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Photodissociation of Acetaldehyde and the Absolute Photoionization Cross Section of HCO† V. Alvin Shubert and Stephen T. Pratt* Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: April 2, 2010; ReVised Manuscript ReceiVed: May 3, 2010
Photodissociation of acetaldehyde (CH3CHO) at 266 nm produced CH3 and HCO radicals, and single-photon vacuum ultraviolet ionization was used to record velocity map ion images of both CH3+ and HCO+. Comparison of the translational energy distributions from both species indicates that secondary fragmentation of HCO is negligible for 266 nm photodissociation. Thus, the relative photoion signals for CH3+ and HCO+ in the mass spectrometer, combined with the recently measured absolute photoionization cross section of CH3, allowed 2.2 the determination of the absolute photoionization cross section of HCO (σ(HCO) ) 4.8 ( 2.0 1.5, 5.9 ( 1.6, and 1.6 3.7 ( 1.2 Mb at 10.257, 10.304, and 10.379 eV, respectively). The observed values are quite small but consistent with the similarly small value at threshold for the isoelectronic species NO. This behavior is discussed in terms of the character of the HOMO in both molecules. I. Introduction Photoionization mass spectrometry is an increasingly popular tool to characterize complex gas-phase mixtures that are often found in investigations of atmospheric chemistry, aerosol chemistry, and combustion.1-3 Absolute photoionization cross sections are therefore needed to quantify the concentrations of the species of interest. These absolute cross sections are difficult to calculate with ab initio methods and provide a stringent test for theory; they can also provide insight into photoionization dynamics.4 Furthermore, in more complex molecules, the differences between photoabsorption and photoionization cross sections can be substantial, and thus the quantum yield for ionization is also of interest.4 Several methods have been developed to measure photoionization cross sections for a wide variety of stable species, usually through the measurement of the absolute photoabsorption cross section and a determination of the ionization quantum yield.4 Once the absolute photoionization cross section for one species is known, the absolute photoionization cross sections for other species can be determined by a relative measurement of both species. For example, Cool et al.5,6 recently determined the absolute photoionization cross sections for a number of stable combustion intermediates by making relative measurements with respect to hydrocarbon molecules with known absolute photoionization cross sections. While these methods work very well for stable molecules, the reactive environments of interest in many applications contain a significant number of radicals and other highly reactive species. For such species, the traditional methods for determining absolute photoionization cross sections are difficult to apply because the absolute concentration of the radicals is generally not well-known. In their measurement of the absolute photoionization cross section of ClO, Flesch et al.7,8 overcame this limitation by using the photodissociation of ClO2 and Cl2O to produce equal amounts of ClO and O or Cl, respectively, and then measured the relative photoionization yields of ClO and O or Cl. Because the absolute cross sections for O and Cl were known, the absolute cross section for ClO was determined from the relative measurement. Neumark and co-workers9-11 extended †
Part of the “Klaus Mu¨ller-Dethlefs Festschrift”. * To whom correspondence should be addressed.
this approach in a significant manner by using photodissociation of hydrocarbon halides and translational spectroscopy with vacuum ultraviolet (VUV) photoionization to compare the photoionization cross sections of momentum-matched partners. Using the known absolute photoionization cross section of the halide (typically chlorine) atom, they determined the absolute photoionization cross sections of a number of species, including the allyl, 2-propenyl, vinyl, propargyl, and phenyl radicals.9-11 Taatjes et al. used two different methods to determine the photoionization cross section of the methyl radical.12 One of these methods used the photodissociation of a precursor with a known absolute photoionization cross section and known quantum yield for the methyl radical (CH3), so that the depletion of the precursor ion signal could be used to determine the CH3 concentration. The other method was closely related to that of Neumark and co-workers,9-11 but ion imaging was used instead of translational spectroscopy, and a laser-based VUV source was used instead of a synchrotron source. The agreement between the two different approaches gave added credibility to the measurement. More recently, Gans et al.13 performed a more comprehensive determination of the absolute photoionization cross section of CH3. In many instances, it is difficult to find a suitable precursor for which photodissociation provides the radical of interest in conjunction with a species with a known photoionization cross section. In the present study, we use the photodissociation of acetaldehyde, CH3CHO, to produce CH3 and the formyl radical (HCO), a significant species in combustion chemistry and an important radical in interstellar chemistry. The absolute photoionization cross section of HCO is determined by using the laser-based VUV photoionization and the imaging method described in ref 12, along with the now known absolute photoionization cross section of CH3.12,13 The cross sections determined here were surprisingly small but comparable to the values for the isoelectronic radical NO.4 The source of these small cross sections is discussed. The present results also provide new information on the photodissociation of acetaldehyde at 266 nm. This process has been investigated previously in a number of studies.14-25 Acetaldehyde absorption between 250 and 350 nm corresponds to an n f π* transition to the S1 state,26 which leads to the
