Tabletop Femtosecond VUV Photoionization and PEPICO Detection

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Tabletop Femtosecond VUV Photoionization and PEPICO Detection of Microreactor Pyrolysis Products David Edward Couch, Grant T Buckingham, Joshua H. Baraban, Jessica P Porterfield, Laura A Wooldridge, G. Barney Ellison, Henry C. Kapteyn, Margaret M Murnane, and William K. Peters J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b02821 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on July 5, 2017

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The Journal of Physical Chemistry

Tabletop Femtosecond VUV Photoionization and PEPICO Detection of Microreactor Pyrolysis Products David E. Couch,1,ø Grant T. Buckingham,2,ø,† Joshua H. Baraban,2 Jessica P. Porterfield,2,‡ Laura A. Wooldridge,1 G. Barney Ellison,2 Henry C. Kapteyn,1 Margaret M. Murnane1 and William K. Peters1,*

AFFILIATION 1

JILA and Department of Physics, University of Colorado, Boulder, Colorado 80309, United

States 2

Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States

ø

DEC and GTB contributed equally to this work.

†Current Address: Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡Current Address: Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, United States and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138

CORRESPONDING AUTHOR *William K. Peters: JILA, University of Colorado 440 UCB Boulder, CO 80309 E-mail: [email protected]. Phone: 303-492-7789. Fax: 303-492-5235.

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ABSTRACT We report the combination of tabletop vacuum ultraviolet photoionization with photoion-photoelectron coincidence spectroscopy for sensitive, isomer-specific detection of nascent products from a pyrolysis microreactor.

Results on several molecules demonstrate two

essential capabilities that are very straightforward to implement: the ability to differentiate isomers, and to distinguish thermal products from dissociative ionization.

Here vacuum

ultraviolet light is derived from a commercial tabletop femtosecond laser system, allowing data to be collected at 10 kHz; this high repetition rate is critical for coincidence techniques. The photoion—photoelectron coincidence spectrometer uses the momentum of the ion to identify dissociative ionization events, and coincidence techniques to provide a photoelectron spectrum specific to each mass, which is used to distinguish different isomers. We have used this spectrometer to detect the pyrolysis products that result from the thermal cracking of acetaldehyde, cyclohexene, and 2-butanol. The photoion—photoelectron spectrometer can detect and identify organic radicals and reactive intermediates that result from pyrolysis. Direct comparison of laboratory and synchrotron data illustrate the advantages and potential of this approach.



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INTRODUCTION

Despite over 100 years of active research, the chemical mechanisms underlying common combustion phenomena are still incompletely understood. Even simple systems display astonishing chemical complexity – the pyrolysis of a single small molecule can lead to several dozen products,1 each of which has its own formation and destruction kinetic pathways. Furthermore, there are difficulties not only in measuring the concentrations of reactive intermediates, but also in merely identifying these transient species. Such measurements must be made before even considering a study of reaction rate coefficients. The use of a wide variety of optically-detected laser spectroscopies (laser-induced fluorescence (LIF), coherent anti-Stokes Raman scattering (CARS), for example)2 has allowed significant progress, but serious challenges still remain in identifying unknown species. Consequently, ion detection schemes are typically favored to detect and identify unknown trace polyatomics (see, for example, the appendix to chapter 2 of ref. 3)

Traditional simple ion-detection schemes, such as photoionization-mass spectrometry (PIMS) with a fixed-wavelength light source, offer high sensitivity and rely on the mass-tocharge ratio (m/z) to identify target molecules.4,5 However, two ambiguities arise in such experiments: constitutional isomers cannot be distinguished, and fragmentation after ionization (known as dissociative ionization) can obscure interpretation of the mass spectrum. Ideally, any additional technique used to address these issues would retain the sensitivity of PIMS. It has recently become popular to perform more sophisticated PIMS experiments at synchrotron facilities,6,7 where the ability to scan the photon energy through the molecule’s


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ionization threshold can typically be used to resolve both of these ambiguities while maintaining excellent sensitivity. An alternative approach to tuning the photon wavelength is photoelectron-photoion coincidence (PEPICO) detection.8–12 In PEPICO spectroscopy, the photoelectron count rate is deliberately kept low (compared to PIMS) so that each detected photoelectron can be registered in coincidence with the corresponding ion. This provides a photoelectron spectrum for each mass, which can be used as a fingerprint for identifying isomers, thus solving one of the two potential problems in a PIMS experiment. In this paper, the phrase “traditional” PIMS will be used to refer to PIMS recorded with a fixed-wavelength light source (such as the 9th harmonic of a Nd:YAG laser), and without detecting coincident electrons.

Dissociative ionization can be identified in a time-of-flight (TOF) PIMS measurement if the instrument is operated in a similar, but dynamically different, configuration. Typically, for example in a Wiley-McLaren13 or reflectron14 design, TOF-PIMS relies on large extraction fields on the order of 500-1000 V/cm to optimize mass resolution. The large electric field minimizes the broadening effect that the initial momentum (i.e. the velocity distributions due to thermal motion or the translational energy from a dissociation event) of the target molecule has on measured time-of-flight. Rather than using a large extraction field to optimize mass resolution, it is possible to keep the electric field relatively low (about 5 V/cm) while collecting a mass spectrum and still retain the dynamical information present in the momentum distribution of the fragment. Indeed, such experiments have been used to study the process of dissociative ionization for almost 50 years,15–17 and can effectively distinguish between dissociative ionization events and parent ions. In these experiments the velocity spread of a dissociation 5 ACS Paragon Plus Environment


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event causes the time of flight peak to broaden significantly, with ions emitted toward the detector arriving sooner than those initially moving away from the detector. The cost of optimizing the instrument for momentum resolution is a loss of mass resolution; the final mass resolution depends on the kinetic energy released in the dissociative ionization event, and we commonly observe a single peak that spans several m/z. We will refer to TOF instruments with large electric fields, optimized for mass resolution, as “mass-resolved” and those with small electric fields, optimized for momentum resolution, as “momentum-resolved.”

