Vibrational energy transfer in the two-channel 1,1-cyclopropane-d2 isomerization system. Krypton bath gas

The study of intermolecular energy transfer in the 1,1-cyclopropane-d₂ system has been repeated for the neat gas at 973 K and has been extended to krypton bath gas at 823 K and 973 K. The method of study is by the competitive collisional activation “spectroscopy” technique for this two-channel competitive isomerization system. Results at 823 K give the relative collisional efficiency of krypton as β ≈ 0.46, at k/k_∞ ≈ 0.02 and yield the average down-jump energy step as 〈ΔE〉 ≈ 1200 cm^?1 on the basis of a stepladder model for the distribution of down-step sizes. At 973 K and k/k_∞ = 0.02, β ≈ 0.07 and 〈ΔE〉 ≈ 500 cm^?8, for both an exponential and stepladder distribution of down-step sizes. Agreement with related earlier data for other bath gases and for neat cyclopropane is good and verifies again a decrease in energy transfer collisional efficiency, and a decrease in 〈ΔE〉, with rise of temperature, as previously reported for this system.     

Modified statistical theory of energy transfer in ion-molecule collisions

The modified statistical theory developed previously for potentials appropriate to interactions in neutral-neutral collisions, is now extended to more strongly attractive potentials involved in ion-neutral collisions. The model system is the collisional deactivation of C₅H₉⁺ by a variety of both polar and non-polar neutral molecules. A 12 - 6 - 4 potential is used for ion interaction with non-polar neutrals, and a 12 - 6 - 4 - 2 potential, as modified by Su and Bowers to take into account the rotational energy of the neutral, for interaction with polar neutrals. Calculated is (ΔE), the average energy lost by the ion in a collision, and compared with experiment. For C₅H₉⁺-CH₄ collisions, the calculated (ΔE) agrees with experiment within 5%. Predictions of the theory, namely that (ΔE) should increase with excitation energy and should decrease with the size of the excited reactant, are found to be in fair agreement with the somewhat ambiguous experimental evidence.     

V → R,T energy transfer processes in vibrationally excited pentafluorobenzene using a Hg tracer absorption method

A new technique based upon the Doppler and Lorentz broadening of the isotopic and hyperfine Hg multiplet lines near 254 nm was used to monitor the translational equilibration of vibrationally excited pentafluorobenzene (PFB). Excitation was achieved with a pulsed CO₂ infrared laser focused into a cell containing PFB and a trace amount of Hg. Rates of V→ R, T energy transfer were found to be linearly dependent on both the excitation energy and the pressure of PFB. Excitation energies were independently determined by the Hg absorption technique and by measuring the change in absorption by the PFB at 254 nm. For PFB¹-PFB¹ collisions, the average energy transferred per collision divided by the average excitation energy of the colliding pair, 〈Δ〉/ 〈E〉, is0.0133±0.0016.     

Relaxation of internal energy and collisional energy transfer

Using the exponential model for the collisional transition probability, it is shown that relaxation of average internal energy is a measure of bulk-average energy transfer ?ΔE?. This is a macroscopic property which is a complicated function of both time and initial excitation and is only distantly related to average energy transferred per collision ?ΔE?, a microscopic property.     

Ir absorption spectrum of CrO2Cl2 molecules for high-lying states of the vibrational quasi-continuum

A new approach to the spectroscopy of highly excited vibrational states of polyatomic molecules has been elaborated. The molecules of CrO₂Cl₂ were prepared in states with a vibrational energy of the ground electronic term A₁ of ≈ 19000 cm⁻¹ by means of internal conversion of electronic energy from the electronic state B₁ excited by laser radiation. The spectroscopy of the vibrationally excited molecules has been carried out in the region of the ν₆ and ν₁ bands with diode and CO₂ lasers. The fwhm of the obtained spectrum was ≈ 15 cm⁻¹. The intermode interaction in CrO₂Cl₂ has been theoretically analyzed, and the calculated spectrum compared with that measured experimentally. The time evolution of the spectrum of vibrationally excited CrO₂Cl₂ molecules has been studied. The average energy transferred per one collision with unexcited CrO₂Cl₂ molecules was equal to 〈δE〉 ≈ 1200 cm⁻¹.     

Direct observation of excited-state dynamics by hot UV absorption spectroscopy after IR multiphoton excitation

Fluoroethylcycloheptatriene has been irradiated by pulses from a TEA CO₂ laser. During and after the pulses, the hot UV absorption of the excited molecules was monitored. At very low gas pressures, time-resolved observation of the rate of unimolecular isomerization of the excited molecules was possible. By adding collision partners, stepwise collisional deactivation of excited molecules was also observed. By analysis of the transient spectra, the intra- and inter-molecular dynamics of the excited molecules was found to be quantitatively consistent with data from single-photon excitation experiments. The dependence of the observed dynamics on the laser fluence is demonstrated.     

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.     

