Photoionization results

Analysis of the spectrum of methyl radical gives the UV /-value, 9.843 0.001 e.v. (38,39), in good agreement with the El value quoted above, 9.95 e.v. (Table I). A PI value also rees closely, 9.82 0.04 e.v. (40). These values seem clearly to refer to the adiabatic ionization potential of methyl radical. Unhappily, this agreement does not extend to other radicals. Additional UV values are not available but recent work by Elder et al. (40) has raised important questions as to the actual significance of the El data quoted so extensively in this section. We have seen how these data form a harmonious and self-consistent tableau nevertheless, the photoionization results of these authors give much lower values for ethyl ( 8.4 compared to 8.78 e.v.), n-propyl ( 8.1 compared to 8.69 e.v.), and isopropyl ( 7.5 compared to 7.90 e.v.) radicals and indicate further that these values may not refer to adiabatic ioniza- 

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Similar photoionization results were observed with 99 and 100 although picosecond (355 nm) and two-color measurements on 100 indicated that photo-... 

Hg and Dg is zero, suggesting that the slight discrepancies are due to the approximate nature of the prediction. An approximate photoionization result, reviewed by Rosenstock et al. (4), deviates from the spectroscopic result by 130 cm". ... 

The adopted spectroscopic IP(Dg) receives strong support from a value calculated indirectly from experimental values (3) of Do (D2> and IP(D) combined with the theoretical value ( ) of DQ (Dg ) this indirect value differs by only 0.4 cm . Rosenstock et al. (4) review several less accurate determinations of IP(Dg) and recommend a value calculated from IP(Hg) using differences in zero-point energies of the diatomic molecules and their ions. Repeating this calculation with JANAF values (2), we predict an IP (Dg) which differs from the observed value by -32 cm . Inclusion of an empirical adjustment for IP(D) - IP(H) overcorrects the prediction, giving a deviation of +28 cra . These slight discrepancies are due (, see HD, g) to the approximate nature of the prediction. Villarejo s average photoionization result (6), IP(Dg) = 124757 180 cm , deviates by +11 cm from the spectroscopic value. 

Janes et al.(2) were the first to apply multistep photoionization to the study of uranium. Their result for the photoionization threshold of 6.187(2) eV is in good agreement with the photoionization result of 6.1912(25) eV obtained by Solarz et al.(D (Throughout this paper, numbers in paranthesis following a numerical value indicate the uncertainty in the last digit of the number.)... 

The difference in the behavior of these four ions is easily rationalized. The ionization energies (lEs) of He and Ne fall in a region where resonant ionization preferentially removes an electron from the 3ai orbital of SiH4. Photoionization studies indicate that ionization from this orbital yields Si+ and SiH " products almost exclusively (Cooper et al., 1990). In contrast, the lEs of Kr and Xe are resonant with the lower-energy 2t2 orbital of SiH4. Photoionization from this orbital results in less dissociation, preferentially forming SiHj and SiH. Thus, the dominant products observed are consistent with the photoionization results for removal of an electron from the two distinct valence molecular orbitals of silane. 

We have studied the application of pulse trains to probe some important aspects of the electronic excitation/deexcitation dynamics coupled with vibrational dynamics, with the LiH system as an example. A train of very short pulses well separated in time including frequency components suited for transfer between multiple electronic states and for photoionization resulted in step-like population transfers that may be recorded in the transient photoelectron signal. 

Fragments are ionized by absorption of a single vacuum ultraviolet photon with an energy just above the ionization threshold. Unlike field ionization which may produce significant amounts of both protonated molecules (MH+) and radical ions (M ) during ionization, photoionization results only in the formation of radical cations. 

The lines of primary interest ia an xps spectmm ate those reflecting photoelectrons from cote electron energy levels of the surface atoms. These ate labeled ia Figure 8 for the Ag 3, 3p, and 3t7 electrons. The sensitivity of xps toward certain elements, and hence the surface sensitivity attainable for these elements, is dependent upon intrinsic properties of the photoelectron lines observed. The parameter governing the relative iatensities of these cote level peaks is the photoionization cross-section, (. This parameter describes the relative efficiency of the photoionization process for each cote electron as a function of element atomic number. Obviously, the photoionization efficiency is not the same for electrons from the same cote level of all elements. This difference results ia variable surface sensitivity for elements even though the same cote level electrons may be monitored. 

Methane-to-methanol conversion by gas-phase transition metal oxide cations has been extensively studied by experiment and theory see reviews by Schroder, Schwarz, and co-workers [18, 23, 134, 135] and by Metz [25, 136]. We have used photofragment spectroscopy to study the electronic spectroscopy of FeO" " [47, 137], NiO [25], and PtO [68], as well as the electronic and vibrational spectroscopy of intermediates of the FeO - - CH4 reaction. [45, 136] We have also used photoionization of FeO to characterize low lying, low spin electronic states of FeO [39]. Our results on the iron-containing molecules are presented in this section. 

Photoionization can also access excited electronic states of the ion that are difficult to study by optical methods. The photoionization yield of FeO increases dramatically 0.36 eV above the ionzation energy. This result corresponds to the threshold for producing low spin quartet states of FeO. These states had not been previously observed, as transitions to them are spin forbidden and occur at inconveniently low energy. Because the FeO + CH4 reaction occurs via low spin intermediates, accurately predicting the energies of high and low spin states is critical. 

Fig.l Ground state photoionization cross section. Full line LG results, broken line VG results. 

Figure 13. Phase lag between the photoionization and photodissociation of vinyl chloride resulting from the Gouy phase of the focused laser beam. The dashed curve shows the results of the analytical model discussed in the text, and the solid curve is a numerical calculation of the phase lag without adjustable parameters.

Reactions (19)-(21) represent the dissociation of benzene and reactions (22)-(26) represent the detection of fragments by VUV laser photoionization. The line-shape images resulted from these reactions. 

One rather unfortunate aspect of the M + hydrocarbon (and M + OX) reactions mentioned thus far is that the products of the reactions were not detected directly, but were instead inferred via the pressure and temperature dependencies of the measured rate constants for metal reactant consumption and by comparison to ab initio calculations. Exceptions are the reactions of Y, Zr + C2H4 and C3H6, for which the Weisshaar group employed the 157 nm photoionization/mass spectrometry technique to identify the products of the reaction as those resulting from bimolecular elimination of H2.45 47 95... 

Figure 7. Depiction of origin of EXAFS. An X-ray photon is absorbed by A, resulting in the photoionization of a core-level electron represented as an outgoing ( + ) photoelectron wave which is backscattered (<- ) by a near neighbor, B.

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