Article pubs.acs.org/JPCA
Photodissociation of Anisole and Absolute Photoionization CrossSection of the Phenoxy Radical Hong Xu and S. T. Pratt* Argonne National Laboratory, Argonne, Illinois 60439, United States ABSTRACT: We have studied the photodissociation dynamics of anisole (C6H5OCH3) at 193 nm and determined the absolute photoionization crosssection of the phenoxy radical at 118.2 nm (10.486 eV) relative to the known cross-section of the methyl radical. Even at this energy, there is extensive fragmentation of the phenoxy radical upon photoionization, which is attributed to ionizing transitions that populate low-lying excited electronic states of the cation. For phenoxy radicals with less than ∼1 eV of internal energy, we find a crosssection for the production of the phenoxy cation of 14.8 ± 3.8 Mb. For radicals with higher internal energy, dissociative ionization is the dominant process, and for internal energies of ∼2.7−3.7 eV, we find a total cross-section (photoionization plus dissociative ionization) of 22.3 ± 4.1 Mb. The results are discussed relative to the recently reported photoionization cross-section of phenol.
93 (phenoxy cation); mass 65 (C5H5+, most likely cyclopentadienyl cation); and mass 15 (methyl cation). Their imaging study clearly shows that the phenoxy and methyl cations are produced by the photoionization of the primary neutral phenoxy and methyl photofragments. In principle, the C5H5+ could be produced in one of two ways (or both): first, it could be formed by the secondary fragmentation of phenoxy radicals to C5H5 + CO, followed by the photoionization of C5H5 (the ionization energy of CO is well above the photon energy used in their work); or second, it could be produced by the dissociative photoionization of neutral phenoxy radicals to produce C5H5+ + CO directly. The multimass imaging technique allows the unambiguous distinction between these two mechanisms,33 and Tseng et al. demonstrated conclusively that the C5H5+ was formed by the dissociative photoionization of phenoxy radicals. The present results are consistent with this conclusion, which is used to help interpret the observed cross-sections. The article is organized as follows. The experimental methods are first described in section II. The background on the photodissociation dynamics anisole is discussed along with the interpretation of the experimental results in section III. Finally, the results are discussed in comparison with recent results on the photoionization cross-section of phenol.
I. INTRODUCTION The pyrolysis of anisole (C6H5OCH3) has recently attracted interest because it produces a number of interesting radicals relevant to combustion and soot formation.1−4 In addition, the phenoxy radical (C6H5O), one of anisole’s primary fragments, is expected to be a significant species produced in the thermal decomposition of biomass.4 The pyrolysis study of Scheer et al.3 employed photoionization mass spectrometry to provide considerable insight into the fragmentation of anisole to produce phenoxy radicals and other species. This powerful tool can be used not only to identify species by mass but also to identify specific isomers by using their different photoionization yield curves.5−8 Knowledge of the absolute photoionization cross-sections would also allow such studies to be performed quantitatively.9 Over the past decade, such cross-sections have been reported for a substantial number of stable molecules.10−15 Approximate methods have also been developed to estimate absolute cross-sections and correlate cross-sections of related molecules.16−18 Although a number of techniques have been developed to determine the absolute photoionization cross-sections for radicals, data are still only available for a small number of these species.19−31 In this article, the photodissociation of anisole is used to produce phenoxy radicals in conjunction with methyl radicals,32−34 allowing the determination of the absolute photoionization cross-section of the phenoxy radical relative to the known cross-section for the methyl radical. The photodissociation of anisole at 193 nm has been studied previously by Tseng et al.33 using multimass ion imaging with photoionization detection at 118.2 nm. The photodissociation of anisole has also been studied by Hadden et al.34 by using time-resolved velocity map ion imaging with two-photon resonant, three-photon ionization detection of the CH3 fragments. Tseng et al.33 reported three dominant ions: mass © 2013 American Chemical Society
II. EXPERIMENTAL SECTION The experimental approach used in the present work has been described previously.35−37 The apparatus consists of a source chamber and a detector chamber separated by a skimmer with a Special Issue: Curt Wittig Festschrift Received: May 13, 2013 Revised: June 28, 2013 Published: July 11, 2013 12075
