Visualizing Intramolecular Vibrational Redistribution in

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Visualizing Intramolecular Vibrational Redistribution in Cyclotrimethylene Trinitramine (RDX) Crystal by Multiplex Coherent Anti-Stokes Raman Scattering Guoyang Yu, Yangyang Zeng, Wencan Guo, Honglin Wu, Gangbei Zhu, Zhaoyang Zheng, Xianxu Zheng, Yunfei Song, and Yanqiang Yang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b00069 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 23, 2017

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The Journal of Physical Chemistry

Visualizing Intramolecular Vibrational Redistribution in Cyclotrimethylene Trinitramine (RDX) Crystal by Multiplex Coherent Anti-Stokes Raman Scattering Guoyang Yu, Yangyang Zeng, Wencan Guo, Honglin Wu, Gangbei Zhu, Zhaoyang Zheng, Xianxu Zheng, Yunfei Song*, and Yanqiang Yang* National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, 621900, P. R. China

ABSTRACT: The femtosecond time-resolved multiplex coherent anti-Stokes Raman scattering (CARS) technique has been performed to investigate the intramolecular vibrational redistribution (IVR) through vibrational couplings in RDX molecules. In the multiplex CARS experiment, the supercontinuum (SC) was used as broadband Stokes light to coherently and collectively excite multiple vibrational modes, and quantum beats arising from vibrational couplings among these modes were observed. The IVR of RDX is visualized by a topological graph of these vibrational couplings. And with analysis of the topological graph, two vibrational modes, both of which are assigned to ring bending, are confirmed to have coupling interactions with most of other vibrational modes, and are considered to have a tendency of energy transfer with these

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vibrational modes. We suggest that the mode at 466 cm-1 is a portal of energy transfer from outside to inside of the RDX molecule and the mode at 672 cm-1 is an important transit point of energy transfer in the IVR.

1. Introduction Energetic materials, in the form of explosives, propellants and pyrotechnics, are widely applied in military affairs and civilian engineering. Thus, a lot of research work has been undertaken to study the reaction mechanism to improve the safety and the property of energetic materials.1-7 According to these studies, it is confirmed that macroscopic properties of energetic materials are closely related to a series of physical and chemical processes on atomic or molecular scale and from femtosecond to nanosecond time scales, and reaction behaviors of energetic materials are diverse under different excitation conditions. On the microscopic level, the shock front or heating usually can be regard as a superposition of phonons, and vibrations of chemical bonds can be treated as a series of vibrons. Before bond breaking, energetic material molecules will undergo a series of energy transfer processes under the shock wave. First, a number of phonons are produced when energetic materials are shocked. Second, the low-frequency vibrational modes are excited through phonon-vibron coupling, which is termed multiphonon up-pumping.8-10 Third, the vibrational energy transfers from one vibrational mode to another due to the vibrational coupling, which is termed IVR.11 At last, the IVR makes energies concentrate on one or a few chemical bonds to give rise to bond breaking, and then the ignition begins. Thus, the IVR can directly influence the direction of vibrational energy flow in molecules and also can decide the cleavage of the first reaction bond which is closely related to the reaction rate and the quantity of energy releasing in the decomposition of energetic materials.


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RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) is a kind of typical high explosive, and is widely employed in military applications because of its insensitivity to extensive stimulation, high performance level and inexpensive synthesis. RDX can exist in five polymorphic phases: α, β, γ, δ, and ε depending on the external conditions, until now three of these phases (α, γ, and ε) can be confirmed to contribute to the decomposition.12-15 And the initial reaction or the decomposition channel of RDX can be deduced in photodissociation16, thermal decomposition17, and shock wave induced decomposition18,19. These studies build solid basis for establishing the reaction mechanism of RDX. However, the reason why RDX has such reaction behaviors on certain conditions is still not very clear. We suggest that the research on the IVR of RDX is helpful for solving this problem. However, tracking the IVR is very difficult, because the IVR is caused by couplings among vibrational modes, and so it is an ultrafast and coherent process.20 In addition, especially for polyatomic molecules such as most of energetic materials, numerous of vibrational modes participate in the IVR which makes this process more complex. According to these difficulties mentioned above, the method which is able to track the IVR must have high temporal resolution, broadband excitation capability, and coherent detection capability. Based on these requirements, the technique named multiplex CARS obviously can be used to investigate the IVR. The multiplex CARS technique can coherently excite numerous of vibrational modes simultaneously because the SC acting as Stokes light is broadband. In this coherent excitation process, the equilibrium of vibrational energy in molecules is broken by pump and Stokes light. In the subsequent IVR, amplitudes and phases of vibrational modes will evolve to establish a new energy equilibrium, which can be regard as vibrational couplings.21,22 Thus, the IVR can be qualitatively evaluated by the multiplex CARS technique.


