Absolute Photoionization Cross Section of the Methyl Radical - The

May 21, 2010 - The absolute photoionization cross section of the methyl radical was determined relative to that of NO at photon energies of 10.54 eV u...
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J. Phys. Chem. A 2010, 114, 6515–6520

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Absolute Photoionization Cross Section of the Methyl Radical Jean-Christophe Loison* Institut des Sciences Mole´culaires, CNRS UMR 5255, UniVersite´ Bordeaux I, 33405 Talence Cedex, France ReceiVed: March 17, 2010; ReVised Manuscript ReceiVed: May 11, 2010

The absolute photoionization cross section of the methyl radical was determined relative to that of NO at photon energies of 10.54 eV using the CH3 + NO2 f CH3O + NO reaction. Kinetics of this reaction was studied in a fast flow reactor coupled with VUV laser photoionization. Simulation of the kinetics of the decrease of the methyl signal and the corresponding increase of the NO signal (in combination with the NO absolute photoionization cross section determined by Watanabe (Watanabe, K. J. Chem. Phys. 1954, 22, 1564; Watanabe, K.; Matsunaga, F. M.; Sakai, H. Appl. Opt. 1967, 6, 391)), yields the absolute photoionization cross section of the methyl radical: σCH3(10.54 eV) ) 5.1 (1.2) × 10-18 cm2 (95% confidence interval). This result is in good agreement with the recently published measurements by Taatjes et al. (Taatjes, C. A.; Osborn, D. L.; Selby, T. M.; Meloni, G.; Fan, H.; Pratt, S. T. J. Phys. Chem. A 2008, 112, 9336) and by Gans et al. (Gans, B.; Mendes, L. A. V.; Boye´-Pe´ronne, 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 2009, 114, 3237). I. Introduction The use of photoionization mass spectrometric measurements in chemical kinetics or combustion measurements requires values of the absolute photoionization cross sections of molecules and radicals to be known. Absolute photoionization cross sections of various molecules (or stable radicals such as NO) have been measured earlier [(see for instance refs 1-7). However there are fewer absolute photoionization cross section determinations for reactive radicals. One of the main difficulties is to determine the absolute concentration of radicals. One way to circumvent this issue is to produce the desired radical in conjunction with the simultaneous appearance or loss of a chemical species with a known ionization cross section. One convenient technique is to use the photodissociation of a suitable precursor, while simultaneously monitoring the appearance of the radical and the depletion of the precursor (for instance Taatjes et al. used acetone or methyl vinyl ketone8) or by comparing the relative appearance of both fragments with one of them having a known ionization cross section. This last technique has been used by Flesh et al.9,10 to determine the absolute photoionization cross section of ClO by photodissociating ClO2 and Cl2O, making use of the known absolute photoionization cross sections of O and Cl. This technique has also been used by Neumark et al. to determine absolute photoionization cross sections of vinyl,11 propargyl,11 allyl,12 2-propenyl,12 and phenyl radicals,13 followed by FitzPatrick et al.14 for the ethyl radical using the known absolute photoionization cross section of Cl. Very recently, Taatjes et al.8 also used this technique to measure the absolute photoionization cross section of CH3 at 10.2, 10.466, 10.471, and 11.0 eV using the known absolute photoionization cross section of I atoms. Instead of photodissociation, Gans et al.15 used a pyrolysis source of radicals to measure the absolute photoionization cross section of CH3 at 10.49 eV comparing CH3 and HI signals with the corresponding depletion of the CH3I precursor signal. The references here were the known absolute photoionization cross * Corresponding author. E-mail: [email protected].

