Energy transfer of highly vibrationally excited naphthalene. II. Vibrational energy dependence and isotope and mass effects

The vibrational energy dependence, H and D atom isotope effects, and the mass effects in the energy transfer between rare gas atoms and highly vibrationally excited naphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity-map ion imaging at various translational collision energies. Increase of vibrational energy from 16 194 to 18 922 cm(-1) does not make a significant difference in energy transfer. The energy transfer properties also remain the same when H atoms in naphthalene are replaced by D atoms, indicating that the high vibrational frequency modes do not play important roles in energy transfer. They are not important in supercollisions either. However, as the Kr atoms are replaced by Xe atoms, the shapes of energy transfer probability density functions change. The probabilities for large translation to vibration/rotation energy transfer (T-->VR) and large vibration to translation energy transfer (V-->T) decrease. High energy tails in the backward scatterings disappear, and the probability for very large vibration to translation energy transfer such as supercollisions also decreases.     

Energy transfer of highly vibrationally excited naphthalene. III. Rotational effects

The rotational effects in the energy transfer between Kr atoms and highly vibrationally excited naphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity map ion imaging at various translational collision energies. As the initial rotational temperature changes from less than 10 to approximately 350 K, the ratio of vibrational to translational (V-->T) energy transfer cross section to translational to vibrational/rotational (T-->VR) energy transfer cross section increases, but the probability of forming a complex during the collisions decreases. Significant increases in the large V-->T energy transfer probabilities, termed supercollisions, at high initial rotational temperature were observed.     

Energy transfer of highly vibrationally excited biphenyl

The energy transfer between Kr atoms and highly vibrationally excited, rotationally cold biphenyl in the triplet state was investigated using crossed-beam/time-of-flight mass spectrometer/time-sliced velocity map ion imaging techniques. Compared to the energy transfer of naphthalene, energy transfer of biphenyl shows more forward scattering, less complex formation, larger cross section for vibrational to translational (V→T) energy transfer, smaller cross section for translational to vibrational and rotational (T→VR) energy transfer, larger total collisional cross section, and more energy transferred from vibration to translation. Significant increase in the large V→T energy transfer probabilities, termed supercollisions, was observed. The difference in the energy transfer of highly vibrationally excited molecules between rotationally cold naphthalene and rotationally cold biphenyl is very similar to the difference in the energy transfer of highly vibrationally excited molecules between rotationally cold naphthalene and rotationally hot naphthalene. The low-frequency vibrational modes with out-of-plane motion and rotationlike wide-angle motion are attributed to make the energy transfer of biphenyl different from that of naphthalene.     

Energy transfer of highly vibrationally excited phenanthrene and diphenylacetylene

The energy transfer between Kr atoms and highly vibrationally excited, rotationally cold phenanthrene and diphenylacetylene in the triplet state was investigated using crossed-beam/time-of-flight mass spectrometer/time-sliced velocity map ion imaging techniques. Compared to the energy transfer between naphthalene and Kr, energy transfer between phenanthrene and Kr shows a larger cross-section for vibrational to translational (V → T) energy transfer, a smaller cross-section for translational to vibrational and rotational (T → VR) energy transfer, and more energy transferred from vibration to translation. These differences are further enlarged in the comparison between naphthalene and diphenylacetylene. In addition, less complex formation and significant increases in the large V → T energy transfer probabilities, termed supercollisions in diphenylacetylene and Kr collisions were observed. The differences in the energy transfer between these highly vibrationally excited molecules are attributed to the low-frequency vibrational modes, especially those vibrations with rotation-like wide-angle motions.     

Supercollisions and energy transfer of highly vibrationally excited molecules

Collisional energy-transfer probability distribution functions of highly vibrationally excited molecules and the existence of supercollisions remain as the outstanding questions in the field of intermolecular energy transfer. In this investigation, collisional interactions between ground state Kr atoms and highly vibrationally excited azulene molecules (4.66 eV internal energy) were examined at a collision energy of 410 cm-1 using a crossed molecular beam apparatus and time-sliced ion imaging techniques. A large amount of energy transfer (1000-5000 cm-1) in the backward direction was observed. We report the experimental measurement for the shape of the energy-transfer probability distribution function along with a direct observation of supercollisions.     

