Molecular vibrational dynamics in water studied by femtosecond coherent anti-Stokes Raman spectroscopy

Molecular vibrational dynamics in water studied by femtosecond coherent anti-Stokes Raman spectroscopy

Chemical Physics Letters 613 (2014) 1–4 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate...

801KB Sizes 1 Downloads 98 Views

Chemical Physics Letters 613 (2014) 1–4

Contents lists available at ScienceDirect

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

Molecular vibrational dynamics in water studied by femtosecond coherent anti-Stokes Raman spectroscopy Yang Zhao a , Sheng Zhang b , Boyang Zhou a , Zhiwei Dong a , Deying Chen a , Zhonghua Zhang a , Yuanqin Xia a,∗ a b

National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China Department of Physics, Harbin Institute of Technology, 150001 Harbin, China

a r t i c l e

i n f o

Article history: Received 10 March 2014 In final form 22 August 2014 Available online 28 August 2014

a b s t r a c t We utilized femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (CARS) to study the ultrafast vibrational dynamics in distilled water at room temperature. The CARS signals from the broad OH-stretching modes between 3100 cm−1 and 3700 cm−1 were obtained and analyzed. The dephasing times of four Raman modes in water were detected and compared. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Coherent anti-Stokes Raman spectroscopy (CARS) has lots of advantages such as excellent high spatial resolution, good temporal resolution and high sensitivity [1]. Femtosecond time-resolved CARS is a method for clarifying the origin of ultrafast dynamics associated with molecular motions in liquid [1] in time domain since 1980s for decades [2,3]. Leonhardt et al. [2,3] used time-resolved CARS for the first time to study the molecular beat phenomena in liquid phase benzene, cyclo-hexane and pyridine. Since then femtosecond time-resolved CARS technique has been extensively used in the investigation of the vibrational dynamic in different liquid samples, such as porphyrin [4,5], liquid with puckered rings [6], toluene [7], methanol-water [8], ethanol [9] and dipicolinic acid [10–12]. Recently, some advances have been achieved in femtosecond time-resolved CARS, such as single-shot [13], hybrid fs/ps [7,14] and in situ heterodyne detection [15]. Water molecules have previously been investigated extensively by Raman and ultrafast spectroscopy. Hope T. Beier et al. [16] obtained micro-scale thermal measurements in water by CARS. Yang and Skinner [17] and Auer and Skinner [18] calculated theoretical Raman line shapes for the OH stretch region of liquid water and their results were in excellent agreement with experiment. Dlott et al. [19–21] studied the dynamics of OH-stretching vibrations in water using ultrafast IR-Raman spectroscopy. Smit and Bakker [22] and Imoto et al. [23] studied the vibrational dynamics in H2 O by femtosecond mid-IR pump-probe spectroscopy and

∗ Corresponding author. E-mail address: [email protected] (Y. Xia). http://dx.doi.org/10.1016/j.cplett.2014.08.055 0009-2614/© 2014 Elsevier B.V. All rights reserved.

two-dimensional infrared spectra. However, little is known about their femtosecond time-resolved CARS spectra. The Raman spectrum of water in this wavenumber range (from 3100 cm−1 to 3700 cm−1 ) can be found in several publications [24,25]. Moskovits and Michaelian [24] presented that band positions of intramolecular stretching in water are 3220 cm−1 , 3450 cm−1 , 3520 cm−1 and 3620 cm−1 in this region. Carey and Korenowski [25] presented that there are four OH stretch modes with A symmetry (3233 cm−1 , 3393 cm−1 , 3511 cm−1 and 3628 cm−1 ) in this region. Baumgartner and Bakker [26] showed that a pure H2 O solution has peak positions at 3223 cm−1 , 3433 cm−1 and 3617 cm−1 in this region. From the above, the Raman spectrum of pure H2 O has four OH stretch modes at 3223 cm−1 , 3433 cm−1 , 3520 cm−1 and 3617 cm−1 in this region. We have performed femtosecond time-resolved CARS to investigate the vibrational dynamics in distilled water. By varying the wavelengths of laser pulses, different dynamics were coherently excited and probed by CARS [27]. A theory model for the liquid is presented briefly. The duration of the laser beams was obtained by “two-beam CARS”. We detected dephasing times of four Raman modes in water and compared them. 2. Experimental In the following, the experimental setup and methodology of time-resolved CARS are described briefly. The experimental setup used for the time-resolved CARS is shown schematically as follows (Figure 1). The laser pulses from a commercial femtosecond laser system Ti:sapphire regenerative amplifier (Micra + Legend, Coherent Inc.) at the central wavelength of 800 nm (2 mJ/pulse, 1 kHz and 40 fs) was split into two parts by a 1:9 beam splitter. The first part,