Recent advances in VUV light source technology allow the above momentum-resolved PEPICO measurements to be performed with a tabletop laser rather than a synchrotron facility. In particular, the process of high harmonic generation yields a tabletop source of ionizing radiation with photons ranging from 5 eV to 1500 eV.18 This process relies upon the very high (roughly 1014 W/cm2) peak intensity of modern amplified femtosecond laser pulses. The high intensity laser field pulls an electron away from an atom, then a half-cycle later the reversed field pushes the electron back to recombine with the original atom.19 The excess energy of the electron caused by acceleration in the laser’s electric field is emitted in a single high-energy photon. Linearly-polarized high harmonic generation in a noble gas produces a set of discrete vacuum-UV wavelengths containing only odd harmonics of the driving laser. In many cases, a single harmonic can be isolated and used to ionize a sample, providing the same mass spectrum that would be recorded using traditional light sources. The wide range of wavelengths available from high harmonic generation provides much-needed flexibility in identifying combustion products. Low harmonics (5-10 eV) can be used to selectively ionize molecules with lower ionization energies, avoiding excess photon energy that can lead to unwanted complications. 6 ACS Paragon Plus Environment

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Alternatively, one can choose higher energy photons to ionize (or even doubly ionize) molecules having higher ionization energies. In either case, the resulting ions can be detected with conventional time of flight mass spectrometry.

Figure 1. Illustration of tabletop VUV-based momentum-resolved PEPICO detection of reactive intermediates. An amplified femtosecond laser pulse is converted to the VUV by high harmonic generation in a hollow waveguide. The VUV pulse ionizes a molecular beam of reactive intermediates generated by pyrolysis of CH3CHO in a heated micro-reactor. PEPICO detection provides a momentum-resolved mass spectrum, as well as a unique photoelectron spectrum for each peak in the mass spectrum.

In this paper, we demonstrate the use of a tabletop VUV harmonic light source combined with momentum-resolved PEPICO detection to identify transient chemical species present in



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pyrolysis processes. We explore the pyrolysis of three species: cyclohexene, acetaldehyde, and 2-butanol. The work shown here uses a microtubular reactor (“Chen nozzle”) as the pyrolysis source. These heated silicon carbide (SiC) capillary tubes (1 mm I.D.), originally used as a source of organic radicals for spectroscopic analysis,20,21 have emerged as a tool for studying the early steps in pyrolytic chemistry.22,23 A key advantage of these micro-reactors is that it is possible to thermally decompose samples over short time scales (25-150 microseconds), so that only the initial decomposition is observed. When combined with computational fluid dynamical modeling of the flow through the capillary, temperature and pressure profiles can be determined and experimental results can be compared to chemical reaction models.24–26 The ability to carry out these measurements with a lab-scale apparatus allows routine execution of experiments and research projects that would otherwise require use of a synchrotron facility.

EXPERIMENTAL

The light source for this experiment is an amplified Ti:sapphire laser (Dragon, KMLabs, Inc.) producing 35 fs pulses at a wavelength of 780 nm with 1.5 mJ energy. The system runs at a repetition rate of 10 kHz; this high rate is critical for coincidence experiments, which require fewer than one event per pulse. Part of the laser output (200-300 μJ) is used to generate VUV pulses by focusing the beam into a xenon-filled hollow waveguide, producing odd harmonics of the driving frequency (5ω to about 19ω, depending on pulse energy, with no amplitude detected between odd harmonics at a signal-to-noise ratio of 1000).27 Harmonics are generated on the leading edge of the laser pulse, which also experiences self-phase modulation and blueshifting due to ionization of the gas. Thus they can shift in energy slightly during the HHG


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process,28,29

and must be independently measured. In this work harmonic energies are

determined from the photoelectron spectra of several known compounds (formaldehyde, acetone and methyl radical). Unwanted harmonics are filtered out by passing the beam through a 10 cm long region filled with an absorbing gas, illustrated in figure 4a. For example, 3 Torr of krypton removes harmonics above 7ω (leaving 5ω = 7.9 eV and 7ω = 11.2 eV) while argon is used to block harmonics above 9ω (leaving 5ω, 7ω and 9ω = 14.4 eV). In this work, we primarily use 11.2 eV (7ω), which has a FWHM bandwidth of 0.25 eV.