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In this work, the femtosecond time-resolved multiplex CARS has been performed to observe vibrational couplings in RDX molecules in order to investigate the IVR. Seventeen vibrational modes in RDX molecules were collectively excited and these vibrational modes are assigned according to the Raman spectrum. The vibrational couplings among these modes are confirmed by Fourier transform power spectra of quantum beats. And the IVR of RDX is visualized by a topological graph of these vibrational couplings. Two vibrational modes, both of which are assigned to ring bending, are confirmed to have coupling interactions with most of other vibrational modes, so they are considered to have a tendency of energy transfer with these modes in the IVR of RDX. We suggest that the mode at 466 cm-1 is a portal of energy transfer from outside to inside of the RDX molecule and the mode at 672 cm-1 is an important transit point of energy transfer. 2. Experiment The experimental setup is shown in Figure 1. A femtosecond pulse (130 fs, 800 nm, 1 kHz) emitted from a Ti: sapphire regeneration amplified laser system (Spitfire, Spectra-Physics) was split into three by two beam splitters. Two of them with wave vector k1 and k3 acted as pump and probe light, respectively. The third one was focused into the distilled water by an off-axis parabolic mirror to generate the SC acted as Stokes light with wave vector k2. The three beams were spatially focused into the RDX crystal in the folded box geometry. The pulse temporal sequence was adjusted by optical delay lines. The interval between the pump and the Stokes pulses (t12) was set to zero and the interval between the pump (Stokes) and the probe pulses (t23) was scanned. In temporal domain, the time when the pump, the Stokes and the probe pulses simultaneously arrived at the sample was defined as zero time. The CARS signal along the


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phase-matching direction k4=k1-k2+k3 was captured by a spectrometer (500IS/SM, Bruker Optics) equipped with a CCD (DU440-BU2, Andor). The energy of the pulse which was used to generate SC was about 1 µJ and the focal spot was 50 µm. Due to the effect of self-focusing, the filament of SC at focal point was about several micrometers. The convergent SC was collimated by a conjugated off-axis parabolic mirror. The spectral region of SC is about from 450 nm to 880 nm. However, in the CARS experiment, this value was about from 820 nm to 880 nm because the long-wavelength pass filter cut off below 820 nm wavelength component. In this wavelength region, the chirp of SC is not obvious, so it has a little effect on CARS experimental results (See the Supporting Information, Figure S1.). In addition, the energy of pump pulse was attenuated to 1 µJ and the energy of probe pulse was set to be much smaller than the one of pump pulse. The RDX crystal sample used in this experiment was prepared by the solvent crystallization in our laboratory. In the sample preparation, the acetone (analytical reagent) as solvent was added to the RDX powder (purity is about 99%) to prepare saturated solution. And then, the beaker which was a container of solution was sealed but left a small hole to allow the solvent vapor to slowly evaporate from the saturated solution at room temperature. Over time, RDX crystals separated out from the solution. (See the Supporting Information, Figure S2.)


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Figure 1. Schematic diagram of the experimental setup. BS: Beam Splitter (10%R : 90%T), M: Mirror, OAPM: Off-Axis Parabolic Mirror, L: Lens, NF: Notch Filters, CCD: Charge Couple Device. 3. Results 3.1 Time- and Frequency-Resolved CARS Spectrum. The time- and frequency-resolved CARS spectrum of RDX can be shown as a contour map of intensity. As shown in Figure 2(a), the excited frequency range is from 250 to 1000 cm-1, and the detecting time range is from -0.5 ps to 2.5 ps. The CARS signal has spectral structures in frequency domain and oscillation structures in temporal domain, which indicates that the CARS signal of RDX is very complicated. However, it still can be analyzed by some methods such as the confirmation of vibrational mode in frequency domain and the Fourier transform in temporal domain. In Figure 2(a), the frequency


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and time-resolved spectra on dash lines are taken as examples to describe general cases of CARS signal in frequency and temporal domain, respectively. And the detailed descriptions are shown in the section 3.2 and 3.3.

