A Theoretical and Mass Spectrometry Study of Dimethyl

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Cite This: J. Phys. Chem. A XXXX, XXX, XXX-XXX

A Theoretical and Mass Spectrometry Study of Dimethyl Methylphosphonate: New Isomers and Cation Decay Channels in an Intense Femtosecond Laser Field G. L. Gutsev,† Derrick Ampadu Boateng,‡ P. Jena,§ and Katharine Moore Tibbetts*,‡ †

Department of Physics, Florida A&M University, Tallahassee, Florida 32307, United States Department of Chemistry and §Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States

‡

S Supporting Information *

ABSTRACT: Using both mass spectrometry with intense femtosecond laser ionization and high-level computational methods, we have explored the structure and fragmentation patterns of dimethyl methylphosphonate (DMMP) cation. Extensive search of the geometries of both neutral and positively charged DMMP yields new isomers that are appreciably lower in total energy than those commonly synthesized using the Michaelis−Arbuzov reaction. The stability of the standard isomer with CH3PO(OCH3)2 topology is found to be due to the presence of high barriers to isomer interconversion that involves several transition states. Our femtosecond laser ionization experiments show that the relative yields of the major dissociation products as a function of peak laser intensity correlate well with the theoretical estimates for the energies of the DMMP+ decay via various channels. In contrast, the peak laser intensities required for observation of minor dissociation products exhibit no correlation with the computed decay energies, which suggests that barrier heights and/or excited electronic states of DMMP+ determine its preferred fragmentation pathways in an intense femtosecond laser field.

1. INTRODUCTION Dimethyl methylphosphonate (DMMP, PO3(CH3)3) is used in practical applications as a flame retardant.1,2 As a simulant for toxic organophosphorus nerve agents such as VX, sarin, and soman, study of DMMP is also useful to develop sensors for chemical warfare agents and catalysts that drive their decomposition.3 Materials including alkanethiolate monolayers, carbon nanotubes, and metal oxides have recently been developed for DMMP sensing applications.4−7 The study of reactions and decomposition of DMMP on the surfaces of various metal oxides and oxide supported catalysts such as metal nanoparticles, both at room and elevated temperatures, has been an active area of research for over 2 decades.8−21 Additionally, multiple studies have demonstrated oxidative22−24 and plasma-mediated25−28 decomposition of DMMP. To assist in interpreting the decomposition mechanisms of DMMP on various heterogeneous catalysts, a number of computational studies of the electronic structure and gas phase unimolecular decomposition pathways of DMMP have recently been performed.29−32 In addition to the extensive research devoted to understanding the decomposition of the neutral DMMP molecule, a number of mass spectrometry studies, going back 5 decades, have revealed a rich array of isomerization and dissociation reactions in its radical cation.33−37 The DMMP radical cation (1) has been found to © XXXX American Chemical Society

spontaneously undergo a series of 1,4-hydrogen atom shifts that results in isomerization to its enol form (3) (see Scheme 1), Scheme 1. Isomerization Reactions of DMMP.+

which is observationally stable over tens of microseconds.36,37 The intermediate 2 can dissociate into PO2C2H7+ (m/z = 94) via low-energy loss of CH2O, and similar dissociation pathways have been observed in related compounds.2,35,36,38 While the complex reactivity of the DMMP radical cation has been known for decades, a thorough computational ab initio investigation of the cation decay pathways involved has not yet been performed. This work presents our computational results on the isomer structures and decay pathways of DMMP radical cation, which revealed a number of novel low-energy isomers, in addition to the previously reported enol form. Received: September 7, 2017 Revised: October 10, 2017 Published: October 16, 2017 A

