Femtosecond Time-Resolved ERE-CARS of CV670 Dye in Solutions

Article pubs.acs.org/JPCC

Femtosecond Time-Resolved ERE-CARS of CV670 Dye in Solutions Deying Chen,*,† Ping He,†,‡ Rongwei Fan,† Yuanqin Xia,† Xin Yu,† Jialing Wang,‡ and Yugang Jiang§ †

National Key Laboratory of Science and Technology on Tunable Laser, Institute of Opto-electronics, Harbin Insitute of Technology, Harbin, 150080, China ‡ College of Foundation Science, Harbin University of Commerce, Harbin, 150028, China § Tianjin Key Laboratory of Optical Thin Films, Tianjin Jinhang Institute of Technical Physics, Tianjin 300192, China

ABSTRACT: We utilize femtosecond time-resolved electronic resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) to study the vibrational dynamics of cresyl violet 670 (CV670) dye molecules. For this purpose, the vibrational properties of CV670 are registered in diluted solutions of ethanol and methanol. By changing the timing (Δt > 0) of the laser pulses of this nondegenerate four wave mixing technique, the wavepacket dynamics on the electronically ground state can be detected as oscillations in the ERE-CARS signal. Two strong Raman vibrational modes with an average period of 0.401 and 0.423 ps in CV670−ethanol solutions are obtained simultaneously, and we observe a qualitative vibrational dephasing time difference of the Raman vibrational modes of CV670 in ethanol and methanol solvents (5 × 10−5 mol/L), which may be due to the influence of the formation of the supramolecular structures in CV670−ethanol and CV670−methanol mixtures on the Raman modes of CV670.

1. INTRODUCTION Laser spectroscopic techniques have been successfully applied to chemistry and biology, providing a rich source of energy structural and dynamical information of condensed matter.1−5 In addition to observing the dynamics of a system, they also open up interesting possibilities for controlling its time evolution and even manipulating a chemical reaction. Coherent Raman spectroscopy has proved to be an efficient tool for studying molecular dynamics and determining molecular vibrational wavenumbers, vibrational rates, and solvent effects, as well as Raman line-broadening mechanisms under different conditions. The study of the photophysical properties of laser dyes is of great scientific and technological interest because of the important implications in photonics (active media of tunable lasers), in optics (mainly in the design of new nonlinear optical devices), and in the development of new fluorescence probes and sensors.6−9 Employing femtosecond spectroscopy techniques, which allow the observation of ultrafast photoreactions in real time, it has been shown that electron injection processes at dye−semiconductor interfaces often take place on an ultrafast sub-picosecond time scale.10−15 For example, an electron injection time as fast as 6 fs has been reported for the system containing alizarin adsorbed on TiO2 nanoparticles in timeresolved experiments.13 Other interesting aspects of ultrafast © 2012 American Chemical Society

interfacial electron-transfer reactions include the nonequilibrium character of the process, the effect of electronic−nuclear coupling,12,16 and the influence of intermediate states localized at the chromophore−substrate interface.17−21 However, to the best of our knowledge, no major effort has been made so far to investigate the vibrational dynamics, beat frequencies, and the solvent effects of cresyl violet dyes using electronic resonanceenhanced coherent anti-Stokes Raman scattering (ERE-CARS) technique with femtosecond time resolution. In the present work, we report the application of femtosecond time-resolved ERE-CARS on the investigation of the dynamics of coherently ground states of CV670 dye in diluted solutions (5 × 10−5 mol/L). We observe two strong Raman vibrational modes with an average of about 0.401 and 0.423 ps in CV670−ethanol solutions, which correspond to the ringbreathing and ring-deformation modes of the six-folded ring in CV670 molecules. To confirm our results and gain a deeper knowledge of the influence of the solvent effects on the dynamics of CV670, we have recorded ERE-CARS transients with the same pump and Stokes wavelengths, keeping the energy difference resonant with the ring-breathing and ringReceived: October 24, 2011 Revised: February 7, 2012 Published: February 8, 2012 5881