After producing the harmonics, the majority of the residual driving laser field is removed by two cylindrical silicon mirrors (Rocky Mountain Instrument Co.), while efficiently reflecting the low-order harmonics. The mirrors are placed 1 m from the waveguide exit and image the harmonics into the ionization region 1 m ahead. To compensate for astigmatism, one mirror has a 4 m radius of curvature while the other has a 0.25 m radius of curvature (both are placed at Brewster’s angle for the driving laser, 76°). Using Fresnel equations with the known complex index of refraction of silicon,30 two bounces at Brewster’s angle should reflect 10-7 of the fundamental (neglecting bandwidth) and 50% of 7ω. We find about 5 mW of the fundamental reflects (roughly 5x10-3 of the 50% that makes it through the waveguide and differential pumping apertures). No ionization events are detected if only this residual driving field is present (tested either by evacuating the waveguide or by inserting a 0.1 μm glass filter, Lebow Inc., into the beam path). Harmonic flux is estimated from the number of ion counts for a molecule of known cross section. Under the conditions used in this paper, where the flux has been kept low to avoid false coincidences, we typically have 1010 photons/sec at 7ω (as noted in the discussion section, it is possible to produce VUV HHG with 104 times more flux31). 9 ACS Paragon Plus Environment


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To measure electron and ion energies in coincidence, we use an imaging photoelectronphotoion coincidence (PEPICO) spectrometer that has been described in detail previously.32 The spectrometer measures the position and time-of-flight of both the imaged ions and electrons. We record each ion-electron pair individually, keeping average count rates low (0.1 counts/pulse or lower) to avoid false coincidences. The detected position and time-of-flight information fully determine the 3D momentum vector for each detected ion and electron. Events caused by ionization of background gas in the chamber can be identified using the position of the ion on the detector, and these events are removed during analysis to leave only events caused by ionization of the gas jet.10 Under the conditions used here, the ion mass spectrometer has a resolving power of m/Δm = 300 for parent ions and covers an m/z range of 0 – 230 amu/q. The electron spectrometer collects electrons from 4π steradians up to a kinetic energy of 25 eV and produces a linewidth of 0.6 eV for a 1.5 eV electron.

Gas-phase samples are made by passing 2 atm helium through the head space of a 2-arm flask containing neat liquid samples; typically a piece of filter paper was placed sticking out of the liquid to provide additional wetted surface area to aid liquid-vapor equilibration. For cyclohexene the flask was kept at room temperature, where cyclohexene has a vapor pressure of 160 Torr, giving an upper limit to the concentration as 10%. We believe the actual delivered concentration to be closer to 3%, as the observed count rate would initially decline but then recover if the flow was temporarily interrupted and the helium/sample mix was allowed to equilibrate. For this reason, we take the equilibrium vapor pressure to be the upper limit for the experimental concentration. Acetaldehyde was held in an ice water bath, giving a vapor pressure of 333 Torr and an upper limit of 22% in helium. 2-butanol was held at room 10 ACS Paragon Plus Environment

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temperature, giving 12.5 Torr and 0.8% in helium. 1,3-butadiene was condensed into the twoneck flask held at -78°C with an isopropanol/dry ice slurry, then warmed up to -13°C with an ethylene glycol bath. At this temperature the vapor pressure was 540 Torr, providing 36% in helium. 1-butyne was condensed similarly, then warmed to 0°C with an ice water bath, giving 547 Torr and 37% in helium. All samples were purchased from Sigma Aldrich (≥98% purity) and used as received.

The fuel/helium mixture was passed through a heated SiC tube, 1 mm ID and approx. 3 cm long, with the heated region about 2 cm long. Due to the high repetition rate of this experiment (10 kHz), the gas was flowed continuously rather than pulsed to better utilize the duty cycle of the laser. To reduce the load on the vacuum pumps and preserve high vacuum in the PEPICO chamber, a 50 μm aperture was inserted into the gas line immediately upstream of the reactor and the gas jet was skimmed with a 300 µm skimmer placed a few millimeters downstream of the reactor exit. The ultimate pressure of the main chamber was 2x10-9 Torr, rising to 1x10-6 Torr under load.

To directly compare our method to traditional techniques, PIMS spectra were recorded with a reflectron time-of-flight instrument using the 9th harmonic of an Nd:YAG laser as described previously.33 Briefly, 355 nm light from a Nd:YAG laser (Quanta Ray PRO-230-10, Spectra Physics, typically 30 mJ pulse-1), is directed into a xenon tripling cell and converted to 118.2 nm radiation (typically 30 nJ pulse-1). The 118.2 nm (10.5 eV) pulses are 5 ns long and the laser fires at 10 Hz. Instead of a single line at 10.5 eV, tunable radiation from a synchrotron has also been used. Time-of-flight photoionization mass spectrometry experiments were carried out at the



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Advanced Light Source Chemical Dynamics Beamline at Lawrence Berkeley National Laboratory.34 The synchrotron provides relatively bright (up to 1014 photons s−1), tunable (8.0 – 15.5 eV), and high resolution (up to 10 meV)35 vacuum ultraviolet radiation to ionize, identify, and quantify species in a molecular beam. The synchrotron operates at 500 MHz and produces pulses 70 ps in width.

RESULTS AND DISCUSSION

A. Recognizing Dissociative Ionization with Momentum-Resolved TOF

We have applied this technique to the pyrolysis of cyclohexene, a molecule frequently used as a standard in high temperature studies.36 The unimolecular decomposition of cyclohexene is dominated by a retro-Diels—Alder reaction producing ethylene and butadiene (Scheme 1), though another pathway is dehydrogenation producing cyclohexadiene and eventually benzene. Any other products observed may be tentatively assumed to arise from dissociative ionization or bimolecular chemistry. We first demonstrate that PIMS recorded with momentum-resolved PEPICO detection can be used to easily distinguish between dissociative ionization and thermal processes.

