The role of excited states in the gas-phase photolysis of acetaldehyde

1973 ◽  
Vol 334 (1598) ◽  
pp. 411-426 ◽  
Keyword(s):  
Triplet State ◽  
Gas Phase ◽  
Excited States ◽  
Product Formation ◽  
Vibrationally Excited ◽  
Liquid Phases ◽  
Radical Decomposition ◽  
A Chain ◽  
Low Pressures

The cis-trans isomerization of butene-2 has been used to measure the triplet state yields in the photolysis of acetaldehyde at various wavelengths between 313 and 254 nm over the temperature range 35 to 140 °C. The results, together with those derived from chemical product formation, are consistent with data from luminescence studies. Dissociation into molecular products occurs rapidly, probably by predissociation, from a non-quenchable excited state formed by absorption. The main free radical decomposition occurs from the triplet state and this, in the absence of additives, such as butene-2, is responsible for the chain decomposition. The intersystem crossing and non-quenchable processes are independent of temperature. Isopentyl radicals formed from methyl addition to butene-2 can also propagate a chain reaction for acetaldehyde decomposition. At high temperatures and low pressures, dissociation of vibrationally excited isopentyl radicals can contribute to the measured isomerization yield. This is shown by the effect of addition of inert gas. Evidence is put forward that geometrical isomerization of the olefin involves a triplet aldehyde-olefin complex that can be decomposed by collision with ground state aldehyde molecules without cis-trans rearrangement of the olefin. This conclusion is consistent with other work in the gas and liquid phases.

Emission and Deactivation of the Excited States of 2,3-Pentanedione

1972 ◽  
Vol 50 (9) ◽  
pp. 1338-1344 ◽  
Author(s):  
A. W. Jackson ◽  
A. J. Yarwood
Keyword(s):  
Triplet State ◽  
Gas Phase ◽  
Excited States ◽  
Singlet State ◽  
Triplet States ◽  
Excited Singlet ◽  
Vibrationally Excited ◽  
Singlet Level ◽  
Singlet And Triplet States ◽  
Fluorescence And Phosphorescence

Vibrationally excited singlet and triplet states of 2,3-pentanedione are formed by photolysis at 365 nm. The processes removing these excited states in the gas phase are studied by measuring the fluorescence and phosphorescence yields. Fluorescence can occur from the vibrationally excited, as well as the vibrationally equilibrated, singlet state. The fluorescence and phosphorescence data are considered in terms of mechanisms which involve either weak or strong collisions. Although the data cannot distinguish between the alternatives, there are two significant conclusions. The fluorescence data require that emission occur from at least two levels in the singlet manifold. To explain the phosphorescence data, the highest emitting singlet level must not lead to a vibrationally equilibrated triplet state.

THE HYDROGEN–CHLORINE SYSTEM IN THE MM PRESSURE RANGE: I. ENERGY DISTRIBUTION AMONG VIBRATIONALLY EXCITED STATES

1964 ◽  
Vol 42 (10) ◽  
pp. 2176-2192 ◽  
Author(s):  
F. D. Findlay ◽  
J. C. Polanyi
Keyword(s):  
Excited States ◽  
Rotational Temperature ◽  
Relative Rate ◽  
Infrared Emission ◽  
Vibrational States ◽  
Vibrationally Excited ◽  
Experimental Conditions ◽  
Reduced Pressure ◽  
A Chain ◽  
First Time

When atomic plus molecular hydrogen coming from a Wood's discharge tube are mixed with molecular chlorine, infrared emission is observed (1). At low reagent pressures, ~10−2 mm Hg, this emission can be related to the relative rate of the reaction H + Cl2 → HCl†ν + Cl proceeding to form HCl in vibrationally excited states ν = 1–6, of the ground electronic state. In the present work this system has been investigated for the first time at ~100 × the reagent pressure (~1 mm Hg). The reaction was shown to proceed by a chain mechanism. The translational–rotational temperature was 1300 ± 100 °K under the experimental conditions normally used. The vibrational distribution was notable for the presence of vibrators in levels ν = 7 and 8, which are respectively 4 and 10 kcal higher in energy than the exothermicity of the H + Cl2 reaction. The population in these levels appeared to be related to that in the levels with [Formula: see text]; it was proposed that vibrational–vibrational exchange among these lower levels was responsible for populating the higher ones. A simple model yielded a collision efficiency for HCl†ν=1 + HCl†ν=6 → HCl†ν=7 + HCl†ν=0, of Z1,6t = 6 × 103 collisions per transfer. Addition of HCl to the reaction mixture brought about a redistribution among vibrationally excited states indicative of a fast vibrational transfer, HClν=0 + HCl†ν=2 → 2 HCl†ν=1.At reduced pressure of HCl† the stationary-state distribution among higher vibrational states approximated closely to that observed at 10−2 mm Hg total pressure (where collisional deactivation is insignificant), suggesting that collisional deactivation was not of major importance even at the pressure used in the present work. In order to account for the high translational–rotational temperature, in the absence of substantial vibrational deactivation, it was necessary to suppose that the greater part of the energy liberated by the reaction H + Cl2 went directly into translational and rotational motion of the products.