dx.doi.org/10.1021/jp2101978 | J. Phys. Chem. C 2012, 116, 5881−5886

The Journal of Physical Chemistry C

Article

deformation modes of the six-folded ring in CV670. We found that the vibrational periods of the ring-breathing and ringdeformation modes show a light solvent dependence, but the vibrational dephasing time for the ring-breathing mode presents a somewhat solvent dependence. The much faster dephasing time of the oscillations belonging to the Raman vibrational mode of CV670 in methanol solution may be linked to the formation of the supramolecular structures in CV670− methanol mixtures. We perform our measurements using a femtosecond ERECARS.4,22−24,31 This technique is currently accepted as a new spectroscopic tool. One may find reports on fs-CARS being used to study different compounds such as iodine,25 H2,26 iodine−benzene complex,27 and dipicolinic acid28 in different phases or on impulsive-stimulated Raman spectroscopy (ISRS) and femtosecond optical Kerr effect (OKE) spectroscopy being more commonly applied to record low-frequency vibrational coherence.29,30 However, ERE-CARS has advantages over these techniques when the medium is in high-pressure combustor environments or when multiple species are present. For instance, to interpret an experimental laser-induced fluorescence spectrum for concentration measurements, the collision partners must be identified.31,32 DWFM can be independent of quenching rates,33 but it loses sensitivity compared to ERECARS at increased pressure levels or at a very low concentration of chromophores.34 Similar to DWFM, OKE can also be affected by collision rates.35 Hence, ERE-CARS technique is a preferable technique at high pressure or when the Raman species is present in low concentration. Moreover, the ERE-CARS technique has significant potential to measure the anharmonic vibrational coupling of the low-frequency modes with high-frequency broadband modes. The essence of the femtosecond time-resolved ERE-CARS is the excitation of coherence at a particular Raman transition or several Raman transitions by a pair of pump/Stokes (dump) pulses, with a subsequent probing by a third laser pulse delayed with respect to the first two (pump/Stokes pulse pair; Figure 2). The probe pulse is scattered from the coherence excited between the molecular vibrational levels with a change of frequency, and the blue-shift scattered light is referred to as the ERE-CARS signal. For the ERE-CARS we have applied threedimensional forward geometry (folded BOXCARS),36 which fulfills the phase matching condition and spatially separates the ERE-CARS signal to enable background-free detection. The time delay between the probe pulse and the pump/Stokes (dump) pulse pair allows one to eliminate the contribution from the strong nonresonant background. In addition, because of the short duration and broad spectral width for femtosecond laser pulses, it can be used to monitor the vibrational coherence dynamics and to observe the quantum beat phenomena on a terahertz scale.

Figure 1. Molecular structure of the compounds used.

2.2. Measurement Setup. The femtosecond ERE-CARS measurements are performed at room temperature. The experimental setup used for the coherent ERE-CARS scheme is shown in Figure 2a. A commercial femtosecond laser system

Figure 2. (a) Experimental setup, femtosecond laser and detecting system; (b) energy diagram; (c) beam path of the BOXCARS arrangement.

Ti:sapphire regenerative amplifier (Coherent Inc.) at 800 nm (2.5 mJ/pulse, 1 kHz, and 40 fs) is used to pump two noncollinear optical parametric amplifiers (NOPAs, Light Conversion). Two independent wavelengths in the range of 570−760 nm can be generated by the two NOPAs. The output of NOPA1 is split into two parts by a splitter to obtain the pump (2.8 μJ/pulse) and probe (3.8 μJ/pulse) pulses, while the output of NOPA2 serves as the Stokes pulse (2.9 μJ/pulse). The ERE-CARS process requires a spatial overlap of the beams in the sample. At the start of the experiment, a temporal overlap of the pulses is required, and the pulses are delayed with respect to each other by means of Michelson interferometer arrangements (with a minimal step size of 2 fs) during the course of the experiment. The temporal overlap of the beams was made using a cross-correlation setup with second harmonic generation, as well as sum frequency mixing in a thin, phase-matched β-barium borate (BBO) crystal. The position of the delay stages at which the beams coincide in time was labeled as time zero. For the ERE-CARS, a folded BOXCARS configuration (see also Figure 2c) was employed to separate the signal from the incoming pump and probe beams. The phase-matching condition is fulfilled in this geometry, and the three beams will pass through the three corners of the front

2. EXPERIMENTAL SECTION 2.1. Preparation of CV670 Dye Sample. CV670 was purchased from Exciton (laser grade) and used as received. The molecular structures of the compounds are shown in Figure 1. Laser dye CV670 was dissolved in the ethanol and methanol solutions, which were placed in an ultrasonic bath in order to mix the dye into the solutions. The volume percentages of the CV670 dye concentration was kept at 5 × 10−5 mol/L. All solvents were of spectroscopy grade and were used without further purifications. 5882



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Figure 3. ERE-CARS and absorption spectra of CV670 in ethanol solvent. The spectra of pump and Stokes excitation pulses are shown for comparison.