Effect of isomer geometry on the steady-state absorption spectra and femtosecond time-resolved dynamics of carotenoids

Steady-state absorption and femtosecond time-resolved optical spectroscopic studies have been carried out on all-trans-beta-carotene, 15,15'-cis-beta-carotene, all-trans-spheroidene, and 13,14-locked-cis-spheroidene. We examine in detail the effect of isomer geometry on the spectroscopic properties and photophysics of the low-lying S(1) (2(1)A(g)(-)) and S(2) (1(1)B(u)(+)) excited states of these molecules. The experiments on 13,14-locked-cis-spheroidene, a molecule incapable of undergoing cis-to-trans isomerization, provide a unique opportunity to examine the role of isomer geometry in controlling excited-state deactivation of carotenoids. The kinetic results have been obtained using both single wavelength transient absorption measurements and global fitting procedures. The overall scheme for the deactivation of these molecules after S(0) --> S(2) photon absorption is decay of S(2) to a vibrationally hot S(1) state, followed by vibrational relaxation within S(1), and finally, S(1) --> S(0) internal conversion back to the ground state. Changes in isomer geometry are shown to lead to small but noticeable alterations in the spectroscopic and kinetic behavior of the molecules. The effects are interpreted in terms of minor alterations in excited-state energy and vibrational coupling upon isomerization that bring about changes in the spectroscopic and kinetic behavior of this biologically important class of pigments.     

Vibrational energy transfer from chemically activated 1,4-cyclohexadiene

The thermal isomerization of cis, anti, cis-tricyclo[3.1.0.0^2,4] hexane was used to produce highly vibrationally excited 1,4-cyclohexadiene. The competition between unimolecular decomposition of the energized diene (to benzene and hydrogen) and collisional stabilization was studied using the parent compound, SF₆, CO₂, N₂, and He as quenching gases. Quenching efficiencies decreased in the order given above. By applying RRKM theory to the isomerization and decomposition reactions, it was possible to calculate the step size in a stepladder model of the deactivation of cyclohexadiene. The step sizes 〈ΔE〉 deduced (at 528 K and in units of kJ/mol) were: parent compound and SF₆, 7; CO₂, 5; N₂, 4; He, 2. The study confirmed the utility of this unimolecular chemical activation system for energy transfer studies.     

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.     

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.     

Incubation in cyclohexene decomposition at high temperatures

A detailed master equation simulation has been carried out for the thermal unimolecular decomposition of C₆H₁₀ in a shock tube. At the highest temperatures studied experimentally [J. H. Kiefer and J. N. Shah, J. Phys. Chem., 91, 3024 (1987)], the average thermal vibrational energy is greater than the reaction threshold and therefore 〈ΔE〉 (up and down steps) is positive for molecules at that energy, rather than negative; the converse is true at lower temperatures. The calculated incubation time, in which the decomposition rate constant rises to 1/e of its steady state value, is found to be only weakly dependent on temperature (at constant pressure) between 1500 K and 2000 K and to depend almost exclusively on 〈ΔE〉_d (down steps, only), and not on collision probability model. Simulations of the experimental data show the magnitude of 〈ΔE〉_d depends weakly on assumed collision probability model, but is nearly independent of temperature. The second moment 〈ΔE〉^½ is found to be independent of both temperature and transition probability model. The experimental data are not very sensitive to the possible energy-dependence of 〈ΔE〉_d for a wide range of assumptions. It is concluded that the observed experimental “delay times” probably can be identified with the incubation time; further experiments are desirable to test this possibility and obtain more direct measures of the incubation time.     

Collisional de-excitation of the metastableD-states of Ba+ by He,Ne, N2 and H2

The rate constants 〈σ · υ〉 for collisional de-excitation of the metastable 5D states of Ba⁺ ions have been determined in an ion trap experiment. TheD-states are selectively populated by pulsed laser excitation of the 6P _1/2 or 6P _3/2 state and the decay at different background pressures is monitored by the change in fluorescence intensity of the excited ions. From the pressure dependence of the decay constants we calculate the de-excitation rate constants for different collision partners, averaged over the velocity distribution of the trapped ion cloud. For He, Ne, H₂ and N₂ we obtain in the c.m. energy range of 0.1–0.5 eV: 〈σ·υ〉 (He)=3.0±0.2·10^?13cm³/s, 〈σ·υ〉 (Ne)=5.1±0.4·10^?13cm³/s, 〈σ·υ〉 (H₂)=3.7±0.3·10^?11cm³/s, 〈σ·υ〉 (N₂)=4.4±0.3·10^?11cm³/s. The results can be understood qualitatively by a consideration of the ion-atom and ion-molecules interaction potential.     

Formation of hot hexafluorobenzene in the 193 nm photolysis

Time-resolved absorption spectra of hexafluorobenzene vapor have been observed with ArF laser (193 nm) excitation. The initial intermediate is postulated to be due to HFB2(S₀) (hot hexafluorobenzene, with internal energy of 639 kj/mol) because the transient spectrum can be simulated as part of the S₃(¹E_1u) ← S₀ transition at 3050 K.     

Level-to-level vibrational energy transfer studies: energy dependence and observation of product species for azulenes(S0, Evib) + CO2

V—V energy transfer from a large molecule excited to vibrational energies of chemical interest has been demonstrated by detection of ≈ 1.5% yield of CO₂(001) due to energy transfer from azulene (E_vib ≈ 30600 cm^?1. Also, the average enery lost per collision by azulene was measured as a function of E_vib, and the rate constant for CO₂(001) deactivation by azulene was determined.     

