Vibrational vave-packet dynamics in CV670 investigated by broadband femtosecond TR-CARS spectroscopy

Vibrational vave-packet dynamics in CV670 investigated by broadband femtosecond TR-CARS spectroscopy

Vibrational Spectroscopy 96 (2018) 106–109 Contents lists available at ScienceDirect Vibrational Spectroscopy journal homepage: www.elsevier.com/loc...

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Vibrational Spectroscopy 96 (2018) 106–109

Contents lists available at ScienceDirect

Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec

Vibrational vave-packet dynamics in CV670 investigated by broadband femtosecond TR-CARS spectroscopy Ping Hea,c,* , Guangming Wanga , Yu Zhanga , Huili Wanga , Ke Hanb,** , Rongwei Fanc , Deying Chenc a Research Center of Ecological Science & Femtosecond Laser Technology, College of Foundation Science, Harbin University of Commerce, Harbin, 150028, China b School of Computer and Information Engineering, Harbin University of Commerce, Harbin, 150028, China c National Key Laboratory of Science and Technology on Tunable Laser, Harbin Insitute of Technology, Harbin, 150080, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 3 November 2017 Received in revised form 21 March 2018 Accepted 24 March 2018 Available online 26 March 2018

Broadband femtosecond time-resolved coherent anti-Stokes Raman scattering ((TR-CARS)) is utilized to investigate the wave-packet dynamics of Cresyl Violet (CV670) dye molecules in ethanol solvent at room temperature. An interesting behavior of wave-packet dynamics phenomena is observed and discussed for the first time, and several unknown Raman vibrational modes with frequency differences of 32 cm1, 38 cm1, 45 cm1, 50 cm1, 55 cm1, 65 cm1, 80 cm1, 95 cm1 and 101 cm1 are excited and obtained at the same time. This work makes possible high efficiency in the structure investigation as well as dynamics of intra-molecular processes by broadband femtosecond (TR-CARS) spectroscopy with fast Fourier transformation (FFT) analysis means. © 2018 Elsevier B.V. All rights reserved.

Keywords: Broadband femtosecond CARS Time-resolved spectroscopy CV670 dye Vibrational dynamics

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . Methodology and experimental Results and discussion . . . . . . . Summary . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . References . . . . . . . . . . . . . . . . .

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1. Introduction In our previous publications, we have demonstrated that a selective excitation of specific vibrational coherences in a gas or liquid phase molecule is possible by using a resonance enhanced method in a coherent anti-Stokes Raman scattering (CARS) process, while a time delayed probe pulse ensures an efficient non-resonant background suppression [1–8]. On the one hand, the potential high selectivity of the proposed technique is based on the

* Corresponding author at: Research Center of Ecological Science & Femtosecond Laser Technology, College of Foundation Science, Harbin University of Commerce, Harbin, 150028, China. ** Corresponding author. E-mail addresses: [email protected] (P. He), [email protected] (K. Han). https://doi.org/10.1016/j.vibspec.2018.03.004 0924-2031/© 2018 Elsevier B.V. All rights reserved.

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fact that the CARS signal can be sensitive to both the electronic and vibrational signature of category under investigation. On the other hand, excitation with high intensity ultra-short (femtosecond) pulses can bring about high optical gain, and associated high brightness of the CARS signal, which offers prospects of high sensitivity and high selectivity of detection [9,10]. The current efforts of numerous groups on femtosecond CARS in organic systems have emphasized separation and recognition of individual vibrational modes. Due to the fact that the ultra-short femtosecond laser pulses are spectrally broad, in most cases several molecular states are coherently excited, resulting in wavepacket excitation and probing [11,12]. In fact, the separation of such individual frequencies and interpretation of these results are challenging. Furthermore, these measurements rely on the controlled shaping of ultra-short laser pulses to excite individual