To illustrate this approach, the ERE-CARS signal of CV670 (Figure 2b) can be well-described with a function of the form39

side of a box. We use a quartz cell (2 mm path, 1 mm thick walls) filled with CV670 dye solution. After interaction with the sample, the ERE-CARS signal emerges out from the fourth corner of the opposite box side. The ERE-CARS signal beam was collimated by a second achromatic lens L2, and the signal could thus be easily separated by a spatial filter. The signal was collected by a silica fiber. After filtering the stray light by means of a monochromator, the signal was detected by a fast photomultiplier tube (PMT). The ERE-CARS signal-to-noise ratio (SNR) was further enhanced using a boxcar integrator (SR250). Figure 2b shows the energy diagram. The excited state |a⟩ corresponds to a particular rotational level J in the ν = 0 vibrational level in the excited electronic state of CV670, and the states |b⟩ and |c⟩ represent particular J levels in the ν = 0, 1 levels of the ground electronic state. Detuning of lasers from their respective transitions is given by δ and Δ, as shown in the figures. For the Raman excitation, a frequency difference of (ωp − ωS)/(2πc) = 1020 cm−1 (near 1000 cm−1 or 30 THz) between the pump and Stokes pulses was chosen. As a result of the spectral width of the femtosecond exciting lasers of more than 300 cm−1, several Raman modes were simultaneously excited in the molecules; there are two strong Raman lines in CV670 molecules near 991 and 984 cm−1, which can be assigned to the ring-breathing modes of the six-folded ring37 and the ring-deformation modes of CV670, respectively.38

ss 2 2 IERE‐CARS ∝ (N ρcc ) L (dac 2dab2/ℏ2δ2)(dab2/ℏ)

(dac 2/ℏ)(|ε1|2 |ε2|2 |ε3|2 /γ2γbc 2)

(1)

where N is the total number density of resonant molecules, where ρccss is the Raman coherence, L is the interaction length, d is the dipole moment operator, ℏ is Planck’s constant, δ represents the detuning of the pump and Stokes beams from the excited electronic transition, γ is the electronic decay rate, and γbc is the Raman coherence dephasing rate. The ERE-CARS signal scales quadratically with (Nρcc), the number density of CV670 molecules in the ground-state level |c⟩ that is directly addressed by the lasers. The ERE-CARS signal does not explicitly depend on the excited-state population. The signal scales as the product of the three applied laser pulse intensities, and the terms in brackets are proportional to cross-sections for the Raman, probe, and signal transitions. To provide a better physical understanding of the ERECARS process, an approximate analytical solution is obtained for the ERE-CARS signal in the weak-field limit as39,40 ss ss ss |ρbc | = |Ω1||Ω2|(ρcc − ρbb )/{δγbc[1 + |Ω3|2 /γγbc]}

(2)

ss where ρbcss , ρbb , and ρccss represent the steady-state ground-state coherence, populations in states b and c, respectively. The detuning of the pump and Stokes beams from the excited electronic transition is denoted by δ, the Rabi frequency is Ωi = 2πdEi/h, d is the dipole moment, h is the Planck constant, and Ei represents the electric field of the laser with i = 1, 2, and 3 for the pump, Stokes, and probe beams, respectively. The Raman and electronic dephasings are given by γbc and γ. The energy levels and the detuning parameters are identified in Figure 2b. Clearly the probe beam E3(ωpr) affects the Raman coherence, which is in contrast to the conventional off-resonant CARS. In the weak field limit, the ERE-CARS signal is shown to scale inversely with the square of the dephasing rates for the electronic and Raman coherences. However, for femtosecond laser pulses, with their short duration and sufficiently high intensity, the effect of laser saturation on homogeneous broadening can overwhelm the effect of collisions by repumping the population to the excited state and, thus, reducing the dependence of the ERE-CARS signal on the collision rate. Since the stronger resonant probe beam drives away more population from the ground state, the ERE-CARS signal intensity is also reduced, but the SNR for the ERE-CARS