10.1021/jp102992b 2010 American Chemical Society Published on Web 05/26/2010
Photodissociation of Acetaldehyde production of CH3 + HCO at wavelengths shorter than 320.5 nm. Dissociation to CH4 + CO, which is produced directly or via a roaming radical mechanism, has also been identified in this region.21,22 Dissociation to CH3 + HCO is thought to occur on both the T1 surface after intersystem crossing from the S1 state and the S0 surface following internal conversion.23 The time scale of the intersystem crossing is ∼10-8 s, resulting in dissociation that is slow with respect to rotation of the parent molecule. At the photodissociation wavelength employed in the present study, 266 nm, there is sufficient excess energy following the dissociation to CH3 + HCO to allow secondary decomposition of the HCO to H + CO, provided the majority of the excess energy is distributed into the internal energy of the HCO radical. At a longer wavelength (∼317 nm), a recent ab initio study by Kurosaki and Yokoyama19 concludes that the excess energy is only partitioned into the translational energy of the two fragments and into rotation of the HCO radical. A recent experimental investigation by Cruse and Softley20 found the vibrational distributions to be colder than those predicted by a statistical model for partitioning of the excess energy at photolysis wavelengths as short as 282.5 nm. In the present study, a comparison of the total translational energy distributions extracted from the ion images for CH3+ and HCO+ indicates that any secondary decomposition is negligible with 266 nm photodissociation. In what follows, a description of the experimental apparatus is provided first, followed by a brief summary of background information on the energetics of the dissociation and ionization processes that could affect the observed results. Next, a discussion of the imaging results on the photodissociation of acetaldehyde is presented. These results are important for the discussion of the determination of the absolute photoionization cross section of HCO that follows. Finally, the results on the photoionization cross section of HCO are compared with those on the isoelectronic species NO, and some general features of the cross section are discussed. II. Experiment The experimental apparatus and approach used for this work have been described in detail previously27-29 and are only briefly described here. The apparatus consisted of two vacuum chambers (source region and interaction/detection region) separated by a skimmer. The molecular beam was produced with a pulsed valve (General Valve, Series 9) and skimmed prior to entering into the interaction region. Standard collinear velocity map imaging ion optics accelerated molecular cations to a channelplate detector coupled to a phosphor screen. In ion imaging mode, a video camera viewing the phosphor screen was interfaced to a computer that was used for data acquisition. In time-of-flight mass spectrometry mode, data were acquired by collecting the signal from the channelplate detector with a computer interfaced to an oscilloscope. The sample gas mixture was ∼20% acetaldehyde (99%, Sigma Aldrich) in He at a backing pressure of ∼1400 Torr. The laser pulses were timed to probe the beginning of the gas pulse under conditions that cooled the acetaldehyde molecules into the zero point vibrational levels but did not produce an observable amount of dimers or large clusters. Methyl iodide calibration images were obtained by creating a sample mixture of ∼5% CH3I (99%, Sigma Aldrich) in He, also at a backing pressure of ∼1400 Torr and monitoring the CH3+ signal. In the interaction region, the lasers intersected the molecular beam at right angles, and the probe laser was counterpropagated with the dissociation laser. The 266 nm output of a frequency-
J. Phys. Chem. A, Vol. 114, No. 42, 2010 11239 quadrupled Nd:YAG laser was used to photodissociate the sample. The resulting fragments were probed by VUV light generated by two-photon resonant, four-wave mixing (2ω1 ω2 ) ω3) in a Kr cell that was separated from the main chamber by a MgF2 lens. Typically, the 266 nm photodissociating light arrived ∼60 ns before the VUV pulse. The 202.315 nm (ω1) light used to pump the two-photon Kr 1S0 f 5p′[1/2]0 transition was generated by frequency tripling the output of an Nd:YAG pumped dye laser. The output of a second Nd:YAG pumped dye laser (660-620 nm, ω2) was used to generate the difference frequency light (10.379-10.257 eV, ω3). The beams used to generate the VUV light were focused into the Kr cell with a 125 mm achromatic lens and the diverging VUV light refocused by the MgF2 lens into the molecular beam. Due to the chromatic aberration of the MgF2 lens, the generating beams were refocused well past the molecular beam and aligning the lens slightly off center effectively separates the VUV light from the generating beams. All laser pulses were ∼10 ns in duration and the wavelengths of the visible light from the dye lasers (i.e., the fundamental wavelengths prior to doubling, tripling, or mixing) were calibrated with a commercial wavemeter (Coherent Wavemaster). Mass spectra were obtained by recording signals with all lasers present, with only the VUV, with only the 266 nm, and with no lasers present. The signal with no lasers present gave the detector background response that was subtracted from all of the other spectra. Spectra were also obtained with only the one or the other of the two beams generating the VUV light present, but these were identical to the spectrum with all lasers blocked. The sum of the signals with only the VUV or 266 nm light present was subtracted from the signal with all of the lasers present to give the final mass spectrum. Depending upon the amount of signal present, mass spectra were obtained by averaging 3600-12000 laser shots for each of the laser conditions described. Because the detector channelplates were operated in analogue mode, the signal must be corrected for the mass dependence of the channelplate gain.30,31 For a given ion-impact energy, lighter ions have higher gains than heavier ions. This behavior has been previously studied and described by Oberheide et al.30 and by Krem et al.31 While the absolute detection efficiencies as a function of ion mass are not in perfect agreement (see Figure 5 of Krems et al.), the detection efficiency of CH3+ is always somewhat higher than that of HCO+. We have used the results of Krems et al.,31 which give a correction factor of 1.08, and we have used the other measurements to estimate an uncertainty in this value of (0.05, which is included in the error analysis below. Thus, in what follows, the ratio of the integrated ion intensities (HCO+/CH3+) used to determine the absolute photoionization cross section of HCO+ has been corrected using 1.08 ( 0.05. Images were obtained by gating the detector around the mass of interest and recording the image with all lasers present, with only the 266 nm present, and with only the VUV present. The sums of the images taken with only either the 266 nm or VUV present were subtracted from the image taken with all lasers present to produce the final image. Depending upon the amount of signal present, centroided images accumulated over 1500045000 laser shots were obtained for each of the laser conditions described. The final images were reconstructed using the pBASEX program.32 The total translational energy distributions were determined by integrating the reconstructed distributions over all angles and calibrating with a distribution for which the
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energetics were known, in this case, the image of CH3+ from dissociation of CH3I. III. Results and Discussion A. Background Data and Energetic Considerations. The photodissociation process used to create the CH3 and HCO radicals in the present experiments can result in significant rotational and vibrational excitation of the radicals. Thus, the dependence of the photoionization cross sections on this excitation must be considered. As has been discussed previously,28 in the absence of resonances, the photoionization cross sections are expected to depend only weakly on the rotational state of the radical. In principle, vibrational excitation of the sample could have a much more substantial effect. The structures of the electronic ground states of HCO and HCO+ have been characterized previously.33 The neutral ground state is strongly bent, with a theoretical HCO angle of 124.6°, and re(CH) and re(CO) of 1.119 and 1.176 Å, respectively. In contrast, the HCO+ ground state is linear, with re(CH) and re(CO) of 1.0932 and 1.1111 Å, respectively.33 Thus, although the bond lengths do not change significantly upon ionization, the bond angle changes substantially. In this case, the Franck-Condon factor from the ground vibrational level of the neutral to that of the ion is extremely small, and photoionization above threshold is expected to populate a long progression in the ν2 bending vibration of the ion. This behavior is exactly what is observed in the He I and Ne I photoelectron spectra of HCO recorded by Dyke et al.34,35 In particular, they observe a long progression, rising from threshold to a peak at V2+ ∼ 10, and extending through the V2+ ∼ 20 band, ∼2 eV above the adiabatic ionization energy (or ∼10.1 eV above the neutral ground state). The VUV photon energies of the present study lie between 10.257 and 10.379 eV and are thus sufficient to access the full Franck-Condon envelope for the ionizing transition from the HCO ground state. Although vibrational excitation of the HCO could expand this envelope somewhat, it is still likely that the major fraction of this envelope will still be accessible. Thus, in the absence of significant non-Franck-Condon/non-BornOppenheimer behavior in the photoionization process, the cross section is expected to be relatively independent of the initial vibrational level of HCO. A similar argument applies to the photoionization cross section of CH3. In this case, the radical and the ion have very similar geometries, and photoionization strongly favors the ∆Vi ) 0 transitions.28 Similarly, ionization from excited vibrational levels are also expected to produce narrow Franck-Condon envelopes. Thus, here again the photoionization cross sections are expected to depend only weakly on the initial vibrational level of CH3. These considerations are important for the present experiments, because the initial vibrational distributions of the two radicals are not precisely known. Nevertheless, the next section indicates that the vibrational excitation of the CH3 and HCO produced by the photodissociation of acetaldehyde at 266 nm is relatively small, further minimizing concerns about the vibrational dependence of the cross section. The energetics of CH3CHO, CH3, HCO, and HCO+ have been relatively well-characterized by experiment and theory.33,36 The ionization potential of the parent acetaldehyde is reported to be 10.229 eV.36 The lowest dissociative ionization threshold lies at 10.50 ( 0.05 eV and results in the production of C2H3O+ + H, while the lowest reported dissociative ionization threshold to produce HCO+ lies at 11.79 eV.36 Thus, dissociative ionization of the parent species acetaldehyde is not expected to
