Tracking the photodissociation dynamics of liquid nitromethane at 266 nm by femtosecond time-resolved broadband transient grating spectroscopy

Tracking the photodissociation dynamics of liquid nitromethane at 266 nm by femtosecond time-resolved broadband transient grating spectroscopy

Chemical Physics Letters 652 (2016) 152–156 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/lo...

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Chemical Physics Letters 652 (2016) 152–156

Contents lists available at ScienceDirect

Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Research paper

Tracking the photodissociation dynamics of liquid nitromethane at 266 nm by femtosecond time-resolved broadband transient grating spectroscopy Honglin Wu a, Yunfei Song b, Guoyang Yu b, Yang Wang a, Chang Wang a, Yanqiang Yang a,b,⇑ a b

Department of Physics, Harbin Institute of Technology, Harbin 150001, China National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China

a r t i c l e

i n f o

Article history: Received 16 March 2016 Revised 12 April 2016 In final form 14 April 2016 Available online 23 April 2016

a b s t r a c t Femtosecond time-resolved transient grating (TG) technique was employed to get insight into the photodissociation mechanism of liquid nitromethane (NM). Broadband white-light continuum was introduced as the probe to observe the evolution of electronic excited states of NM molecules and the formation of photodissociation products simultaneously. The reaction channel of liquid NM under 266 nm excitation was obtained that NM molecules in excited state S2 relax through two channels: about 73% relax to low lying S1 state through S2/S1 internal conversion with a time constant of 0.24 ps and then go back to the ground state through S1/S0 internal conversion; the other 27% will dissociate with a time constant of 2.56 ps. NO2 was found to be one of the products from the experimental TG spectra, which confirmed that C–N bond rupture was the primary dissociation channel of liquid NM. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Energetic materials are widely applied to military and industry. The macroscopic properties of energetic materials such as detonation performance, thermal stability and sensitivity are closely related to the molecular dissociation mechanisms. The study of photodissociation dynamics plays an important role in understanding the dissociation mechanisms of energetic materials on the molecular level. As a prototype, the photodissociation of nitromethane (NM) has been studied experimentally [1–12] and theoretically [13–18] in the past few decades. Depending on the excitation energies and experimental conditions, different dissociation mechanisms have been suggested. NM has two absorption bands centered at 198 nm and 270 nm which are assigned to p⁄ p transition localized on the NO2 moiety and p⁄ n transition involving a nonbonding electron of O atom, respectively [19,20]. Under p⁄ p excitation, C–N bond rupture has been generally accepted to be the primary photodissociation channel of NM [1–5]. As for the case of p⁄ n excitation, besides the major photodissociation channel of C–N bond rupture [6–8], low yield products such as O atom [9] and OH were also found in some works [10,11]. It should be noted that, in most of the existing ⇑ Corresponding author at: National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China. E-mail address: [email protected] (Y. Yang). http://dx.doi.org/10.1016/j.cplett.2016.04.048 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.

works, gas phase NM is usually used as the sample. However, in most applications, NM is mainly in the liquid phase and the intermolecular interaction cannot be ignored. Then whether the reaction channel of NM molecule in liquid phase is the same as in gas phase needs to be figure out. So an in-depth understanding of the photodissociation mechanism of liquid NM will be necessary. However, very few works have been done on photodissociation of liquid NM. Limited by the excitation light source, early experimental studies were performed to detect the photodissociation products but no dynamics information about the electronic excited-state energy relaxation could be obtained [21,22]. Timeresolved studies of photodissociation of NM were not started until the ultrashort pulsed laser was used in experiments. The earliest time-resolved study of the photodissociation of liquid NM was reported by Faust et al. in 1978 [23]. Picosecond 266 nm laser pump/white light continuum probe transient absorption (TA) experiment was carried out in their work and NO2 was found to be one product. Limited by the time resolution, this work did not give the initial dynamics of the photodissociation. Actually, before the rupture of chemical bonds, intramolecular energy relaxation processes may have taken place, which determines the subsequent dissociation channels. To our knowledge, only the work reported by Rajchenbach et al. in 1995 provided the electronic excitedstate lifetime of liquid NM [24]. They observed the electronic excited-state energy relaxation by monitoring the population in the electronic ground state with coherent anti-Stokes Raman