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reconstructed by using the pBASEX program,40 and the total translational energy distributions were determined by integrating the reconstructed distribution over all angles. The CH3+ image from the photodissociation of CH3I at 193 nm was recorded under the same conditions to allow the calibration of the translational energy distributions. Mass spectra were recorded for a range of ArF laser pulse energies, as well as for a range of delays between the photodissociation and photoionization pulses. For each mass spectrum, background spectra were also recorded with only 193 and 202.315 nm light present and with only VUV light present. These spectra were separately subtracted from the full spectrum to give two spectra with different background subtractions. The true 193 nm + VUV signal could be readily identified by comparing the two background subtracted images. The mass peaks of interest were integrated and corrected for the mass dependence of the channelplate response by using the results of Krems et al.41 These corrected intensities were then used to determine the relative signal strengths for momentum-matched pairs of photofragments. These relative intensities were then used with the known cross-section of the methyl radical29 to determine the unknown cross-section. To allow the comparison of the phenoxy cross-section with the known cross-sections of related species, we have performed electronic structure calculations of the molecular orbitals for the phenol and phenoxy using the Gaussian09 Program Suite.42 Unrestricted Hartree−Fock calculations at the 6-311G level were performed to visualize the highest occupied molecular orbital (HOMO), HOMO−1, and HOMO−2 of phenoxy. For the closed-shell phenol molecule, analogous restricted Hartree−Fock calculations were performed. Gaussview 5.0.9 was used to display the symmetry and nodal structure of the molecular orbitals of each species.43
2 mm diameter aperture. The sample is expanded into the source chamber via a 0.5 mm diameter nozzle by using a pulsed valve (General Valve, Series 9) to form a molecular beam. The samples are prepared by using the room temperature vapor pressure of anisole and diluting it in helium to a total pressure of 800 to 1200 Torr. This resulted in a sample concentration of approximately 0.3% of anisole. The molecular beam passes through the skimmer and into the interaction region, where it is crossed by the photodissociation laser beam and the photoionization (detection) laser beam. Molecules in the beam were photodissociated by using an ArF excimer laser, whose output was passed through an air-spaced, MgF2 Rochon polarizer, providing linearly polarized light. This beam was attenuated to energies of 1− 70 μJ/pulse and loosely focused into the interaction region. The dependence of the photodissociation signal on the pulse energy was determined, and the conditions were chosen to minimize both the multiphoton processes and background signals caused by the excimer laser alone. The photofragments generated by the ArF laser beam were photoionized by using a vacuum ultraviolet (VUV) laser beam that was counterpropagating to the photolysis beam. Two Nd:YAG-pumped dye laser systems were used to generate the VUV light by four-wave difference frequency mixing. The two beams are made colinear by using a dichroic mirror and focused into a cell containing Kr. The first beam, ω1, is operated at a fundamental wavelength of 606.945 nm and then frequency tripled to 202.315 nm.38 This beam was used to pump the twophoton transition to the (2P1/2)5p[1/2]0 level of Kr. The second dye laser output, ω2, is tuned between 675 and 705 nm to generate light at the difference frequency, 2ω1 − ω2 (118.0 to 119.0 nm). A commercial wavemeter was used to determine the actual wavelength of the dye lasers. The generated VUV light was refocused into the interaction region by using an offaxis LiF lens, which serves to separate the VUV light from the ω1 and ω2 beams. The photoions produced by the VUV light were accelerated by a set of standard optics for velocity map ion imaging39 oriented with the time-of-flight axis colinear with the molecular beam axis. The linear polarizations of the ArF beam and the ω1 and ω2 beams were all parallel with each other, perpendicular to the spectrometer axis, and parallel to the face of the ion detector. This detector was an 80 mm diameter dual channelplate coupled to an imaging phosphor screen. A standard video camera was used to record the ion images on a shot-to-shot basis. Mass spectra were also recorded by retuning the ion optics and sending the signal directly from the multichannel plate to a digital oscilloscope. The synchronization of the photolysis laser, VUV laser, molecular beam valve, and detection electronics was accomplished by using a series of digital delay generators. The actual delay time between the photolysis laser pulse and the ω1 and ω2 pulses was determined at the experimental chamber by using a fast photodiode. Photoion images and time-of-flight mass spectra were recorded at a VUV wavelength of 118.2 nm. In particular, images were recorded for each of the photofragments of interest, gating the detector voltage to the time-of-flight of specific photofragments, and summing the results from 10 000 laser shots. Additional images were recorded to allow the removal of background. In particular, background images were recorded with only the 193 nm beam and 202.315 nm beams present. These background images were subtracted from those recorded with all beams present. The final images were
III. RESULTS AND DISCUSSION The dissociation energy for C6H5OCH3 → C6H5O + CH3
(1)
is reported to be 2.754 ± 0.030 eV. At 193 nm, the photon energy is 6.4241 ± 0.0149 eV, leaving an excess energy of 3.666 eV, which can go into translational, rotational, or vibrational energy of the fragments. The secondary dissociation process 44
C6H5O → C5H5 + CO
(2)