P. He et al. / Vibrational Spectroscopy 96 (2018) 106–109

vibrational modes selectively, and thus require highly controlled conditions, such as laser-pulse configuration optimizing, laser polarization controlling, etc [13–16]. In this paper, we use a hybrid technique that combines 33 fs CARS spectroscopy with the advantages of time-resolved technique to directly observe the vibrational wave-packet dynamics of CV670 dye molecules in ethanol solvent (5  105 mol/L). By detecting the CARS signal of CV670 molecules, an interesting behavior of wave-packet dynamics phenomena is observed. By taking the fast Fourier transformation (FFT) of the 33 fs (TR-CARS) trace, the beat frequency spectrum with frequency differences of 32 cm-1, 38 cm-1, 45 cm-1, 50 cm-1, 55 cm-1, 65 cm-1, 79 cm-1, 95 cm1 and 101 cm-1 are obtained at the same time. The results show that 33 fs laser pulses allow a multitude of processes for preparation and investigation of molecular coherences with (TR-CARS) spectroscopy technique: no complicated controlled conditions are required for the 33 fs laser pulses. The possibility to excite and investigate several vibrational modes by broadband femtosecond (TR-CARS) spectroscopy with FFT analysis method, it opens up clear and quick pathways for the structure analysis as well as dynamics of intra-molecular processes. 2. Methodology and experimental The basic idea of the broadband femtosecond (TR-CARS) experiment is shown in Fig. 1. As shown in Fig. 1a) three laser pulses interact with the low concentration of CV670 molecules. Two laser pulses called the pump (vpu) and probe (vpr) pulses have the same frequencies, vpu = vpr. The third laser (Stokes) is tuned to a lower frequency, vS, in such a manner that the difference in tuning between pump and Stokes laser frequencies is responsible for resonance with someone vibrational transition in the ground state of CV670 molecules. The pump-Stokes laser pulses create a coherent excitation of molecules (historically called the Raman coherence) in the sample at time t0. The third laser pulse (probe: vpr) interacts with this coherent state at time t1, creating a signal called the anti-Stokes pulse (vCARS) that can be used to map out the molecular resonances that identify particular chemical species or vibrational mode. For a homodyne detection scheme, the ensemble CARS signal (Fig. 1a), given by the third-order polarization Pð3Þ [17,18], Z 1  2 dtpð3Þ ðt; DtÞ ð1Þ SðDtÞ ¼ 1

where t is the time and Dt denotes the positive temporal separation between the single pump pulse and the Stokes-pump

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pulse pair, reduces to the damped quantum beats of the prepared vibrational coherence: X   a a 0 cos v  v 0 eðg n þg n0 ÞDt ð2Þ SðDtÞ / n;n0

n n

n

n

with phenomenological dephasing rates g n;n0 that arise from the within sample averaging indicated by the angle brackets in Eq. (1). Fig. 1b) shows the broad spectral bandwidth of the pump and Stokes frequency pairs that contribute to the excitation of the Raman coherence for the transitions near the (vpuvS)/ 2pc = 1064 cm1 vibrational band of CV670. The schematic of our experimental setup is shown in Fig. 2. The experimental setup is described briefly, as following: The laser pulses from a commercial femtosecond laser system Ti: sapphire regenerative amplifier (Coherent Inc.) at the central wavelength of 800 nm (6 mJ/pulse,1 kHz and 33 fs) is split into two parts to pump two optical parametric amplifiers (TOPAS: Topas-Prime 1 and Topas-Prime 2 in Fig. 2). Two independent wavelengths in the range of 266–1150 nm can be generated by the TOPAS. The output of Topas-Prime 2 is split into two parts by a splitter to obtain the pump and probe pulses, while the output of Topas-Prime 1 serves as the Stokes pulse. For the 33 fs TR-CARS, a folded BOXCARS configuration is employed to separate the signal from the incoming pump-Stokes and probe beams. The generated beams of blueshifted photons (CARS signal), whose propagation directions are determined by the phase-matching conditions [19], ~ kpr þ ð~ kpu  ~ kS Þ kCARS ¼ ~

ð3Þ

The CARS signal is filtered by a spatial filter and collected by a silica fiber, dispersed in a spectrometer (ANDOR SR-750-B1-R) and detected by a fast photo- multipliter tube (PMT). The CARS signalto-noise ratio (SNR) is further enhanced by using a boxcar integrator (SR250). 3. Results and discussion The Fig. 3 shows the wavelength arrangement of the laser beams together with the CARS spectrum of CV670 in ethanol solvent (5  105 mol/L). The CARS signal at 594 nm is generated by a Stokes wavelength at 681 nm and a pump and probe wavelength at 635 nm. From the figure, it can determined that the half-width of the anti-Stokes line of the CARS spectrum of CV670 in ethanol solvent amounts to 545 cm-1,while the half-width of the pumpStokes lines at 635 nm and 681 nm are about 441 cm-1 and 455 cm1 , respectively.