2

Y. Zhao et al. / Chemical Physics Letters 613 (2014) 1–4





S(T13 ) =

Ipr (t − T13 )(Pnres + Pres )2 dt

(3)

−∞

Pres (t) and Pnres (t) are the resonant and nonresonant polarization, respectively. Ipr is the intensity of the probe pulses (Gaussian pulse). Epu (t) and Est (t) are the time-dependent electric-field amplitudes of the pump and Stokes pulses (Gaussian pulse). The ˛ and ˇ are arbitrary scaling factors used to adjust the ratio of resonant and nonresonant contributions. T2j is the dephasing time of the vibrational mode. When T13 is fixed to 0 fs and the Stokes pulse arrives with a variable delay time, which is the same as the two-beam CARS. The theoretical CARS signal as a function of T12 is calculated using the following equations. Pnres (t, T12 ) = ˛Epu (t − T12 )Est (t)



Pres (t, T12 ) = ˇ

Figure 1. Femtosecond time-resolved CARS experimental device.

 taking up 90% of the total optical power, is used to pump an optical parametric amplifier (OPA; TOPAS, Light Conversion). The output of OPA was split into two parts by a 1:1 beam splitter to obtain the pump (pu , kpu ) and probe (pr , kpr ) beams. The second and weaker part of the amplifiers beam serves as the Stokes beam (st , kst ). The central wavelength of Stokes is st = 800 nm. The generation of CARS signal requires spatial and temporal overlap of the beams in the samples. The relative delay time among the different beams was changed by computer-controlled delay stages (Sigma Koki, SGSP20-85 with a minimal step size of 6.7 fs). The delay time between the pump and Stokes pulses defines T12 . The delay time between the probe and pump pulses defines T13 . The three beams were aligned parallel to each other and spatially overlapped in the folded-BOXCARS beam geometry (4 mm spot diameter, square with 10 mm sides) by the lens L1 with focal length 200 mm. This folded-BOXCARS beam geometry ensures that the CARS signal propagates in a direction different from the incoming beams and therefore can be background-free collected. The femtosecond CARS signal travels in a direction determined by the phase-matching condition (kCARS = kpr − kst + kpu ). The signal was then collimated by the lens L2 of focal length 200 mm. The CARS signals were filtered by a spatial filter (a pinhole) and were detected. It was spectrally dispersed in the monochromator (Newton, Andor) and detected by a photo multiplier tube (PMT) with a lock-in amplifier (SR830, Stanford Research Systems). The distilled water molecules were used as the sample. The sample was purchased and used without further purification. The sample was liquid contained in a quartz cuvette (10-mm path, 1mm thick walls). All laser polarizations and the signal polarization were parallel to each other. The temporal overlap (time zero) of the beams was examined by self-diffraction in coverslip. Depending on the sample, the average power of three beams was attenuated with power of 0.1–0.5 mW by a variable neutral-density filter. All the experiments were performed at the room temperature about 293 K. 3. Theory The theory model for the liquid is presented in detail by Kiefer et al. [4]. Briefly, the theoretical CARS signal as a function of T13 can be calculated using the following equations. Pnres (t) = ˛Epu (t)Est (t)



t

Pres (t) = ˇ −∞

(1) 

Epu (t  )Est (t  )e(t −t)/T2j dt 

t

(2)

(4) 

Epu (t  − T12 )Est (t  )e(t −t)/T2j dt 

(5)

−∞ ∞

S(T12 ) =

Ipu (t − T12 )(Pnres + Pres )2 dt

(6)