Scheme 1


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In Figure 2, we directly compare the momentum-resolved spectrometer (figure 2B) with a reflectron (mass-resolved) mass spectrometer (figure 2A), using cyclohexene as a target molecule. The measured37 ionization energy of cyclohexene is 8.94 ± 0.02 eV. For the momentum-resolved spectrometer, we ionize with 11.2 eV photons, while 10.9 eV photons from the ALS synchrotron were used with the reflectron spectrometer. Variation in integrated peak intensity between these two experiments can be attributed to the difference in photon energy, accessing different absorption cross sections for ionization and dissociative ionization; flow conditions and exact reactor temperature may also cause differences between spectra. The room-temperature traces in each panel (black lines) show the parent molecular ion at mass/charge (m/z) ratio of 82 as well as peaks at m/z 67 and 54. Since these are room temperature samples, cyclohexene is the only species present, and the fragment peaks at 67 and 54 must be assigned to dissociative ionization processes (Scheme 1). This assignment is confirmed by the broad linewidths observed with the momentum-resolved spectrometer (Figure 2B), with no appeal to chemical arguments. This data shows a narrow peak at m/z 82 (FWHM 0.3 amu), indicating a parent molecule, and broad peaks at m/z 67 and 54 (FWHM 3 13 ACS Paragon Plus Environment


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amu) caused by the momentum spread from the fragmentation of the cyclohexene ion. Molecules present in the reactor form a collimated beam with narrow transverse velocity spread, yielding a narrow peak in the mass spectrum like that of m/z 82. Any molecules produced during pyrolysis have this same narrow velocity spread and narrow time of flight distribution. In contrast, additional momentum created during dissociative ionization causes wide time-of-flight peaks, since initial momentum toward the detector causes a shorter time of flight while initial momentum away from the detector causes a longer time of flight. The difference between the wide peaks caused by dissociative ionization and the narrow peaks corresponding to pyrolysis products is immediately apparent in Figure 2B.

Figure 2. Comparison of mass-resolved PIMS (A) to momentum-resolved PIMS (B) in the pyrolysis of cyclohexene. PIMS data in panel A were recorded at the Advanced Light Source using 10.9 eV synchrotron radiation and a traditional reflectron mass spectrometer. PIMS data in panel B were recorded using 11.2 eV photons generated from a tabletop Ti:sapphire laser and a low-field momentum-resolved mass spectrometer. The ionization energy of cyclohexene is 14 ACS Paragon Plus Environment

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8.94 eV. Black traces show unheated cyclohexene and red traces show the pyrolysis products heated to 1300 K. The broad linewidth in the momentum-resolved data immediately reveals that the m/z=67 channel (methyl loss) arises from dissociative ionization, not pyrolysis.

The ability to distinguish the ions of the nascent pyrolysis products from those resulting from dissociative ionization becomes important in pyrolysis mixtures, where multiple species are present and several dissociative ionization pathways may be possible from each pyrolysis product.33,38 The red traces in figure 2 show PIMS spectra of pyrolyzed cyclohexene (Scheme 1). The reflectron data (figure 2A) show sharp peaks at m/z 28, 54, 67, 80, and 82. The momentumresolved data (figure 2B) shows sharp peaks at m/z 28, 54, 78, 80, and 82, along with broad features at m/z 39 and 67. Here we immediately see that the peak at m/z 67 arises from dissociative ionization, while the other peaks are the pyrolysis products: ethylene (28), butadiene (54), and cyclohexadiene (80). The peak at m/z 39 in the 11.2 eV data results from dissociative ionization of the pyrolysis product, CH2=CHCH=CH2. The momentum-resolved data also shows m/z 78, which is likely benzene (Scheme 1). The presence of benzene and increased peak height of cyclohexadiene (m/z 80) compared to the parent (m/z 82) in Figure 2B in contrast to Figure 2A is likely due to the higher concentration of fuel in this measurement (3% vs 0.1%), which will increase the importance of bimolecular reactions with hydrogen atoms.

These assignments from the spectra in Figure 2 confirm the processes in Scheme 1. The dominant pathway for cyclohexene pyrolysis is to ethylene (m/z 28) and butadiene (m/z 54); a minor pathway to cyclohexadiene (m/z 80) and benzene (m/z 78) is observed. There are also dissociative ionization processes producing ions at m/z 54 and m/z 67. These results agree with earlier electron impact studies39 that observed dissociative ionization of cyclohexene to produce daughter ions m/z 54 and m/z 67 at 10.27 eV. We typically observe dissociative 15 ACS Paragon Plus Environment


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ionization peaks to be about 10 times broader than parent ions or nascent pyrolysis products, due to the kinetic energy released during fragmentation. For example, the m/z 67 fragment has an average translational energy of 22 meV (directly calculated from the observed TOF distribution in Figure 2B). Since we know the mass of the undetected neutral fragment (in this case 15 amu, Scheme 1), we can use conservation of momentum to calculate the momentum and therefore kinetic energy of the undetected neutral fragment. We then deduce the average total kinetic energy released by the dissociation process ([cyclohexene]+ → [CH2=CHCHCH=CH2]+ + CH3) to be 120 meV. The momentum-broadening of the peak can be detected if the kinetic energy of the fragment is larger than a few hundred µeV, although the precise limit depends on the mass of the fragment ion. It is worth noting that if this momentum broadening of a fragment ion from one molecule overlaps the parent ion of another, the counts from the fragment ion will limit the sensitivity of detection for the parent ion. The limit of detection will be equal to the noise on the daughter ion peak shape (the roughness of the broad peaks in figure 2b). Also note that a peak in PIMS measurements may be broadened by the production of a metastable ion that dissociates part-way down the flight tube; this form of broadening would affect both mass-resolved and momentum-resolved measurements, and for the purposes of this paper would lead to the same conclusion: a broad line indicates a fragment ion.

