Transients in the vibrational excitation of cyclobutane decomposition using the variable encounter method

The probability of reaction of cyclobutane molecules in a fixed time interval after experiencing a known variable number of collisions with a hot surface at temperatures between 749 K and 1126 K has been determined using the Variable Encounter Method. Calculations utilizing exponential or gaussian models for energy transfer enable the average amounts of energy transferred for deactivating collisions, <ΔE′?, to be estimated. The exponential model fits the experimental data the best and, using this model, <ΔE′? is 2430 cm^?1 at 749 K and decreases to 1470 cm^?1 at 1126 K. Incubation times are derived from the mean first passage times.     

Gas/gas and gas/wall average energy transfer from very low-pressure pyrolysis

It is shown that data obtained using very low-pressure pyrolysis (VLPP) on the pressure and temperature dependence of unimolecular rate coefficients of reactants with several reaction channels yield average energies transferred in gas/gas and gas/wall collisions (the wall being seasoned quartz at 800–1200 K). The downward average energy transferred, «ΔEå, for chlorocyclobutane/ethylene collisions is found to be 1600 cm^?1 at 970 K; «ΔEå for chlorocyclobutane/wall collisions varies from 5000 cm^?1 (wall efficiency β_w = 0.8) at 930 K to 3500 cm^?1 (β_w = 0.4) at 1150 K; similar values are found from published data on cycloheptatriene and cyclopropane-d₂. This indicates that the assumption of unit wall efficiency usually used in fitting VLPP experiments to RRKM theory needs revision.     

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.     

Vibrattonal excitation of molecules by collisions with “hot” hydrogen atoms from laser photolysis

Excimer laser (ArF) photolysis of diatomic and triatomic hydrides produces hydrogen atoms with translational energies in excess of 15000 cm^?1 per atom. Subsequent collisions of these “hot” atoms with CO₂ and N₂O produces vibrationally excited molecules which can be detected by their characteristic infrared emission.     

The exciting oxidation of CO on Pt

An uncollimated molecular beam (free jet) of CO and O₂ molecules incident on a polycrystalline Pt surface at 775 K in vacuo produces vibrationally hot CO₂ molecules at a density sufficient for infrared emission spectrometry. Analysis of spectra at a resolution of 0.1 cm^?1 clearly shows that nascent product molecules have much more internal excitation than would be the case for equilibrium at the surface temperature.     

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.     

Temperature dependence of vibrational energy transfer in the “low” pressure competitive two-channel 1,1-cyclopropane-d2 system

Intermolecular energy transfer has been studied in the two-channel competitive isomerization of 1,1-cyclopropane-d₂, both in the neat system and in the presence of helium bath gas at values of k/k^? centered around 0.02. The competing path ways differ in threshold energy by ≈ 0.6 kcal. The temperature range 773 K to 973 K was covered. Several methods of treating the data, whether by isotopic ratios of rate constants or by temperature dependence of fall off, are each independent of a knowledge of collision cross sections. Used in conjunction, they provide measurements of these quantities. Cyclopropane is an operationally strong collider (β_ω = 1) for itself at 773 K with an average down step size, <ΔE/s> >/ 10 kcal mole ^?1 (>/ 3500 cm^?1). At 973 K the substrate is no longer a strong collider; β_ω declines to ≈ 0.55 with <ΔE/s> ≈ 5.2 kcal mole^?1. For helium the corresponding quantities are β_ω ≈ 0.078 <ΔE/s> ≈ 1.1 kcal mole ^?1 declining to β_ω ≈ 0.010 and <ΔE/s> ≈ 0.53 kcal mole^?1. The several methods of measuring these quantities give excellent independent agreement. Comparison with the earlier theoretical formulation of Tardy and Rabinovitch gives good agreement, the temperature dependence of β_ω for the weak collider, helium, follows the relation β_ω ∝ T^?m, where m /s> 2.     

Nature and location of excited charge-transfer states in the porphine-magnesium porphine dimer

The ground and excited states of a cofacial porphine-magnesium porphine dimer with a ring separation of 5.35 Å are investigated by ab initio configuration interaction calculations, using a floating gaussian basis. A pair of charge-transfer states are found ≈23000 cm^?1 above the ground state, but are lowered by ≈7400 cm^?1 upon coordination of the Mg atom with chloride ion.     

The frequency,fluence, and pressure dependence of the absorption of pulsed CO2-laser radiation by SF6 at 140 K

We report measurements of absorption of pulsed CO₂ laser radiation by SF₆ at 140 K. These measurements cover a broad range of fluence (10^?6 to 0.8 J/cm²), SF₆ gas density (2 × 10¹⁵ to 6 × 10¹⁶ cm^?3), and frequency (six CO₂-laser frequencies within the SF₆ v₃ band). We employ two methods of data reduction including one that gives a simple phenomenological function of the three principal independent variables. We conclude that at low fluence a small fraction of the SF₆ molecules absorb the laser radiation and that collisions and higher fluence both increase that fraction. At higher fluence absorption by vibrationally excited molecules becomes increasingly important.     

