Two-dimensional Fourier transform electronic spectra of the cyanine dye IR144 in methanol are used to explore new aspects of optical 2D spectroscopy on a femtosecond timescale. The experiments reported here are pulse sequence and coherence pathway analogs of the two-dimensional magnetic resonance techniques known as COSY (correlated spectroscopy) and NOESY (nuclear Overhauser effect spectroscopy). Noncollinear three pulse scattering allows selection of electronic coherence pathways by choice of phase matching geometry, temporal pulse order, and Fourier transform variables. Signal fields and delays between excitation pulses are measured by spectral interferometry. Separate real (absorptive) and imaginary (dispersive) 2D spectra are generated by measuring the signal field at the sample exit, performing a 2D scan that equally weights rephasing and nonrephasing coherence pathways, and phasing the 2D spectra against spectrally resolved pump–probe signals. A 3D signal propagation function is used to correct the 2D spectra for excitation pulse propagation and signal pulse generation inside the sample. At relaxation times greater than all solvent and vibrational relaxation timescales, the experimental 2D electronic spectra can be predicted from linear spectroscopic measurements without any adjustable parameters. The 2D correlation spectra verify recent computational predictions of a negative region above the diagonal, a displacement of the 2D peak off the diagonal, and a narrowing of the 2D cross-width below the vibrational linewidth. The negative region arises from 4-level four-wave mixing processes with negative transition dipole products, the displacement off the diagonal arises from a dynamic Stokes shift during signal radiation, and the narrow 2D cross-width indicates femtosecond freezing of vibrational motion.

Two different electronically resonant two-dimensional spectroscopies are described. The first, two-color photon echo peak shift spectroscopy, is sensitive to correlations in transition frequency between the initial and probed (final) states. It provides new insight into the mechanism of ultrafast solvation and should prove useful for characterizing dynamics in inhomogeneous systems in general. The second technique, fifth order threepulse scattering, contains two coherence periods whose durations are controlled. The entire two-dimensional surface was recorded for a dye molecule in dilute solution and a photosynthetic light-harvesting complex. The data provide insight into the short-time dynamics of solvation and exciton relaxation, respectively.

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Laser-induced breakdown spectroscopy (LIBS) is examined as a potential method for detecting airborne biological agents. A spectrally broadband LIBS system was used for laboratory measurements on some common biological agent simulants. These measurements were compared to those of common, naturally occurring biological aerosol components (pollen and fungal spores) to determine the potential of LIBS for discriminating biological agents from natural background aerosols. A principal components analysis illustrates that linear combinations of the detected atomic lines, which are present in different ratios in each of the samples tested, can be used to discriminate biological agent simulants from other biological matter. A more sensitive, narrowband LIBS instrument was used to demonstrate the detection of single simulant (Bg) particles in the size range 1-5 microns. Ca, Mg, and Na, which are present in varying concentrations between 0.3 and 11% (by mass) in the Bg particles, were observed in single particles using LIBS.