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scattering (CARS) technique. However, in their work, the lifetime of the excited state was measured indirectly. Besides, information about the photodissociation products was not obtained. So far, there is still a lack of understanding about the excited-state relaxation, dissociation channel and branching ratio of liquid NM, especially the direct experimental evidence of the photodissociation channels. For a comprehensive understanding of the reaction mechanism of liquid NM, further studies on intramolecular energy relaxation in photodissociation process are necessary. Our previous work has proved that the femtosecond 266 nm pump/white-light continuum probe transient grating (TG) is a powerful technique for the study of photodissociation mechanism in liquid materials [25]. In this work, TG is employed to observe the ultrafast photodissociation process of liquid NM. The use of white-light continuum provides a broadband spectral range of probe, which makes it feasible to monitor the reactants and products simultaneously. Compared with TA, the background free TG technique has better signal to noise ratio (SNR) and sensitivity that even tiny amount of products can be detected effectively.

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558 nm, which means the population relaxation of S2 state can be detected by monitoring the diffraction signal arising from the excited-state absorption of S2 state. The schematic diagram of TG signal generation is shown in Fig. 1(a). In addition, not only the information of the reactant but also of the photodissociation products will be obtained if they have absorption/emission in the probe spectral range. 2.2. UV-TG experimental configuration The experimental configuration is basically the same as our previous work [25]. The excitation laser pulses at 266 nm used in our UV-TG experiments were the third harmonic of the 800 nm (110 fs, 1 kHz) laser pulses which were generated from a Ti:sapphire regenerative amplifier (Spitfire, Spectra-Physics). The output 800 nm laser beam from Spitfire was split by a 9:1 beam splitter. The laser beam with 90% energy was led into a femtosecond tripler (TP-1A, Photop Technologies) to get the third harmonic at 266 nm. The 266 nm beam was then split into two equal beams to serve as pump light and each beam was attenuated to 300 nJ per pulse

2. Experiment 2.1. Generation of TG signal In a transient grating experiment, two time-coincident laser pulses (with wave vectors k1 and k2, respectively) of the same frequency are crossed inside the sample and set up an optical interference pattern. The periodic distribution of light field results in a periodic modulation of the complex refractive index of the sample, thus a diffraction grating is created [26,27]. This transient grating can be detected by monitoring the diffraction of a probe pulse (k3) in the phase matching direction ks = k1  k2 + k3. In a typical time-resolved TG experiment, the probe pulse is physically delayed and the ultrafast dynamic process excited by the writing pulses can be obtained from the time-dependent diffraction signal. NM has two absorption bands centered at 198 nm and 270 nm, corresponding to S0 ? S2 and S0 ? S3 transitions, respectively. (S0 ? S1 transition is symmetry forbidden). In this work, NM is excited by 266 nm and populated on S2 state. Broadband whitelight continuum (WLC) which covers almost the visible spectral range is introduced as the probe. The spectral component that is resonant with the electronic transition of NM will be diffracted most efficiently. In all the possible transitions of excited NM, only S2 ? S3 lies in the visible spectral range. According to the result of quantum chemical calculation, the absorption of S2 ? S3 is about

Fig. 2. (a) Time- and wavelength-resolved diffraction efficiency (with intensity shown on the logarithmic scale) of transient grating of liquid NM. (b) Normalized diffraction efficiency at 520 nm as a function of delay time.