Shubert and Pratt interfere with the present measurements, which are performed at VUV photon energies at or below 10.379 eV. This expectation is supported by the translational energy distributions reported in the following section. The bond dissociation energy of CH3CHO to produce CH3 + HCO is 3.623 eV.37 Thus, for photodissociation at 266 nm (4.661 eV), there is ∼1.037 eV in excess energy to be distributed among the internal degrees of freedom of the CH3 and HCO and translational energy of the fragments. The bond dissociation of HCO is ∼0.609 eV,33 so that secondary dissociation of HCO to H + CO is possible if a sufficient fraction of the available energy from the primary process goes into the internal degrees of freedom of the HCO. The ionization potential of HCO is 8.15022 ( 0.00006 eV.38 Because HCO+ is a closed shell species, it is quite stable. The lowest dissociative ionization threshold produces H+ + CO, and lies 14.207 eV above the ground state neutral. Note that even if the neutral HCO is excited all the way to its dissociation threshold, the VUV photon energies of the present experiments (e10.379 eV) are insufficient to allow dissociative ionization. Similarly, the ionization potential of CH3 is 9.83899 ( 0.000018 eV,39 and as in the case of HCO, dissociative ionization is not possible in the present experiments. In summary, for the VUV photon energies employed in the present study, we expect the photoionization cross section to be relatively independent of the initial rotational and vibrational state of the CH3 or the HCO, and dissociative ionization of CH3 and HCO is not expected to be energetically accessible. The possible secondary dissociation of HCO following the photodissociation of acetaldehyde at 266 nm is discussed in the context of the imaging experiments in the next section. B. Photofragment Translational Energy Distributions. The photodissociation of acetaldehyde has been studied previously by numerous researchers.14-25 For the most part, these studies have focused on photodissociation wavelengths between ∼320 and 300 nm, and emphasized the determination of the translational, rotational, and vibrational distributions of the CH3 and HCO fragments. Most of these studies concluded that dissociation occurs following internal conversion from the optically excited S1 state to the lowest triplet T1 state. The T1 surface has a barrier in the exit channel, which has a significant effect on the product state distributions. Close to threshold, most of the excess energy has been found to go into the relative translation of the two fragments, with some rotational excitation primarily in the HCO. At wavelengths below ∼302 nm, Cruse and Softley20 observed a small amount of vibrational excitation in the CH3, primarily in the ν2 umbrella mode. Their work used a resonant detection scheme for the HCO that selected only vibrationless radicals. Gejo et al.17 have examined the vibrational excitation of the ground state HCO fragment and found the production of the (010) and (001) states at wavelengths shorter than ∼308.7 and 302.6 nm, respectively. They argue that the vibrational state distributions are in accord with statistical models if it is assumed that the energy required to overcome the barrier (∼24 kJ/mol) goes into rotation and translation of the fragments.17 Lee and Chen18 have observed similar thresholds for the vibrational excitation in the HCO but in general find lower rotational excitation of the HCO than Gejo et al.17 Lee and Chen also found that the rotational excitation increases with decreasing photodissociation wavelength, while Gejo et al. indicate the rotational excitation does not change significantly with wavelength. No detailed measurements of the HCO vibrational distribution have been made at 266 nm. In the present context,
Photodissociation of Acetaldehyde
Figure 1. Total translational energy distributions (left column) obtained from reconstructions of the velocity map images (right column) obtained by monitoring CH3+ or HCO+ produced from the photodissociation of acetaldehyde at 266 nm. Single-photon ionization of the CH3 and HCO radical products was performed with 10.379 eV VUV light. The total energy available following photodissociation is marked by the arrow labeled Eavail, where Eavail ) hν - D0, where hν is the energy of the photodissociation laser and D0 ) 29240 cm-1 is the dissociation energy for the reaction (ref 37). Note that both translational energy distributions are similar in shape. The arrows on the images show the laser polarization with respect to the face of the detector. Outside the intense, isotropic central portion of the CH3+ image is a faint ring due to the photodissociation of residual methyl iodide producing CH3 correlated with spin-orbit excited iodine. This ring occurs at a diameter associated with a kinetic energy greater than that possible for CH3 fragments from acetaldehyde dissociation and did not affect the results.