−∞

From Eqs. (1)–(6), the CARS signal as a function of T12 and T13 can be obtained by simulation. The effect of experimental parameters (the ratio of resonant and nonresonant contributions, duration of laser beams and the T2j ) on the two types of signal can be analyzed by the theory model. 4. Results and discussion In our femtosecond time-resolved CARS experiment, the pump and probe pulses have the same wavelength, pr = pu = 638 nm, 630 nm, 625 nm, 622 nm. The Stokes pulse is set to a longer wavelength st = 800 nm in such a way that the difference between pump and Stokes pulses is resonant with a vibrational Raman transition in 3100–3700 cm−1 . The time-resolved CARS signal is at CARS = 530 nm, 520 nm, 513 nm, 509 nm. This is a result of timeresolved femtosecond CARS measurements for vibration modes in water at Raman shift of 3170 cm−1 , 3370 cm−1 , 3500 cm−1 , 3570 cm−1 . 4.1. Time-integrated CARS intensity of water as a function of T12 For all the experiments and simulations described in the section, T13 is fixed to 0 fs and the Stokes pulse arrives with a variable delay time, which is the same as the two-beam CARS. The time-integrated CARS intensity as a function of T12 was simulated by Eqs. (4)–(6). The ratio of resonant and nonresonant contributions and the T2j will not influence the time-integrated CARS intensity as a function of T12 . Duration of laser beams defines as intensity full width at half maximum (FWHM) of laser beams, which will influence the time-integrated CARS intensity as a function of T12 . Time-integrated CARS intensity of water as a function of T12 is shown in Figure 2, when pr = pu = 638 nm, 630 nm, 625 nm, 622 nm (a, b, c, d). Time-integrated CARS intensities of water as a function of T12 have similar FWHM for different pump laser pulses. The signal of water for 3570 cm−1 shows a weak peak at about −100 fs. This is due to a little chirp in pump pulses. Rocha-Mendoza et al. [28] clearly showed that CARS response for Gaussian pulses ∝ exp(−4t2 /2.16 2 )/ 2 , where t is the time delay and  is FWHM of the Gaussian pulses. To fit dependence of CARS signal on time delay by using CARS response ∝ exp(−4t2 /2.16 2 )/ 2 , we can get FWHM of pump and stokes beams for 80 ± 3 fs, 75 ± 3 fs, 78 ± 3 fs and 72 ± 3 fs (ignoring the weak peak), respectively. This result is similar to FWHM of laser pulse near the laser system. From the

Y. Zhao et al. / Chemical Physics Letters 613 (2014) 1–4

3

Figure 2. Time-integrated CARS intensity of water as a function of T12 when T13 = 0 fs. (a) pr = pu = 638 nm and a Stokes wavelength st = 800 nm detecting the CARS signal at ␭CARS = 530 nm. (b) pr = pu = 630 nm, ␭CARS = 520 nm. (c) pr = pu = 625 nm, ␭CARS = 512 nm. (d) pr = pu = 622 nm, ␭CARS = 509 nm. (Solid curves represent best-fitted results.)

above, different pump laser pulses have the similar temporal characteristics. 4.2. Time-integrated CARS intensity of water as a function of T13 In order to generate the vibrational coherence, the difference frequencies of the pump and Stokes laser pulses must be tuned to match the Raman resonance. The vibrational coherence due to Raman transition can be monitored in real time by changing the delay time between the pump and probe beams [29]. In this measurement, the pump pulses and the Stokes pulses are coincident in the time scale, the probe pulse arrives with a variable delay time. By fitting the time-integrated CARS intensity as a function of T13 using Gaussian functions, the FWHM of signals can be got. The FWHM of signals corresponds to the T2j . Furthermore, the T2j was given by comparing with the simulation from Eq. (3). The time-integrated CARS intensity as a function of T13 was simulated by Eqs. (1)–(3). The ratio of resonant and nonresonant contributions, duration of laser beams and the T2j will influence the time-integrated CARS intensity as a function of T13 . The simulation of the time-integrated CARS intensity as a function of T13 with different ratio of resonant and nonresonant contributions (14:1, 7:1, 3.5:1 and 1.7:1) is shown in Figure 3(a). The FWHM of pump and stokes beams is 80 fs. T2j equals to 100 fs. The shape of the curve is similar with each other. To fit dependence of simulation on time delay by using Gaussian functions, we can get FWHM of signals 133.6 fs, 133.1 fs, 131.4 fs and 126.5 fs, respectively. Astinov and Georgiev [30] showed the ratio between the resonant and nonresonant part of the third-order susceptibility in water is 2:1. The water will absorb more energy in 800 nm than