Energy transfer in collisions of highly vibrationally excited tetrahedral molecules

Model trajectory calculations of the energy transfer processes in collisions of Ar with highly vibrationally excited CH₄, CD₄, SiH₄ and CF₄ are performed. Special attention is payed to the calculation of the energy transferred to active (vibrational) degrees of freedom. The results support the diffusion model of excitation-dissociation and give the low pressure collision efficiency β_c which qualitatively agrees with experiment in magnitude and temperature dependence.     

Energy transfer of highly vibrationally excited naphthalene: collisions with CHF3, CF4, and Kr

Energy transfer of highly vibrationally excited naphthalene in the triplet state in collisions with CHF(3), CF(4), and Kr was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. Highly vibrationally excited naphthalene (2.0 eV vibrational energy) was formed via the rapid intersystem crossing of naphthalene initially excited to the S(2) state by 266 nm photons. The shapes of the collisional energy-transfer probability density functions were measured directly from the scattering results of highly vibrationally excited naphthalene. In comparison to Kr atoms, the energy transfer in collisions between CHF(3) and naphthalene shows more forward scatterings, larger cross section for vibrational to translational (V → T) energy transfer, smaller cross section for translational to vibrational and rotational (T → VR) energy transfer, and more energy transferred from vibration to translation, especially in the range -ΔE(d) = -100 to -800 cm(-1). On the other hand, the difference of energy transfer properties between collisional partners Kr and CF(4) is small. The enhancement of the V → T energy transfer in collisions with CHF(3) is attributed to the large attractive interaction between naphthalene and CHF(3) (1-3 kcal/mol).     

Energy transfer of highly vibrationally excited 2-methylnaphthalene: Methylation effects

The methylation effects in the energy transfer between Kr atoms and highly vibrationally excited 2-methylnaphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity-map ion imaging at a translational collision energy of approximately 520 cm(-1). Comparison of the energy transfer between naphthalene and 2-methylnaphthalene shows that the difference in total collisional cross section and the difference in energy transfer probability density functions are small. The ratio of the total cross sections is sigma(naphthalene): sigma(methylnaphthalene)=1.08+/-0.05:1. The energy transfer probability density function shows that naphthalene has a little larger probability at small T-->VR energy transfer, DeltaE(u)<300 cm(-1), and 2-methylnaphthalene has a little larger probability at large V-->T energy transfer, -800 cm(-1)相似文献   

Near-resonant energy transfer from highly vibrationally excited OH to N2

The probability per collision P(T) of near-resonant vibration-to-vibration energy transfer (ET) of one quantum of vibrational energy from vibrational levels nu=8 and nu=9 of OH to N(2)(nu=0), OH(nu)+N(2)(0)-->OH(nu-1)+N(2)(1), is calculated in the 100-350 K temperature range. These processes represent important steps in a model that explains the enhanced 4.3 microm emission from CO(2) in the nocturnal mesosphere. The calculated energy transfer is mediated by weak long-range dipole-quadrupole interaction. The results of this calculation are very sensitive to the strength of the two transition moments. Because of the long range of the intermolecular potential, the resonance function, a measure of energy that can be efficiently exchanged between translation and vibration-rotation degrees of freedom, is rather narrow. A narrow resonance function coupled with the large rotational constant of OH is shown to render the results of the calculation very sensitive to the rotational distribution, or the rotational temperature if one exists, of this molecule. The calculations are carried out in the first and second orders of perturbation theory with the latter shown to give ET probabilities that are an order of magnitude larger than the former. The reasons for the difference in magnitude and temperature dependence of the first- and second-order calculations are discussed. The results of the calculations are compared with room temperature measurements as well as with an earlier calculation. Our calculated results are in good agreement with the room temperature measurements for the transfer of vibrational energy for the exothermic OH(nu=9) ET process but are about an order lower than the room temperature measurements for the exothermic OH(nu=8) ET process. The cause of this discrepancy is explored. This calculation does not give the large values of the rate coefficients needed by the model that explains the enhanced 4.3 microm emission from CO(2) in the nocturnal mesosphere.     