B. Isomer Identification Using Mass-Specific Photoelectron Spectra and Wavelength Selectivity


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Frequently m/z information alone is insufficient to identify a molecule. An exception in cyclohexene decomposition is that the only stable hydrocarbon with m/z=28 is CH2=CH2. However, the peak at m/z 54 from cyclohexene pyrolysis (in Figure 2) is assumed to be butadiene (CH2=CHCH=CH2) based on the chemical argument illustrated in Scheme 1; observation of the mass alone permits other assignments. Besides 1,3-butadiene40 (IE 9.082 ± 0.004 eV), isomeric C4H6 hydrocarbons could also be cyclobutene41 (IE 9.42 ± 0.02 eV), 1,2butadiene42 (IE 9.23 ± 0.02 eV), and 1-butyne37 (IE 10.20 ± 0.02 eV). PEPICO detection provides a direct method to distinguish amongst isomers via their photoelectron spectra.

In Figure 3 we overlay the photoelectron spectra coincident with m/z 54 with those recorded from 1,3-butadiene (CH2=CHCH=CH2) at the same temperature for comparison, and also electrons from 1-butyne (HC≡CCH2CH3). The pyrolysis data (m/z 54 from cyclohexene) clearly overlaps well with CH2=CHCH=CH2, although with some deviation toward the highbinding energy side of the peak, and does not resemble the overlaid HC≡CCH2CH3 spectrum. (Room-temperature 1-butyne is used to avoid isomerization to butadiene, which has been described in previous work.43) The data in figure 3 do not rule out all possible m/z = 54 isomers due to limited resolution, but in many cases chemical arguments could reduce the number of possibilities such that our instrument would successfully make the distinction (such as between 1,3-butadiene and 1-butyne shown here). With higher resolution the other isomers could be distinguished (as documented in other work9); the resolution limits for our technique are addressed in the discussion section.



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It should be mentioned that the photoelectron spectrum in coincidence with a specific mass is not entirely equivalent to a photoelectron spectrum of the appropriate pure compound. In the PEPICO experiment in Figure 3, the photoelectrons are only collected if they are associated with ions at m/z 54. For the case of 1,3-butadiene, photoelectrons at the ionization threshold are all collected in coincidence with [CH2=CHCH=CH2]+ ions. At higher energies, dissociative ionization causes problems. If the ionizing light produces [CH2=CHCH=CH2]+ cations that fragment, then no m/z 54 ions will be detected and none of the corresponding photoelectrons will be registered in coincidence with m/z 54. The photoelectron spectrum coincident with a parent ion, then, extends from the molecule’s ionization energy up to the lowest fragment appearance energy.

Figure 3. Photoelectron spectrum taken in coincidence with m/z=54 ions in the pyrolysis of cyclohexene, compared to 1,3-butadiene and 1-butyne. The photoelectrons from the pyrolysis product of cyclohexene (thick black trace) match those from 1,3-butadiene (red trace), except at high binding energies, confirming the assignment of m/z=54 as 1,3-butadiene. The deviation at high binding energies may arise from differences in internal energy between the pyrolysis product and the neat sample. The cyclohexene and 1,3-butadiene samples were run with a 1300 K microreactor temperature, while the 1-butyne sample (thin blue trace) was run with a 300 K microreactor in order to prevent isomerization to butadiene. 11.2 eV photons generated from a Ti:sapphire laser were used for all three data sets.


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The light source used in this work – high harmonics produced from an amplified femtosecond laser – provides additional flexibility beyond the momentum-resolved PEPICO described above. The process of high harmonic generation produces a set of discrete frequencies, with spectral components at every odd harmonic from 5ω to 19ω (7.9 to 30.2 eV in steps of 3.2 eV) under the conditions used here. Individual spectral components can be selected using a combination of transmission filters and specially-coated mirrors. In the present work, we use mirrors that suppress only the driving laser frequency and use gas-phase media as filters to absorb all harmonics above their ionization energy. The effects of two different gas filters are illustrated schematically in Figure 4 (insets). If argon (IE 15.76 eV) is chosen as a filter (Figure 4A), then 11ω and all higher harmonics are absorbed. If krypton (IE 14.00 eV) is chosen (Figure 4B), then 9ω and higher harmonics are absorbed, although a small amount of the low-energy tail of 9ω lies below the ionization energy of krypton and thus does pass through the filter.



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Figure 4. Selection of different harmonics and associated mass spectra. These PIMS spectra show pyrolysis products of acetaldehyde (CH3CHO, mass 44) with a microreactor temperature of 1200 K, recorded with a low-field momentum-resolved mass spectrometer. For panel A, 11.2 and 14.4 eV photons generated from a Ti:sapphire laser (7ω and 9ω) are used. The inset gives an illustration of the spectrum produced by high harmonic generation, along with a shaded region indicating the spectral range (11ω and higher) blocked by the argon gas filter. Carbon monoxide44 (m/z 28, IE 14.0 eV) is the major product. For panel B, a krypton filter was used to additionally block the majority of 9ω, leaving only 11.2 eV photons (illustration shown in inset). A small concentration of ethylene45 (m/z 28, IE 10.5 eV) is observed in panel B that had been masked by carbon monoxide in panel A.