DOI: 10.1021/acs.jpca.7b08889 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A In order to determine which isomer and dissociation pathways were observed experimentally, we performed mass spectrometric measurements of DMMP, ionized by optical excitations with strong field near-infrared femtosecond laser pulses. The large electric field strengths (in excess of ∼1010 V/m) attainable with femtosecond laser pulses enable molecular ionization, even when the photon energy is significantly lower than a molecule’s ionization potential. Furthermore, the short pulse duration (∼10−100 fs) ensures that the molecular ion is formed intact; i.e., that any cation decay reactions proceed af ter the ionization process is complete.39 These two features render femtosecond laser excitation as an attractive “soft” ionization technique that can be used to produce a stable parent molecular ion in a variety of polyatomic molecules, particularly when near-infrared wavelengths (e.g., 1150−1600 nm) are employed.40−48 This favorable behavior has been attributed to an adiabatic tunnel ionization process,40,41 wherein limited energy transfer to the molecule following electron loss results in predominant formation of a “cold” molecular ion in its electronic ground state.49,50 Near-infrared excitation can also result in multiply charged molecular ions due to electron recollision,42 fragmentation when a low-lying electronic excited state in the ion is accessible,45−48 and both multiple ionization and fragmentation due to excitation of multiple electrons at sufficiently high intensities.40,41 Nevertheless, the predominant formation of ground state molecular ion at moderate laser intensities for strong field adiabatic ionization will enable the direct study of the energetics of DMMP cation decay pathways in our experiments. We have also performed computations of a number of isomers of both DMMP and DMMP+ and their fragments that can be formed in the process of the decay via various channels. In addition, transition states for the isomer interconversions were located and were found to be quite high in energy. The computations were performed at several levels of theory in order to confirm the consistency of the results obtained, as well as to determine the most reliable method to be used in forthcoming work. Our experimental results, using strong field adiabatic ionization, point to intriguing differences in the decay pathways as compared to electron impact ionization and suggest the importance of transition state barrier heights and electronically excited states in DMMP+ for understanding its decay processes.

Figure 1. Experimental setup: (a) optical beam path and TOF-MS.; (b) schematic of sample inlet and ion extraction geometry.

tion plates (biased at +4180 and +3910 V, respectively), where the latter plate has a 500 μm slit that restricts sampling over the laser focal volume to the region of highest intensity. The molecular sample DMMP (Sigma-Aldrich) is introduced into the vacuum chamber as an effusive molecular beam through a 1/16 in. diameter stainless steel tube placed ∼1 cm from the laser focus (Figure 1b). In this configuration, a pressure of 1.2 × 10−6 Torr is maintained in the extraction region, while differential pumping produces a pressure of 4.5 × 10−7 Torr at the Z-gap microchannel plate detector. A 1 GHz digital oscilloscope (LeCroy WaveRunner 610Zi) was used to record the spectral data, where spectra over 10 000 laser shots were averaged to produce the raw data. The peak laser intensity at each pulse energy was calibrated by measuring the ion intensities of Xen+, where up to n = 3 was observed at high pulse energies. For the Xe measurements, the recorded pressure at the detector was maintained at 2.0 × 10−8 Torr to avoid space-charge effects. The sum of Xen+ signals was recorded as a function of pulse energy and fit to tunnel ionization rates of a rare gas atom according to the well-established procedure52 (Supporting Information, Figure S2).

3. DETAILS OF COMPUTATIONS Our computations were performed using four different generalized exchange−correlation functionals within the density functional theory (DFT-GGA). These include BPW91 exchange− correlation functional composed of the Becke exchange53 and the Perdew−Wang correlation.54 Hybrid Hartree−Fock DFT (HFDFT) methods were considered using the B3LYP functional,55 where the exchange−correlation functional is composed of the Becke’s hybrid three-parameter exchange functional and Lee−Yang−Parr’s correlation functional56 and the Truhlar’s M062X functional.57 We also used the long-range corrected functional given by the ωB97XD method.58 The results obtained using these methods were compared to the results obtained with the use of the coupled-cluster method with singles and doubles and noniterative inclusion of triples [CCSD(T)].59 We have chosen the 6-311+G* basis set60 of triple-ζ quality in geometry optimizations by the four chosen methods and a larger basis set 6-311+G(3df) in our single-point CCSD[T] calculations, which were performed using the optimized B3LYP geometrical configurations. All our computations were performed using the Gaussian 09 suite of programs.61 The performance of each method can be assessed via comparing the computed adiabatic ionization energies (IEad) with experimental values. The IEad values were computed according to the equation