sections of CH3I and HI. Their results are in good agreement with the measurements of Taatjes et al.8 The goal of the present work is the measurement of absolute photoionization cross section of CH3 at 10.54 eV using VUV laser photodissociation in conjunction with the CH3 + NO2 f CH3O + NO reaction in a fast flow reactor, using the known absolute photoionization cross section of NO1,2 as a reference. The CH3 radical was produced in situ using the rapid CH4 + F f CH3 + HF reaction. The fast flow reactor technique was chosen as it allows us to easily control the kinetic parameters and gives us the opportunity to furnish an alternative measurement of the absolute photoionization cross section. II. Experimental Section The experimental setup used in this study has been detailed elsewhere,16 and only a brief description is thus given. It is comprised of a fast flow reactor coupled to a time of flight mass spectrometer (TOFMS) from R.M Jordan Co., D850 Reflectron. A. Experimental Setup. The fast flow reactor consists of a main tube and two concentric injectors. The main tube is a 24 mm internal diameter/65 cm long quartz tube, inside which was mounted a sliding movable double injector. The double injector consists of two pieces. The outer injector is a 12 mm internal diameter/90 cm long glass tube ending 2-4 cm before the end of the inner injector. The outer injector is used to introduce CH4 molecules. The inner injector is a 6 mm internal diameter/ 90 cm long glass tube with a showerhead mixer and is used to introduce NO2 molecules. The F atoms are produced by a microwave discharge at 2450 MHz (Sairem GMP 03 KSM) in a mixture of 1% F2 in He (Linde), introduced into the 24 mm main reactor 50 cm before the reaction zone. Using electron beam ionization, we measure that 100% of the F2 molecules were dissociated and a typical F atom initial concentration present in the flow tube ranged from (2 to 10) × 1012 molecules cm-3. Methane gas (Linde 5.5) is premixed in a secondary flow of He gas in large excess with respect to the F concentration (typically [CH4] ) (1-10) × 1014 molecules cm-3) and is introduced through the medium tube (12 mm). NO2 gas (1% in He, AlphaGaz; typically [NO2] ) 1 × 1014 molecules cm-3) is

10.1021/jp1024312  2010 American Chemical Society Published on Web 05/21/2010

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premixed in a third flow of He gas and is introduced through the injector (tube of 6 mm with a showerhead mixer). The flow in the injector is kept constant for the experiment with and without NO2, by adjusting the He flow, to ensure constant flow conditions in the reactor. Gas flow rates were measured with carefully calibrated Tylan FC-2900 mass flow controllers. The pressure in the flow reactor was measured using an Edwards capacitance manometer. Typical flow velocities ranged from 7 to 31 m s-1. The total exiting gas was pumped away through a two-stage Edwards primary pump (300 m3 h-1). The double injector was used to produce CH3 from CH4 + F f CH3 + HF before injection of NO2 to avoid not only the cross reaction between F atoms and NO2 but to ensure that CH3 radicals had undergone several collisions to equilibrate to ambient temperature before reacting with NO2. The energy partitioning of the F + CH4 f HF + CH3 (∆H ) -31.85 kcal/ mol)17 reaction has been experimentally studied.18 Most of the energy is localized in the HF vibration with very little in the CH3 vibrations or rotations. At 1.2 Torr of He in the presence of 1 mTorr of CH4, we can assume that all internal degrees of freedom of the CH3 radical have been thermalized before reacting with NO2 molecules. In our conditions of first order kinetics with CH4 introduced in large excess, we can neglect secondary reactions between the F atom and CH3 radicals. We verify experimentally this hypothesis by attempting to observe CH2, CH2F, or CHF radicals. In our usual experimental conditions, we were not able to detect any of these species, with these three radicals being the products of the CH3 + F reaction19 with ionization potentials lower than the 10.54 eV photons used in this work (CH2 (10.3864 ( 0.0004 eV),20,21 CH2F (9.04 ( 0.10 eV),22 and CHF (10.06 ( 0.05 eV)23). The interface between the reactor and the mass spectrometer consists of a differentially pumped orifice-skimmer combination. The reacting gases at 0.6-2 Torr total pressure are expanded through a 1.0 mm diameter homemade Kel-F skimmer (a small cone with a 2 cm base and a 2 cm height with a sampling orifice of 1 mm at the end of the cone)24 into a region where the background pressure is maintained at 1 × 10-3 Torr using a Alcatel T550 turbo pump (450 L s-1 for He). The centerline portion of the expanded jet passed through a 2.1 mm skimmer (Beam Dynamics) aperture into the ionization chamber, which was maintained at less than 10-5 Torr by a Varian turbo pump (550 L s-1 for He). Ionized particles are extracted and sent to the detector through a 820 mm reflectron, maintained at the pressure of 1 × 10-7 Torr, by a Varian turbo pump (150 L s-1 for He). Ions are detected through microchannel plate detectors (MCP, C-726; 40 mm active area). A mass resolution R50% of 1200 m/z (measured at 30 m/z) was achieved. Electrical signals produced by ion detection are acquired by a numerical oscilloscope (TDS 3000 from Tektronix) and are sent to a microcomputer. B. Ionization Method. The photons (117.6 nm (10.54 eV)) used in this study are generated by tripling a UV laser beam in a rare gas (30 Torr of Xe). To produce 117.6 nm photons we first produced 706 nm photons by pumping a dye (LDS720) laser (Quantel TDL 50) with the second harmonic of a 10 Hz Nd:YAG laser (Quantel YG 581C) with a 8 ns pulse. The resultant photons from the dye laser (706 nm) are doubled to 353 nm using a BBO crystal, the residual 706 nm wavelength being separated using a dichroic mirror. This UV beam is focused using a quartz lens (f ) 150 mm) in a tripling cell made with a quartz tube. The entrance window is a quartz window. The exit window of the tripling cell is in fact a MgF2 lens glued on the tripling cell. The tripling cell is filled with a rare gas (30