Figure 2. (a) Time- and frequency-resolved CARS spectrum of RDX shown by the contour map. (b) Frequency-resolved CARS spectrum at 600 fs (solid line), spontaneous Raman (dash line) and calculated Raman (dot line) spectra. (c) Time-resolved CARS spectrum at 350 cm-1 (solid circles). The dynamics curve contains three components: Gaussian curve (short dash line), exponential decay curve (dash line and dash dot line), and oscillation curve (dot line). 3.2 Frequency-Resolved CARS Spectrum. The frequency-resolved CARS spectrum at 600 fs is chosen to describe the general case of CARS signal in frequency domain for avoiding the effect of non-resonant electronic background around zero time. As shown in Figure 2(b), this spectrum has several peaks which have broad spectral widths. The femtosecond pulse used as


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probe pulse is broadband, resulting in the decrease of spectral resolution. For each vibrational mode, the linewidth of signal peak in the CARS spectrum is equal to the linewidth of probe pulse, which is about 150 cm-1 [Full Width at Half Maximum (FWHM)] in our experiment. So, when numerous vibrational modes were excited simultaneously, the corresponding signal peaks overlapped with each other, which results in the spectral congestion. In addition, the pump and Stokes pulses both had spectral structures which also modulated the intensity of CARS spectrum. In our previous work, we observed similar cases in IR780/NM solution, and demonstrated that the bad spectral resolution did not influence the analysis of vibrational coupling.23 Table 1. Raman Modes of α-RDX

a

Frequency (cm-1)

Mode

Experiment

b

Assignmentc

Calculation

ν1

210

molecular b

ν2

231

N-NC(2) u (eq)

ν3

350

329

molecular st

ν4

418

406

ring b (flattening)

ν5

466

441

ring b (f)

ν6

491

469

ring b (f)

ν7

594

588

ring tw

ν8

609

609

ring r

ν9

672

672

ring b

ν10

741

756

N-NO2 u (eq)

ν11

760

770

N-NO2 u (ax)

ν12

790

798

N-C-N sci, NO2 sci

ν13

851

854

C-N st, NO2 sci (eq)

ν14

861

868

C-N st, NO2 sci


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a

ν15

887

896

C-N st

ν16

947

952

CH2 r

ν17

1034

1015

N-N st (eq), CH2 r

The vibrational modes are named by νn (n=1, 2, 3…).

b

In the spontaneous Raman experiment, the vibrational modes ν1 and ν2 cannot be observed because the notch filter which was used to filter out the Rayleigh scattering to cut off frequency components below 300 cm-1. c

Abbreviation: b, bending; u, umbrella; st, stretching; f, folding; tw, twisting; r, rocking; sci, scissoring; eq, equatorial; ax, axial. For Raman modes assignment, the Raman spectrum of α-RDX was calculated by Ab initio calculations using the B3LYP Density Functional Theory method with the Sadlej’s mediumsized polarized basis set (SadlejpVTZ). Calculations were performed with the Gaussian 03 computation package.24 This calculation work concerning the Raman spectrum of RDX was described in detail in our previous work.25 And here we only used a part of the results to analyze the CARS experimental results. The calculated and experimental Raman spectra were shown in Figure 2(b). And Table 1 shows Raman modes of α-RDX with the frequency range from 200 cm1

to 1100 cm-1. It is confirmed that the calculated and experimental results accord fairly well,

which indicates that the calculation is reliable. According to the Raman spectra, we suggest that seventeen vibrational modes were simultaneously excited in the CARS experiment. 3.3 Time-Resolved CARS Spectrum. The time-resolved CARS spectrum at 350 cm-1 is taken as an example to describe the general case of dynamics because time-resolved CARS spectra at other wavelengths have similar behaviors. As shown in Figure 2(c), the Gaussian curve near zero time in the dynamic curve is the non-resonant electronic background which is related to the electron response to the electric field component of light. Usually, the relaxation of timeresolved CARS spectrum is described by26


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S AS (t ) ∝ C12 exp(− 2t τ 1 ) + C 22 exp(− 2t τ 2 ) + 2C1C 2 exp[− t (1 τ 1 + 1 τ 2 )]cos[(ω1 − ω2 )t − (φ1 − φ2 )]