2. EXPERIMENTAL SECTION A commercial Ti:sapphire regenerative amplifier (Astrella, Coherent, Inc.) produces 7 mJ, 30 fs, 800 nm pulses. 1.9 mJ of the output is used to pump an optical parametric amplifier (OPA, TOPAS Prime). The output signal beam (1500 nm, 120 μJ) was directed through an achromatic half-wave plate and Wollaston polarizer (ThorLabs, Inc.) for energy attenuation. The beam was then passed through a 5× expander consisting of gold mirrors with f = −10 cm and f = 50 cm to increase the beam diameter (1/e2) from 4.5 mm to 22 mm. The beam expansion serves to decrease the focal spot size and thus increase the peak intensity at the focus. The expanded beam was then focused with an f = 20 cm fused silica lens into the vacuum chamber (Figure 1a). The polarization of the laser electric field was parallel to the TOF axis. The pulse duration was measured prior to the lens with a homebuilt frequency resolved optical gating (FROG)51 setup and found to be 18 fs (Supporting Information, Figure S1). Mass spectra were recorded with a custom-built 1 m linear time-of-flight mass spectrometer (TOF-MS) (Jordan TOF, Inc.) with a vacuum base pressure of 2 × 10−9 Torr. The focus of the laser beam is centered between the ion extraction and accelera-

IEad(M) = [Etot el(M+) + E0(M+)] − [Etot el(M) + E0(M)] (1)

Etotel

where is the total Born−Oppenheimer energy and E0 is the zero-point vibrational energy. The results are presented in Table 1. B

DOI: 10.1021/acs.jpca.7b08889 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Table 1. Comparison of Computed Adiabatic Ionization Energies of DMMP Fragments with Experimenta 6-311++G* CH3 PO CH4 OC2H5 C2H6 OCH3 C2H4 H2 CH2O OC2H6 C3H8 H2O OH

6-311++G(3df)

BPW91

B3LYP

M062X

wB97XD

CCSD(T)

exptl

10 8.51 12.33 6.71 10.95 7.61 10.23 15.25 10.71 9.79 10.33 12.5 13.1

9.92 8.6 12.54 6.83 11.25 7.68 10.28 15.47 10.83 9.99 10.61 12.54 13.18

9.78 8.45 12.59 6.77 11.43 7.57 10.24 15.34 10.92 10.19 10.79 12.51 13

9.78 8.55 12.51 6.73 11.31 7.56 10.22 15.36 10.79 10.08 10.68 12.48 12.97

9.67 8.3 12.48 6.64 11.41 7.37 10.34 15.21 10.72 10.43 10.69 12.38 12.83

9.8380 ± 0.0004 8.39 ± 0.01 12.61± 0.01 6.67 11.52 ± 0.04 7.562 ± 0.004 10.51 15.42 10.88 10.41± 0.05 10.96 12.65 ± 0.05 13.0170 ± 0.0002

a

All values are in eV. The experimental data are from http://webbook.nist.gov/. Our computed IEad values of the rest of fragments are presented in Table S1 of the Supporting Information.

Figure 2. Isomers of neutral PO3(CH3)3 optimized at the B3LYP/6-311+G* level. Total energy shifts are given with respect to the lowest total energy state.

self-consistently at the CCSD(T) level. This was confirmed by the CCSD(T)//CCSD(T) results for small species presented in Table 1. The adiabatic electron affinity (EAad) values of PO3 computed at the BPW91, B3LYP, M062X, ωB97XD, and CCSD(T) levels are 4.37, 4.77, 5.16, 4.94, and 4.94 eV, respectively. The experimental value of the PO3 EAad measured62 from the photoelectron spectra is 4.95 ± 0.06 eV; that is, the ωB97XD and CCSD(T)

As can be seen, the results obtained using the B3LYP, M062X, and ωB97XD methods are rather similar and the largest deviations from the experimental values were obtained for the values computed at the BPW91 level: 0.53, 0.58, and 0.63 eV for C2H6, CH2O, and C3H8, respectively. The CCSD(T) values agree with experiment within 0.2 eV; therefore, one can assume that the use of the B3LYP geometries does not lead to significant deviations of the CCSD(T)//B3LYP total energies from those obtained C

DOI: 10.1021/acs.jpca.7b08889 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 3. Isomers of the PO3(CH3)3+ cation optimized at the B3LYP/6-311+G* level. Total energy shifts are given with respect to the lowest total energy state.