Energy transfer of highly vibrationally excited azulene. III. Collisions between azulene and argon

The energy transfer dynamics between highly vibrationally excited azulene molecules (37 582 cm(-1) internal energy) and Ar atoms in a series of collision energies (200, 492, 747, and 983 cm(-1)) was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. The angular resolved collisional energy-transfer probability distribution functions were measured directly from the scattering results of highly vibrationally excited azulene. Direct T-VR energy transfer was found to be quite efficient. In some instances, nearly all of the translational energy is transferred to vibrational/rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy (V-T). Significant amount of energy transfer from vibration to translation was observed at large collision energies in backward and sideway directions. The ratios of total cross sections between T-VR and V-T increases as collision energy increases. Formation of azulene-argon complexes during the collision was observed at low enough collision energies. The complexes make only minor contributions to the measured translational to vibrational/rotational (T-VR) energy transfer.     

Quantum state-resolved collision relaxation of highly vibrationally excited SO2

Collision depopulation cross sections of 13 single, highly vibrationally excited levels with 45,000 cm(-1) energy in the electronic ground state of SO(2) in collision with CO in a supersonic jet have been measured. The measurements for these single highly excited quantum states are conducted through pressure dependence of the decay of the fluorescence quantum beat resulted from their coupling with the rovibronic levels in the optically allowed transitions to the (140), (210), and (132) C(1)B(2) levels. The relaxation cross sections of these highly excited states, each with well-defined energy and symmetry, range from 27 to 187 A(2) with an average of 71 A(2). This average cross section is much larger than the hard sphere cross section of 48 A(2). The relaxation cross section is also found to be larger for the quantum states with a larger matrix element in coupling with the "bright" electronically excited level. Both observations suggest a substantial contribution from long range interactions in collision relaxation of highly excited molecules.     

Energy transfer of highly vibrationally excited azulene: collisions between azulene and krypton

The energy-transfer dynamics between highly vibrationally excited azulene molecules and Kr atoms in a series of collision energies (i.e., relative translational energies 170, 410, and 780 cm(-1)) was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. "Hot" azulene (4.66 eV internal energy) was formed via the rapid internal conversion of azulene initially excited to the S4 state by 266-nm photons. The shapes of the collisional energy-transfer probability density functions were measured directly from the scattering results of highly vibrationally excited or hot azulene. At low enough collision energies an azulene-Kr complex was observed, resulting from small amounts of translational to vibrational-rotational (T-VR) energy transfer. T-VR energy transfer was found to be quite efficient. In some instances, nearly all of the translational energy is transferred to vibrational-rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy (V-T). The shapes of V-T energy-transfer probability density functions were best fit by multiexponential functions. We find that substantial amounts of energy are transferred in the backward scattering direction due to supercollisions at high collision energies. The probability for supercollisions, defined arbitrarily as the scattered azulene in the region 160 degrees 2000 cm(-1) is 1% and 0.3% of all other collisions at collision energies 410 and 780 cm(-1), respectively.     

Intermode energy transfer in vibrationally excited O3

The laser excited fluorescence method has been employed to determine the rate constants for vibrational relaxation of the O₃ (010), O₃ (100) and O₃ (001) levels at 298 K. The fluorescence observations from the O₃ (010) level provide direct measurements of the rate for intermode vibrational energy transfer from the coupled ν₁ and ν₃ modes to the ν₂ mode. The slowness of this process indicates the likelihood that the ν₁ and/or ν₃ modes (rather than the ν₂ mode) play a predominant role in the laser enhanced reaction between O₃² and NO at 298 K.     