this is important because this distribution determines the possibility for secondary dissociation of HCO. Recently, Heazlewood et al.23 have performed new measurements both below and above the threshold for radical dissociation on the triplet surface. They found substantial evidence for some dissociation to CH3 + HCO following internal conversion from the S1 state to the S0 state. Dissociation on the S0 state is expected to be barrierless and to result in fragments with low translational energy and high internal (vibrational and rotational) energy of the fragments. Thus, this process is readily distinguishable from the T1 process for which the vibrational excitation is weak. Heazlewood et al.23 found that the S0 process decreased in importance at shorter wavelengths where the T1 process was allowed, but detailed studies of the wavelength dependence of the branching between the two processes have not been performed. The S0 process would be more likely to produce highly vibrationally excited HCO that could undergo secondary decomposition. In principle, velocity-map ion imaging provides the means to determine the internal state distributions of the fragments. Figure 1 shows the images obtained following the photodissociation of acetaldehyde at 266 nm, and monitoring the CH3+ or HCO+ ion signals at a VUV photoionization wavelength of 10.379 eV. Similar images are observed at the other VUV energies reported here. Also shown are the corresponding total translational energy distributions of the fragments that were obtained following reconstruction of the images. Although the
J. Phys. Chem. A, Vol. 114, No. 42, 2010 11241 signal is weak, the most important point for the present study is that the total translational energy distribution derived from the CH3+ and HCO+ images are nearly identical. Thus, secondary dissociation of the HCO is not significant in the present context. More specifically, for an HCO dissociation energy of 0.609 eV,33 only those fragments with total Etrans < 0.428 eV (3450 cm-1) (i.e., Eavail - Etrans > 0.609 eV) even have the possibility of secondary dissociation. If secondary dissociation of HCO was important, the signal in the HCO+ translational energy distribution for Etrans between 0.0 and 3450 cm-1 in Figure 1 would be depleted relative to the same portion of the CH3+ distribution. In summary, the mass spectra in section III.B should reflect a 1:1 ratio of the neutral CH3 and HCO. This observation is important because the VUV energies are too low to ionize either H or CO, so a depletion in the HCO from secondary dissociation would go undetected in the mass spectrum. Because the present method does not selectively detect CH3 and HCO in specific rotational and vibrational states, the images and distributions in Figure 1 correspond to a convolution of the distributions for all of the states populated in the dissociation process. At 266 nm, these states are expected to include a number of different vibrational levels of both CH3 and HCO. These different vibrational states will contribute to the observed distribution over different ranges of Etrans. The similarity of the distributions from the CH3+ and HCO+ images supports the argument of section III.A that, at the VUV energies of the present study, the photoionization cross sections are only weakly dependent on the internal state of CH3 and HCO. Thus, the absolute cross section of HCO can be determined simply by measuring the relative intensity of HCO+ and CH3+. While the nonselective nature of the detection methods precludes a detailed discussion of the photodissociation dynamics, a few observations are possible. First, the isotropic character of the images in Figure 1 suggests that the dissociation is slow with respect to the rotational period of the parent. Second, the translational energy distribution spans almost 7000 cm-1, indicating a wide range of internal states of the CH3 and HCO are populated with similar probabilities. While dissociation on the T1 surface is expected to dominate at 266 nm, the distribution is broad enough that the S0 process could also play a role. Unfortunately, the combination of the lack of specificity in the