TiO2 anatase (001)65 and (110)66,67 surfaces, in a study of TiO2mediated gas-phase detoxification of DMMP,68 in simulations of DMMP adsorption and reactivity on amorphous silica surfaces69 and when interpreting the data obtained using two electron spectroscopy methods.32 In this work, we explored if the MA CH3PO(OCH3)2 topology corresponds to the geometrical structure of the ground states of dimethyl methylphosphonate and performed an extensive search using different ligand compositions around the phosphorus atom. Geometrical structures of all isomers in both the DMMP and DMMP+ series generated were optimized using the BPW91, B3LYP, M062X, and ωB97XD methods. 4.1. Isomers of DMMP and DMMP+. The geometrical structures of 20 lowest total energy states found for neutral DMMP and 24 states of DMMP+ are presented in Figures 1 and 3. All these states correspond to the minima on the potential energy surfaces, since all computed harmonic vibrational frequencies are

values match the experimental value within the experimental uncertainty. PO3 possesses such a large EAad because it belongs to the class of superhalogens.63

4. RESULTS OF COMPUTATIONS Dimethyl methylphosphonate is typically prepared from trimethyl phosphite and a halomethane via the Michaelis−Arbuzov (MA) reaction.64 Obtained in this way, the DMMP has the chemical formula CH3PO(OCH3)2 with the formal valence of 5 of the central phosphorus atom. Therefore, the previous computational studies have mainly concentrated on the study of the DMMP conformers of CH3PO(OCH3)2. Yang et al.30 have found four isomers corresponding to different orientations of the OCH3 ligands with respect to each other. These conformers lie within the 1.9 kcal/mol total energy range. The MA geometrical topology was assumed in computations of the DMMP absorption on the D

DOI: 10.1021/acs.jpca.7b08889 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Table 2. Comparison of Computed Vertical Ionization Energies of Two DMMP Isomers with Experimenta 6-311++G*

a

6-311++G(3df)

isomer

BPW91

B3LYP

M062X

wB97XD

CCSD(T)

exptlb

calcdb

I IV V VI

9.83 10.00 9.74 9.59

10.29 10.48 10.24 10.12

10.77 10.95 10.68 10.7

10.54 10.74 10.47 10.38

10.63 10.86 10.68 10.52

10.64

10.71 10.6

All values are in eV. bValues taken from ref 32.

IEvert of isomers I and IV−VI at all levels of theory chosen in this work and compared our IEvert values to experiment in Table 2. As may be seen in Table 2, the IEvert of the states with structures I and V are quite close to each other and the CCSD(T) values practically match the experimental value. This means that one cannot discriminate between states of isomers on the basis of their IEvert values. In most cases, where the experimental geometrical structure is unavailable, the results of measurements by several methods are required for unambiguous identification of isomers.74 4.2. Interconversion of Isomers. Since the states of DMMP isomers produced when using the MA reaction are higher in total energy than the ground state, one may wonder: What is the path from these isomers to the ground state isomers in the neutral and cation cases? In the neutral isomer series, the MA isomers have structures V and VI whose states are degenerate in total energy. A pathway V → I is presented in Figure 4

positive. We have optimized also the triplet states for a number of isomers, but they were found to be always appreciably higher in total energy than the corresponding singlet states. As can be seen in Figure 2, the ground state of DMMP (structure I) is of the enol type and can be described as CH3POOH(OC2H5), followed by the C2H5POOH(OCH3) (structure III), OC3H7PHOOH (structure IV), and two CH3PO(OCH3)2 isomers. That is, isomers V and VI, with the canonical MA structures, are higher in total energy by 0.47 eV. We note that the relative total energies in the figures are computed at the B3LYP/6-311+G* levels. The values computed at three other levels are slightly different (by 0.01−0.08 eV; see Table S2 in the Supporting Information), but the order of the states remained the same. In addition, we performed computations of total energies of all DMMP isomers at the CCSD(T)/ 6-311+G(3df)//B3LYP/6-311+G* level and found that isomers V and VI are higher in total energy than isomer I by 0.51 eV at this level. Structure I of DMMP is similar to the structure70 of the nerve agent VX, where an OH ligand is replaced with a SNC8H18 ligand. Structure IV of DMMP contains a C3H7 ligand and can be transformed into the structure of sarin70−72 by replacing an OH group with CH3 and an H atom attached to P with a F atom. One can notice that the phosphorus atom is formally pentavalent in the states of isomers corresponding to the lowest total energy states, whereas it can be tri- or tetravalent in the higher total energy states. Detachment of an electron from DMMP leads to the formation of cation isomers, which are ordered quite differently from those of the neutral isomers (cp. Figure 2 and Figure 3). The detachment of an electron from the ground state of DMMP results in the cation state with geometrical structure XIII which is higher in total state by 0.86 eV with respect to the cation ground state. Moreover, there is no neutral isomer with geometrical structure I of the cation. Optimization of the neutral, beginning with cation structure I, led to a transition state. Reoptimization of the geometry of this state, following the imaginary mode, led to the neutral ground state I. One can notice that structure I of the cation in Figure 3 formally corresponds to a tetravalent phosphorus atom. According to the population analysis,73 performed using natural atomic orbitals, the P atom in the ground state of neutral DMMP with structure I carries a charge of +2.24e and the P charge in the corresponding cation state with structure XIII is +2.16e; i.e., the charges are nearly equal. On the contrary, the P charge of +2.82e in the cation ground state with structure I is essentially larger. The charge increase can be related to a tetravalent configuration around the phosphorus atom in structure I of the cation. The isomers formed after an electron detachment from the neutral states with structures V and VI correspond to cation isomers XIV−XVI which are practically degenerate in total energy and are higher than the ground state of DMMP+ by 0.99−1.00 eV. Since the experimental vertical ionization energy (IEvert) of neutral DMMP is known, we have performed computations of