Fig. 1. (a) Schematic diagram of the generation of TG signal. Two time-coincident pulses at 266 nm resonant with the S0 ? S2 transition set up a transient grating which can be detected by a delayed WLC probe pulse resonant with the excited-state absorption of NM. (b) Beam arrangement in the BOXCARS configuration. The diffraction signal can be detected in the direction k1  k2 + k3.

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before the sample. The other 800 nm beam with 10% energy was focused into a 5 mm thick Al2O3 crystal to generate WLC with wavelength range from 430 nm to 1200 nm to serve as the probe. The three beams were combined at the sample in the folded BOXCARS geometry (shown in Fig. 1(b)) by a lens with a focal length of 200 mm. The two pump beams were crossed at an angle of about 4.2° resulting in a grating constant of about 3.6 lm. In order to obtain the time-dependent transient grating signal, the two pump pulses were kept temporally overlapped and the probe pulse was delayed with respect to the pump pluses by a motorized translational stage with a step of 4.167 fs. The diffraction signal beam was spatially selected and directed to a spectrometer (Bruker Optics 500 IS/SM). The signals were then detected by a CCD (Andor DU440-BU2) and recorded by a computer for analysis. The analytical pure liquid nitromethane was used in the experiments. The sample was placed in an ultraviolet quartz flow cell with a 1-mm optical path length. The UV-TG experiments were carried out at ambient temperature and pressure.

3. Results and discussion The time- and wavelength-resolved TG signal (diffraction efficiency, the ratio of diffracted to incident WLC light) is shown in Fig. 2(a). The TG signal is dispersion corrected with the dispersion curve extracted from the TG signal of acetone under the same experimental conditions. The absolute diffraction efficiency is relatively small, so it is shown on logarithmic scale. As shown in the contour map, a diffraction signal appears in a broad wavelength range centered at about 520 nm. As mentioned above, this signal originates from S2 ? S3 transition. This can also be supported by transient absorption experiment (see the supplementary material). The diffraction signal shows obvious decay which is considered to be the depopulation of S2 state. The corresponding dynamic curve at 520 nm is shown in Fig. 2(b). The signal decays obviously with two time constants and then almost comes to a plateau at a very low intensity level which arises from the thermal grating formed by laser energy deposition. The decay process indicates that there are at least two relaxation channels in the S2 state. The signal in short wavelength range centered at about 480 nm shows more complex dynamics. The rise of this signal in the range of 2.5–4 ps indicates that some species that can be probed by this wavelength range is increasing. The species are considered to be the photodissociation products. According to previous works, the most possible products include CH3, NO2, NO, O, OH, CH3O and CH3NO. Among these possible products, only NO2 has absorption

Fig. 4. (a) Intensity normalized TG spectra at different delay time. (b) Peak positions at different delay time.

in the spectral range around 480 nm [23,28]. Thus the rise signal at around 480 nm is considered to come from the formation of NO2. In other works [6–8], NO2 was also reported to be the primary reaction product in some photodissociation experiments on NM. In order to extract the energy relaxation process from the experimental data, we need to know about the energy level structure of NM. The critical energy values on the global potential energy surfaces (PESs) of NM have been calculated theoretically as reported in Refs. [15,16]. Here, we quote some theoretical calculation results in the form of an energy-level diagram, as shown in Fig. 3. The excitation energy 4.67 eV (266 nm) used in our experiment is greater than the energy of transition state (TS) in S2, thus a part of molecules can pass over the TS. In the following relaxation process, the molecules that haven’t passed the TS barrier will lose their energy due to the rapid intramolecular vibrational energy redistribution (IVR) and no longer have enough energy to pass over the TS. The possible processes of these molecules are radiationless relaxation to low-lying state S1 through conical intersections CI2 and CI3. Also due to IVR, CI3 with higher energy cannot be a main relaxation channel. Most of the molecules will relax to S1 state

Fig. 3. Schematic diagram of NM energy-state structure where CI is the abbreviation for conical intersection [16]. The violet dotted line represents the excitation energy 4.67 eV in our experiments. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 6. Energy relaxation path diagram of liquid NM under 266 nm excitation.