The flexibility of high harmonic generation is demonstrated in Figure 4, showing a PIMS trace recorded for the pyrolysis of acetaldehyde (CH3CHO). The pyrolysis of dilute acetaldehyde is well-studied23,46–59 and known to produce the following products: CH3, CO, H, H2, CH2=C=O, CH2=CHOH, H2O, and HC≡CH. To detect these products, harmonics up to 9ω (14.4 eV) are used in order to ionize carbon monoxide (IE 14.01 eV) (Figure 4A). Observed here are m/z 15, 16, 28 and 44, assigned as methyl radical, methane, carbon monoxide and unreacted acetaldehyde. Dissociative peaks are observed at m/z 29 (HCO+) and 43 (CH3CO+). The peak at m/z 16 (CH4+) is 20 ACS Paragon Plus Environment

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particularly interesting, since methane has been proposed to be produced in two ways: either through bimolecular chemistry23 or through dissociative ionization.60 In the present experiment the m/z 16 peak is narrow and clearly not the result of dissociative ionization, consequently methane is the result of bimolecular chemistry: CH3 + CH3CHO → CH4 + CH3CO. This was further confirmed by comparing peak intensities while changing the dilution (see supporting information). Bimolecular rates of reaction depend more strongly on concentration than unimolecular processes, so upon dilution fragments resulting from bimolecular processes will decrease more rapidly than parent signal and unimolecular fragmentation products. Following the conclusions found in previous pyrolysis studies23,46 of acetaldehyde, the presence of bimolecular chemistry suggests that ethylene may have also been produced but was masked by the strong signal at m/z 28 from CO. To test for ethylene, the experiment is repeated with 7ω only (11.2 eV) by switching the gas in the filter from argon to krypton, a process which takes a few minutes and can be done without affecting the flow and pyrolysis conditions for the acetaldehyde. The results are shown in Figure 4B. Compared to panel A, we observe that m/z 16 and m/z 28 have both been reduced, m/z 28 significantly, but both still exist. For m/z 16, resulting from methane61 (IE 12.618 ± 0.004 eV), we attribute its continued presence in the data to the small amount of 9ω that leaks through the krypton filter (Figure 4B inset). We cannot, however, attribute the persistence of m/z 28 to CO being ionized by spectral contamination, as the IE of CO is above the IE of krypton, and therefore any photon reaching the sample cannot ionize CO. The remaining peak at m/z 28, then, is assigned to ethylene. The bimolecular chemistry also produced H2, which can be detected by allowing higher harmonics to ionize the sample. Another simple advantage of this experimental design compared to 118.2 21 ACS Paragon Plus Environment


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nm Nd:YAG PIMS is the ability to ionize all molecules, not only those with I.E. ≤ 10.5 eV. For this pyrolysis experiment, traditional, fixed-wavelength PIMS would require secondary techniques to detect CO (IE 14.0136 ± 0.0005 eV44), CH4 (IE 12.6 eV), CH2CH2 (IE 10.5127 ± 0.0003 eV45), H atom (IE 13.6 eV), H2 (IE 15.425805 eV62), and H2O (IE =12.61737 eV63).

C. Pyrolysis of 2-butanol In Figure 5 we demonstrate the capabilities of the instrument on the pyrolysis of 2-butanol. Alcohols frustrate traditional, fixed-wavelength PIMS studies due to an extreme propensity toward dissociative ionization.64,65


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Figure 5. Pyrolysis of 2-butanol. 2-butanol (m/z 74) has an ionization energy66 of 9.88 ± 0.03 eV. Pyrolysis products recorded using a reflectron mass spectrometer and the 9th harmonic of a Nd:YAG, 10.49 eV, are shown in panel A. Spectral congestion makes it difficult to assign lines as either true pyrolysis products or dissociative ionization fragments. Pyrolysis products recorded with a low-field, momentum-resolved PEPICO and the 7th harmonic of a Ti:sapphire, 11.2 eV, are shown in panel B. Peaks at m/z= 31, 45, and 59 are clearly identified as arising from dissociative ionization. Panel C shows photoelectrons recorded in coincidence with the m/z=44 peak overlaid on a neat acetaldehyde sample (IE = 10.23 eV) for comparison. The extra amplitude at low binding energy implies the presence of vinyl alcohol, CH2=CHOH (IE67,68 = 9.17 eV), the enol isomer of acetaldehyde.



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Figure 5A shows a PIMS spectrum recorded using 10.49 eV photons derived from a nanosecond Nd:YAG laser and a traditional reflectron spectrometer. The large number of observed peaks indicates the fearsome challenge of separating the pyrolytically-produced fragments from the fragments resulting from dissociative ionization. Even though the 118.2 nm laser is only 0.6 eV above the ionization threshold, there are 4 ions at 300 K that must result from dissociative ionization (m/z 44, 45, 58, and 59). A traditional analysis69 would predict the parent cation, m/z 74 [CH3CH2CHOHCH3]+, would fragment to CH3CH2CH=OH+ (m/z 59), CH3CHCH=OH+ (m/z 58), CH3CH=OH+ (m/z 45), and CH2CH=OH+ (m/z 44). As the micro-reactor is heated to 1200 K, the pyrolysis of 2-butanol begins. The parent alcohol would be expected to dissociate to two different radical pairs: [CH3CH2 + CH(OH)CH3] and [CH3CH2CH(OH) + CH3]. These radicals will continue to fragment in the micro-reactor to produce [CH2=CH2, H atom, CH3CHO, CH3CH2CHO, CH3]. Instead of dissociating to radicals, CH3CH2CHOHCH3 might simply dehydrate to H2O and a mixture of butenes (CH3CH2CH=CH2 and cis or trans CH3CH=CHCH3). The 10.49 eV spectrum in Figure 5A might be assigned as m/z 56 (butenes), m/z 44 (CH3CHO), m/z 28 (CH2=CH2), and m/z 15 (CH3). In addition to unimolecular decomposition, the low symmetry of 2-butanol means that several chemically distinct hydrogen abstraction pathways are available via bimolecular reactions. The small features in Figure 5A result from this bimolecular chemistry, further thermal dissociation of the initial pyrolysis products, or by their dissociative ionization, and are consistent with past work on the detailed mechanisms of 2-butanol decomposition in shock tubes,58,70,71 flames,72–74 and pyrolysis ovens.75,76 In Figure 5B we show a PIMS spectrum using 11.2 eV photons derived from our Ti:sapphire laser and recorded by our momentum-resolved mass spectrometer. Peaks at 31, 45, and 59 can


