Nitric oxide (NO) is a major chemical byproduct of many photochemically active nitrogen-containing compounds. As a prototypical free radical with a very well characterized high-resolution spectrum, NO provides a standard spectroscopic fingerprint for indirect quantitative analysis and detection of a number of low vapor pressure nitroaromatic compounds in air through either direct photochemical decomposition of a parent molecule or from its relatively high vapor pressure chemical constituents. In this paper, we will discuss applications of picosecond laser spectroscopy for measurements and detection of NO and the nascent NO generated from photolysis of nitrobenzene. We will give a general overview of our tunable picosecond laser and detection system that we routinely use for probing and exciting the NO gamma band. This broad wavelength tuning capability of our laser allows us to set up pump-probe type experiments for detecting blue shifted rovibronic bands and probing the relative population distribution for NO. In all cases, experiments were performed using UV laser pulses of duration less than 20 ps. Also, we studied the effect of N<sub>2</sub> collisions on the photoframentation spectrum of nitrobenzene in 1000 mbar of N<sub>2</sub> buffer gas.
The standoff detection of energetic materials via laser-induced fluorescence of vapors has received relatively little
attention due to spectrally broad fluorescence emission from aerosols and unwanted background molecules. This
unwanted broad emission can obscure fluorescence from the molecule of interest. When multiphoton excitation is used,
the problem can be avoided by blue-shifting the emission from the target molecule relative to the unwanted broad
emission. As a precursor to the detection of explosives, we demonstrate coherent multiphoton excitation via stimulated
Raman adiabatic passage (STIRAP) on sodium vapor in an argon buffer gas as a function of argon pressure. Results
indicate that STIRAP can be performed in a buffer gas at atmospheric pressure with a minimal eduction in STIRAP
efficiency. The 15 ps long light pulses used for the pump and Stokes pulses were produced by two synchronously
pumped OPO/OPAs tuned to the 3p (<sup>2</sup>P<sub>1/2</sub>) ← 3s (<sup>2</sup>S<sub>1/2</sub>) transition for the pump pulse and the 5s (<sup>2</sup>S<sub>1/2</sub>) ← 3p (<sup>2</sup>P<sub>1/2</sub>) for
the Stokes pulse.
Termed Special Nuclear Material (SNM) by the Atomic Energy Act of 1954, fissile materials, such as <sup>235</sup>U and <sup>239</sup>Pu, are
the primary components used to construct modern nuclear weapons. Detecting the clandestine presence of SNM
represents an important capability for Homeland Security. An ideal SNM sensor must be able to detect fissile materials
present at ppb levels, be able to distinguish between the source of the detected fissile material, i.e., <sup>235</sup>U, <sup>239</sup>Pu, <sup>233</sup>U or
other fission source, and be able to perform the discrimination in near real time. A sensor with such capabilities would
provide not only rapid identification of a threat but, ultimately, information on the potential source of the threat. For
example, current detection schemes for monitoring clandestine nuclear testing and nuclear fuel reprocessing to provide
weapons grade fissile material rely largely on passive air sampling combined with a subsequent instrumental analysis or
some type of wet chemical analysis of the collected material. It would be highly useful to have a noncontact method of
measuring isotopes capable of providing forensic information rapidly at ppb levels of detection. Here we compare the
use of Kr, Xe and I as "canary" species for distinguishing between <sup>235</sup>U and <sup>239</sup>Pu fission sources by spectroscopic
Proc. SPIE. 7304, Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing X
KEYWORDS: Biosensing, Standoff detection, Weapons of mass destruction, Laser induced fluorescence, Explosives, Picosecond phenomena, Biological and chemical sensing, Sodium, Multiphoton fluorescence microscopy, Current controlled current source
Laser detection technologies offer obvious benefits for the standoff detection of hazardous or energetic materials where safe detection at a distance is the goal. Of the many optical standoff detection methods available, multiphoton fluorescence techniques have been studied less extensively. Multiphoton fluorescence allows high selectivity relative to the background while preserving the larger signal of laser induced fluorescence (LIF). Using sodium vapor as a test system, we demonstrate that stimulated Raman adiabatic passage (STIRAP) is capable of providing more than a factor of ten improvement in population transfer efficiency to the final state when compared to stimulated emission pumping (SEP). The two sodium transitions used are the 3p (<sup>2</sup>P<sub>1/2</sub>) ← 3s (<sup>2</sup>S<sub>1/2</sub>) and 5s (<sup>2</sup>S<sub>1/2</sub>) ← 3p (<sup>2</sup>P<sub>1/2</sub>). The light used to couple the states was produced with two synchronously pumped OPG/OPAs pumped by the 355 nm light from a picosecond tripled Nd:YAG.
In the implementation of laser-induced fluorescence (LIF) for the detection of vapor-phase organic compounds that
accompany hazardous materials, multiphoton excitation offers a significant advantage over single photon methods. In
particular, if the absorption spectra of unwanted background molecules overlap that of the target molecule, single photon
LIF is plagued by false positives. Multiphoton methods alleviate this difficulty by requiring that the target molecule be
in resonance with multiple molecular transitions. A promising multiphoton method is stimulated Raman adiabatic
passage (STIRAP). This method involves a counterintuitive sequence of laser pulses which is capable of transferring
100% of the target molecules to the desired excited state from which fluorescence is to be observed.