Loison Torr of xenon (from Linde)) and allows frequency tripling of the 353 nm beam by third harmonic generation. The exit MgF2 lens (f ) 75 mm at 121.6 nm) allows the 117.6 nm beam to be collimated into the ionization region. To avoid residual photons at the 353 nm wavelength participating in the ionization process, we locate the tripling cell 120 cm from the ionization region (in an evacuated side arm) and use the difference of the dispersion coefficient of the MgF2 lens to disperse the 353 nm beam and minimize its effect. By pumping the tripling medium (leading to a complete disappearance of the ion signal), we ruled out the multiphoton ionization with 353 nm alone. Also, by varying the position of the quartz lens (f ) 150 mm) as well as the Xe pressure in the tripling cell, i.e., mean varying the focus of the VUV and residual UV beam as well as the VUV intensity, we checked that that the small amount of residual UV photons do not interfere with the ionization process as the ratio of ion signals (CH3+ and NO+ for instance) is constant for the various conditions. C. Measurement of the Mass Discrimination Factor of the Detection System. In our experimental setup, the ion signal of a species X, SX, is given by SX ) N[X]FXσX, where N is the normalization factor of the apparatus function, [X] is the concentration, FX is the mass discrimination factor, and σX is by the absolute ionization cross section. To determine the mass discrimination effect25 that depletes the concentrations of light mass molecules relative to heavier mass molecules, we calibrate the response of our molecular beam TOFMS system in the 4-136 amu mass range using atoms and molecules ionized with an electron beam at 50 eV or using molecules photoionized at 10.54 eV with known photoionization cross sections. The concentrations of the various gases are determined using known gas flow rates measured with carefully calibrated Tylan FC2900 mass flow controllers (precision of (5%). For electron ionization calibration we use He, σ(He, 50 eV) ) (0.22 ( 0.02) × 10-16 cm2;26 CH4, σ(CH4 f CH4+, 50 eV) ) (1.63 ( 0.08) × 10-16 cm2 27 and σ(CH4 f CH3+, 50 eV) ) (1.29 ( 0.06) × 10-16 cm2;27 NH3, σ(NH3 f NH3+, 50 eV) ) (1.30 ( 0.30) × 10-16 cm2)28 and σ(NH3 f NH2+, 50 eV) ) (1.12 ( 0.20) × 10-16 cm2;28 NO, σ(NO f NO+, 50 eV) ) (2.00 ( 0.40) × 10-16 cm2;29,30 Ar, σ(Ar, 50 eV) ) (2.53 ( 0.15) × 10-16 cm2 26 and Kr, σ(Kr, 50 eV) ) (3.45 ( 0.20) × 10-16 cm2.26 The value of the mass discrimination factor for He should be used with care as He is the main flow. For photoionization calibration we use NH3 (σ(NH3,10.54 eV)) (2.9 ( 0.4) × 10-18 cm2,31 NO (σ(NO, 10.54 eV) ) (2.1 ( 0.2) × 10-18 cm2,1,2 propene (σ(C3H6,10.54 eV) ) (11.0 ( 1.0) × 10-18 cm2,5 butene (σ(C4H8,10.54 eV) ) (10.0 ( 1.0) × 10-18 cm2,7 and acetone (C3H6O, 10.54 eV) ) (11.2 ( 1.0) × 10-18 cm2.6 The mass discrimination factor has been found to be slightly dependent on the fast flow reactor pressure, and the values for a typical pressure of 1.0 Torr are shown in Figure 1 with data from laser ionization as 0 and data from electron beam ionization as b. III. Results Typical VUV (10.54 eV) SPI mass spectra of the CH3 radical with and without NO2 are shown in Figure 2. The spectrum in gray represents the mass spectrum of NO2 alone (shifted by 0.4 g/mol) to indicate the amount of NO+ as a result of NO2 photodissociation (see below). The only products observed in this experiment are CH3+ at m/z 15.02, NO+ at m/z 30.00, and NO2+ at m/z 45.99. The other products of the CH3 + NO2 reaction, CH3O and CH3NO2 were never detected as their ionization energies (IE) are above the 10.54 eV photons used in this study, 10.73 eV for CH3O32,33 and 10.92 eV for