Fig. 1. Methodology of the bandwidth femtosecond (TR-CARS) experiment: a) The Energy diagram of the (TR-CARS) process, b) Schematic illustration of the pump-Stokes frequency pairs that contribute to the excitation of the Raman coherence near the 1064 cm1 of CV670.

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P. He et al. / Vibrational Spectroscopy 96 (2018) 106–109

Fig. 2. 33 fs TR-CARS experimental setup: femtosecond laser and detecting system.

Fig. 3. Spectra of CARS signal, the probe and pump/Stokes excitation pulses in CV670 solution.

Examples of time traces that are characteristic of 33 fs (TRCARS) signal in the CV670-ethanol mixtures are shown in Fig. 4(A), together with the signal of ethanol solvent recorded under identical conditions 4(B). Comparing curve A with curve B, it’s easy to see an interesting vibrational wave-packet dynamics phenomenum of fluctuation and nodes appearing alternately as presented in Fig. 4(A). The node on the envelope of the response function is caused by the beat frequency between different vibrational transitions in the CV670 molecules. It’s apparent that the ethanol solvent signal decays in 1 ps (Fig. 4 curve B), and the signal does not fluctuate, while the signal of CV670-ethanol mixture solution persists for the 10 ps duration of the measurement. This indicates that the vibrational wave-packet dynamics prepared and excited belonging to CV670 molecules under lpu=lpr = 635 nm (15,748 cm1), lS = 681 nm (14,684 cm1), laS = 594 nm (16,835 cm1), and when the frequency difference (vpu-vS)/2pc = 1064 cm1 is detuned from the ethanol solvent molecular resonance. As shown in Fig. 4(A), the main extended periods of oscillations show an average period of T = 0.432 ps. The frequency difference (cm1) is 1012/(C*Dt), where Dt is the delay time interval (ps) and C is the velocity of light (cm/s). Thus, the

Fig. 4. TR-CARS traces in the CV670-ethanol mixture solution (A) together with the signal of the ethanol solvent recorded under identical conditions (B).

P. He et al. / Vibrational Spectroscopy 96 (2018) 106–109

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Fig. 5. FFT power spectra of the TR-CARS trances from Fig. 4.

average period of the oscillations corresponds to an average vibrational wave-number spacing of about 77 cm1, corresponding to several vibrational wavepacket dynamics in the ground state of CV670 molecules. Due to the anharmonicity of the potential, the wavepacket spreads or dephases but undergoes a series of rephasings and partial rephasings, leading to revival (fluctutation) and fractial revival (node) structures [20]. In order to further analyze the experimental data shown in Fig. 4(A) and (B), we give the results of a fast Fourier transform (FFT) of the transients in the corresponding Fig. 5(A) and (B). In both FFT spectra, peaks in Fig. 5(A) and no peaks in 5(B) confirm that the oscillations in the transient of CV670 molecules result from multiple coherent superposition of vibrational eigenstates near 1046 cm1 with energy spacings centered around 80 cm1, 65 cm1, 95 cm1, 101 cm1, 55 cm1, 50 cm1, 45 cm1, 38 cm1, and 32 cm1, etc. As is known, the fundamental peak in the FFT shows coherences between nearest-neighbor vibrational levels and is centered on the average vibrational frequency of the wavepacket (vn). When resolved in the FFT [21,22], each peak corresponds to the vibrational energy level splitting between n and its nearest-neighbor n + 1. The peaks at this fundamental frequency are separated by the vibrational anharmonicity term vexe [18]. Comparing the FFT power spectra of the Fig. 5(A) and 5(B), the FFT power spectra of the (TR-CARS) signals illustrate that when the frequency difference (vpu-vS)/2pc = 1064 cm-1 is detuned from the ethanol solvent molecular resonance, only the constant background of the three-beam electronic CARS of CV670 molecule remains. In Fig. 5(A) FFT spectra, peaks at low wavenumbers (about 32 cm1, 38 cm1, 45 cm1, 50 cm1, 55 cm1, 65 cm1, 80 cm1, 95 cm1 and 101 cm1 for the ground-state transients), appear. We assign these features to the vibrational dynamics of the CV670 molecules. As expected, selecting 33 fs (TR-CARS) spectrum with FFT fits to monitor the wave-packet dynamics of the target molecules. 4. Summary In this paper, vibrational wave-packet dynamics of CV670 molecules in ethanol solvent (5  105 mol/L) were first studied by broadband femtosecond (TR-CARS) spectroscopy technique simply