As a precursor to more complex molecules, we demonstrate the STIRAP technique on sodium vapor using the 3p (<sup>2</sup>P<sub>1/2</sub>)
← 3s (<sup>2</sup>S<sub>1/2</sub>) and 5s (<sup>2</sup>S<sub>1/2</sub>) ← 3p (<sup>2</sup>P<sub>1/2</sub>) transitions. This is the first time STIRAP has been achieved on a vapor using
picosecond lasers. We produced light to couple the states using two synchronously pumped OPG/OPAs (pumped by the
355 nm light from a picosecond YAG). We measured the fluorescence from the 5s state to both 3p states (<sup>2</sup>P<sub>1/2</sub>, <sup>2</sup>P<sub>3/2</sub>)
and from both 3p states to the 3s state with monochromator using a gated CCD to eliminate Rayleigh scattered light.
Our results indicate a four to five-fold increase in the transfer efficiency to the 5s state when the laser pulse that couples
the 3p and 5s states precedes the laser pulse tuned to the 3p ← 3s transition.
A theoretical model is presented to account for the experimental observation that infrared tissue ablation is optimized by the use of wavelengths near the amide II band of proteins. The model recognizes the partitioned absorption of IR photons between protein and water due to overlapping spectral features along with the dynamics of biopolymers, the loss of mechanical integrity in proteins, and the explosive role played by the vaporization of water. The theoretical foundation for this model can be found in previous accounts of thermal confinement, multicomponent models, and selective photothermolysis.
Theoretical considerations of thermal lens effect due to linear and nonlinear optical absorption is presented. Based on this model, Z-scan technique, especially two-color Z-scan due can be used to detect very low level of impurities or defects in optical materials. Depending upon the optical cross section of the particular species being probed, two-color Z-scan can detect impurities, for example, the OH groups in fused silica at sub-ppm level by weight or better.
The Vanderbilt free-electron laser has been operational for several years. This extended collaboration has been investigating outstanding problems in biological physics and medical physics with several research goals in mind. Our most fundamental goal is to improve the understanding of intermolecular and intramolecular vibrational energy transfer mechanisms in biopolymers. Our approach is to pursue both experimental and theoretical research addressing vibrational energy transfer in biological physics. The remaining goals can be summarized as the application of our fundamental advancements in polymer physics to molecular biology and to clinical and surgical medicine.
The vibrational dynamics of O-H groups in fused silica have been examined by a time- resolved pump-probe technique using the Vanderbilt Free Electron Laser (FEL). We consider two effects, local heating and transient thermal lensing, which can influence measured T<SUB>1</SUB> values in one color pump-probe measurements. The dependence of these two effects on both the micropulse spacing and the total number of micropulses delivered to the sample are analyzed in detail for the O-H/SiO<SUB>2</SUB> system. The results indicate that transient thermal lensing can significantly influence the measured probe signal. The local heating may cause thermally induced changes in the ground state population of the absorber, thereby complicating the analysis of the relaxation dynamics.
Tunable, pulsed radiation sources in the ultraviolet, visible, and infrared wavelength ranges offer novel opportunities for investigating laser-induced biomedical effects. Free-electron lasers (FELs) deliver continuously tunable, pulsed radiation in the infrared, providing the capability to selectively target radiation into the vibrational modes of water or other biopolymers. Experimental techniques for measuring the absorption spectra of biological samples are described. These spectra indicate wavelengths that potentially serve as the basis for laser-induced biomedical effects. Some practical considerations for infrared, visible, and UV spectroscopy of biological samples are summarized, and the connection between biomedical research and more fundamental investigations of vibrational energy transfer are emphasized.
Free-electron lasers (FELs) provide tunable, pulsed radiation in the infrared. Using the FEL as a pump beam, we are investigating the mechanisms for energy transfer between localized vibrational modes and between vibrational modes and lattice or phonon modes. Either a laser-Raman system or a Fourier transform infrared (FTIR) spectrometer will serve as the probe beam, with the attribute of placing the burden of detection on two conventional spectroscopic techniques that circumvent the limited response of infrared detectors. More specifically, the Raman effect inelastically shifts an exciting laser line, typically a visible frequency, by the energy of the vibrational mode; however, the shifted Raman lines also lie in the visible, allowing for detection with highly efficient visible detectors. With regards to FTIR spectroscopy, the multiplex advantage yields a distinct benefit for infrared detector response.
Our group is investigating intramolecular and intermolecular energy transfer processes in both biopolymers and more traditional materials. For example, alkali halides contain a number of defect types that effectively transfer energy in an intermolecular process. Similarly, the functioning of biopolymers depends on efficient intramolecular energy transfer. Understanding these mechanisms will enhance our ability to modify biopolymers and materials with applications to biology, medecine, and materials science.