3. RESULTS AND DISCUSSION In Figure 3a we show the wavelength arrangement of the laser beams together with the ERE-CARS spectrum of the CV670− ethanol solution (5 × 10−5 mol/L) at room temperature. The wavelength of the pump pulse is adjusted to 628 nm, which is within the absorption band of the CV670 sample (Figure 3b). The Stokes is tuned to 671 nm, resulting in a strong ERECARS signal centered at 590 nm. We can see that under electronic resonance-enhanced conditions, the signal from the CV670−ethanol solutions is clearly detectable and that it can be seen by the naked eye, whereas that from the pure four wave mixing (FWM) signal from the quartz sample cell or ethanol is practically absent. We explain this difference by increased optical gain at the mixing frequency, ωCARS = 2ωp − ωS, in the CV670 sample, brought about by the electronic resonance condition. It is seen from these dependences that the half-width of the anti-Stokes line of the ERE-CARS spectrum of CV670 amounts to ∼253 cm−1, while the half-widths of the pump/ Stokes lines at 628 and 671 nm are about 422 and 467 cm−1, respectively. 5883



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Figure 4. Dependence of femtosecond ERE-CARS (λpu = λpr = 628 nm, λS = 671 nm) and R-CARS (λpu = λpr = 601 nm, λS = 640 nm) signals on probe pulse delay for 5 × 10−5 mol/L of CV670 in ethanol and methanol solutions.

It should be mentioned that all of the experimental conditions in the above experiment are the same as the process of investigating vibrational eigenstates of the CV670−ethanol solution except the solvent. For the CV670−methanol solution, the extended periods of oscillations present a period of about 0.424 ps, corresponding to a wavepacket motion prepared by coherent two-photon pumping (pump and Stokes) between ν = 0 and ν = 1 of the ground state of the ring-breathing modes in CV670 (compare ERE-CARS signals b with a of Figure 4). The average period of the oscillations corresponds to a vibrational wavenumber spacing of about 79 cm−1. This agrees with a particular vibrational energy spacing between ν = 0 and ν = 1 levels of the ground electronic state in the ring-breathing modes of CV670 as observed from the ERE-CARS of CV670− ethanol solution experiments. A further analysis of these oscillations is given below. Compared to the femtosecond time-resolved ERE-CARS signals b with a of Figure 4, the main vibrational periods of Raman vibrational modes in CV670 show a light solvent dependence. However, the vibrational dephasing time for the Raman vibrational modes of CV670 in methanol solutions is faster than in ethanol solutions. Up to now we do not have a conclusive explanation for the fast dephasing at short time delays. The much faster dephasing of the oscillations belonging to the Raman vibrational modes in CV670−methanol solutions needs careful analysis. This intriguing behavior may have its roots in the formation of extended supramolecular structures that will affect the CV670 and ethanol/methanol molecules. In 2004, Dougan et al. have reported the mixtures of methanol and water form extended structures despite the components being fully miscible in all proportions.43 According to ref 42, the extended structures characterizing the CV670−ethanol/ methanol system are very dynamic, with rapid shedding and reforming of cluster members. The formation of complex supramolecular structures can actually speed up the relaxation because the bigger structure experiences a larger number of collisions. The increase of the relaxation rate depends on the

