95, 9302-93 I O IO ' -I 0 : * IO6 I-2 U Im z 0 0 W ta: u lo5 lo4 12 12 4 12 8 13.2 E N E R G Y , eV Figure 5. Comparison of calculated VTST k ( E ) curves (dashed) with experiment (continuous curve). The calculations are for a critical energy 2.83 eV (= 65.26 kcal/mol).The loosening parameters a = 2 and a = 1 correspond to early and late loosening of the transitional modes, respectively. Early loosening is basically the phase space theory (PST) result. The experimental curve RSP is from coincidence experimentszzb by Rosenstock, Stockbauer, and Parr.demonstrates that the present calculations for the J = 0 approximation may be compared with experiment. We compared calculated VTST time-resolved photoionization efficiency curves with experimental ones from ref 5. The method for calculating PIE curves has been de~cribed.~ We employed a critical energy Eo = 2.76 eV (= 63.65 kcal/mol) which originally gave agreement,5 on the basis of a fixed RRKM transition state with a 1000 K activation entropy AS* = +8.07 eu. The result for late loosening with a = 1 and Eo = 2.76 eV is seen to be in disagreement with experimental results (Figure 2). A slightly higher critical energy, Eo = 2.83 eV (= 65.26 kcal/mol), and early loosening ( a = 2), which is the PST result, are in excellent agreement with experiment ( Figure 3). Keeping Eo constant at 2.83 eV and assuming late loosening of the transitional modes ( a = 1) has a very minor effect on the agreement with experiment (Figure 4). We next compared the calculated microcanonical VTST rate coefficients with the photoelectron photoion coincidence (PEPICO) results of Rosenstock, Stockbauer, and Parr22b (RSP, Figure 5 ) . The coincidence data are the ones that were fittedub by an RRKM model with Eo = 2.76 eV and AS* = 8.07 eu. Figure 5 shows that the RSP results are in good agreement with early loosening PST results ( a = 2, Eo = 2.83 eV) between IO5 and IO6 s-l above and below which the curves diverge slightly, while the late loosening curve ( a = 1, Eo = 2.83 eV) is in fair agreement with the RSP result between IO4 and IO5 s-I and diverges considerably at energies higher than 12.4 eV ( E > 3.43 eV).Comparison of the VTST results with experiments is still incomplete. More accurate experiments giving k ( E ) over wider energy ranges, particularly in the range k(E) I IO7 s-', are desirable. The present data indicate best agreement for a totally loose OTS, Le., early loosening of the transitional modes.Pratt and C h~p k a~~ were the first to suggest the use of VTST for the halobenzene ions but have not actually performed the calculations. In agreement with their results, we find the OTS to fit experimental data and to give a slightly higher critical energy than a fixed TTS (Eo(OTS) = 2.83 eV versus Eo(TTS) = 2.76 eV) and as a result a slightly higher heat of formation for the phenyl cation,27 AHfoo(C,H5+) = 1148 kJ mol-'.The comparison of VTST calculations with microcanonical rate coefficients performed here demonstrates the necessity of more refined ab initio potential energy surfaces in...
The following work gives a discussion on hydrogenated amorphous silicon (a‐Si:H) film preparation by laser induced chemical vapor deposition (LICVD) using a pulsed CO2‐laser in a parallel configuration. Deposition rate and initiation of polymerization were studied as a function of total pressure, gas flow, gas mixture (buffer gas, silane and/or disilane), substrate temperature, and radiation flux. The results lead to a reaction model where higher silane homologues play an important role for the film production. The film properties strongly depend on the substrate temperature Ts. The activation energy Ea of the dark conductivity decreases from 1.1 eV to 0.7 eV for films prepared at Ts = 230°C to 450°C. In the same temperature range, the dark conductivity σd increases from ∼10−14 to ∼10−10 (Ω cm)−1, whereas the photo conductivity σph shows a maximum of ∼10−6 (Ω cm)−1 at Ts = 320‐330°C. These values, especially their temperature behaviour, are compared with the results of other CO2‐LICVD works and further production methods. It is shown that the film formation depends on the gas phase chemistry as well as on the substrate temperature.
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