Reply to Comment by E. Pollak on “Photoisomerization of trans-Stilbene in Moderately Compressed Gases:  Pressure-Dependent Effective Barriers” (J. Phys. Chem. A 1999, 103, 10528−10529)

Meyer, A.; Schroeder, J.; Troe, J.; Votsmeier, Martin

doi:10.1021/jp0042112

Cited by 5 publications

(5 citation statements)

References 17 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…21,40 Stilbene crystals were introduced into the heated stainless steel sample cell shown in Figure 1 such that the concentration was determined by the vapor pressure of stilbene of about 10 µbar. The cell could be evacuated to pressures below 50 µbar, and its temperature was controlled to within 0.5°C between 20°C and 100°C.…”

Section: Methodsmentioning

confidence: 99%

“…This is the falloff regime of the unimolecular photoisomerization reaction of trans-stilbene in the S 1 state and it is located well below the densities of the so-called Kramers turnover region which was investigated in great detail in the past. 14,18,[28][29][30][31][32] Whereas determinations of microcanonical rate coefficients k(E) from supersonic jets, 21,23,24,[33][34][35][36] fluorescence decay measurements under thermal collision-free conditions, [37][38][39][40] and studies of the density dependence of the photoisomerization rate coefficients at bath gas pressures above 5-10 bar in different solvents are available, 18,28,29,31,35,41 systematic studies at lower pressure are scarce. 39,40 As one may expect to find essential clues to the discussed problem in this density regime, we investigated trans-stilbene fluorescence decays in a variety of fluid solvents in the relevant pressure range in great detail.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Photoisomerization of trans-Stilbene in Moderately Compressed Gases: Pressure-Dependent Effective Barriers

1999

Self Cite

View full text Add to dashboard Cite

A systematic experimental study of the bath gas and pressure dependence of the photoisomerization of transstilbene in the low to intermediate pressure regime is presented. The analysis of the results by a detailed numerical master equation simulation reveals specific bath gas influences in the effective specific rate constants for isomerization that are already observable at pressures of about 1 bar. The low-pressure regime of the unimolecular reaction in the S 1 state can be located in the pressure range well below 1 bar for most of the bath gases. The effective "high-pressure limit" of the photoisomerization rate constant that can be extracted from the simulation is found to be pressure dependent, approaching a bath gas specific plateau value in the 10 bar range for bath gases such as methane, ethane, propane, or xenon. The existence of a bath gas dependent plateau of the effective high-pressure limit is consistent with the proposition of a pressure-or density-dependent effective barrier of the reaction. The values obtained for the pressure-dependent barrier height agree with the trend observed in earlier experiments in highly compressed gases and liquids and confirm a substantial contribution of "static" lowering of the reaction barrier. Additional "dynamic" lowering of the effectice barrier due to collision-induced intramolecular vibrational energy redistribution cannot be ruled out, however, and the relative importance of these two contributions remains an open question. The recently proposed effect of vibrational Franck-Condon cooling upon optical excitation of trans-stilbene definitely is not consistent with the experimental results.

show abstract

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Photoisomerization of trans-Stilbene in Moderately Compressed Gases: Pressure-Dependent Effective Barriers

1999

Self Cite

View full text Add to dashboard Cite

show abstract

“…Gershinski and Pollak 1 noted that if the molecule is initially cooled, then immersion in a liquid would reheat the molecule and thus lead to faster decay and shorter lifetimes. This explanation has been the the topic of rather-heated debate, because of the uncertainty in the details of the excited-state potential energy surface of stilbene. − …”

Section: Introductionmentioning

confidence: 99%

“…This explanation has been the the topic of ratherheated debate, because of the uncertainty in the details of the excited-state potential energy surface of stilbene. [9][10][11][12] Wadi and Pollak 2 gave an in-depth study of the conditions that lead to the cooling phenomenon, in the absence of Dushinskii rotations. 14 Within the Condon approximation and using harmonic potentials for the ground and excited potential energy surfaces, they showed that cooling typically will occur when the vibrational frequencies in the excited electronic state are somewhat weaker than the frequencies in the ground state.…”