Ultraviolet Photodissociation Dynamics of 1-Pentyl Radical?

The ultraviolet (UV) photodissociation of jet-cooled 1-pentyl radical is investigated in the wavelength region of 236-254 nm using the high-n Rydberg-atom time-of-flight (HRTOF) technique. The H-atom photofragment yield spectrum of the 1-pentyl radical shows a broad UV absorption feature peaking near 245 nm, similar to the 2p_z→3s absorption bands of ethyl and n-propyl. The center-of-mass translational energy distribution, P(E_T), of the H+C₅H₁₀ product channel is bimodal, with a slow peak at ~5 kcal/mol and a fast peak at ~50 kcal/mol. The fraction of the average translational energy release in the total available energy, 〈f_T〉, is 0.30, with those of the slow and fast components being 0.13 and 0.58, respectively. The slow component has an isotropic product angular distribution, while the fast component is anisotropic with an anisotropy parameter ~0.4. The bimodal translational energy and angular distributions of the H+C₅H₁₀ products indicate two H-atom elimination channels in the photodissociation of 1-pentyl:(ⅰ) a direct, prompt dissociation from the electronic excited state and/or the repulsive part of the ground electronic state potential energy surface; and (ⅱ) a unimolecular dissociation of internally hot radical in the ground electronic state after internal conversion from the electronic excited state.     

Rotation of p-dimethylaminobenzonitrile molecules during relaxation

The Stokes shift of time-resolved p-dimethylaminobenzonitrile luminescence spectra excited at the long-wavelength edge of absorption band was studied. The time evolution of the emission anisotropy was recorded, and its dependence on the luminescence wavelength was determined. The effect is explained by the conversion of molecular interaction energy into heat. Calculations based on a rotating body hydrodynamics model suggest that neighboring solvent molecules adhere to excited luminophore molecule during their rotation     

Bonded excimer in stacked adenines:Semiclassical simulations

The nonradiative decay of a π-stacked pair of adenine molecules,one of which was excited by an ultrafast laser pulse,is studied by semiclassical dynamics simulations.This simulation investigation is focused on the effect of the formation of bonded excimer in stacked adenines on the mechanism of ultrafast decay.The simulation finds that the formation of the bonded excimer significantly lowers the energy gap between the LUMO and HOMO and consequently facilitates the deactivation of the electronically excited molecule.On the other hand,the formation of the chemical bond between two stacked adenines restricts the deformation vibration of the pyrimidine of the excited molecule due to the steric effect.This slows down the formation of the coupling between the HOMO and LUMO energy levels and therefore delays the deactivation process of the excited adenine molecule to the electronic ground state.     

The different photoisomerization efficiency of azobenzene in the lowest n pi* and pi pi* singlets: the role of a phantom state

Azobenzene E<==>Z photoisomerization, following excitation to the bright S(pi pi*) state, is investigated by means of ab initio CASSCF optimizations and perturbative CASPT2 corrections. Specifically, by elucidating the S(pi pi*) deactivation paths, we explain the mechanism responsible for azobenzene photoisomerization, the lower isomerization quantum yields observed for the S(pi pi*) excitation than for the S1(n pi*) excitation in the isolated molecule, and the recovery of the Kasha rule observed in sterically hindered azobenzenes. We find that a doubly excited state is a photoreaction intermediate that plays a very important role in the decay of the bright S(pi pi*). We show that this doubly excited state, which is immediately populated by molecules excited to S(pi pi*), drives the photoisomerization along the torsion path and also induces a fast internal conversion to the S1(n pi*) at a variety of geometries, thus shaping (all the most important features of) the S(pi pi*) decay pathway and photoreactivity. We reach this conclusion by determining the critical structures, the minimum energy paths originating on the bright S(pi pi*) state and on other relevant excited states including S1(n pi*), and by characterizing the conical intersection seams that are important in deciding the photochemical outcome. The model is consistent with the most recent time-resolved spectroscopic and photochemical data.     

Location of Solubilization of 2′-ethylhexyl Salicylate in Micelles

Absorption and excited state intramolecular proton transfer (ESIPT) fluorescence of 2′-ethylhexyl salicylate (EHS) were examined in the presence of cationic, non-ionic, and anionic surfactants. It was found that linear EHS molecule was solubilized in micelles with its flexible and hydrophobic 2′-ethylhexyl chain toward the micellar core and with its rigid salicyl moiety toward the micelle-water interface. The UV absorption of EHS was improved and the intramolecular hydrogen bonding formation of EHS was favored, resulting in greatly enhanced ESIPT fluorescence. The excited EHS molecules decay via visible luminescence and non-radiative deactivation. The binding sites of EHS in micelles were explained at a molecular level in terms of molecular structures and sizes of EHS and surfactants. Dynamic fluorescence quenching and spectral measurements of ester hydrolysis of EHS provide further evidences for the binding sites of EHS in different micelles.