has a significant barrier. The experimental activation energy is 1.904 ± 0.039 eV,45 but theoretical determinations give a value approximately 0.35 to 0.48 eV higher.1,4,46 Using the theoretical asymptotic energy of C5H5 + CO relative to C6H5O from Carstensen and Dean4 and the reported ionization energy for cyclopentadienyl (C5H5) of 8.42711 ± 0.0005 eV,47 we find the energetic threshold for dissociative ionization of C6H5O to C5H5+ + CO to be 9.50−9.52 eV. Of course, where this onset is actually observed will depend on both the absorption spectrum and the initial internal energy of the phenoxy radical. Note that in the present experiments the methyl fragment is stable with respect to both neutral dissociation and dissociative ionization. In particular, the total excess energy in the anisole photodissociation is less than the methyl dissociation energy, and even if all of the excess energy went into the methyl fragment, the VUV photon energy is insufficient to reach the dissociative ionization threshold.48 Additional evidence in support of dissociative ionization as the mechanism for C5H5+ formation comes from consideration 12076
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on the ground state surface; the fast component is consistent with direct dissociation from the optically prepared state. The C6H5O+ distribution in Figure 1 shows only the fast component of the distribution. The high-energy edge of the distribution occurs at the same energy as the CH3+ distribution, as seen in the magnified trace of the latter. Indeed, the distributions overlap quite well above 2.0 eV. This region corresponds to internal energies of the fragments of less than 1.67 eV. At lower energies (higher internal energies), dissociative ionization of the C6H5O takes over, and the C6H5O+ distribution falls gradually to zero. Figure 2 shows the translational energy distribution of the C5H5+ ions plotted as a function of the C5H5+ translational
of the kinetics of the alternative possibility, secondary fragmentation of hot, neutral C6H5O to C5H5 + CO followed by photoionization of the C5H5. In particular, the available energy following the 193 nm photodissociation of anisole to C6H5O + CH3 is 3.666 eV.44 Some of this energy goes to the total translational energy, and we use the value at the peak of the CH3 distribution, 0.230 eV.33 Although the product state distribution of the C6H5O and CH3 are not known, statistical dissociation would lead to branching of the internal energy approximately proportional to the relative number of vibrational modes. Thus, the C6H5O would receive ∼5/6 of the excess internal energy, or 2.863 eV. This analysis assumes that no energy ends up in rotational motion of the fragments, which would only reduce the vibrational energy further. Liu et al. have calculated the dissociation rate of C6H5O and found that the first step corresponds to the isomerization
which has a rate of ∼3 × 105 s−1 at the expected internal energy (see Figure 7 of ref 1). Indeed, even at internal energies of 3.47 eV, the rate is only 107 s−1, still too small to significantly affect the signal.1 Figure 1 shows the translational energy distributions obtained from the present CH3+ and C6H5O+ images. Note
Figure 2. Translational energy distribution of the secondary C5H5+ produced following the 193 nm photodissociation of anisole. The C5H5+ is produced by the dissociative photoionization of the primary C6H5O photoproduct and is plotted versus the C5H5+ fragment translational energy.
energy alone. As discussed above, the C5H5+ is formed by dissociative ionization of C6H5O. Unfortunately, because the C5H5+ gains translational energy both from the parent neutral and from the VUV photon energy used to ionize the fragments, it is not possible to plot the C5H5+ against the total translational energy. However, the strong maximum near zero translational energy is consistent with dissociative ionization in which the parent ion is formed with substantial internal energy and undergoes statistical dissociation on the electronic ground state surface. Even though it is difficult to momentum-match the C5H5 (and the CO that is not detected) with the CH3 cofragment, the overall simplicity of the dissociation products and the form of the translational energy distributions provide good evidence for their association. Assuming the C5H5+ has CO as its cofragment in the dissociation of C6H5O+, it is possible to plot the distribution as a function of the total translational energy of the secondary fragments. This distribution is shown in Figure 3. While the C6H5O+ distribution is negligible at low energy as a result of near complete fragmentation upon ionization, the CH3 distribution can be used to show how the phenoxy distribution would look in the absence of fragmentation. In particular, Figure 3 also shows the CH3 data plotted as a function of the kinetic energy of its C6H5O cofragment from the primary dissociation process. Comparison of these data with the distribution determined from the C5H5+ image shows that the
Figure 1. Translational energy distributions following the 193 nm photodissociation of anisole. The traces show the distributions determined from the reconstructed CH3+ and C6H5O+ velocity map ion images and plotted against the total translational energy of the CH3 and C6H5O fragments. An expanded version of the CH3+ trace is also shown in which the intensity is multiplied by a factor of 11.