Figure 4. Transition path from “Michaelis−Arbuzov” isomer V to isomer I corresponding to the ground state of neutral DMMP. Structures and energies are calculated at the B3LYP/6-311++G(d,p) level.

and proceeds via three transition states, with the reaction barrier of 3.99 eV obtained at the B3LYP/6-311+G* level of theory. Clearly, this is not the preferred way for producing the groundstate isomer and a specific route for its synthesis is desirable. There are three MA isomers XIV−XVI of the cation, which are nearly degenerate in total energy (see Figure 3). Two pathways XV → I found in our search for transition states are shown in Figure 5. The transition barrier of 1.59 eV obtained using the B3LYP/6-311+G* approach is essentially smaller than that in the neutral case. Note that the electron detachment from the groundstate neutral results in isomer XIII of the cation, whose state is lower in total energy than the states of MA isomers XIV−XVI by 0.13−0.14 eV. The keto-to-enol isomerization of DMMP+ (cf., Scheme 1) has been studied using deuterium labeling and semiempirical E

DOI: 10.1021/acs.jpca.7b08889 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A PO3(CH3)3 → PO3CH5 + C2H4

0.73 eV

PO3H + C3H8

1.05 eV

PO3CH3 + C2H6

1.28 eV

PO2 C2H 7 + CH 2O 1.35 eV PO2 C3H 7 + H 2O

1.68 eV

PO2 H + OC3H8

1.76 eV

PO3C2H5 + CH4

1.84 eV

PO2 CH3 + OC2H6 1.85 eV

The energies of dissociation via all other computed channels are larger than 3.0 eV and are given in Table S3 of the Supporting Information. The dissociation energy of DMMP with the canonical MA structures were computed previously at the CBS-QB3 level for the PO3(CH3)3 → PO2C2H5 + CH3OH and PO3(CH3)3 → PO3C2H7 + CH2 channels.30 Their computed CBS-QB3 values of 2.59 and 4.09 eV are to be compared to our CCSD(T) values of 2.55 and 3.65 eV, respectively. Note that −0.60 eV correction was done to the values presented in Table S3. There is a good agreement between computed values for the first channel, whereas there is a discrepancy of 0.41 eV for the second channel. One can assume that this could be due to the fact that a larger basis set was used in the present work. The decay energies of the DMMP+ for 45 channels are presented in the Supporting Information, Table S4. All energies are given with respect to the total energy with structure I. The energies and product structures of the decay channels observed in our experiments will be discussed below in section 5.

Figure 5. Transition paths from “Michaelis−Arbuzov” isomer V to isomers VII and I of the DMMP+ cation. Structures and energies are calculated at the B3LYP/6-311++G(d,p) level.

calculations.36 The authors concluded that the energy barrier for keto-to-enol isomerization is below the internal energy required for decomposition of the cation into PO2C2H7+ and CH2O. Their conclusion is consistent with the results of our search of a transition state between the keto and enol isomers, i.e., XV → VII, according to which the transition barrier of 1.41 eV is slightly below 1.43 eV required for loss of CH2O (see Figure 6).

