Studies On The Effect Of Solution Concentration On Raman Scattering Cross Section Of Single Molecule

Raman spectroscopy, as a powerful tool for analysis and detection of trace amount, is receiving comprehensive study. Subject of doctoral research of mine mainly centers on experimental and theoretical studies on effect of solution concentration on Raman scattering cross section (RSCS) and frequency shift at lower concentrations by using liquid-core optical fiber (LCOF) technique. The major fruits are listed below: 1. The study on the RSCS of single molecule by using LCOF technique Using the liquid-core optical fiber technique which can enhance Raman intensity 10³ times, we obtained the RSCSs of CCl4 in CS2 at low concentrations and the dependence of the RSCSs on solution concentration. The results show that the RSCSs of CCl4 bands increase with decreasing concentration of CCl4 over the whole range measured. It was unexpected that the RSCSs show different changes in various ranges, that is, a flat linear change at higher concentrations (>4%) and a strong change at lower concentrations (<4%). On the basis of the experimental results, an empirical equation was obtained, which is σ_R(C ) = a₁C+a₂+a₃exp(-C/a₄), a₁ ,a₂,a₃and a₄ are fitted parameter. The equation is not only in excellent agreement with our results but also contains the former work. With the decrease of the concentration from 90% to 0.05% (0.1% for 314 cm-1 band), the RSCSs of 459 and 314 cm-1 bands increase 2 orders of magnitude. When the concentration is below 10%, the RSCS of the 218cm-1 band show a 1 order of magnitude increase compared with the RSCS of CCl4 at initial concentration, 90%. Two kinds of RSCS values of the 459 cm-1 band, σRAand σRH, were obtained for each concentration. Both kinds of RSCS values and their changing tendencies with decreasing solution concentration are in agreement with each other. The theoretical studies on the results are performed with the Onsager model and molecular interaction theory, and a qualitative interpretation is proposed, which are the initiatory work on these studies at so low concentrations. According to Onsager's model, the local field correction factor increases with decreasing solution concentration, that is, the electric fields produced by surrounding molecules become enhanced. This enhancement makes the effective polarizability large so that the RSCS increase with decrease in CCl4 concentration. However, Onsager's model can't explain the experimental results at low concentrations. This is because the dielectric continuum model adopting the average field method only shows the whole effect, which can't be applied to calculated concrete properties of solvent effect. When the concentration is < 10%, the local field correction factor L (ν0) is almost equal. In addition, theabsolute value of slope of RSCS with respect to concentration which was calculated by using Onsager's model is smaller than the experimental one, especially at concentrations below 4%. All above-mentioned suggest that Onsager's model can't be accounted completely for our results. Hence, a tentative supplement for Onsager's model was provided. The dispersion forces are introduced into Onsager's model. This is because the dispersion forces are not present in the average field methods, and furthermore, as nonpolar solute is dissolved in nonpolar solvent, only dispersion forces contribute to the solvention of solute. Therefore the incorporating dispersion force into Onsager's model is reasonable. For more anisotropyic and polarizable molecule, CS2, the dispersion forces of interactions in molecules are especially strong. They are extremely short-range in action (depending on 16R p, R p is the molecular distance). With decreasing concentration, the number of CS2 molecule around single CCl4 molecule increase relatively. Therefore the effective distance of interaction between solute molecule and solvent molecule become short, which induces further enhancement in dispersion forces, that is, induced dipole moments become large. The lower the concentration is, the more rapid the enhancement in induced dipole moment or polarizability( P = αE)is. Hence the RSCS of CCl4 band would increase rapidly with decreasingsolution concentration after introducing dispersion force into Onsager's model. Additionally, some likely error sources in the RSCS measurements are analyzed, and the effect of NA on RSCS can be neglected according to the theoretical analysis. The experimental results obtained are positive, and suggest that the light-scattering capacity of solute molecule is enhanced at lower concentrations. Thus the Raman signal generation in LCOF is expected to provide a new way to investigate Raman spectra at low concentrations, which are almost unobserved under the normal condition. 2. The study on Raman spectra of single molecule by using LCOF-FTRS The technique combining the LCOF and Fourier transition Raman scattering (LCOF-FTRS) was devised and applied to Raman measure at lower concentrations, which can enhance Raman intensity 2⁴ orders of magnitude. We have obtained Fourier transition Raman spectra of β-carotene and Rhrodamine-B in CS2 from 10-7 to 10-12 mol/L, respectively, and have found that the characteristic peak of β-carotene and Rhrodamine-B, 1520 cm-1, shifts red and the linewidth narrows with decreasing concentration. Theoretical study on the experimental result was presented in our related article. The technique combining the merits of LCOF and Fourier transition Raman scattering will be well applied in the lower detection and analysis with its sensitivity and signal-to-noise. 3. The study on Raman spectra of single molecule by using LCOF-RRS Using the resonance Raman scattering (RRS) in liquid-core optical fiber (LCOF-RRS) which can raise Raman spectra intensity by 109 times, wemeasured the 514.5 nm excited RR spectra of p-benzoquinone (p-BQ) and β-carotene in CS2 with the range from 10-3 to 10-14 mol/L and from 10-6 to 10-18 mol/L, respectively. Firstly, a weak visible absorption spectrum of p-BQ in CS2 due to n-Ï€* singlet-triplet transition was measured, which is around 507 nm. we have obtained the RR spectra of p-benzoquinone (p-BQ) and have demonstrated that the new characteristic RR band, 1439 cm-1, is attributed to the symmetric C=O stretch (νC=O) of n-Ï€* singlet–triplet transition of p-BQ. This experimental result is in excellent agreement with theoretical calculation of other group. At the same time, the effect of solution concentration on the RR band was investigated. The result shows that the RR peak spreads towards short wavelength side by 13 cm-1 with decreasing concentration ranging from 10-3 to 10-11 mol/L, whereas the blue-shift isn't obvious when the concentration is lower than 10-11 mol/L. This research shows a lot of structural information of molecule, which is expected to offer some helps for further single molecule detection and single molecule study. Secondly, the absorption spectrum of β-carotene in CS2 was measured, which is around 509 nm. We have developed LCOF-RRS technique to measure the β-carotene in CS2 when the concentration is ranging from 10-6 to 10-18 mol/L. The clearly distinguishable 514.5 nm excited RR spectra of β-carotene superimposed on a fluorescence background are detected. Raman cross sections approaching that of solution fluorescence of β-carotene were obtained from a comparison of the intensities of LCOF-RRS with solution fluorescence of β-carotene, which will become a...