Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 270−273
Unravelling the Electronic State of NO2 Product in Ultrafast Photodissociation of Nitromethane Shunsuke Adachi,† Hiroshi Kohguchi,‡ and Toshinori Suzuki*,† †
Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima, Hiroshima 739-8526, Japan
‡
S Supporting Information *
ABSTRACT: The primary photochemical reaction of nitromethane (NM) after ππ* excitation is known to be C−N bond cleavage (CH3NO2 + hν → CH3 + NO2). On the other hand, NO2 can be formed in both the ground and excited states, and identification of the electronic state of the NO2 product has been a central subject in the experimental and theoretical studies. Here we present time-resolved photoelectron spectroscopy using vacuum-ultraviolet probe pulses to observe all transient electronic states of NM and the reaction products. The result indicates that ultrafast internal conversion occurs down to S1 and S0 within 24 fs, and the dissociation proceeds on the S1 surface (τdiss ≲ 50 fs), leading to comparable product yields of NO2(A) and NO2(X). The overall dissociation quantum yield within our observation time window ( 200 fs. The population transfer from NO2(A) to NO2(X) appears to be minor, and hence Pathway F plays a less significant role in the cascading reactions (or it may possibly complete within 200 fs). We may estimate a quantum yield (QY) for the CH3 + NO2 dissociation from the excitation (3.5%) and dissociation probabilities. The latter is evaluated to be 1.0% from the photoelectron spectra for the negative and positive time delays in Figure S2. These values provide the dissociation QY of 0.29 (= 1.0/3.5). It is similar to that for the nπ* state (0.24) evaluated in a nonadiabatic molecular dynamics calculations.21 This might suggest that the dynamics on the S1 surface play a crucial role in both cases. It is worth mentioning the reaction pathways that are not considered in our experiments. In addition to the fast reaction pathways shown in Figure 1, various thermal reaction pathways are open in the S0 state (Pathway E), because of a large internal energy (6.0 eV). Isomerization to methyl nitrite (CH3ONO) and three-body dissociation (CH3NO2* → CH3 + NO + O) are the examples.22,23 However, there is no peak assignable to CH3ONO (Ie ∼ 10.9 eV24) or NO (Ie ∼ 9.3 eV25) in the experimental spectrum (Figure 3a). These thermal reactions take much longer time than our observation time window in the present work (∼2 ps). Note that our reported dissociation QY do not include hot ground state reactions that might affect “final” photoproduct QYs on longer time scales. In fact, hot NM will eventually dissociate as is evidenced by nearly unity QY of the total dissociation channels at a photolysis wavelength of 193 nm.10 In conclusion, TRPES with the VUV supports the theoretical study by Isegawa et al. in that ππ* electronic excitation leads to ultrafast cascading S3 → S2 → S1 + S0 IC within τex = 24 fs prior to dissociation into CH3 + NO2 fragments. Dissociation predominantly proceeds on the S1 PES (Pathways D and D′; τdiss ≲ 50 fs), leading to comparable production of NO2(A) and NO2(X), while that on the S3 and S2 PESs (Pathways B and C) plays only an insignificant role. The overall dissociation QY via Pathways D and D′ within 2 ps was evaluated to be 0.29.
Figure 4. (a) Time profile of photoelectron intensity at eBE = 9.9 eV (symbols) and the result of least-squares fitting using a linear combination of exponential decay and rise components (solid curve). Dashed curves show the two components individually. (b) Ratio between NO2(X) and NO2(A) populations.
data was performed using a linear combination of exponential decay and rise components (a solid curve). The first component (a red dashed curve, an exponential decay) expresses the excited state population of NM. The decay time constant of τdecay = 26 ± 1 fs obtained from the fitting agrees with the τex = 24 fs determined from the time profiles in the lower eBE region discussed earlier. Meanwhile, the second component in Figure 4a (a blue dashed curve, an exponential rise) is attributed to population of the CH3 product. As is shown in Figure S4 in the Supporting Information, the fitting error increases monotonically when we assume τdiss greater than 50 fs. Therefore, τdiss is considered to be ≲50 fs, and the fragments are formed in a similar time scale to the excited-state deactivation (τex = 24 fs). It is noted that all dissociation pathways lead to formation of CH3. Therefore, the result indicates that the dissociation is completed within 50 fs. Isegawa et al. argued that a major fraction of S1 populations makes IC to the S0 via S1/S0 conical intersections with nearly equilibrium C−N bond length (1.45 Å) and/or with almost dissociated C−N bond length (∼3 Å).8 The former gives hot NM in the electronic ground state, while the latter may facilitate bifurcation into CH3+NO2(A) and CH3+NO2(X) products (Pathways D and D′, respectively). Individual contributions from the four species (NM, CH3, NO2(X), and NO2(A)) are separately shown in Figure 3c. The populations of NO2(X) and NO2(A) appear comparable with each other, although one needs to consider the difference in their photoionization cross sections more quantitatively. On the
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b03032. Further information on time profiles of photoelectron intensity for the lower eBE region, pump−probe photoelectron spectra for negative and positive delays, reproduced spectra by simpler models, and goodness of the fit (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Toshinori Suzuki: 0000-0002-4603-9168 Notes
The authors declare no competing financial interest. 272
DOI: 10.1021/acs.jpclett.7b03032 J. Phys. Chem. Lett. 2018, 9, 270−273
Letter
The Journal of Physical Chemistry Letters
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(20) Worner, H. J.; Bertrand, J. B.; Fabre, B.; Higuet, J.; Ruf, H.; Dubrouil, A.; Patchkovskii, S.; Spanner, M.; Mairesse, Y.; Blanchet, V.; et al. Conical Intersection Dynamics in NO2 Probed by Homodyne High-Harmonic Spectroscopy. Science 2011, 334, 208−212. (21) Nelson, T.; Bjorgaard, J.; Greenfield, M.; Bolme, C.; Brown, K.; McGrane, S.; Scharff, R. J.; Tretiak, S. Ultrafast Photodissociation Dynamics of Nitromethane. J. Phys. Chem. A 2016, 120, 519−526. (22) Wodtke, A. M.; Hintsa, E. J.; Lee, Y. T. The Observation of CH3 O in the Collision Free Multiphoton Dissociation of CH3NO2. J. Chem. Phys. 1986, 84, 1044−1045. (23) Dey, A.; Fernando, R.; Abeysekera, C.; Homayoon, Z.; Bowman, J. M.; Suits, A. G. Photodissociation Dynamics of Nitromethane and Methyl Nitrite by Infrared Multiphoton Dissociation Imaging with Quasiclassical Trajectory Calculations: Signatures of the Roaming Pathway. J. Chem. Phys. 2014, 140, 054305. (24) Ogden, I. K.; Shaw, N.; Danby, C. J.; Powis, I. Completing Dissociation Channels of Nitromethane and Methyl Nitrite Ions and the Role of Electronic and Internal Modes of Excitation. Int. J. Mass Spectrom. Ion Processes 1983, 54, 41−53. (25) Pratt, S. T. Electric Field Effects in the Near-threshold Photoionization Spectrum of Nitric Oxide. J. Chem. Phys. 1993, 98, 9241−9250.
ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Nos. JP16K17528 and 15H05753.
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DOI: 10.1021/acs.jpclett.7b03032 J. Phys. Chem. Lett. 2018, 9, 270−273