that the higher the translational energy, the smaller the internal energy of the fragments. The two distributions appear quite different. The CH3+ distribution is similar to that of Hadden et al.34 obtained by photodissociation at 200 nm, with a strong peak near zero energy and a long tail extending to almost the total available energy of 3.666 eV. The distribution is also similar to that of Tseng et al.,33 but the high-energy tail in the present spectrum is considerably weaker than in theirs. As discussed previously by Tseng et al.33 and Hadden et al.,34 the distribution is consistent with the presence of two decay mechanisms: the slow component is consistent with internal conversion of the initially excited state followed by dissociation 12077
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ionization cross-sections. In this case, the sum is over the full range of fragment internal energies, so that only the ion signals, and not the ion images, are required. The only additional step is that the C5H5+ detector efficiency has to be taken into account along with the CH3+ and C6H5O+ efficiencies. Figure 4 shows a typical mass spectrum. The relative intensities of the ion signals for CH3+, C5H5+, and C6H5O+ are
Figure 3. Translational energy distribution of the secondary C5H5+ produced following the 193 nm photodissociation of anisole. The C5H5+ is plotted against the total translational energy of the fragments following dissociative ionization, i.e., the sum of the C5H5+ and CO translational energies. For comparison, the translational energy distribution obtained from the CH3+ image, but plotted against the C6H5O translational energy, is also shown. The dissociative ionization process imparts significant translational energy to the fragments. Figure 4. Time-of-flight mass spectrum following photodissociation of anisole at 193 nm, followed by single-photon photoionization at 118.22 nm. The spectrum shown corresponds to the spectrum obtained with both the 193 and 118.22 nm light minus the spectrum recorded with only the 193 nm light and the 202.315 nm light used in the difference-frequency generation scheme (i.e., the visible light in the difference frequency scheme is blocked). This subtraction removes the background signal generated by the two UV beams. To prevent damage to the detector, the detector is gated to eliminate the very intense anisole parent-mass peak.
secondary fragments gain significant translational energy in the dissociative ionization process. The absolute photoionization cross-section of the phenoxy radical can be addressed in two fashions. In the first, we consider only the photoionization of phenoxy radicals that lead to the production of stable phenoxy cations. As seen in Figure 1, there is good agreement between the CH3+ and C6H5O+ translational energy distributions for 2.0 eV ≤ ET ≤ 3.1 eV. In this energy region (i.e., for low internal energy), the phenoxy ion signal provides a measure of the total cross-section because dissociative ionization is not occurring. This energy range accounts for 5% and 62% of the CH3+ and C6H5O+ signals, respectively. These percentages can then be used to scale the ratio of the C 6 H 5 O + and CH 3 + signals in the mass spectrometer. This scaling leads to a ratio of the C6H5O+ and CH3+ signals of 1.585 ± 0.331. Finally, this ratio is corrected by the relative detector efficiencies41 for mass 15 and 93, giving a final C6H5O+:CH3+ ratio of 2.605 ± 0.632. As discussed above, because the CH3 radicals are stable with respect to both secondary dissociation and dissociative ionization, the CH3+ signal, combined with the known CH3 photoionization crosssection,29 gives a measure of the true number of radicals produced. Using the photoionization cross-section of CH3 at 118.2 nm (σ = 5.68 ± 0.67 Mb),29 the absolute photoionization cross-section of phenoxy at this wavelength is 14.8 ± 3.8 Mb. The second approach to the photoionization cross-section of phenoxy is to determine the total cross-section for photoionization and dissociative ionization. In general, dissociative ionization of larger polyatomic molecules is not a direct process, but rather occurs in two steps corresponding to ejection of the electron (i.e., photoionization of the parent molecule) and subsequent fragmentation of the parent ion. Using the observation that the C5H5+ results from the dissociative ionization of the radical (i.e., photoionization of C6H5O followed by dissociation to C5H5+ + CO), we can simply compare the sum of the C6H5O+ and C5H5+ ion signals to the CH3+ ion signal to determine the relative photo-
