Trace analysis of aromatic-compounds in natural samples by Shpolskii spectroscopy

Abstract

component appear together in the spectrum and lead to congestion. The nonlinear, four-wave mixing spectroscopy was applied to see if one could produce spectra from the individual sites without contributions from the other sites. We found that one could obtain single site spectra for the vibrational, electronic and vibronic states separately with high selectivity. There were, in fact, no contributions visible in the spectra from even the nearby sites.The pentacene in benzoic acid doped with p-terphenyl system was studied as a model to see if one could eliminate inhomogeneous broadening. We used both parametric and nonparametric techniques to test the characteristic of each. We found that both methods could eliminate inhomogeneous broadening to the limit of our laser resolution.This result was a surprise because the theory for the method predicts that only nonparametric methods could successfully eliminate inhomogeneous broadening. The explanation for the observation of parametric nonlinear mixing removing inhomogeneous broadening could only rest on the possibility that the character of the mixing changes as a function of the laser power because of the participation of excited state populations. A new theory was developed that treated saturation effects correctly and described excited state populations. The theory predicts intensity dependent changes in the relative intensity of the narrowed line to the non-narrowed line. It also predicts that the relative intensities are dependent upon the amount of inhomogeneous broadening. These predictions are consistent with the experimental observations. At this point, we have succeeded in developing a new class of high resolution laser spectroscopy methods based on nonlinear mixing. The methods do not require that the sample emit light. We have applied the methods to molecular and atomic spectroscopy and are now ready to extend the methods to realistic samples and mixtures. The methods are capable of measuring both bulk concentrations and surface concentrations. Surface specific analysis is possible because nonlinear mixing processes that are based on three wave mixing are only allowed for noncentrosymmetric systems. Thus, three wave mixing will not be observed for the bulk of a material but it will be observed at the interfaces between materials. It is also important to realize that the selectivity is determined on the time scale of the lasers and is not limited by the lifetime of excited states. An environment can change while the excited state is occupied and the changes will act to broaden the spectral line. However, if a picosecond excitation is used in the line narrowing experiment, the motion wvill be frozen and one can eliminate the broadening. This idea has important implications for solution spectroscopy because the motion within a solvent acts to broaden all of the lines that one would normally observe. There is, in fact, a rich field of opportunity for the development of these spectroscopies that is only being hinted at in our present work and it will be excitin...