Section: Introductionmentioning

confidence: 99%

Photoinduced Cooling of Polyatomic Molecules in an Electronically Excited State in the Presence of Dushinskii Rotations

2004

View full text Add to dashboard Cite

We present a theoretical study of the effect of Dushinskii rotations on the vibrational population created in an excited electronic state through photoexcitation. Special attention is given to the effect of Dushinskii rotations on the possibility of cooling the vibrational population in the excited state, relative to the thermal distribution in the ground state. The absorption spectrum and corresponding average energy in the excited state are calculated using a closed-form expression for the harmonic correlation function between the ground and excited electronic states, which includes the effects of Dushinskii rotations, equilibrium position shifts, and frequency shifts between the excited-and ground-electronic-state normal modes. We investigate numerically the separate and joint effects of rotation, position shifts, and frequency shifts on the absorption spectrum and average vibrational energy in the excited electronic state. We find that, although the Dushinskii rotations generally diminish the cooling effect, the effect does not disappear and, in some cases, may also increase slightly.

show abstract

“…There are a few extant claims, but almost all have problems. Of the more convincing examples there is the isomerization of trans-stilbene, 6 a recent shock-tube study of dissociation in H 2 O 2 , 7 and a large reported difference between predicted and extrapolated k ∞ in methyl recombination, 8 again at high temperatures. Although some indications of non-RRKM behavior are thus beginning to appear, this statistical theory has proven to be remarkably successful over a truly broad range of application.…”

Section: Introductionmentioning

confidence: 99%

A Shock-Tube, Laser-Schlieren Study of the Dissociation of 1,1,1-Trifluoroethane: An Intrinsic Non-RRKM Process

et al. 2004

View full text Add to dashboard Cite

We report a shock-tube, laser-schlieren investigation of the molecular dissociation of the title trifluoroethane, CF3CH3 → CH2CF2 + HF, over very high temperatures, 1600−2400 K, and a wide range of sub-atmospheric pressures, 15−550 Torr. The density gradients are well fit by a simple two-reaction mechanism and accurate dissociation rates obtained. The results are compared with a k ∞ calculated from a G3 TS for this molecular elimination, which is a superb fit to the available lower-T data and a reliable extrapolation of k ∞ to high temperatures. The derived rate constants show a very deep falloff from this extrapolation but surprisingly little variation with pressure. This peculiarity is so severe that RRKM calculations dramatically fail to account for the behavior. The dissociation seems to be a clear example of an intrinsic non-RRKM process (nonstatistical dissociation). This conclusion is strongly supported by the observation of double vibrational relaxation at both dissociating and nondissociating temperatures, an unambiguous demonstration of slow IVR. Using a simple model with division into two groups of states, the deep falloff is found to be consistent with a rate-controlling slow IVR, not with low collision efficiency. The model suggests an IVR rate of ∼108 s-1 for dissociation energies.

show abstract

Reply to Comment by E. Pollak on “Photoisomerization of trans-Stilbene in Moderately Compressed Gases: Pressure-Dependent Effective Barriers” (J. Phys. Chem. A 1999, 103, 10528−10529)

Cited by 5 publications

References 17 publications

Photoisomerization of trans-Stilbene in Moderately Compressed Gases: Pressure-Dependent Effective Barriers

Photoisomerization of trans-Stilbene in Moderately Compressed Gases: Pressure-Dependent Effective Barriers

Photoinduced Cooling of Polyatomic Molecules in an Electronically Excited State in the Presence of Dushinskii Rotations

A Shock-Tube, Laser-Schlieren Study of the Dissociation of 1,1,1-Trifluoroethane: An Intrinsic Non-RRKM Process

Contact Info

Product

Resources

About