Photoionization Cross Section of Methyl Radical

Figure 1. Calibration of the mass discrimination factor of the TOF mass spectrometer relative to NO. 0 represent the experimental values obtained with laser ionization and b represent experimental values obtained with electron beam ionization. Errors bars represent 95% confidence intervals.

Figure 2. Mass spectra obtained at 10.54 eV for the CH3 radical alone (top) and in the presence of NO2 (bottom). The spectrum in gray (bottom) represents the mass spectrum of NO2 alone (shift by 0.4 g/mol) to indicate the amount of NO+ coming from the NO2 photodissociation process.

CH3NO2.34 The absence of signals at m/z 14.01, at m/z 33.01, and at m/z 32.01 shows the absence of CH2, CH2F, and CHF species, which have IEs lower than 10.54 eV, indicating that the CH3 + F reaction does not interfere in this study. To measure the ionization cross section of the CH3 radical by the ratio of CH3 depletion and NO appearance, we choose to study the kinetics of the reaction CH3 + NO2 f CH3O + NO by simulating the various effects involved in this kinetic process. Typical kinetics of CH3 disappearance and NO appearance are shown in Figure 3 together with their simulations. As the ion signal of a species X, SX, is given by SX ) N[X]FXσX, the experimental concentration points plotted in Figure 3 are the area time of flight peaks of the ions (SX) divided by the product of the mass discrimination factor (FX) times the absolute

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Figure 3. Typical kinetics of CH3 disappearance and NO appearance with their respective simulations. 0 represent experimental CH3 concentrations when no NO2 is added, O represent experimental CH3 concentrations when NO2 is added ([NO2] ) 8 × 1013 molecules cm-3) and b represent experimental NO concentrations. The solid line represents the simulation of CH3 concentration when no NO2 is added, the dashed line represents the simulation of CH3 disappearance when NO2 is added, and the dot-dashed line represents the simulation of NO appearance. To compare the kinetics of CH3 radicals and NO, the experimental CH3 points are multiplied by (σNOFNO)/(σCH3FCH3) relative to the NO signal (F is the mass discrimination factor).