1.00, 3.01 ± 0.22, and 0.14 ± 0.03, respectively. By correcting for the detector efficiencies, summing the C5H5+ and C6H5O+ signals, and using the CH3 cross-section29 of 5.68 ± 0.67 Mb at 118.2 nm, we find that the total photoionization cross-section of C6H5O is 22.3 ± 4.1 Mb. As discussed below, that this value is somewhat larger than the value based on stable C6H5O+ is not so surprising. One important distinction with such a determination is that it also corresponds to the cross-section for phenoxy radicals with a broad distribution of internal energy (∼0.0−3.6 eV). As is evident from the CH3+ cofragment distribution in Figure 1, the internal energy distribution of the C6H5O is heavily weighted toward the higher end of this range, that is, for internal energies of ∼2.7−3.7 eV. We can try to rationalize the observed value of the phenoxy cross-section using a simplified, single-electron, central field picture for the photoionization process. In general, photoionization cross-sections can be affected by a wide variety of factors, including autoionization, shape resonances, Cooper minima, interchannel coupling, etc.49 Estimations of the crosssections based on the single-electron picture are bound to be approximate, and may disagree substantially from the true cross-sections. Nevertheless, ignoring fine structure and using a one electron, central potential model in the dipole approximation, one finds after summing over final states and averaging over initial states that the photoionization cross-section is proportional to the number of electrons in the subshell of interest.50 This dependence is often used in photoelectron 12078
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9.90 eV (9.70 eV), respectively.52 Ionization out of the nonbonding π ring/oxygen lone pair orbital of phenoxy gives rise to a triplet and a singlet state of the cation with ionization energies of 9.76 eV (9.70 eV) and 9.96 eV (10.00 eV), respectively. The corresponding orbital of phenol has an ionization energy above 10.486 eV, and all of the other orbitals of phenol and phenoxy also have ionization energies above the photon energy range of this work. Thus, at 10.486 eV, a total of five electrons will contribute to the ionization signal for phenoxy, whereas only four electrons will contribute to the cross-section for phenol. At 21.2 eV, the HOMO and HOMO−1 bands in the He I photoelectron spectrum of phenol have nearly equal intensities.53 The HOMO−2 band appears somewhat stronger, but it is overlapped with several other bands making it difficult to assess quantitatively. The photoionization cross-section of phenol has been reported recently by Eschner and Zimmerman,54 who used a novel source with a photon energy of 9.8 eV. (The profile of the output is nearly Gaussian with a fwhm of 0.7 eV.) The full envelope of the source is above the entire first photoelectron band of phenol. The center wavelength is also above nearly the full second photoelectron band (ionization from HOMO-1), but the lower half of the envelope covers only a portion of this band.52,53 A convolution of the second photoelectron band with the source envelope indicates that the effective cross-section will include only 86% of the full second band. As a result, two electrons from the HOMO and ∼1.72 electrons from the HOMO−1 contribute to the crosssection determined by Eschner and Zimmerman.54 With a measured cross-section of 22.25 ± 0.72 Mb, this analysis corresponds to 5.98 ± 0.19 Mb per electron. Assuming this value also hold for the orbitals of the phenoxy radical and that each of the photoelectron bands has similar intensity, the phenoxy cross-section at 10.486 eV is estimated to be approximately 5 × (5.98 ± 0.19) Mb = 29.9 ± 1.0 Mb. In addition to the approximations mentioned above, the estimate is also based on the assumption that all ionization processes are included in the cross-section. Thus, the value is best compared to the cross-section for the total ionization signal (including dissociative ionization). With this in mind, the agreement with the