Fig. 5. The calculated potential energy surface of the ground and the lowest excited state of NO2. The black curve schematically shows the evolution direction on the ground state PES with the arrow point to the final molecular structure.

through the relative lower CI2 and then go back to the ground state through another internal conversion. Meanwhile, the molecules that have passed the TS barrier will move along the repulsive PES and dissociate to CH3 and ground state NO2 finally. Therefore, the molecules are divided into two parts by transition state at the very beginning and relax through respective channels, which results in two exponential decay components in the relaxation curve. In the resonant TG experiment, the intensity of the diffraction efficiency is proportional to the square of the population in the excited state. In this experiment, two parts of molecules contribute to the population grating: molecules that will relax to low lying state through internal conversion and molecules that will dissociate. Therefore, the experimental data can be fitted by the following equation:

    2t 2t þ A22 exp  þ I0 ; IðtÞ ¼ A21 exp 

s1

s2

where the two exponential decay terms represent the two contributions from the two parts of molecules. Considering that the excess energy will heat the bath when the excited molecules relax to the ground state through radiationless transition, the signal also contains the contribution from thermal grating. But the relaxation time of thermal grating is relatively long, typically on the time scale of nanoseconds to microseconds. So within the ultrafast time scale in our experiment, the contribution from thermal grating hardly changes over time and can be described by a constant I0. The fitting result is shown as the red1 curve in Fig. 2(b). The time constants for radiationless relaxation and dissociation are 0.24 ± 0.02 ps and 2.56 ± 0.22 ps, respectively, and the ratio between the two parts is about 0.73:0.27. It should be mentioned that the triplet states are not considered in this work even though the triplet states play an important role in the photodissociation of many organic molecules [29]. In Ref. [16], Arenas listed several lowest intersystem crossing (ISC) points, including S0/T1 (ISC1), S1/T2 (ISC2), S1/T3 (ISC3) and S2/T4 (ISC4). ISC2 and ISC3 can be easily ruled out because two conical intersections of singlet states coexist with these two ISCs and internal con1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.

version seems to be more efficient than intersystem crossing in this case. In principle, ISC4 is a possible relaxation channel of S2 state. But the excitation energy of 4.67 eV is much lower than ISC4, which rules out the relaxation through ISC4. Besides, in our experiment, there is also not enough indication to show the existence of intersystem crossing. Considering above, the triplet states are not considered here. Besides the relaxation of S2 state, the formation of photodissociation product of NM is also observed, from the contour map shown in Fig. 2(a). It is generally accepted that C–N bond rupture is the primary dissociation channel for gas NM, but there is lack of experimental evidence to show whether such an opinion is also appropriate for liquid NM. Based on the results of this experiment, the spectral range of the product signal helps us to deduce that this signal comes from the photodissociation product NO2 formed after C–N bond rupture. In order to further verify this conclusion, more detailed analyses have been performed on the spectra of photodissociation product. As can be seen from Fig. 2(a), the spectra of the product exhibit blue-shift in 2.5–4 ps. To make it clearer, spectra at different delay time (from 2.4 to 3.3 ps) are normalized and compared together as shown in Fig. 4(a). At 2.4 ps, there is only one obvious peak which is assigned to the excited-state absorption of S2 state. Starting from 2.5 ps, a new spectral component appears from the short wavelength side of S2 state signal, which corresponds to the formation of the photodissociation product. The peak position of this new spectral component can be extracted by multiple-peak fitting. As shown in Fig. 4(b), the peak of the product signal shifts from 495 nm to 480 nm in 2.5–4.0 ps. This blue-shift of spectra can be attributed to the change of the molecular structure of the initial product. For a NM molecule, if its dissociation channel is C–N bond rupture, the dissociation product will change gradually from the structure of nitro group to the structure of NO2 molecule. The bond length and bond angle are 1.25 Å and 153° for nitro group at the transition state of NM [16] and 1.19 Å and 134° for NO2 molecule, respectively. The potential energy surface (PES) of the ground state (from the structure of initial nitro group at TS2 of NM to the free NO2 molecule) and the PES of the corresponding lowest excitedstate were calculated with Gaussian 03 program [30]. The calculated results are shown in Fig. 5. It is obvious that the energy of ground state decreases when NO2 evolves from nitro group to free molecule as shown in Fig. 5. The evolution path is marked as the black line with an arrow toward NO2 molecule. In this evolution process, the energy of the lowest excited-state increases monotonically, resulting in a blue-shift of the absorption of NO2. According to the calculated results, the absorption of NO2 will exhibit

