K for the reactions CF3+Br2→CF3Br+Br, CF3+ClNO→CF3Cl+NO, CF3+O3→CF3O+O2, and CF3+NO2→CF3O+NO yield (7.8±1.3) ×108, (3.5±0.5) ×108, (5.6±0.8) ×108, and (1.6±0.3) ×109 M−1 s−1, respectively. This new technique shows great promise for production of other free radicals of interest and measurement of thermal absolute rate constants over a wide temperature range.

Isotope effects in the vibrational deactivation of large molecules

Collisional deactivation of highly vibrationally excited gas phase toluene‐d8 and benzene‐d6 pumped at 248 nm, has been investigated by monitoring the time resolved infrared fluorescence from the C–D stretch modes near 4.3 μm. For toluene‐d8, energy transfer data were obtained for about 20 collider gases, including unexcited toluene‐d8; for benzene‐d6, only a few colliders were investigated. For both systems the data were analyzed by an inversion technique that converts the fluorescence decay to the bulk average energy, from which is calculated the average energy transferred per collision, 〈〈ΔE〉〉inv. Data obtained earlier for benzene‐d0 were reanalyzed and the revised results are reported. Results for both normal and deuterated excited species show 〈〈ΔE〉〉inv to be nearly directly proportional to the vibrational energy 〈〈E〉〉inv of the excited molecule from 5 000 to 25 000 cm−1. However, for pure toluene‐d8, benzene‐d6, and a few other collider gases at high energies, the slope of the 〈〈ΔE〉〉inv vs 〈〈E〉〉inv ...