(a)

14:1 7:1 3.5:1 1.7:1

1.0 0.8

(b)

1.0

CARS intensity(a.u.)

CARS intensity(a.u.)

in 533 nm (st = 533 nm used by Astinov and Georgiev). So the ratio of resonant and nonresonant contributions in our experiment will be larger than 2:1. Furthermore, the error of the FWHM of signals due to the ratio of resonant and nonresonant contributions will be smaller than ±6 fs. The simulation of the time-integrated CARS intensity as a function of T13 with different T2j (40 fs, 70 fs, 100 fs and 150 fs) is shown in Figure 3(b). The FWHM of pump and stokes beams is 80 fs. The ratio of resonant and nonresonant contributions is 3:1. To fit dependence of simulation on time delay by using Gaussian functions, we can get FWHM of signals 108 fs, 119 fs, 131 fs and 150 fs, respectively. By fitting dependence of signal on time delay with using Gaussian functions, the different T2j can be compared. Figure 4 shows the time-integrated CARS intensity of water as a function of delay time T13 , when pr = pu = 638 nm, 630 nm, 625 nm, 622 nm (a, b, c, d). To fit dependence of CARS signal on time delay by using Gaussian functions, we can get FWHM of signals 158 ± 2 fs, 128 ± 3 fs, 137 ± 5 fs and 116 ± 10 fs, respectively. The temporal characteristics are similar for different pump laser pulses. The error of the signals’ FWHM due to the ratio of resonant and nonresonant contributions will be smaller than ±6 fs. The T2j will influence the time-integrated CARS intensity as a function of T13 . So the slight differences as observed in the measurement are due to the characteristic of the Raman modes in water. The dephasing times for four OH stretch modes at 3223 cm−1 , 3433 cm−1 , 3520 cm−1 and 3617 cm−1 define as T1 , T2 , T3 and T4 . Comparing the FWHM of signals with the simulation from Eq. (3), T1 is close to 150 fs. T2 and T3 are close to and a little smaller than 100 fs. T4 is close to 70 fs. The coherence vibrational relaxation time is measured as less than 100 fs for OH modes in water by Kozai et al. [31]. Our results are consistent with their results.

0.6 0.4 0.2

40fs 70fs 100fs 150fs

0.8 0.6 0.4 0.2 0.0

0.0 -100

0

100

T13(fs)

200

300

-100

0

100

200

300

T13(fs)

Figure 3. (a) The theoretical CARS signal as a function of T13 with different ratio of resonant and nonresonant contributions (14:1, 7:1, 3.5:1 and 1.7:1); (b) The theoretical CARS signal as a function of T13 with different T2j (40 fs, 70 fs, 100 fs and 150 fs).

4

Y. Zhao et al. / Chemical Physics Letters 613 (2014) 1–4

Figure 4. Time-integrated CARS intensity of water as a function of T13 when T12 = 0 fs. (a) pr = pu = 638 nm, ␭CARS = 530 nm. (b) pr = pu = 630 nm, ␭CARS = 520 nm. (c) pr = pu = 625 nm, ␭CARS = 512 nm. (d) pr = pu = 622 nm, ␭CARS = 509 nm. (Solid curves represent best-fitted results.)