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(1)

where Ci, τi, ωi, and φi (i = 1, 2) represent the intensity of vibrational transition, the population relaxation time of vibrational state, the angular frequency and the phase, respectively. And the variable t denotes the delay time. As shown in the inset of Figure 2(c), the dynamic curve at 350 cm-1 has a fast decay component with 0.73 ps and a slow decay component with 1.2 ps, both of which are caused by the population relaxation. In addition, the dynamic curve also has an oscillation which is result of the vibrational coupling and is usually called quantum beats. The quantum beat frequency in this curve is about 120 cm-1 which corresponds to the vibrational coupling between the molecular stretching (ν3) and the ring bending modes (ν5). Thus, the timeresolved CARS spectrum has three components: the Gaussian curve, the exponential decay, and the oscillation, which are corresponding to the non-resonant electronic background, the population relaxation, and the vibrational coupling, respectively. 4. Discussion 4.1 Confirmation of Vibrational Couplings in RDX Molecules. Vibrational couplings in RDX molecules can be shown by a two-dimensional (2D) frequency spectrum which is formed by Fourier transform power spectra at every excited frequency. According to Eq. (1), the quantum beat frequency is equal to the frequency difference (ω1- ω2) between vibrational modes which participate in vibrational coupling. And the carrier frequency is located at (ω1 +ω2)/2.27 Actually, as shown in Figure 3, due to the effect of the bandwidth of signal, Fourier transform power spectra of quantum beats are a series of carrier frequency-centered long stripes which parallel to the Raman shift axis. According to this 2D frequency spectrum, the beat frequency and the corresponding participant modes are listed in Table 2.


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Figure 3. 2D frequency spectrum formed by Fourier transform power spectra of quantum beats. The mark “QB” denotes quantum beat. The Raman spectrum (solid line) is observed from the spontaneous Raman experiment. Table 2. Assignment of the Vibrational Coupling between the Vibrational Modes of RDX Vibrational couplinga

Beat frequency (cm-1)

Participant mode (cm-1)

QB1

115

ν1(210)&ν3(329)

QB2

120

ν3(350)&ν5(466)

QB3

45

ν4(418)&ν5(466)

QB4

80

ν4(418)&ν6(491)

QB5

33

ν5(466)&ν6(491)

QB6

66

ν8 (609)&ν9(672)

QB7

100

ν6(491)&ν7(594)

QB8

146

ν5(466)&ν8(609)

QB9

94

ν7(594)&ν9(672)

QB10

171

ν8(609)&ν12(790)


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QB11

211

ν5(466)&ν9(672)

QB12

100

ν9(672)&ν11(760)

QB13

140

ν9(672)&ν12(790)

QB14

42

ν10(741)&ν12(790)

QB15

100

ν11(760)&ν14(860)

QB16

44

ν12(790)&ν13(851)

QB17

44

ν13(851)&ν15(887)

QB18

56

ν15(887)&ν16(947)

QB19

150

ν15(887)&ν17(1034)

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a

The dynamics within excited frequency range exists nineteen vibrational couplings, marked QBn (n=1, 2, 3, …). 4.2 Description of the IVR in the RDX Molecule. The IVR can be qualitatively analyzed from the information of vibrational couplings because the IVR is caused by coupling interactions among vibrational modes. To facilitate the discussion of the tendency of the IVR, the vibrational couplings listed in Table 2 are shown by a topological graph. As shown in Figure 4, the behaviors of the vibrational modes can roughly be divided into two categories in the IVR of RDX. The first is that the vibrational mode can interact with two or more other vibrational modes. And most of these modes such as the vibrational modes ν4, ν5, ν6, ν7, ν8 and ν9 are corresponding to the hexatomic ring in the RDX molecule or the N-C-N bond in the hexatomic ring. According to some previous research work,16-18 it is confirmed that the ring-opening reaction usually does not occur in the initial reaction of RDX. Thus, we suggest that these modes could be energy transit points in the IVR. The second is that the vibrational mode only can be coupled with one other vibrational mode. And most of these modes such as the vibrational modes ν10, ν14, ν16 and ν17 are corresponding to the N-O bond in the NO2 group, the C-H bond in the CH2 group, or the N-N bond, which are usually most susceptible to break first during the