ionization process and the resolution of the apparatus does not allow the assignment of the vibrational bands in the translational energy distribution or even the branching between rotational and vibrational excitation. C. The Absolute Photoionization Cross Section of HCO. Mass spectra following the photodissociation of acetaldehyde at 266 nm were obtained at three different ionization energies: 10.257, 10.304, and 10.379 eV. These spectra are shown in Figure 2. The absolute photoionization cross sections were calculated from the mass spectra by first integrating the area under the peaks corresponding to CH3 and HCO. The areas of the HCO peaks were then multiplied by 1.08 ( 0.05 to correct for the greater detection efficiency of the smaller CH3+ ion,30,31 as discussed in section II, to give the ratio of the HCO and CH3 cross sections. Using the known absolute photoionization cross section of CH3 from Gans et al.,13 the absolute photoionization cross section of HCO can then be determined. These 1.6 to absolute photoionization cross sections range from 3.7 ( 1.2 2.2 5.9 ( 1.6 Mb and are given in Table 1. The error bars are primarily determined by those of the CH3 cross sections. That the photoionization cross section of HCO is somewhat smaller than that of CH3, which is about half the size,12,13 was
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Figure 2. Mass spectra obtained following photodissociation of acetaldehyde at 266 nm and photoionization at (a) 10.379, (b) 10.304, and (c) 10.257 eV. The area under each peak was integrated and used to calculate the ratio of the HCO+ signal to the CH3+ signal. The negative peaks observed at ∼28 and ∼26 amu result from multiphoton absorption of the 266 nm laser light that generates HCO+ and C2H3+. These peaks result from incomplete background subtraction and occur at ∼26 and ∼28 amu in the mass spectrum reported because the 266 nm intersects the molecular beam ∼70 ns before the VUV pulse, so that ions produced by the 266 nm laser reach the detector earlier in time than ions of the same mass produced by the VUV laser.
TABLE 1: Absolute Photoionization Cross Sections Determined for HCO absolute photoionization cross section (Mb) VUV energy (eV)
σ(CH3)a
σ(HCO)b
10.257 10.304 10.379
5.8 ( 2.1 1.5 6.3 ( 2.3 1.7 6.4 ( 2.3 1.7
4.8 ( 2.1 1.5 5.9 ( 2.2 1.6 3.7 ( 1.6 1.2
a Reference 13. Uncertainties computed by scaling those reported for σ(CH3) ) 6.7 ( 2.4 1.8 with a photoionization energy of 10.48 eV. b Uncertainties computed by propagation of uncertainties between those for CH3 cross sections from ref 13 and those for the HCO+/ CH3+ ratio from Figure 2.
at first somewhat surprising. For example, the photoionization cross sections of N2 and CO are both approximately 20 Mb at threshold.4 These species have two electrons in their HOMOs, and HCO has only one, so the latter might be expected to have a cross section of ∼10 Mb, which is twice what is observed. However, the threshold photoionization cross section4,40 of NO, a radical that is isoelectronic with HCO, is only 2-3 Mb at threshold, much closer to the present values. Figure 3 shows plots of the HOMOs for both NO and HCO that were obtained from simple Hartree-Fock SCF calculations.41 The similarity between the two HOMOs is clear. The HOMO of O2 also has a similar character, and the cross section for direct photoionization is also quite small (2-3 Mb) close to threshold.42 The appearance of these HOMOs is similar to that of an atomic dzx or dyz orbital and suggests an explanation for the small cross sections. In particular, the photoionization matrix elements for such an atomic d orbital would result in εp and εf photoelectrons, with the latter having greater weight.40 However, it is well-known that the centrifugal barrier produced by the l ) 3, f wave often prevents significant wave function amplitude at short range.40 Thus, the d f f matrix elements are often small at threshold and only attain their maxima at higher energies above threshold. For example, the photoionization cross
Shubert and Pratt
Figure 3. Molecular orbitals for the HOMOs of NO and HCO obtained from simple Hartree-Fock SCF calculations (ref 41).