There are two methods for more precisely measuring the dephasing times of water. First, the ratio of resonant and nonresonant contributions is improved by electronic resonance-enhanced and polarization-sensitive resonance methods. Second, the shorter laser pulses (shorter than 10 fs) are used for measurement. 5. Conclusion In this letter, through the femtosecond time-resolved CARS experimental platform, ultrafast dynamics process of the OHstretching modes between 3100 cm−1 and 3700 cm−1 in water was studied. A theory model for water was presented briefly. The duration of the laser beams was obtained by two-beam CARS. The dephasing times of four Raman modes between 3100 cm−1 and 3700 cm−1 in water were detected and compared. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant nos. 11174068, 61275157, 11004042), the Research Fund for the Doctoral Program of Higher Education (RFDP20102302120022), CERS (Cers-1-43) and the HIT. NSRIF (HIT. NSRIF.2009011). References [1] F. El-Diasty, Vib. Spectrosc. 55 (2010) 1. [2] R. Leonhardt, W. Holzapfel, W. Zinth, W. Kaiser, Rev. Phys. Appl. 22 (12) (1987) 1735.

[3] R. Leonhardt, W. Holzapfel, W. Zinth, W. Kaiser, Chem. Phys. Lett. 133 (5) (1987) 373. [4] M. Heid, T. Chen, U. Schmitt, W. Kiefer, Chem. Phys. Lett. 334 (1–3) (2001) 119. [5] M. Heid, et al., J. Raman Spectrosc. 32 (9) (2001) 771. [6] A. Scaria, J. Konradi, V. Namboodiri, M. Sackmann, A. Materny, Chem. Phys. Lett. 433 (1–3) (2006) 19. [7] B.D. Prince, A. Chakraborty, B.M. Prince, H.U. Stauffer, J. Chem. Phys. 125 (4) (2006) 44502. [8] D. Pestov, et al., J. Raman Spectrosc. 37 (1–3) (2006) 392. [9] Y.H. Wang, X. Du, X. He, Y.Q. Yang, Vib. Spectrosc. 50 (2) (2009) 303. [10] M. Zhi, et al., JOSA B 24 (5) (2007) 1181. [11] Y. Huang, et al., J. Appl. Phys. 100 (12) (2006) 124912. [12] D. Pestov, et al., Proc. Natl. Acad. Sci. U. S. A. 102 (42) (2005) 14976. [13] Y. Paskover, I.S. Averbukh, Y. Prior, Opt. Express 15 (4) (2007) 1700. [14] D. Pestov, et al., Science 316 (5822) (2007) 265. [15] A. Shalit, Y. Paskover, Y. Prior, Chem. Phys. Lett. 450 (4) (2008) 408. [16] H.T. Beier, G.D. Noojin, B.A. Rockwell, J. Biomed. Opt. 17 (2012) 80501. [17] M. Yang, J.L. Skinner, Phys. Chem. Chem. Phys. 12 (4) (2010) 982. [18] B.M. Auer, J.L. Skinner, J. Chem. Phys. 128 (2008) 224511. [19] J.C. Deàk, S.T. Rhea, L.K. Iwaki, D.D. Dlott, J. Phys. Chem. A 104 (21) (2000) 4866. [20] A. Pakoulev, Z. Wang, D.D. Dlott, Chem. Phys. Lett. 371 (5) (2003) 594. [21] Z. Wang, A. Pakoulev, Y. Pang, D.D. Dlott, Chem. Phys. Lett. 378 (3) (2003) 281. [22] W.J. Smit, H.J. Bakker, J. Chem. Phys. 139 (20) (2013) 204504. [23] S. Imoto, S.S. Xantheas, S. Saito, J. Chem. Phys. 139 (4) (2013) 44503. [24] M. Moskovits, K.H. Michaelian, J. Chem. Phys. 69 (6) (1978) 2306. [25] D.M. Carey, G.M. Korenowski, J. Chem. Phys. 108 (7) (1998) 2669. [26] M. Baumgartner, R.J. Bakker, Mineral. Pet. 95 (1–2) (2009) 1. [27] A. Materny, et al., Appl. Phys. B: Lasers Opt. 71 (3) (2000) 299. [28] I. Rocha-Mendoza, W. Langbein, P. Borri, Appl. Phys. Lett. 93 (2008) 201103. [29] R.P. Lucht, Science 316 (5822) (2007) 207. [30] V.H. Astinov, G.M. Georgiev, Appl. Phys. B 63 (1) (1996) 62. [31] T. Kozai, et al., Chem. Phys. Lett. (2012).