Rotational Diffusion of Microconfined Liquids
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a transient orientational anisotropy in a liquid. The time dependence of this anisotropy is measured by a second laser pulse, thus providing direct information about the rotational diffusion of the liquid. The OKE signal can be related directly to the orientational correlation function of the liquid, and as a result can provide detailed and quantitative information about both dynamics and populations of molecules. Here we employ OKE spectroscopy to study the effects of confinement on liquids that interact very weakly or very strongly with the confining material. In particular, we will explore the behavior of methyl iodide (CH 3I) as a prototypical weakly wetting liquid and acetonitrile (CH 3CN) as a prototypical strongly wetting liquid. EXPERIMENT The OKE setup is similar to that reported previously [18]. Briefly, a Ti:sapphire laser produces mode-locked pulses at a repetition rate of 76 MHz with a center wavelength of 800 nm. The pulses are externally recompressed to a duration of approximately 45 fs. A 90/10 beam-splitter is used to divide the beam into a strong pump beam and a weak probe beam, each of which is chopped at a different frequency. The pump beam is then passed through a halfwave plate and a polarizer that is set at 450 relative to the vertical before impinging on the sample. The pump pulse creates a small degree of alignment of the liquid molecules, thus producing 33 Mat. Res. Soc. Symp. Proc. Vol. 543 01999 Materials Research Society
a transient birefringence in the sample. The time dependence of this birefringence is measured by the probe pulse, whose arrival time at the sample is controlled by an optical delay line. The probe beam passes through a quarter-wave plate and a polarizer that is set in the vertical position before reaching the sample, and any pump-induced birefringence is measured by passing the probe beam through a crossed polarizer after the sample and using a low-noise amplified photodiode to detect any light that makes it through the polarizer. The two polarizers in the probe path are adjusted to maximize extinction with the pump beam blocked, and then the first polarizer is rotated by a small amount to induce a local oscillator for use in optical heterodyne detection [20]. A small portion of the probe beam is picked off and sent to a matched low-noise amplified photodiode. The resultant signal is used to subtract the local oscillator from the heterodyned signal. Differential detection is accomplished at the inputs of a low-noise preamplifier, which also employs a high-pass filter. The preamplifier signal is sent to a lock-in amplifier that
is referenced to the sum of the chopping frequencies. Data are taken at both positive and negative heterodyne angles so that the homodyne portion of the signal can be removed later [21]. The pump beam is also frequency-doubled after the sample, and the second-harmonic signal is used to correct for any laser intensity fluctuations over the course of the experiment. Monolithic silicate sol-gel glass samples with various pore sizes were prepared
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