ionization cross section (σX) times the normalization factor of the apparatus function (N), which include the intensity of the ionizing photon beam. The normalization factor of the apparatus function (N) is determined for each scan by fitting the NO+ signal to the [NO] theoretical curve given by the simulation. The only parameter to be determined is the ionization cross section of the methyl radical. The simulations are performed using the rate constants summarized in Table 1. To determine the ionization cross section of CH3 radicals using the CH3 + NO2 f CH3O + NO reaction, we need to know the branching ratio of this channel. Indeed, the CH3 + NO2 reaction has four possible exit channels, two pressure-dependent ones leading to CH3NO2 and CH3ONO and two pressure-independent ones yielding H2CO + HNO and CH3O + NO.35,36 However, experimental and theoretical studies35-38 have shown that only two exit channels play a role, the formation of CH3O + NO and, for a minor fraction, the formation of CH3NO2. The CH3NO2 molecule is not detectable in this study as its IE (11.08 eV)39 is higher that the photon energy used in this work. However, McCaulley et al.37 have measured third-order recombination rate coefficients yielding a branching ratio of CH3NO2 equal to 7 ( 2% at a total pressure of 1 Torr of He. Wollenhaupt and Crowley38 have performed a simultaneous fit on all experimental determinations of the rate constant yielding an higher branching ratio of CH3NO2 equal to 23% at a total pressure of 1 Torr of Ar. Some of the discrepancy is related to the fact that Ar is a more efficient third-body collisional quencher than He. However there still exists some controversy over the difference between the strong pressure dependence of the theoretical estimation and the fact that the experimental rate constant is dominated by the pressure independent bimolecular reaction even at low pressure. As our experimental measurements of the CH3 ionization cross section show no real pressure dependence between 0.6 and 2 Torr in agreement with rate constant measurement between 1 and 7 Torr,36 we use a

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TABLE 1: Reaction Mechanism Used in Simulations of the Kinetics for the CH3 + NO2 System at 1 Torr Total Pressure of He reaction

k298K (cm3 molecule-1 s-1)