Collisionless intramolecular energy transfer in vibrationally excited SF6

Infared-infrared double resonance measurements of the ν₃ absorption band of SF₆ using a CO₂ TEA pump laser (0.3 J/cm² maximum excitation) and a cw CO₂ probe laser have been carried out. These measurements probe both the coherent and quasi-continuum phases of the multiphoton dissociation process in SF₆. The spectral and temporal dependences of the induced absorption are obtained. The results give the first direct measurement of collisionless intramolecular vibrational energy redistribution times in a vibrationally excited complex molecule; for an excess energy of three quanta deposited in the ν₃ mode of SF₆, the ν₃ mode comes into equilibrium with all the remaining vibrational degrees of freedoom with a 3 ± 1 μs time constant.     

Relaxation of highly vibrationally excited molecules

The rate constant for V-V relaxation of diatomic homonuclear molecules is determined from collisions of unexcited molecules with molecules near the dissociation threshold. It is shown that a quasi-resonant transition through several levels dominates in this process so that the energy difference between the initial and final states of the excited molecule is approximately equal to the transition energy from the zero level to the first one. The relaxation kinetics of excited molecules is studied. Absorption of IR radiation by polyatomic molecules is discussed taking into account collisions. A criterion for the negligibility of energy loss is obtained, and the dissociation rate of molecules under the action of IR laser irradiation found. The computational results are compared with experimental data. A self-consistent procedure is formulated for a gas irradiated by a quasi-continuously operating IR laser, in order to determine simultaneously the dissociation rate, dissipation energy flux and temperature. The existence of an optimum radiation region for dissociation is found.     

Collisional energy transfer in highly vibrationally excited molecules: A very low-pressure pyrolysis study of acetyl chloride

The average downward energy transfer (〈Δ E_down〉) is obtained for highly vibrationally excited acetyl chloride with Ne and C₂H₄ bath gases at ca. 870 K. Data are obtained by the technique of very low-pressure pyrolysis (VLPP). Fitting these data by solution of the appropriate reaction-diffusion integrodifferential master equation yields the gas/gas collisional energy transfer parameters: 〈Δ E_down〉 values are 220 ± 10 cm^?1 (Ne bath gas) and 330 ± 20 cm^?1 (C₂H₄). These energy transfer quantities are much less than those predicted by statistical theories, or those observed for similar sized molecules such as CH₃CH₂Cl. These results are explained by the qualitative predictions of the biased random walk model wherein the fundamental mechanism of energy transfer is the multiple interactions between the bath gas and the individual atoms of the reactant molecule, during the course of the collision event. The charge distribution of acetyl chloride decreases the number of such interactions, thereby reducing the amount of energy transferred per collision.     

High temperature collisional energy transfer in highly vibrationally excited molecules. III: Isotope effects in tert-butyl bromide systems

Changes in the magnitude of 〈ΔE_down〉, the average downward collisional energy transferred between a highly vibrationally excited reactant molecule and an inert bath gas, upon perdeuteration of the substrate are reported for tert-butyl bromide dilute in Ar, Kr, N₂, and CO₂. The technique of pressure-dependent very low-pressure pyrolysis (VLPP) was used to obtain the absolute values of 〈ΔE_down〉, which are for C₄H₉Br, 230 (Ar), 285 (Kr), 270 (N₂), and 365 (CO₂) while for C₄D₉Br, 200 (Ar), 250 (Kr), 220 (N₂), and 335 (CO₂), all in cm^?1 at ca. 720 K. The estimated uncertainties in these values are ca. ± 10%. These observed 〈ΔE_down〉, values and trends found with results from this series of isotope studies, are compared with current theoretical models. Extrapolated high-pressure temperature-dependent rate coefficients (s^?1) for the thermal decomposition of reactant are 10^13.8±0.3 exp(?175 ± 8 kJ mol^?1/RT) for C₄H₉Br and 10^14.3±0.3 exp(?183 ± 8 kJ mol^?1/RT) for C₄D₉Br. These results are in accord with other studies and the expected isotope effect.     