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be assigned as dissociative ionization, removing them from consideration for the pyrolysis mechanisms for 2-butanol. Interestingly, PEPICO data taken for m/z 44 shows a very broad photoelectron spectrum, inconsistent with the vertical transition known for ionizing acetaldehyde. Figure 5C shows these electrons (red trace), overlaid with electrons from a sample of neat acetaldehyde (IE77 = 10.2295 ± 0.0007 eV) under similar conditions (blue trace). It can be seen that m/z 44 from butanol pyrolysis produces electrons that extend to much lower ionization energy than electrons from acetaldehyde ionization, suggesting the formation of vinyl alcohol. The IE(anti-CH2=CHOH) has been measured67,68 to be 9.17 ± 0.05 eV. The production of vinyl alcohol has been observed in the combustion chemistry of several fuels,78 including all four butanol isomers,72 as well as in plasma discharges7 and in the pyrolysis of acetaldehyde itself,23 although at much higher temperatures. The importance of enol chemistry in butanol combustion has been discussed in shock tube studies.58 The mechanism of pyrolysis of butanol will be discussed in greater detail in a subsequent paper.

CONCLUSIONS

The technique described here takes the power of a synchrotron facility (demonstrated in past work6,7) for universal, multiplexed, fast, sensitive, tunable, and isomer-specific detection of pyrolysis products and implements it in a university laboratory. Although the photoelectron kinetic energy resolution here is insufficient to resolve vibrational structure in the photoelectron spectrum, future improvements in the spectrometer, radiation bandwidth and high harmonic flux promise diagnostic utility that approaches that of synchrotron-based PEPICO experiments. In addition to the capabilities used here, tunable femtosecond high harmonic



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generation has been demonstrated in the 20-100 eV energy range79 and can be implemented in ionization studies. Higher count rates than those demonstrated here could be achieved with higher repetition rate lasers,80 particularly if combined with sophisticated data acquisition techniques used at synchrotron-based PEPICO experiments.81,82 Synchrotron beams generally35 provide bandwidths on the order of 2.5% (250 meV for 10 eV photons) that are narrowed to 0.1%, (10 meV for 10 eV photons) by a monochromator. Femtosecond lasers, due to the short temporal duration and Heisenberg uncertainty, have bandwidths that are very large compared to typical lasers, but not much larger than that offered by synchrotrons in this energy range. For example, a transform-limited (minimum uncertainty) laser pulse with 35 fs duration has a bandwidth of 50 meV. The harmonics used in this work had a full-width half-max bandwidth of 250 meV (11.20 ± 0.11 (1σ)), but this width should not be considered a limiting case. High harmonic generation tends to provide narrower harmonics if driven with shorter wavelengths (which also dramatically increases photon flux, by a factor of 104),31 with temporally longer initial laser pulses, or with chirped or shaped pulses.83 Very

recently,

angle-resolved

photoemission

spectroscopy

from

solids

has

been

demonstrated84 using low-order harmonics with sub-150-meV energy resolution. For improved resolution, monochromators similar to those used at synchrotrons35 could be added, although at a cost of a significant reduction in flux (which could be compensated by driving the harmonics with shorter wavelengths).31

This novel application of ultrafast lasers combines some of the best aspects of many other tools used to study combustion. The ability to ionize every molecule retains the flexibility of


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traditional PIMS. Our mass-selected photoelectron spectra combined with multiple available ionizing photon energies give this technique enough information to unambiguously identify many molecules, and with improved electron energy resolution could identify all but a few select cases (isomers with very similar photoelectron spectra, or non-bound cations). This level of information content is common in LIF or CARS experiments, but atypical of traditional ionization-based techniques. The PEPICO spectrometer used here was built a decade ago32 and is capable of measuring the three-dimensional momentum vector of several charged particles per laser pulse; however, the full vector information is not needed for the experiments presented here. As we are demonstrating a new approach to studying complex chemical reaction networks, it is worthwhile to consider what the minimum requirements would be to carry out these experiments. First, as only the time of flight of the ions was used in this analysis, the ion half of the experiment could be simplified by removing the electronics associated with the position information – in this case a delay line anode but in other systems a phosphor screen and camera. Interestingly, these electronics also limit the repetition rate of the experiment, so this simplification could improve performance while reducing cost. Second, the electron detector could also be replaced with a 1-dimensional detector. A simple time-of-flight detector would not be recommended, as collection efficiency is at a premium in PEPICO experiments, but a magnetic bottle spectrometer would be sufficient. For an additional cost savings, PEPICO experiments have been demonstrated that use a single detector.85 This can be done with fast switching electronics to reverse the electric field after the electrons have hit the detector, and can be combined with magnetic bottle designs.86 27 ACS Paragon Plus Environment


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The combination of light source and detection scheme presented here – tabletop ultrafast VUV lasers and PEPICO spectroscopy – is not limited to pyrolysis experiments but could prove valuable to a wide variety of problems. It would be straightforward, for example, to adapt the setup with a flame sampling apparatus or a plasma discharge source. Recent work demonstrating the use of a miniature, high-repetition rate shock tube87 at a synchrotron facility suggests this setup could be applied to shock-wave chemistry. The wide variety of systems to which this setup could be adapted suggests applications not just in combustion phenomena but also in atmospheric chemistry and astrochemistry.