ref

Main Reactions

CH3 + NO2 f CH3O + NO

2.1(0.5) × 10-11

Yamada et al.,47 Biggs et al.,36 Wollenhaupt and Crowley38

CH3 + NO2 f CH3NO2

2.1(1.1) × 10-12

McCaulley et al.,37 Wollenhaupt and Crowley38

CH3 + CH3 f C2H6

3.6(0.4) × 10-11

Wang et al.48

CH3O + NO2 f H2CO + HONO

Secondary Reactions McCaulley et al.,41 Wollenhaupt and Crowley38 2.0 × 10-13

CH3O + NO2 f CH3ONO2

1.8 × 10-12

CH3 + NO f CH3NO

Minor Cross Reactions Kaiser49 1.1 × 10-13

CH3 + CH3O f H2CO + CH4

4.0 × 10-11

Tsang and Hampson50

CH3 + CH3O f CH3OCH3

1.0 × 10-11

Tsang and Hampson50

CH3O + CH3O f H2CO + CH3OH

1.0 × 10-10

Tsang and Hampson50

CH3O + NO f H2CO + HNO

3.0 × 10-12

Caralp et al.,51 Atkinson et al.52

CH3O + NO f CH3ONO

8.0 × 10-13

Caralp et al.51

branching ratio measurement for the CH3 + NO2 f CH3O + NO reaction equal to 90 ( 5% at a total pressure of 1 Torr of He. In addition to CH3O and CH3NO2, products of the CH3 + NO2 reaction with IEs above 10.54 eV, the products of the most important secondary reaction, CH3O + NO2, which are CH3ONO2,36,38,40 H2CO,38,41 and HONO,38,41 are also undetectable for the same reason. Indeed, the IEs are 11.53 eV34 for CH3ONO2, 10.88 eV39 for H2CO, and 10.97 eV42 for HONO. If these products cannot be ionized in this study, they can absorb one photon leading to dissociation. It is important to estimate the effect of the CH3 or NO radicals produced by these photodissociations on the measurement of ionization cross section. We can see in Figure 2, a small amount of NO+ signal coming from NO2 photodissociation. This NO+ signal cannot be a result of NO2+ fragmentation as the NO2+ f NO++ O limit is 12.38 eV43 and the NO2+ f NO+ + O- limit is 10.92 eV.43,44 The only two possibilities come from small NO impurities in the NO2 mixture and/or VUV photodissociation of NO2 at 10.54 eV, leading to NO + O, followed by NO photoionization at 10.54 eV. As the NO+ ion signal is dependent on the VUV beam intensity and focusing conditions, the photodissociation process plays a role. We use the amount of NO+ signal coming from NO2 photodissociation to estimate the role of the eventual photodissociation of other molecules or radicals produced in this study. The photoabsorption cross section of NO2 at 10.54 eV is measured to be equal to 10.3 × 10-18cm2,45 leading mainly to NO + O.46 The photoionization cross section of NO2 at 10.54 eV leading to NO2+ has been measured by Au and Brion45 equal to 0.13 × 10-18 cm2,45 and by comparison with C3H6 and NO, I have determined the photoionization cross section of NO2 equal to (0.24 ( 0.06) × 10-18 cm2 (the difference could come from the fact that Au and Brion45 used dipole (e, e) spectroscopy with 1 eV fwhm and as 10.5 eV is the appearance threshold of NO2+; the deconvolution of the photoionization cross section leads to large uncertainties). Considering the photoionization cross section of NO2 and NO and the mass discrimination factor and considering also that all the NO+ signal coming from NO2 is due to photodissociation, we measure in our experimental

Biggs et al.,36 Wollenhaupt and Crowley,38 Martinez et al.40

VUV focus condition that we photodissociate 1% at most of the NO2. We can now estimate the role of the photodissociation of the various products in our system. As simulations of the kinetics give a good fit to the CH3 concentration, particularly for high NO2 concentrations for which CH3 concentrations reach near zero values, production of the CH3 radical by photodissociation of products is very limited and can therefore be neglected. The problem is different for NO however, as it is difficult to deconvolute its production via primary product channels from its production via secondary reactions. However, simulations of the kinetics gives concentrations of species susceptible to produce NO by photodissociation, such as CH3NO2, CH3ONO2, HONO, and CH3NO, as being always inferior to 10% of the NO concentration from the CH3 + NO2 reaction. In our VUV beam conditions we photodissociate at most 1% of the NO2, NO2 having a photoabsorption cross section of 10.3 × 10-18 cm2 45 at 10.54 eV. So considering that the various species producing (eventually) NO by photodissociation have photoabsorption cross sections inferior to 100 × 10-18 cm2 (seems more likely), the eventual NO production by photodissociation is always inferior to 1% of the NO concentration from the CH3 + NO2 reaction and can be completely neglected. We therefore neglect all photodissociation processes except the NO2 f NO + O which we measure by recording the NO+ signal without CH3 production in each experiment. The direct and indirect (by photodissociation) effect of secondary reactions is very low, the only one being measurable is the CH3 + CH3 reaction when no NO2 is added. The influence of secondary processes, including photodissociation, is estimated to be inferior to 5%. We perform kinetics on the disappearance of CH3 and the appearance of NO for various conditions (for example, varying the CH3 concentration in the 2-10 × 1014 molecules cm-3 range). We subtract the NO signal coming from NO2 photodissociation, and we fit the CH3 and NO signals using the rate constants in Table 1, the only parameter being adjustable is the photoionization cross section of the CH3 radical relative to the photoionization cross section of NO. The result of our measurements yields the photoionization cross section of the CH3 radical relative to the photoionization cross section of NO at 10.54 eV

Photoionization Cross Section of Methyl Radical

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TABLE 2: Comparison of the Various Values of the Ionization Cross Section of the CH3 Radical reaction

reference for the ionization cross section

energy (eV)