signal is significantly increased. For a more detailed physical explanation, we refer the reader to refs 31 and 41 Figure 4a shows a typical selective exciting transient in CV670−ethanol solution obtained for a pump wavelength λpu = λpr = 628 nm (15 923 cm−1) and a Stokes wavelength λS = 671 nm (14 903 cm−1), detecting the ERE-CARS signal at λERE‑CARS = 590 nm. The frequency difference (ωp − ωS)/(2πc) = 1020 cm−1 (∼30 THz) matches a set of low-frequency Raman transitions (near 1000 cm−1 or 30 THz), which correspond to the ring-breathing and ring-deformation modes of the sixfolded ring in CV670 at wavenumber 991 and 984 cm−1.42 As shown in Figure 4a, on the one hand, the extended periods of oscillations show an average period of T = 0.423 ps. This average period of the oscillations corresponds to a vibrational wavenumber spacing of about 79 cm−1, corresponding to a beating at the particular J levels in the ν = 0, 1 levels of the ground electronic state of CV670 molecules. This vibrational dynamics may be linked to the ring-breathing modes of the sixfolded ring in CV670. In the femtosecond time-resolved ERECARS signal (Figure 4a), peaks at another wavenumber with a period of approximately T′ = 0.401 ps (83 cm−1) appear. We assign this vibrational dynamics to the ring-deformation modes of the six-folded ring in CV670. As a result, the ERE-CARS technique is sensitive to investigation of the vibrational dynamics of laser dyes in solution, and it provides a tool for obtaining time-resolved information at a very low concentration of chromophores. On the contrary, it opens up interesting possibilities to measure the anharmonic vibrational coupling of the frequency modes (such as 991 and 984 cm−1) dependence of the vibrational coherence by the ERE-CARS technique. To confirm our results and gain a deeper knowledge of the influence of the solvent effects on vibrational eigenstates of CV670, we have recorded coherent anti-Stokes Raman scattering (CARS) with the same pump and Stokes wavelengths, keeping the energy difference (ωpu − ωS)/(2πc) = 1020 cm−1 (∼30 THz) resonant with the Raman active vibrations of the six-folded ring in CV670−methanol solution. 5884



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Figure 5. Fourier transform (FFT) power spectra of the CARS signal from Figure 4.

coherences in low concentration and for selective extracting of the intrinsic characteristics of molecular motion. When applied to CV670 dye, which is a classical class of dyes with excellent photophysical and lasing characteristics, this technique allows one to track and determine the coherence vibrational periods (frequencies) and the wavenumber differences between the excited Raman transitions. It also makes it possible to observe the influence of solvent effects on the Raman vibrational properties of dye molecules. For such effects, to our knowledge, this is the first using femtosecond time-resolved ERE-CARS technique to investigate the vibrational eigenstates and the influence of the solvent effects on the Raman vibrational dynamics reported for CV670. In general, we contend that femtosecond ERE-CARS may become a new tool for studying the chemical nature of dye molecules in organic solutions of ethanol and other amphiphiles. As mentioned above, the setup will be used to find an optimal algorithm for the detection of solvent effects on dye molecules, such as CV670, which is widely used in spectroscopic research and in technology because of its unique characteristics.

coupling between the Raman vibration modes in the complex. If the coupling is strong, the relaxation rate is enhanced; otherwise it does not change significantly. Hence, we can see that the decrease of the relaxation for the molecular vibrations can be explained as a manifestation of the formation of clusters.4 We consider that the nontrivial dependence of the dephasing behavior observed in our experiments is similar and related to the formation of clusters reported in ref 42. Further research is currently being carried out in our group to explain the chemical nature of this phenomenon. In Figure 5 we show the Fourier transform (FFT) power spectra of the date from Figure 4. Again, these power spectra show that the Δt > 0 regions (positive time delays) correspond to observation of the vibrational wave packet dynamics in the CV670−ethanol and CV670−methanol solutions, respectively. It is worth noting that the wavenumber spacing 81 cm−1 showing in Figure 5a is the average vibrational wavenumber spacing of the 79 and 83 cm−1. In contrast to ERE-CARS, the conventional resonant coherent anti-Stokes Raman scattering (R-CARS) of the dye CV670 in ethanol and methanol solutions (5 × 10−5 mol/L) are shown in Figure 4c,d, and the Fourier transform power spectra of Figure 4c,d are shown in Figure 5c,d. In Figures 4 and 5, we compare the results from femtosecond time-resolved ERE-CARS and R-CARS spectroscopy on CV670 solutions. We can see that the ERE-CARS signal can be enhanced by more than 3 orders of magnitude when the probe beam is tuned to the electronic resonance of the molecule, and it is obvious that drastic changes in the transient shape occur, pointing to the fact that the ERE-CARS has advantages over the conventional R-CARS technique when the medium is present in low concentration, while the R-CARS technique loses sensitivity and accuracy compared to ERE-CARS, primarily due to electronic quenching and background interference.44

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by National Natural Science Foundation of China (Grant Nos.60878018 and 61008023). REFERENCES

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4. CONCLUSION The femtosecond time-resolved ERE-CARS technique has proved to be useful for studying dye molecular vibrational 5885



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Femtosecond Time-Resolved ERE-CARS of CV670 Dye in Solutions

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