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blue-shift from 843 nm to 420 nm. Actually, the absorption of nitro group near transition state cannot be observed in experiment, because the binding between nitro group and methyl cannot disappear completely. Only when approaching free NO2 molecule, the diffraction signal arising from the absorption of NO2 can be detected. This is basically consistent with the product formation process we detected in the experiment. Therefore, by comparing with the theoretical calculation, we can confirm that the photodissociation product observed in our experiment is NO2, and C–N bond rupture is also the primary dissociation channel for liquid NM. In summary, the dynamics after 266 nm excitation can be depicted by the model shown in Fig. 6. After excitation, about 27% NM molecules will evolve toward the transition state and then dissociate with a time constant of about 2.56 ps. The photodissociation products were confirmed to be CH3 and ground state NO2. The other 73% will undergo fast IVR process and relax to S1 state with a time constant of about 240 fs through the S2/S1 conical intersection. Instead of dissociation, these molecules will go back to the ground state through the S1/S0 internal conversion. So far, to our knowledge, only Rajchenbach et al. have reported the excited-state lifetime of liquid nitromethane and the lifetime they got was 1.1 ± 0.3 ps [24]. In their work, the excited-state lifetime of liquid NM was measured indirectly by monitoring the population in the electronic ground state with CARS. The repopulation time of the ground state would imply the lifetime of the excitedstate. In their model, NM molecules were believed to be excited to the first excited state and then part of the molecules go back to the ground state through radiationless transition. However, according to the theoretical calculations and our experimental result, NM molecules will undergo two internal conversion processes after p⁄ n excitation. Thus, the time constant 1.1 ± 0.3 ps should be the lifetime of the lowest excited-state S1. In this work, the S2 state was directly monitored by TG, so the measured lifetime 240 fs belongs to S2, and it is not conflict with Rajchenbach’s result. In addition, the dissociation ratio (27%) we got is also close to their result (24%). It indicates that the process they observed is identical with what we observed. In contrast, the white light continuum was used as the probe in our experiment and both excited state and the product can be detected directly and simultaneously. Therefore, more information about the photodissociation dynamics of liquid NM can be obtained by our UV-TG technique. 4. Conclusion The femtosecond 266 nm pump/white-light continuum probe transient grating technique provides us an effective experimental approach to study the photodissociation dynamics of energetic materials directly. Liquid nitromethane was used as the sample in this work. It was found that after excitation at 266 nm, about 73% of the excited NM molecules in S2 state decay rapidly through internal conversion to the low lying S1 state with a time constant of 0.24 ps followed by another internal conversion to the ground