σCH3 (10-18 cm2)

ref

acetone + hν

acetone

10.2

5.9 ( 1.4

Taatjes et al.8

methyl vinyl ketone + hν

methyl vinyl ketone

10.2

5.6 ( 1.1

Taatjes et al.8

CH3I + hν

I

10.460

5.4 ( 2.0

Taatjes et al.8

CH3I + hν

I

10.466

5.5 ( 2.0

Taatjes et al.8

CH3I + hν

I

10.471

4.9 ( 2.0

Taatjes et al.8

CH3I + hν

CH3I

10.49

6.7+2.4-1.8

Ganz et al.15

CH3 + NO2

NO

10.54

5.1 ( 1.2

this study

acetone + hν

acetone

11.0

6.0 ( 1.8

Taatjes et al.8

equal to (2.43 ( 0.51). Using the 1967 Watanabe et al.2 absolute photoionization cross section of NO at 10.54 eV (σ ) 2.1 ( 0.2 × 10-18 cm2), we obtain the absolute photoionization cross section of the methyl radical at 10.54 eV: σCH3(10.54 eV) ) (5.1 ( 1.2) × 10-18 cm2. The errors bars (95% confidence interval) reflect not only the statistical uncertainty of the measurement but also the three major sources of uncertainty in this study, the uncertainty of the branching ratio of the CH3 + NO2 f CH3O + NO reaction (90 ( 5%), the uncertainty of the mass discrimination factor (FCH3/FNO ) 0.69 ( 0.08), and the estimated uncertainty of the absolute photoionization cross section of NO at 10.54 eV (σ ) 2.1 ( 0.2 × 10-18 cm2).1,2 Considering the (rather small) exothermicity of the CH3 + NO2 f NO + CH3O reaction (∆H ) -17.21 kcal/mol),17 we neglect the eventual effect of the vibrational excitation of NO on the absolute photoionization cross section of NO. As the photoionization spectrum of the CH3 radical shows no structure and almost no variation between 10.3 and 11.0 eV,8,15 we can directly compare our measurement with those of Taatjes et al.8 and Gans et al.15 The various results are compared in Table 2 showing good agreement between all measurements. The method developed in this study is therefore a reliable alternative method to use photodissociation to measure absolute ionization cross section of radicals and will be used in further studies to determine absolute ionization cross section of C2H5 and C3H7 radicals using the C2H5 + NO2 and C3H7 + NO2 reactions. References and Notes (1) Watanabe, K. J. Chem. Phys. 1954, 22, 1564. (2) Watanabe, K.; Matsunaga, F. M.; Sakai, H. Appl. Opt. 1967, 6, 391. (3) Koizumi, H. J. Chem. Phys. 1991, 95, 5846. (4) Person, J. C.; Nicole, P. P. J. Chem. Phys. 1968, 49, 5421. (5) Person, J. C.; Nicole, P. P. J. Chem. Phys. 1970, 53, 1767. (6) Cool, T. A.; Wang, J.; Nakajima, K.; Taatjes, C. A.; McLlroy, A. Int. J. Mass Spectrom. 2005, 247, 18. (7) Wang, J.; Yang, B.; Cool, T. A.; Hansen, N.; Kasper, T. Int. J. Mass Spectrom. 2008, 269, 210. (8) Taatjes, C. A.; Osborn, D. L.; Selby, T. M.; Meloni, G.; Fan, H.; Pratt, S. T. J. Phys. Chem. A 2008, 112, 9336. (9) Flesch, R.; Schu¨rmann, M. C.; Plenge, J.; Hunnekuhl, M.; Meiss, H.; Bischof, M.; Ru¨hl, E. Phys. Chem. Chem. Phys. 1999, 1, 5423. (10) Flesch, R.; Plenge, J.; Kuhl, S.; Klusmann, M.; Ruhl, E. J. Chem. Phys. 2002, 117, 9663. (11) Robinson, J. C.; Sveum, N. E.; Neumark, D. M. J. Chem. Phys. 2003, 119, 5311. (12) Robinson, J. C.; Sveum, N. E.; Neumark, D. M. Chem. Phys. Lett. 2004, 383, 601. (13) Sveum, N. E.; Goncher, S. J.; Neumark, D. M. Phys. Chem. Chem. Phys. 2006, 8, 592. (14) FitzPatrick, B. L.; Maienschein-Cline, M.; Butler, L. J.; Lee, S. H.; Lin, J. J. J. Phys. Chem. A 2007, 111, 12417.

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