5. EXPERIMENTAL RESULTS AND DISCUSSION Mass spectra of DMMP obtained upon excitation with 1500 nm, 18 fs laser pulses at intensities of 5 × 1013 and 1.6 × 1014 W cm−2 are shown in Figure 7a. The fragment ions produced in highest yields are found at m/z = 109, 94, 93, 79, 63, 47, and 15. These fragments have been identified in previous mass spectral studies of DMMP using a variety of ionization methods.2,33−36 The associated decay energies for these channels, along with the corresponding structures of the products, are shown in Figure 7b. On the basis of previous experimental reports that the fragment at m/z = 94 arises from CH2O loss following keto−enol isomerization35,36 and the calculated energy barriers in Figure 6, we assign the observed m/z = 94 fragment to PO2C2H7+ instead of PO3CH3+ (loss of C2H6). This assignment is consistent with the dissociation energy via C2H6 loss being approximately 0.5 eV higher in energy (Supporting Information, Table S4). We also assign the m/z = 79 fragment to PO2CH4+ instead of PO3+ based on previous reports of the fragment structure2 and because an additional ∼6 eV of energy is required to produce PO3+ (Table S4). In order to determine the relationship between the yields of the major fragmentation products and their computed dissociation energies, mass spectra of DMMP were measured over a range of laser intensities from 2 × 1013 W cm−2 (threshold for observation of ion signal) to 2.5 × 1014 W cm−2. Figure 8 displays the fractional yields (i.e., relative to the total ion signal normalized to unity) of DMMP+ and the major dissociation products in Figure 7b as a function of laser intensity. With the exception of DMMP+ and PO2C2H7+, which exhibit monotonically decreasing fractional yields as a function of laser intensity (Figure 8, upper plot), the remaining fragmentation products increase in yield with intensity until reaching a particular maximum yield,

Figure 6. Energy profile for the keto-to-enol isomerization of DMMP+ and its dissociation into PO2C2H7+ and CH2O. Structures and energies are calculated at the B3LYP/6-311++G(d,p) level.

Note also that the enol isomer VII dissociates to PO2C2H7+ + CH2O via a transition state that is slightly less than the dissociation limit (Figure 6). 4.3. Fragmentation of DMMP and DMMP+. In order to gain insight into thermodynamic stability of DMMP and DMMP+, we have computed the energies Ebi of their decay via various channels i. The decay energies of DMMP have been computed according to the equation Ebi(DMMP) =

∑ [Etotel(Fk) + E0(Fk)] k el − [Etot (DMMP) + E0(DMMP)]

(2)

Eeltot

where is the total Born−Oppenheimer energy, Fk are fragments, and E0 is the zero-point vibrational energy. The decay energies of DMMP+ were computed using an equation analogous to eq 2 with one of the products Fk being a cation. The Ebi values obtained at the CCSD(T)/6-311++G(3df)// B3LYP/6-311+G* level for the smallest energy decay channels of neutral DMMP are F

DOI: 10.1021/acs.jpca.7b08889 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 7. (a) TOF mass spectra of DMMP taken with 1500 nm pulses at peak intensities of 5 × 1013 W cm−2 (blue) and 1.6 × 1014 W cm−2 (red). (b) Calculated energies of DMMP+ dissociation channels corresponding to the formation of the major products observed in the mass spectra.

yields with computed decay energies both serves to validate the computational results and suggests that thermodynamic considerations determine the relative contributions of decay channels in radical cations. We wish to emphasize that all observed fragmentation products originate from the DMMP+ radical cation initially prepared via strong field adiabatic ionization, as opposed to ionization of neutral fragmentation products of DMMP. At sufficiently high laser intensities, additional decay channels identified in Table S4 corresponding to fragments with m/z = 14, 16, 28, 29, 30, 31, 32, 44, 45, 46, 60, 61, 64, 65, 77, 78, 80, 81, 92, 95, 96, 110, and 123 are also visible in small yields. This is seen in the magnification of the mass spectrum, taken at 1.6 × 1014 W cm−2 in Figure 9. However, the products of decay channels presented in Table S4 that correspond to fragments with m/z = 43, 59, 106, 107, 108, or 122 were not observed, even at the highest intensity of 2.5 × 1014 W cm−2. The lack of formation of these fragments is surprising, considering their relatively low decay energies (