A consistent model for the thermal decomposition of NCN3 and the singlet– triplet relaxation of NCN

The thermal decomposition of cyanogen azide (NCN₃) and the subsequent collision‐induced intersystem crossing (CIISC) process of cyanonitrene (NCN) have been investigated by monitoring excited electronic state ¹NCN and ground state ³NCN radicals. NCN was generated by the pyrolysis of NCN₃ behind shock waves and by the photolysis of NCN₃ at room temperature. Falloff rate constants of the thermal unimolecular decomposition of NCN₃ in argon have been extracted from ¹NCN concentration–time profiles in the temperature range 617 K <T< 927 K and at two different total densities: k(ρ ≈ 3 × 10^?6 mol/cm³)/s^?1=4.9 × 10⁹ × exp (?71±14 kJ mol^?1/RT) (± 30%); k(ρ ≈ 6 × 10^?6 mol/cm³)/s^?1=7.5 × 10⁹ × exp (‐71±14 kJ mol^?1/RT) (± 30%). In addition, high‐temperature ¹NCN absorption cross sections have been determined in the temperature range 618 K <T< 1231 K and can be expressed by σ /(cm²/mol)= 1.0 × 10⁸ ?6.3 × 10⁴ K^?1 × T (± 50%). Rate constants for the CIISC process have been measured by monitoring ³NCN in the temperature range 701 K <T< 1256 K resulting in k_CIISC (ρ ≈ 1.8 ×10^?6 mol/cm³)/ s^?1=2.6 × 10⁶× exp (‐36±10 kJ mol^?1/RT) (± 20%), k_CIISC (ρ ≈ 3.5×10^?6 mol/cm³)/ s^?1 = 2.0 × 10⁶ × exp (?31±10 kJ mol^?1/RT) (± 20%), k_CIISC (ρ ≈ 7.0×10^?6 mol/cm³)/ s^?1=1.4 × 10⁶ × exp (?25±10 kJ mol^?1/RT) (± 20%). These values are in good agreement with CIISC rate constants extracted from corresponding ¹NCN measurements. The observed nonlinear pressure dependences reveal a pressure saturation effect of the CIISC process. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 30–40, 2013     

Optical studies of the electronphonon interaction in the pyrene-PMDA complex

Low temperature phosphorescence spectra of pyrene-PMDA (pyromellitic acid dianhydride) imbedded in a naphthalene-PMDA host crystal are reported. The spectra exhibit resolved zero-phonon and multi-phonon structure which is significant since the phosphorescent state is characterized by ≈36% charge-transfer character. Several different phonons contribute to the structure with the dominant phonon having ground and excited state (from hot band analysis) frequencies of 25 and 15 cm^?1. The linear electronphonon coupling strength for the 25cm^?1 phonon is computed. This phonon is tentatively assigned to rotational motion of the rigid complex. Linewidth data yield a relaxation time of 0.4 ps at 2 K for the 25 cm^?1 phonon which is believed to be a pseudolocalized or resonant mode.     

State-to-state total cross sections for ion-pair formation

Relative total cross sections are reported for the production of electrons and anions in collisions between an alkali atom (Na, K, Cs) and the molecules O₂ and NO. The formation of electrons is due to autoionization of vibrationally excited O₂^? (gu′ ? 4) respectively NO^? (υ′ ? 1) molecules. The experiments have been performed with fast alkali atom beams in the laboratory energy range of 30–300 eV.     

Ultrafast investigations of vibrationally hot molecules after internal conversion in solution

The creation of vibrationally very hot molecules after internal conversion is seen for the first time in a liquid environment. Cooling of the excited molecules occurs within several 10⁻¹¹ s. Excitation of azulene with photons of 19000 cm⁻¹ leads to a transient internal temperature of 1200 K. The excess population of vibrational modes of 700 cm⁻¹ decays with a time constant of 40 ps.     

Interpretation of the optical dephasing time and lineshape function in the S0 → T1 exciton transitions of 1,4-dibromonaphthalene

The lineshape function for the S₀ → T₁ absorption in 1,4-dibromonaphthalene (DBN) is analyzed in terms of exchange theory. It is shown that the dominant optical dephasing mechanism for the electric dipole transition to the k = 0 state in the band results from the absorption and emission of a low energy optic phonon. This process dephases the optical absorption because of frequency differences of the phonon in the ground and excited state. In addition, it is shown how to extract the energy of the phonon responsible for dephasing, the phonon absorption rate, and the lifetime in the phonon promoted state from the data. The analysis of the data for DBN shows that very little dephasing of the optical transition occurs before ≈ 15 K but from 15 K to ≈ 40 K the singlet-triplet transitions to site I (20192 cm^?1) and site II (20245 cm^?1) are dephased by absorption and emission of an ≈ 38 cm^?1 and 45 cm^?1 phonon respectively. The phonon absorption rates by the k = 0 state in the exciton band are similar for both sites being 5 × 10⁶ s^?1 and 3 × 10⁵ s^?1 at 4 K and 7 × 10¹¹ s^?1 and 4 × 10¹¹ s^?1 at 30 K for site I and II respectively. Finally, the lifetimes in the phonon promoted state for sites I and II are 0.23 and 0.28 ps over the range 15–40 K.     