state, and the other 27% dissociate finally with a time constant of about 2.56 ps. By combining the peak shift in TG spectra and the calculated structure evolution of NO2, the photodissociation product was confirmed to be NO2. It has been generally accepted that C–N bond rupture is the primary photodissociation channel of gas NM. The observation of NO2 in this work provides direct evidence that such a dissociation channel also applies to liquid NM when excited by 266 nm. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21173063), NSAF (Grant No. U1330106), and the Special Research Project of National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics (Grant No. 2012-S-07). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.04. 048. References [1] L.J. Butler, D. Krajnovich, Y.T. Lee, G. Ondrey, R. Bersohn, J. Chem. Phys. 79 (1983) 1708. [2] N.C. Blais, J. Chem. Phys. 79 (1983) 1723. [3] D.B. Moss, K.A. Trentelman, P.L. Houston, J. Chem. Phys. 96 (1992) 237. [4] E.A. Wade, K.E. Reak, S.L. Li, S.M. Clegg, P. Zou, D.L. Osborn, J. Phys. Chem. A 110 (2006) 4405. [5] Y.Q. Guo, A. Bhattacharya, E.R. Bernstein, J. Phys. Chem. A 113 (2009) 85. [6] K.G. Spears, S.P. Brugge, Chem. Phys. Lett. 54 (1978) 373. [7] P.E. Schoen, M.J. Marrone, J.M. Schnur, L.S. Goldberg, Chem. Phys. Lett. 90 (1982) 272. [8] J.C. Mialocq, J.C. Stephenson, Chem. Phys. 106 (1986) 281. [9] M.S. Park, K.H. Jung, H.P. Upadhyaya, H.R. Volpp, Chem. Phys. 270 (2001) 133. [10] S. Zabarnick, J.W. Fleming, A.P. Baronavski, J. Chem. Phys. 85 (1986) 3395. [11] X.F. Yue, J.L. Sun, Q. Wei, H.M. Yin, K.L. Han, Chin. J. Chem. Phys. 20 (2007) 401. [12] H.S. Kwok, G.Z. He, R.K. Sparks, Y.T. Lee, Int. J. Chem. Kinet. 13 (1981) 1125. [13] R.P. Saxon, M. Yoshimine, Can. J. Chem. 70 (1992) 572. [14] W.F. Hu, T.J. He, D.M. Chen, F.C. Liu, J. Phys. Chem. A 106 (2002) 7294. [15] J.F. Arenas, J.C. Otero, D. Pelaez, J. Soto, J. Chem. Phys. 119 (2003) 7814. [16] J.F. Arenas, J.C. Otero, D. Pelaez, J. Soto, J. Chem. Phys. 122 (2005) 084324. [17] R.S. Zhu, M.C. Lin, Chem. Phys. Lett. 478 (2009) 11. [18] M. Isegawa, F.Y. Liu, S. Maeda, K. Morokuma, J. Chem. Phys. 140 (2014) 244310. [19] N.S. Bayliss, E.G. McRae, J. Chem. Phys. 58 (1954) 1006. [20] S. Nagakura, Mol. Phys. 3 (1960) 152. [21] R.E. Rebbert, N. Slagg, Bull. Soc. Chim. Belg. 71 (1962) 709. [22] S. Paszyc, Photochem. Photobiol. 4 (1965) 841. [23] W.L. Faust, L.S. Goldberg, T.R. Royt, J.N. Bradford, R.T. Williams, J.M. Schaur, P. G. Stone, R.G. Weiss, in: Chemical physics, Picosecond Phenomena, vol. 4, Springer-Verlag, New York, 1978, p. 43. [24] C. Rajchenbach, G. Jonusauskas, C. Rullière, Chem. Phys. Lett. 231 (1995) 467. [25] Y. Wang, Y.F. Song, W.L. Liu, Y.Q. Liu, L.P. Duo, L.L. Jiang, Y.Q. Yang, Chem. Phys. Lett. 633 (2015) 126. [26] M.D. Fayer, Ann. Rev. Phys. Chem. 33 (1982) 63. [27] C. Högemann, M. Pauchard, E. Vauthey, Rev. Sci. Instrum. 67 (1996) 3449. [28] R.M. Mihalcea, D.S. Baer, R.K. Hanson, Appl. Opt. 35 (1996) 4059. [29] K.L. Han, G.Z. He, J. Photochem. Photobiol. C 8 (2007) 55. [30] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, et al., Gaussian 03, Revision E.01, Gaussian Inc, Wallingford CT, 2004.