section of NO starts to grow ∼2-3 eV above threshold.40 Calculations of the photoionization matrix elements for NO reflect this substantial growth in the d f f matrix elements within the first few electronvolts of the ionization threshold.43 The somewhat greater value of the present cross section for HCO could reflect the beginning of this rise, or it may simply result from the “less pure” atomic d character of the HOMO in Figure 3. Unfortunately, measurements at significantly lower photon energies are problematic because, as the ionization energy of CH3 is approached, the assumption that the cross section is independent of the initial vibrational state of the CH3 will break down. IV. Conclusions We have presented measurements of the absolute photoionization cross section of HCO within ∼2 eV of the ionization threshold. The cross section in this region is quite small and can be rationalized by a consideration of the character of the HOMO and comparison with analogous species. In particular, the correspondence of the angular character of the HOMO with an analogous atomic orbital provides useful insight for understanding the present observations. To our knowledge, there has been no measurement of the wavelength dependence of the relative single-photon ionization cross section in the threshold region, although there has been extensive work using double resonance techniques to look at excited-state photoionization spectra.38,44 Given the relatively large energy range of interest (at least 3 eV above threshold), synchrotron-based measurements of the single-photon ionization spectrum would be most beneficial. New measurements of the photoionization spectra of many radicals are now being performed, and HCO would be a prime candidate for future study. One difficulty of such studies, however, will be the poor Franck-Condon factor for direct photoionization to the vibrationless ion exacerbated by the problem of the small electronic matrix elements, especially near threshold. In addition, the spectrum is likely to be quite complex owing to the presence of a Rydberg series converging to multiple vibrational levels of the ion. Synchrotron measurements of the CO+ signal near its ionization threshold at ∼14.0 eV would also be useful to confirm the minimal amount of secondary HCO fragmentation observed here. The present results also provide some insight into the photodissociation of acetaldehyde, the precursor used to generate the HCO radicals. The primary result is that there appears to
Photodissociation of Acetaldehyde be little, if any, secondary HCO fragmentation following photodissociation at 266 nm. The present imaging results also indicate the dissociation process is slow on the time scale of the parent dissociation, as the fragments are distributed isotropically. The translational energy distribution is quite broad and, although dissociation on the T1 surface is expected to dominate at 266 nm, we cannot rule out that dissociation on the S0 surface may play a more significant role in the photodissociation dynamics than has been previously thought. Acknowledgment. We thank S. T. Manson for helpful conversations about the photoionization cross sections of NO and HCO. 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. References and Notes (1) Oktem, B.; Tolocka, M. P.; Johnston, M. V. Anal. Chem. 2004, 76, 253–261. (2) Taatjes, C. A. J. Phys. Chem. A 2006, 110, 4299–4312. (3) Taatjes, C. A.; Hansen, N.; Osborn, D. L.; Kohse-Ho¨inghaus, K.; Cool, T. A.; Westmoreland, P. R. Phys. Chem. Chem. Phys. 2008, 10, 20– 34. (4) Berkowitz, J. Atomic and Molecular Photoabsorption; Academic Press: San Diego, 2002. (5) Cool, T. A.; Wang, J.; Nakajima, K.; Taatjes, C. A.; McIlroy, A. Int. J. Mass Spectrom. 2005, 247, 18–27. (6) Wang, J.; Yang, B.; Cool, T. A.; Hansen, N.; Kasper, T. Int. J. Mass Spectrom. 2008, 269, 210–220. (7) Flesch, R.; Schu¨rmann, M. C.; Plenge, J.; Hunnekuhl, M.; Meiss, H.; Bischof, M.; Ru¨hl, E. Phys. Chem. Chem. Phys. 1999, 1, 5423–5428. (8) Flesch, R.; Plenge, J.; Ku¨hl, S.; Klusmann, M.; Ru¨hl, E. J. Chem. Phys. 2002, 117, 9663–9670. (9) Robinson, J. C.; Sveum, N. E.; Neumark, D. M. Chem. Phys. Lett. 2004, 383, 601–605. (10) Robinson, J. C.; Sveum, N. E.; Neumark, D. M. J. Chem. Phys. 2003, 119, 5311–5314. (11) Sveum, N. E.; Goncher, S. J.; Neumark, D. M. Phys. Chem. Chem. Phys. 2006, 8, 592–598. (12) Taatjes, C. A.; Osborn, D. L.; Selby, T. M.; Meloni, G.; Fan, H. Y.; Pratt, S. T. J. Phys. Chem. A 2008, 112, 9336–9343. (13) Gans, B.; Vieira Mendes, L. A.; Boye-Peronne, S.; Douin, S.; Garcia, G.; Soldi-Lose, H.; Cunha de Miranda, B. K.; Alcaraz, C.; Carrasco, N.; Pernot, P.; Gauyacq, D. J. Phys. Chem. A 2010, 114, 3237–46. (14) Stoeckel, F.; Schuh, M. D.; Goldstein, N.; Atkinson, G. H. Chem. Phys. 1985, 95, 135–144. (15) Goldstein, N.; Atkinson, G. H. Chem. Phys. 1986, 105, 267–279.