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chemical decomposition of RDX.16-18 Thus, we suggest that these modes could be the energy centralization points in the IVR. However, we can not confirm that which the two categories the rest of vibrational modes in Figure 4 belong to because the direction of energy transfer in the IVR can not be determined by the multiplex CARS technique. In the first category of the vibrational mode, especially for vibrational modes ν5 and ν9, they can interact with most of vibrational modes and with each other, which indicates that they have a tendency of energy transfer with these vibrational modes. The vibrational mode ν5 has a relatively low frequency, which is relatively easy to match the frequency of phonon28, so we suggest that the vibrational mode ν5 at 466 cm-1 might be a portal of energy transfer from outside to inside of the RDX molecule in the multiphonon up-pumping. And the vibrational mode ν9 has a relatively high frequency, so we suggest that the vibrational mode ν9 at 672 cm-1 might be an important transit point which makes the energy transfer to higher vibrational states in the IVR. Thus, the two vibrational modes play an important role in the initial reaction of RDX. According to the above analysis, we deduce the IVR of RDX as shown in the inset of Figure 4. The multiphonon up-pumping makes the energy in the environment transfer to the hexatomic ring of the RDX molecule. Then, through the IVR, the energy on the hexatomic ring transfers to some chemical bonds such as the N-O bond, the C-H bond, or the N-N bond, which makes the energy concentrate on these bonds and ultimately could result in the cleavages of these bond in the initial reaction of RDX.


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Figure 4. Topological graph of vibrational couplings in the RDX molecule. The double arrow represents coupling between two vibrational modes. Inset: schematic diagram of energy transfer in the RDX molecule. The oriented arrow denotes the direction of energy transfer in the multiphonon up-pumping and the IVR.


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5. Summary In summary, the femtosecond time-resolved multiplex CARS technique is an efficiency method to investigate the IVR. The information of vibrational couplings was provided by Fourier transform power spectra of quantum beats, and the IVR of RDX is visualized by the topological graph of vibrational couplings. According to the results, the vibrational modes at 466 cm-1 (ν5) and 672 cm-1 (ν9) are consider to have a tendency of energy transfer with most of the vibrational modes, and both of them are assigned to ring bending. We suggest that the mode ν5 is a portal of energy transfer from outside to inside of the RDX molecule and the mode ν9 is an important transit point of energy transfer in the IVR. Thus, the two vibrational modes play an important role in the initial reaction of RDX. In our future work, the multiplex CARS spectroscopy will be combined with a diamond anvil cell (DAC) and a heating equipment to observe the effect of pressure and temperature on the IVR of energetic materials, which will be used to research the reaction behavior under different conditions relevant to the shock wave induced and the thermal decompositions. ASSOCIATED CONTENT Supporting Information Characterization of the SC, and Sample Preparation. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ACKNOWLEDGMENT


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This work was financially supported by the NSFC (Grant number 11404307) and the SCP (Grant number JCKY2016212A501). REFERENCES (1) Dreger, Z. A.; Gruzdkov, Y. A.; Gupta, Y. M.; Dick, J. J. Shock Wave Induced Decomposition Chemistry of Pentaerythritol Tetranitrate Single Crystals: Time-Resolved Emission Spectroscopy. J. Phys. Chem. B. 2002, 106, 247-256. (2) Zhurova, E. A.; Tsirelson, V. G.; Stash, A. I.; Yakovlev, M. V.; Pinkerton, A. A. Electronic Energy Distributions in Energetic Materials: NTO and the Biguanidinium Dinitramides. J. Phys. Chem. B. 2004, 108, 20173-20179. (3) Zhu, W.; Huang, H.; Huang, H.; Xiao, H. Initial Chemical Events in Shocked Octahydro1,3,5,7-Tetranitro-1,3,5,7-Tetrazocine: A New Initiation Decomposition Mechanism. J. Chem. Phys. 2012, 136, 044516. (4) Ge, N. N.; Wei, Y. K.; Ji, G. F.; Chen, X. R.; Zhao, F.; Wei, D. Q. Initial Decomposition of the Condensed-Phase β-HMX under Shock Waves: Molecular Dynamics Simulations. J. Phys. Chem. B. 2012, 116, 13696-13704. (5) Guo, Y. Q.; Greenfield, M.; Bhattacharya, A.; Bernstein, E. R. On the Excited Electronic State Dissociation of Nitramine Energetic Materials and Model Systems. J. Chem. Phys. 2007, 127, 154301. (6) Yu, Z.; Bernstein, E. R. Experimental and Theoretical Studies of the Decomposition of New Imidazole Based Energetic Materials: Model Systems. J. Chem. Phys. 2012, 137, 114303.


