Airborne contaminants, e.g., bacterial spores, are usually analyzed by time-consuming microscopic, chemical, and biological assays. Current research into real-time laser spectroscopic detectors of such contaminants is based on e.g., resonance fluorescence. The present approach derives from recent experiments in which atoms and molecules are prepared by one (or more) coherent laser(s) and probed by another set of lasers. However, generating and using maximally coherent oscillation in macromolecules having an enormous number of degrees of freedom is challenging. In particular, the short dephasing times and rapid internal conversion rates are major obstacles. However, adiabatic fast passage techniques and the ability to generate combs of phase-coherent femtosecond pulses provide tools for the generation and utilization of maximal quantum coherence in large molecules and biopolymers. We call this technique FAST CARS (femtosecond adaptive spectroscopic techniques for coherent anti-Stokes Raman spectroscopy), and the present article proposes and analyses ways in which it could be used to rapidly identify preselected molecules in real time. There is an urgent need for the rapid assay of chemical and biological unknowns, such as bioaerosols. Substantial progress toward this goal has been made over the past decade. Techniques such as fluorescence spectroscopy (1, 2) and UV resonant Raman spectroscopy (3-7) have been successfully applied to the identification of biopolymers, bacteria, and bioaerosols.At present, field devices are being engineered (1) that will involve an optical preselection stage based on, e.g., fluorescence radiation as in Fig. 1. If the fluorescence measurement does not give the proper signature then that particle is ignored. Most of the time the particle will be an uninteresting dust particle; however, when a signature match is recorded, then the particle is selected for special biological assay (see Fig. 1b). The relatively simple fluorescence stage can very quickly sort out some of the uninteresting particles whereas the more time-consuming bio-tests will be used for only the ''suspects.''The good news about the resonance fluorescence technique is that it is fast and simple. The bad news is that although it can tell the difference between dust and bacterial spores it cannot differentiate between spores and many other organic bioaerosols (see Fig. 1c).However, despite the encouraging success of the abovementioned studies, there is still interest in other approaches to, and tools for, the rapid identification of chemical and biological substances. To quote from a recent study (8): ''Current [fluorescence-based] prototypes are a large improvement over earlier stand-off systems, but they cannot yet consistently identify specific organisms because of the similarity of their emission spectra. Advanced signal processing techniques may improve identification.'' Resonant Raman spectra hold promise for being spore specific as indicated in Fig. 2b. This is the good news, the bad news is that the Raman signal i...

Flash photoconductivity studies are presented for a liquid organic solution. It was found that a dilute solution of N,N,N′,N′-tetramethylpharaphenylenediamine in 3-methylpentane at room temperature could be photoionized by light quanta with energies of the order of one-half those required for gas-phase ionization. This is fully reminiscent of the low-energy photoionizations seen when this and similar solutions are cooled to a very rigid state at 77°K. These ionizations are now known to be biphotonic, and the possibility of a similar mechanism holding for the liquid, room-temperature system is explored. A detailed kinetic analysis of one class of flash-biphotonic phenomena is examined. The general category treated consists of all cases in which a one-photon-produced intermediate state is subsequently ionized by a second photon. The charge-carrier density (and peak photocurrent) is proportional either to [ ∫ 0T I(t)dt]2or to ∫ 0T I2(t)dt, depending on whether the intermediate-state lifetime is either long or short compared to the flash duration T, respectively. Experimental studies verify proportionality of the peak photocurrent to both of these expressions, which, it happens, are equivalent under the conditions employed. The biphotonic character of the ionization is confirmed. A molecular-weight study of the solute molecule verifies the monomer nature of the photosensitive species, and a linear concentration dependence of the photocurrent at low concentrations eliminates a bimolecular (solute–solute) mechanism in this instance. The sublinear dependence of photocurrent on concentration at higher concentrations is analyzed as a bimolecular quenching of the intermediate state. The range of possible intermediate-state lifetimes obtained suggests (but does not prove) that, just as in the rigid solution at 77°K, the triplet state serves as the intermediate state in this fluid solution as well. The physical basis of the peak photocurrent as a meaningful parameter is discussed in an appendix.

A technique is reported which allows simultaneous two-wavelength selection in the N2 laser-pumped dye laser. The two wavelengths are continuously tunable anywhere within the optical gain region of the particular dye solution used, and the two output beams are collinear and collimated, have mutually orthogonal polarizations, and in general can be temporally synchronized. Following the description of this device, the extension of this technique to other lasers and a few potential applications will be briefly discussed.

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