High temperature collisional energy transfer in highly vibrationally excited molecules: Isotope effects in tert-butyl chloride systems

The average downward collisional energy transfer (<ΔE_down>) is obtained for highly vibrationally excited tert-butyl chloride, both undeuterated and per-deuterated, with Kr, N₂, CO₂, and C₂H₄ bath gases, at ca. 760 K. Data are obtained using the technique of pressure-dependent very low-pressure pyrolysis. Reactant internal energies to which the data are sensitive are in the range 200–250 kJ mol^?1. For C₄H₉Cl, the <ΔE_down> values (cm^?1) are 255 (Kr), 265 (N₂), 440 (CO₂), and 585 (C₂H₄), and for C₄D₉Cl, 245 (N₂), 370 (CO₂), and 540 (C₂H₄). The uncertainties in these values are ca. 20% (40% for Kr); the uncertainties in the deuteration ratios are 10–15%. The value for Kr is in agreement with theoretical predictions of a biased random walk model for internal energy change in monatomic/substrate collisions. The effect of deuteration of <ΔE_down> is also in accord with that predicted by a modification of the theory. Extrapolated highpressure rate coefficients for the thermal decomposition of reactant are 10^13.6 exp(-187 kJ mol^?1/RT) s^?1 (C₄H₉Cl) and 10^14.2 exp(?196 kJ mol^?1/RT) s^?1 (C₄D₉Cl), in accord with other studies and the expected isotope effect.     

High-temperature collisional energy transfer in highly vibrationally excited molecules II: Isotope effects in isopropyl bromide systems

Values for 〈ΔE_down〉, the average downward energy transferred from the reactant to the bath gas upon collision, have been obtained for highly vibrationally excited undeuterated and per-deuterated isopropyl bromide with the bath gases Ne, Xe, C₂H₄, and C₂D₄, at ca. 870 K. The technique of pressure-dependent very low-pressure pyrolysis (VLPP) was used to obtain the data. For C₃H₇Br, the 〈ΔE_down〉 values (cm^?1) are 490 (Ne), 540 (Xe), 820 (C₂H₄), and 740 (C₂D₄), and for C₃D₇Br, 440 (Ne), 570 (Xe), 730 (C₂H₄), and 810 (C₂D₄). The uncertainties in these values are ca. ±10%. The 〈ΔE_down〉 values for the inert bath gases Ne and Xe show excellent agreement with the theoretical predictions of the semi-empirical biased random walk model for monatomic/substrate collisional energy exchange [J. Chem. Phys., 80 , 5501 (1984)]. The relative effects of deuteration of the reactant molecule on 〈ΔE_down〉 also compare favorably with the predictions of this theoretical model. Extrapolated high-pressure rate coefficients (s^?1) for the thermal decomposition of reactant are 10^13.6±0.3 exp(?200 ± 8 kJ mol^?1/RT) for C₃H₇Br and 10^13.9±0.3 exp(?207 ± 8 kJ mol^±1/RT) for C₃D₇Br, which are consistent with previous studies and the expected isotope effect.     

Energy transfer of highly vibrationally excited azulene. II. Photodissociation of azulene-Kr van der Waals clusters at 248 and 266 nm

Photodissociation of azulene-Kr van der Waals clusters at 266 and 248 nm was studied using velocity map ion imaging techniques with the time-sliced modification. Scattered azulene molecules produced from the dissociation of clusters were detected by one-photon vacuum ultraviolet ionization. Energy transfer distribution functions were obtained from the measurement of recoil energy distributions. The distribution functions can be described approximately by multiexponential functions. Fragment angular distributions were found to be isotropic. The energy transfer properties show significantly different behavior from those of bimolecular collisions. No supercollisions were observed under the signal-to-noise ratios S/N=400 and 100 at 266 and 248 nm, respectively. Comparisons with the energy transfer of bimolecular collisions in thermal systems and the crossed-beam experiment within detection limit are made.