ACKNOWLEDGEMENTS We gratefully acknowledge support from the U.S. Department of Energy Office of Basic Energy Sciences (DE-FG02-99ER14982). GTB, JHB and GBE acknowledge support from the National Science Foundation (CBET-1403979). GTB was also funded by the Marion L. Sharrah Fund at the University of Colorado Boulder. DEC acknowledges support by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE 1650115. JPP is supported by a Joseph Addison Sewall Scholarship and a CU Graduate School Dissertation Completion Fellowship. We thank Musahid Ahmed for his assistance for the measurement of the 10.9 eV photoionization mass spectrum of cyclohexene in Figure 2. This spectrum was measured on Beamline 9.0.2 at the Advanced Light Source that is supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, and Chemical Sciences Division of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.


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DESCRIPTION OF SUPPORTING INFO In order to distinguish bimolecular chemistry from unimolecular decomposition, an additional experiment examining the effect of dilution of the target molecule is included here.

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Figure 1. Illustration of tabletop VUV-based momentum-resolved PEPICO detection of reactive intermediates. An amplified femtosecond laser pulse is converted to the VUV by high harmonic generation in a hollow waveguide. The VUV pulse ionizes a molecular beam of reactive intermediates generated by pyrolysis of CH3CHO in a heated micro-reactor. PEPICO detection provides a momentum-resolved mass spectrum, as well as a unique photoelectron spectrum for each peak in the mass spectrum. 139x109mm (300 x 300 DPI)


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Figure 2. Comparison of mass-resolved PIMS (A) to momentum-resolved PIMS (B) in the pyrolysis of cyclohexene. PIMS data in panel A were recorded at the Advanced Light Source using 10.9 eV synchrotron radiation and a traditional reflectron mass spectrometer. PIMS data in panel B were recorded using 11.2 eV photons generated from a tabletop Ti:sapphire laser and a low-field momentum-resolved mass spectrometer. The ionization energy of cyclohexene is 8.94 eV. Black traces show unheated cyclohexene and red traces show the pyrolysis products heated to 1300 K. The broad linewidth in the momentum-resolved data immediately reveals that the m/z=67 channel (methyl loss) arises from dissociative ionization, not pyrolysis. 101x135mm (300 x 300 DPI)



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Figure 3. Photoelectron spectrum taken in coincidence with m/z=54 ions in the pyrolysis of cyclohexene, compared to 1,3-butadiene and 1-butyne. The photoelectrons from the pyrolysis product of cyclohexene (thick black trace) match those from 1,3-butadiene (red trace), except at high binding energies, confirming the assignment of m/z=54 as 1,3-butadiene. The deviation at high binding energies may arise from differences in internal energy between the pyrolysis product and the neat sample. The cyclohexene and 1,3butadiene samples were run with a 1300 K microreactor temperature, while the 1-butyne sample (thin blue trace) was run with a 300 K microreactor in order to prevent isomerization to butadiene. 11.2 eV photons generated from a Ti:sapphire laser were used for all three data sets. 55x40mm (300 x 300 DPI)


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Figure 4. Selection of different harmonics and associated mass spectra. These PIMS spectra show pyrolysis products of acetaldehyde (CH3CHO, mass 44) with a microreactor temperature of 1200 K, recorded with a low-field momentum-resolved mass spectrometer. For panel A, 11.2 and 14.4 eV photons generated from a Ti:sapphire laser (7ω and 9ω) are used. The inset gives an illustration of the spectrum produced by high harmonic generation, along with a shaded region indicating the spectral range (11ω and higher) blocked by the argon gas filter. Carbon monoxide36 (m/z 28, IE 14.0 eV) is the major product. For panel B, a krypton filter was used to additionally block the majority of 9ω, leaving only 11.2 eV photons (illustration shown in inset). A small concentration of ethylene37 (m/z 28, IE 10.5 eV) is observed in panel B that had been masked by carbon monoxide in panel A. 91x109mm (300 x 300 DPI)



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Figure 5. Pyrolysis of 2-butanol. 2-butanol (m/z 74) has an ionization energy45 of 9.88 ± 0.03 eV. Pyrolysis products recorded using a reflectron mass spectrometer and the 9th harmonic of a Nd:YAG, 10.49 eV, are shown in panel A. Spectral congestion makes it difficult to assign lines as either true pyrolysis products or dissociative ionization fragments. Pyrolysis products recorded with a low-field, momentum-resolved PEPICO and the 7th harmonic of a Ti:sapphire, 11.2 eV, are shown in panel B. Peaks at m/z= 31, 45, and 59 are clearly identified as arising from dissociative ionization. Panel C shows photoelectrons recorded in coincidence with the m/z=44 peak overlaid on a neat acetaldehyde sample (IE = 10.23 eV) for comparison. The extra amplitude at low binding energy implies the presence of vinyl alcohol, CH2=CHOH (IE46,47 = 9.17 eV), the enol isomer of acetaldehyde. 152x304mm (300 x 300 DPI)


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Tabletop Femtosecond VUV Photoionization and PEPICO Detection

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