value of 22.3 ± 4.1 Mb determined above is not so bad. The significantly smaller cross-section for photoionization of phenoxy radicals with less internal energy and leading to parent ions is also understandable. In particular, the threshold given above for dissociative ionization of the phenoxy radical is ∼9.51 eV, and both the singlet component of the HOMO−1 and both the components of the ring-π/oxygen-lone-pair orbital have ionization energies above this threshold. Thus, even for vibrationally cold phenoxy radicals, at 10.486 eV some fraction of the cations are expected to be produced with more than sufficient energy to dissociate to C5H5+ + CO. Unfortunately, collecting on the C5H5+ produced only from C6H5O with low internal energy is not possible in the present experiments and cannot be distinguished from C5H5+ produced from hot C6H5O because the translational energy of the C5H5+ is changed in the dissociative ionization process. In short, photoionization of even cold C6H5O at 10.486 eV is expected to produce some C5H5+, and the absolute photoionization cross-section to produce stable C6H5O+ is considerably smaller than one might first expect. Note that the observation that the CH3+ and C6H5O+ translational energy distributions match over a range of energies indicates that, for these energies, the fraction of
spectroscopy to determine the number of electrons in the molecular orbitals responsible for particular photoelectron bands.51 While this dependence is certainly approximate, it is a useful starting point for qualitative estimates of photoionization cross-sections. In particular, an unknown cross-section can be estimated by determining the number of photoelectron bands that are accessible at the photon energy of interest and then estimating the cross-section for each band by comparison with similar species for which the cross-sections are known. Information on the phenoxy cation comes from the experimental photoelectron spectrum and theoretical calculations reported some time ago by Dewar and David.52 The calculations were semiempirical, but the agreement between the observed and calculated ionization energies was quite good. Dewar and David also showed the correlation between the valence orbitals of phenol and phenoxy.52 Figure 5 shows the
Figure 5. Relevant molecular orbitals for phenol and the phenoxy radical, showing the strong similarities between the HOMOs and (HOMO−1)s of the two molecules, as was concluded by Dewar and David (ref 51). The HOMO−2 in phenol has a combination of π character from the ring and lone pair character on the oxygen atom. The two components of the HOMO−2 in phenoxy show different character, and the calculated energy for the α component is actually below that of the HOMO−3. This may be due to mixing of the α component of the HOMO−2 with the HOMO and the approximate nature of the single-electron model used in the discussion.
molecular orbitals of phenol and phenoxy resulting from the present calculations. The figure is consistent with the discussion of Dewar and David, but the oxygen lone pair orbital is also found to have substantial π character from the ring. In particular, the doubly occupied π(b1) HOMO of phenol correlates with the singly occupied HOMO of phenoxy. The experimental (theoretical) ionization energy from the HOMO of phenoxy is 8.56 eV (8.68 eV).52 Similarly, the doubly occupied π(a2) HOMO−1 of phenol correlates with the doubly occupied HOMO−1 of phenoxy. Ionization out of the HOMO−1 of phenoxy leads to a triplet and a singlet state of the cation, with ionization energies of 9.42 eV (9.36 eV) and 12079
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Hydrocarbons in the Vacuum Ultraviolet Range. J. Electron Spectrosc. Relat. Phenom. 2002, 123, 225−238. (13) Cool, T. A.; Wang, J.; Nakajima, K.; Taatjes, C. A.; McIlroy, A. Photoionization Cross Sections for Reaction Intermediates in Hydrocarbon Combustion. Int. J. Mass Electron. 2005, 247, 18−27. (14) Wang, J.; Yang, B.; Cool, T. A.; Hansen, N.; Kasper, T. NearThreshold Absolute Photoionization Cross-Sections of Some Intermediates in Combustion. Int. J. Mass Spectrom. 2008, 269, 210−220. (15) Yang, B.; Wang, J.; Cool, T. A.; Hansen, N.; Skeen, S.; Osborn, D. L. Absolute Photoionization Cross-Sections of Some Combustion Intermediates. Int. J. Mass Spectrom. 