Singlet exciton interactions in crystalline naphthalene

The decay of prompt fluorescence in crystalline naphthalene at 300 K, excited by a picosecond 266 nm pulse, has been studied as a function of excitation intensity. Experimental decay curves can be fitted only when the exponential distribution in depth of excitation and the radial (gaussian) intensity profile of the excitation are both taken into account. From an analysis of the decay at early time (?5 ns) a best fit value of the singlet—singlet annihilation rate constant is found γ_SS = (4 ± 1) × 10^?10 cm³ s^?1. If the reaction is diffusion-limited, this rate implies an average singlet diffusivity D_S = (2 ± 1) × 10^?4 cm² s^?1.     

Optically detected magnetic resonance study of the lowest excited triplet state of aromatic thioketones: Xanthione

The aromatic thioketone xanthione has been investigated by means of the optically detected magnetic resonance (ODMR) technique in a n-hexane matrix at ≈ 1.1 K. It was established that the short-lived red emission, which is characteristic for many thione molecules, is phosphorescence. At high temperatures (77 K) this phosphorescence originates mainly (>80%) from the T₁^z (n, π*) sublevel (k_z^(r) >k_x^(r), k_y^(r). At low temperature (1.1 K) the intersystem crossing following S₂ (π, π*) ← S₀ excitation is a highly spin-sublevel selective process which populates predominantly the T₁^x and T₁^y, levels. Hence, the slow spin—lattice relaxation phosphorescence at low temperature originates from these sublevels. A value of 0.0611 cm^?1 was obtained for the zero-field parameter |E|/hc. A lower limit of 0.66 cm^?1 has been found for the zero-field parameter |D|/hc. This value is considerably larger than those observed for ketones, and it is shown that spin—orbit coupling contributes strongly to the zero-field splitting.     

Reexamination of the phosphorescence of Ba2Pt2(H2P2O5)4 via thermally modulated emission spectroscopy

Reexamination of the phosphorescence of Ba₂Pt₂(H₂P₂O₅)₄ reveals that the ≈10 K spectrum is a superposition of two electronic transitions [³A₂u(E_u,A_1u → A_1g] separated by ≈40 cm^?1. Each band displays a prominent 110 cm^?1 vibrational progression. Franck-Condon analysis yields a ≈0.25 Å distortion of the PtPt bond in the excited states, interpreted as a contraction.     

The effect of vibrational excitation in the reactions of OH with H2

Upper limits have been deduced at T ≈ 298 K for the rates of chemical reactions of hydroxyl, OH(υ = 0) and OH³ (υ = 1), with vibrationally excited hydrogen, H³₂(υ = 1). These are 3.3. × 10¹² and 5.7 × 10¹² cm³/mole s, respectively.     

Migrational quenching in a system of centers with cross relaxation

The concept of quenching pairs is proposed for a system of randomly distributed centers which can quench one another through an energy cross-relaxation mechanism. The formula for the quenching rate is derived for the case of an exciton migrating over the centers. Comparison is made with experimental data for cross-relaxational quenching of the vibrationally excited molecule CH₃CCl₃. The micro-interaction parameters are estimated to be C_DD = 2 × 10^?33 cm⁶/s and C_DA = 3 × 10^?35 cm⁶/s.     

Perturbations of molecular extravalence excitations by rare-gas fluids

In this paper we report the results of an experimental study of the vacuum ultraviolet absorption spectra of molecular impurity states of methyl iodide in Ar (density range ? = 0–1.4 g cm^?3) and in Kr (? = 0–2.3 g cm^?3), of carbon disulphide in Ar (? = 0–1.4 g cm^?3) and of formaldehyde in Ar (? = 0–1.25 g cm^?3). The experimental results provide new information regarding medium perturbations of intravalenc transitions, of the lowest extravalence transitions and of transitions to mixed valence—Rydberg configurations, which serve as a diagnostic tool to distinguish between different types of electronic excitations. All the lowest extravalence molecular excitations exhibit appreciable blue spectral shifts at moderate and at high fluid densities, intravalence transitions are practically insensitive to medium effects, while excitations to mixed valence—Rydberg configurations are characterized by a moderate blue spectral shift. New information has been obtained concerning the energetics of molecular ionization processes in a dense fluid. The high n = 2–5 Rydberg states of CH₃l exhibit a large red shift at moderate (? = 0–0.5 cm^?3) Ar densities. The ionization potential E_g and the effective Rydberg constant G for CH₃I in Ar was found to decrease from G = 13.6 eV and E_g = 9.55 eV at ? = 0 and E_g = 9.08 eV and constant G for CH₃l in Ar was found to decrease from G = 13.6 eV and E_g = 9.55eV at ? = 0 and E_g = 9.08 eV and G ≈ 7.15 eV at ? = 0.5 g cm^?3. Experimental evidence was obtained for the identification of n = 2 molecular Wannier impurity states of CH₃I and of CH₂O in liquid Ar. These spectroscopic data result in E_g ≈ 8.6 eV for CH₃I in liquid Ar and E_g ≈ 10.2 eV for CH₂O in liquid Ar.