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Chapter 12
Vibrational Dynamics in Porous Silica Glasses Studied by Time-Resolved Coherent Anti-Stokes Raman Scattering 1,*
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Keisuke Tominaga , Hiroaki Okuno , Hiroaki Maekawa , Tadashi Tomonaga , Brian J. Loughnane , Alessandra Scodinu , and John T. Fourkas 1
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Department of Chemistry, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan Eugene F. Merkert Chemistry Center, Boston College, 140 Commonwealth Avenue, Chestnut Hill, MA 02467
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We have studied pore size dependence on the vibrational dephasing oftheCDstretching of CDCl in porous silica glasses by time-iesolved coherent anti-Stokes Raman scattering (CARS) technique. The CARS signal decays almost single -exponentially for all the porous silica glasses, and the decay time constant becomes smaller by decreasing the pore size. There is almost no spectral shift in the IR spectra of the CH stretching of CHCl when the liquid is confined in the glasses. We explain these observations in terms of a two-state model in which liquid molecules inside the pore are classified into "surface molecules" and "bulk molecules". 3
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Poious silica glasses aie materials with nano-scale pores that have several characteristic features; the diameters of the poresrangefroma few tens ofangstroms to several thousands of angstroms, depending on its preparation method; the pore sizes can be controlled and are quite uniform; many kinds of liquid molecules can go into the glass and be trapped in the poies. Because of these characteristic
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© 2002 American Chemical Society
In Liquid Dynamics; Fourkas, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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161 features, porous silica glasses are a suitable system to investigate the confinement effect on molecular motions. So far, confinement effects on molecular dynamics i n porous silica glasses have been studied by means of N M R (1-4), Raman scattering (5-8), and femtosecond optical Kerr effect (9-12). Local structures and dynamics in liquids are often studied by vibrational spectroscopy. This is because vibrational spectra are veiy sensitive to the environment around the oscillator. The transition frequency and lineshape of the spectrum, or vibrational dephasing, aie good "detectors" to monitor microscopic details in liquids. In this work we study vibrational dynamics of liquids confined in porous silica glasses by time-iesolved C A R S measurement and FT-IR measurements.
Experimental The preparation of the porous silica glasses was reported elsewhere in detail (10). The porous silica glasses wereheatedat 450 °C forafewhours, cooled down in the desiccater, and filled with the solvent. The sample cell was sealed with an epoxy glue. The C A R S experiment was performed with a pair of synchronously pumped dye lasers. A schematic picture of the system is shown i n Figure 1. One oscillator gave pulses with a duration of about 90 fsand a center wavelength of 600 nm. The duration of the other oscillator was set to be about 8 ps to avoid a timing jitter between the two lasers. The wavelength of the picosecond laser is tunable from 600 to 750 nm. The details ofthe oscillators (13) and the amplifier (14) for the femtosecond laser pulse were already mentioned elsewhere. The energy of the amplified femtosecond pulse is about 5 μΙ/pulse. The picosecond pulse was amplified by a standard method using a three-stage dye amplifier. The green output from the N d : Y A G regenerative amplifier pumps both the amplifiers. The fiist and second stages consist of flow cells with a 1 mm optical path length and solutions of Pyridine 1 in a mixture of ethanol and water. The third stage, which has a flow cell of a 10 mm optical path length and the same solution, is pumped from both the sides. The final output energy ofthe amplified pulse is about 10 μΙ/pulse. The femtosecond pulse is split into two portions, one being a pump pulse, and the other a probe pulse. These two pulses are focused into the sample together with the picosecond pulse which woiks as a Stokes pulse. The input angles ofthe pulses satisfy the phase-matching condition. The C A R S signal is collected by a photomultiplier and amplified by a lock-in amplifier (SRS830). IR measurements were made with a Perkin-Elmer Model S P E C T R U M 1000 FT-IR spectrometer.
In Liquid Dynamics; Fourkas, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Two-Col or Ultrafast Laser System
Figure L A schematic picture of the two-color aye laser system. EM (end mirror), OC (output coupler), ML (mode locker), and PC(Pockels cell).
In Liquid Dynamics; Fourkas, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Results Time-Resolved CARS Measurements Figure 2 shows a CARS signalfromthe CD stretching of CDC1 confined in a porous silica glass of a diameter of 24 Â. Since the polarizations of the pulses are under the magic angle condition, only the pure vibrational dephasing contributes to the signal. There is an instantaneous response due to an electronic polarizability at t=0, which is followed by a nuclear response at £>400 fs. The nuclear response at 0