2012, 309, 118−128. (16) Cox, P. A.; Orchard, F. A. On Band Intensities in the Photoelectron Spectra of Open-Shell Molecules. Chem. Phys. Lett. 1970, 7, 273−275. (17) Koizumi, H. Predominant Decay Channel for Superexcited Organic Molecules. J. Chem. Phys. 1991, 95, 5846−5852. (18) Bobeldijk, M.; van der Zande, W. J.; Kistemaker, P. G. Simple Models for the Calculation of Photoionization and Electron Impact Ionization Cross Sections of Polyatomic Molecules. Chem. Phys. 1994, 179, 125−130. (19) Flesch, R.; Schürmann, M. C.; Plenge, J.; Hunnekuhl, M.; Meiss, H.; Bischof, M.; Rühl, E. Absolute Photoionization Cross Sections of the Primary Photofragments of Chlorine Dioxide and Dichlorine Monoxide. Phys. Chem. Chem. Phys. 1999, 1, 5423−5428. (20) Robinson, J. C.; Sveum, N. E.; Neumark, D. M. Determination of the Absolute Photoionization Cross Sections for the Isomers of C3H5: Allyl and 2-Propenyl Radicals. Chem. Phys. Lett. 2004, 383, 601−605. (21) Robinson, J. C.; Sveum, N. E.; Neumark, D. M. Determination of Absolute Photoionization Cross Sections for Vinyl and Propargyl Radicals. J. Chem. Phys. 2003, 119, 5311−5314. (22) Sveum, N. E.; Goncher, S. J.; Neumark, D. M. Determination of the Absolute Photoionization Cross Section of the Phenyl Radical. Phys. Chem. Chem. Phys. 2006, 8, 592−598. (23) Gross, R. L.; Liu, X.; Suits, A. G. The Ultraviolet Photodissociation of 2-Chlorobutane: an Imaging Study Comparing StateResolved and Universal Probe Techniques. Chem. Phys. Lett. 2002, 362, 229−234. (24) Taatjes, C. A.; Osborn, D. L.; Selby, T. M.; Meloni, G.; Fan, H. Y.; Pratt, S. T. Absolute Photoionization Cross-Section of the Methyl Radical. J. Phys. Chem. A 2008, 112, 9336−9343. (25) Loison, J. C. Absolute Photoionization Cross Section of the Methyl Radical. J. Phys. Chem. A 2010, 114, 6515−6520. (26) Gans, B.; Vieira Mendes, L. A.; Boyé-Péronne, S.; Douin, S.; Garcia, G.; Soldi-Lose, H.; Cunha de Miranda, B. K.; Alcaraz, C.; Carrasco, N.; Pernot, P.; et al. Determination of the Absolute Photoionization Cross Sections of CH3 and I Produced from a Pyrolysis Source by Combined Synchrotron and Vacuum Ultraviolet Laser Studies. J. Phys. Chem. A 2010, 114, 3237−3246. (27) Gans, B.; Garcia, G. A.; Boyé-Péronne, S.; Loison, J. C.; Douin, S.; Gaie-Levrel, F.; Gauyacq, D. Absolute Photoionization Cross Section of the Ethyl Radical in the Range 8−11.5 eV: Synchrotron and Vacuum Ultraviolet Laser Measurements. J. Phys. Chem. A 2011, 115, 5387−5396. (28) FitzPatrick, B. L.; Alligood, B. W.; Butler, L. J.; Lee, S. H.; Lin, J. J. M. Primary Photodissociation Pathways of Epichlorohydrin and Analysis of the C−C Bond Fission Channels from an O(3P) + Allyl Radical Intermediate. J. Chem. Phys. 2010, 133, 094306. (29) Savee, J. D.; Soorkia, S.; Welz, O.; Selby, T. M.; Taatjes, C. A.; Osborn, D. L. Absolute Photoionization Cross Section of the Propargyl Radical. J. Chem. Phys. 2012, 136, 134307. (30) Shubert, V. A.; Pratt, S. T. Photodissociation of Acetaldehyde and the Absolute Photoionization Cross Section of HCO. J. Phys. Chem. A 2010, 114, 11238−11243. (31) Xu, H.; Pratt, S. T. Photoionization Cross Section of the Propargyl Radical and Some General Ideas for Estimating Radical Cross Sections. J. Phys. Chem. A 2013, DOI: 10.1021/jp309814q. (32) Kajii, Y.; Obi, K.; Nakashima, N.; Yoshihara, K. ArF Laser Flash Photolysis of Phenol and Anisole. J. Chem. Phys. 1987, 87, 5059−5063.
dissociative ionization does not depend significantly on the initial internal energy of the radical. In summary, we have used a combination of velocity map imaging and mass spectrometry to study the photodissociation of anisole and to determine the photoionization cross-section of the phenoxy radical at 10.486 eV (118.2 nm). At this energy, photoionization of the phenoxy radical is found to result in significant fragmentation of the parent ion, even when the initial state of the phenoxy radical has little internal energy. This situation most likely results from the presence of low-lying excited states of the cation that can be populated by excitation from the HOMO−1 and oxygen lone-pair orbital. This situation is probably not so uncommon for medium- to largesized radicals.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under contract No. DE-AC02-06CH11357.
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