combing FFT analysis means. Multiple Raman vibrational modes with frequency differences of 32 cm1, 38 cm1, 45 cm1, 50 cm1, 55 cm1, 65 cm1, 80 cm1, 95 cm1 and 101 cm1 in CV670 dye molecules were excited and obtained simultaneously. This investigation demonstrates that broadband femtosecond (TRCARS) spectroscopy with FFT analysis method is more efficient to obtain the molecular vibrational wave-packet dynamics and intramolecular processes at the same time. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No JJ20140239) and the Support Program for Young Academic Key Teacher of Higher Education of Heilongjiang Provence (Grant No 1254G030). References [1] P. He, H. Wang, R. Fan, D. Chen, Y. Xia, X. Yu, J. Wang, Y. Jiang, Sci. China-Phys. Mech. Astron. 55 (2012) 2351. [2] Y. Zhao, S. Zhang, B. Zhou, Z. Dong, D. Chen, Z. Zhang, Y. Xia, Vib. Spectrosc. 73 (2014) 24. [3] P. He, S. Li, R. Fan, X. Li, Y. Xia, X. Yu, D. Chen, Chin. Phys. B 21 (2012) 027801. [4] P. He, S. Li, R. Fan, Y. Xia, X. Yu, Y. Yao, D. Chen, Opt. Commun. 284 (2011) 4677. [5] D. Chen, P. He, R. Fan, et al., J. Phys. Chem. C 116 (2012) 5881. [6] P. He, M. Wang, H. Wang, R. Fan, D. Chen, X. Yu, Sci. China-Phys. Mech. Astron. 55 (2012) 1808. [7] P. He, R. Fan, D. Chen, et al., Opt. Laser. Technol. 43 (2011) 1458. [8] R. Fan, P. He, D. Chen, et al., Opt. Laser. Technol. 45 (2013) 540. [9] V. Kumar, R. Osellame, R. Ramponi, et al., Opt. Express 19 (16) (2011) 15143. [10] A. Rebane, M. Drobizhev, M. Kruk, et al., J. Mod. Opt. 52 (9) (2005) 1243. [11] N. Dudovich, D. Oron, Y. Silberberg, Nature 418 (2002) 512. [12] A. Materny, T. Chen, M. Schmitt, et al., Appl. Phys. B 71 (2000) 299. [13] D. Oron, N. Dudovitch, Y. Silberberg, Phys. Rev. Lett. 90 (2003) 2139025. [14] S. Zhang, J. Shi, H. Zhang, et al., Phys. Rev. A 82 (2010) 043841. [15] A. Laubereau, W. Kaiser, Rev. Mod. Phys. 50 (3) (1978) 607. [16] H. Wu, Y. Song, G. Yu, et al., J. Raman Spectrosc. 47 (2016) 1213. [17] S. Mukamel, Principles of Nonlinear Optical Spectroscopy, Oxford Univ. Press, 1995. [18] L. Rune, S. Olga, J.S. Benjamin, et al., J. Chem. Phys. 128 (2008) 244310. [19] V.F. Kamalov, Y.P. Svirko, Chem. Phys. Lett. 194 (1992) 13. [20] O. Rubner, M. Schmitt, et al., J. Phys. Chem. A 102 (1998) 9734. [21] M.J.J. Vrakking, D.M. Villeneuve, A. Stolow, Phys. Rev. A 54 (1996) R37. [22] I.S. Averbukh, M.J.J. Vrakking, D.M. Villeneuve, A. Stolow, Phys. Rev. Lett. 77 (1996) 3518.