The Reaction of O3 with CCl2CH2

1973 ◽  
Vol 51 (10) ◽  
pp. 1504-1510 ◽  
Author(s):  
Leslie A. Hull ◽  
I. C. Hisatsune ◽  
Julian Heicklen
Keyword(s):  
Gas Phase ◽  
Infrared Absorption ◽  
Product Formation ◽  
Phase Reaction ◽  
Chain Mechanism ◽  
Rate Law ◽  
Gas Phase Reaction ◽  
Lower Olefin ◽  
Consumption Ratio ◽  
A Chain

The gas-phase reaction of O3 with CCl2CH2 at 25 °C was studied by monitoring O3 consumption by ultraviolet absorption and product formation and olefin consumption by infrared absorption. In the absence or presence of excess N2, and for initial olefin-to-O3 ratios, [CCl2CH2]0/[O3]0, in excess of 20 ([O3]0 ~ 1.0 Torr), the rate law is[Formula: see text]with k = 2.4 × 106 M−2 s−1. At lower olefin-to-O3 ratios, the rate is initially more rapid than predicted by the above equation, but follows the equation after the reaction has proceeded for some time. In the presence of excess O2, the rate is markedly reduced, and the rate law becomes[Formula: see text]with k′ = 2.2 M−1 s−1.The products of the reaction are CCl2O, HCOOH, CH2ClCCl(O), CO, O2, HCl, and presumably H2O. In the absence of O2, the CCl2CH2-to-O3 consumption ratio approaches 2, but the CCl2O produced per O3 consumed is between 0.25 and 0.4. With excess O2, the latter ratio becomes unity, but the CCl2CH2-to-O3 consumed approaches 5.The results are interpreted in terms of a chain mechanism with CCl2O2 as the chain carrier. The mechanism developed explains the main features of the reaction.

Photosensitization by Cd(3P0,1) atoms. III. Gas phase decomposition of cyclopentane

1976 ◽  
Vol 54 (1) ◽  
pp. 77-84 ◽  
Author(s):  
Bansi L. Kalra ◽  
Arthur R. Knight
Keyword(s):  
Gas Phase ◽  
Vapor Phase ◽  
Product Formation ◽  
Time Dependent ◽  
Primary Process ◽  
Phase Decomposition ◽  
Unimolecular Decomposition ◽  
Hydrogen Atoms ◽  
Volatile Products ◽  
Radical Decomposition

The triplet cadmium photosensitized decomposition of cyclopentane in the vapor phase has been studied at 355 °C and has been shown to give rise to cyclopentyl radicals and hydrogen atoms with close to unit efficiency in the primary process. Subsequent reactions of these species, including an important contribution from unimolecular decomposition of cyclopentyl radicals, yield the observed volatile products, hydrogen, methane, ethylene, ethane, propylene, and cyclopentene. As a result of significant olefin scavenging of H-atoms product yields are strongly time dependent. The system has been shown to be unaffected by addends. The temperature dependence of the rate of product formation is consistent with the known energetics of cyclopentyl radical decomposition.

scholarly journals New Insights into the State Trapping of UV-Excited Thymine

2016 ◽  
Author(s):  
Ljiljana Stojanovic ◽  
Shuming Bai ◽  
Jayashree Nagesh ◽  
Artur F. Izmaylov ◽  
Rachel Crespo-Otero ◽  
...  
Keyword(s):  
Gas Phase ◽  
Excited States ◽  
Electron Correlation ◽  
Dynamics Simulation ◽  
Energetic Cost ◽  
Multiple Time ◽  
Nonadiabatic Dynamics ◽  
The Difference ◽  
Uv Excitation

After UV excitation, gas phase thymine returns to ground state in 5 to 7 ps, showing multiple time constants. There is no consensus on the assignment of these processes, with a dispute between models claiming that thymine is trapped either in the first (S1) or in the second (S2) excited states. In the present study, nonadiabatic dynamics simulation of thymine is performed on the basis of ADC(2) surfaces, to understand the role of dynamic electron correlation on the deactivation pathways. The results show that trapping in S2 is strongly reduced in comparison to previous simulations considering only non-dynamic electron correlation on CASSCF surfaces. The reason for the difference is traced back to the energetic cost for formation of a CO p bond in S2.

Photochemical studies of unimolecular processes V. The photolysis and quenching of cis- and trans hexa-1, 3, 5-triene and of cyclohexa-1, 3-diene

1974 ◽  
Vol 337 (1609) ◽  
pp. 243-256 ◽  
Keyword(s):  
Quantum Yield ◽  
Gas Phase ◽  
Pressure Dependence ◽  
Experimental Error ◽  
Internal Conversion ◽  
Vibrationally Excited ◽  
Excited Molecules ◽  
Cis And Trans ◽  
Low Pressures

The photolyses of cis- and trans -hexa-1, 3, 5-triene at wavelengths of 228.8 and 253.7 nm and of cyclohexa-1, 3-diene at 265.4 and 280.4 nm have been studied in the gas phase at various pressures and in the presence of added gases. At low pressures, the limiting quantum yield for hexatriene photolysis is unity within experimental error, and it is shown that the pressure dependence of all the observed interconversions of C 6 H 8 isomers and of benzene formation can be explained by reactions of vibrationally excited molecules produced by very rapid internal conversion.

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