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(16) Guo, Y. Q.; Greenfield, M.; Bernstein, E. R. Decomposition of Nitramine Energetic Materials in Excited Electronic State: RDX and HMX. J. Chem. Phys. 2005, 122, 244310. (17) Maharrey, S.; Jr. Behrens, R. Thermal Decomposition of Energetic Materials. 5. Reaction Processes of 1,3,5-Trinitrohexahydro-s-Triazine below Its Melting Point. J. Phys. Chem. A. 2005, 109, 11236-11249. (18) Patterson, J. E.; Dreger, Z. A.; Miao, M.; Gupta, Y. M. Shock Wave Induced Decomposition of RDX: Time-Resolved Spectroscopy. J. Phys. Chem. A. 2008, 112, 7374-7382. (19) Miao, M.; Dreger, Z. A.; Patterson, J. E.; Gupta, Y. M. Shock Wave Induced Decomposition of RDX: Quantum Chemistry Calculations. J. Phys. Chem. A. 2008, 112, 73837390. (20) Nesbitt, D. J.; Field, R. W. Vibrational Energy Flow in Highly Excited Molecules: Role of Intramolecular Vibrational Redistribution. J. Phys. Chem. 1996, 100, 12735-12756. (21) Felker, P. M.; Zewail, A. H. Dynamics of Intramolecular Vibrational Energy Redistribution (IVR). I. Coherence Effects. J. Chem. Phys. 1985, 82, 2961-2974. (22) Felker, P. M.; Zewail, A. H. Dynamics of Intramolecular Vibrational Energy Redistribution (IVR). II. Excess Energy Dependence. J. Chem. Phys. 1985, 82, 2975-2993. (23) Wu, H.; Song, Y.; Yu, G.; Chen, X.; Yang Y. Vibrational Dynamics of Nitromethane Mixed with IR780 Dye Studied by Coherent Anti-Stokes Raman Spectroscopy. J. Raman Spectrosc. 2016, 47, 1213-1219.


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(24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Jr. Montgomery,

J. A.; Vreven, T. ;Kudin, K. N.; Burant, J. C., et al. Gaussian 03;

Gaussian, Inc.: Wallingford, CT, 2004. (25) Zeng, Y.; Song, Y.; Yu, G.; Zheng, X.; Guo, W.; Zhao, J.; Yang, Y. A Comparative Study of 1,3,5-Trinitroperhydro-1,3,5-Triazine (RDX) and Octahydro-1,3,5,7-Tetranitro-1,3,5,7Tetrazocine (HMX) under High Pressures Using Raman Spectroscopy and DFT Calculations. J. Mol. Struct. 2016, 1119, 240-249. (26) Leonhardt, R.; Holzapfel, W.; Zinth, W.; Kaiser, W. Terahertz Quantum Beats in Molecular Liquids. Chem. Phys. Lett. 1987, 133, 373-377. (27) Paskover, Y.; Shalit, A.; Prior, Y. Four Wave Mixing Spectroscopy at the Interface between the Time and Frequency Domains. Opt. Comm. 2010, 283, 1917-1923. (28) Hooper, J. Vibrational Energy Transfer in Shocked Molecular Crystals. J. Chem. Phys. 2010, 132, 014507.


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TOC Graphic


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Figure 1. Schematic diagram of the experimental setup. BS: Beam Splitter (10%R : 90%T), M: Mirror, OAPM: Off-Axis Parabolic Mirror, L: Lens, NF: Notch Filters, CCD: Charge Couple Device. 109x66mm (300 x 300 DPI)



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Figure 2. (a) Time- and frequency-resolved CARS spectrum of RDX shown by the contour map. (b) Frequency-resolved CARS spectrum at 600 fs (solid line), spontaneous Raman (dash line) and calculated Raman (dot line) spectra. (c) Time-resolved CARS spectrum at 350 cm-1 (solid circles). The dynamics curve contains three components: Gaussian curve (short dash line), exponential decay curve (dash line and dash dot line), and oscillation curve (dot line). 108x65mm (300 x 300 DPI)


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Figure 3. 2D frequency spectrum formed by Fourier transform power spectra of quantum beats. The mark “QB” denotes quantum beat. The Raman spectrum (solid line) is observed from the spontaneous Raman experiment. 177x124mm (300 x 300 DPI)



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Figure 4. Topological graph of vibrational couplings in the RDX molecule. The double arrow represents coupling between two vibrational modes. Inset: schematic diagram of energy transfer in the RDX molecule. The oriented arrow denotes the direction of energy transfer in the multiphonon up-pumping and the IVR. 178x178mm (300 x 300 DPI)


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TOC Graphic 40x35mm (300 x 300 DPI)


Visualizing Intramolecular Vibrational Redistribution in

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