J. Phys. Chem. A, Vol. 114, No. 42, 2010 11243 (16) Kono, T.; Takayanagi, M.; Hanazaki, I. J. Phys. Chem. 1993, 97, 12793–12797. (17) Gejo, T.; Takayanagi, M.; Kono, T.; Hanazaki, I. Chem. Phys. Lett. 1994, 218, 343–348. (18) Lee, S. H.; Chen, I. C. J. Chem. Phys. 1996, 105, 4597–4604. (19) Kurosaki, Y.; Yokoyama, K. Chem. Phys. Lett. 2003, 371, 568– 575. (20) Cruse, H. A.; Softley, T. P. J. Chem. Phys. 2005, 122, 124303. (21) Houston, P. L.; Kable, S. H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16079–16082. (22) Rubio-Lago, L.; Amaral, G. A.; Arregui, A.; Izquierdo, J. G.; Wang, F.; Zaouris, D.; Kitsopoulos, T. N.; Banares, L. Phys. Chem. Chem. Phys. 2007, 9, 6123–6127. (23) Heazlewood, B. R.; Rowling, S. J.; Maccarone, A. T.; Jordan, M. J. T.; Kable, S. H. J. Chem. Phys. 2009, 130, 054310. (24) Chen, S. L.; Fang, W. H. J. Chem. Phys. 2009, 131, 054306. (25) Lee, S. H. J. Chem. Phys. 2009, 131, 174312. (26) Huan, X.; Wentworth, P. J.; Howell, N. W.; Joens, J. A. Spectrochim. Acta, Part A 1993, 49A, 1171–8. (27) Aguirre, F.; Pratt, S. T. J. Chem. Phys. 2003, 118, 1175–1183. (28) Aguirre, F.; Pratt, S. T. J. Chem. Phys. 2005, 122, 234303. (29) Eppink, A. T. J. B.; Parker, D. H. ReV. Sci. Instrum. 1997, 68, 3477–3484. (30) Oberheide, J.; Wilhelms, P.; Zimmer, M. Meas. Sci. Technol. 1997, 8, 351–354. (31) Krems, M.; Zirbel, J.; Thomason, M.; DuBois, R. D. ReV. Sci. Instrum. 2005, 76, 093305. (32) Garcia, G. A.; Nahon, L.; Powis, I. ReV. Sci. Instrum. 2004, 75, 4989–4996. (33) See: van Mourik, T.; Dunning, T. H.; Peterson, K. A. J. Phys. Chem. A 2000, 104, 2287–2293, and references therein. (34) Dyke, J. M.; Jonathan, N. B. H.; Morris, A.; Winter, M. J. Mol. Phys. 1980, 39, 629–636. (35) Dyke, J. M. J. Chem. Soc., Faraday Trans. 2 1987, 83, 69–87. (36) National Institute of Science and Technology Chemistry WebBook at http://webbook.nist.gov/. (37) Chuang, M. C.; Foltz, M. F.; Moore, C. B. J. Chem. Phys. 1987, 87, 3855–3864. (38) Mayer, E.; Grant, E. R. J. Chem. Phys. 1995, 103, 10513–10519. (39) Schulenburg, A. M.; Alcaraz, C.; Grassi, G.; Merkt, F. J. Chem. Phys. 2006, 125, 104310. (40) Berkowitz, J. Photoabsorption, Photoionization, and Photoelectron Spectroscopy; Academic Press: New York, 1979. (41) Hyperchem release 6.03 for MS Windows; Hypercube, Inc., 2000. (42) Holland, D. M. P.; Shaw, D. A.; McSweeney, S. M.; MacDonald, M. A.; Hopkirk, A.; Hayes, M. A. Chem. Phys. 1993, 173, 315–331. (43) Wiedmann, R. T.; White, M. G.; Wang, K.; McKoy, V. J. Chem. Phys. 1993, 98, 7673–7679. (44) See, for example: Robinson, J. D.; Foltynowicz, R. J.; Prentice, K.; Bell, P.; Grant, E. R. J. Chem. Phys. 2002, 116, 8384–8395, and references therein.
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