Peak Absorption Cross-Section

A significant challenge in using vibrational spectroscopy for explosive detection (especially in the vapor phase) arises because of the combination of low vapor pressures and relatively low cross-section for absorption in the IR and the low scattering cross-section for Raman spectroscopy. For example, typical peak absorption cross-sections, a, for the NOz stretching modes are near 1 x 106 cm2/mole in the IR (for comparison, peak UV absorption cross-sections for TNT approach 50 x 106 cm2/mole). For Raman spectroscopy, scattering cross-sections in the UV may approach 1 x 10 2 cm2/mole [3,7],... 

The constant C was initially calculated for the nuclide zqzPu based on the first major resonance at 259 kJ/mol (2.68 eV) (12). For some nuclides, values of C were assumed based on the peak absorption cross section in the major resonance. Others were assumed based on proportionality to the resonance integral (which can be measured empirically without knowing the detailed energy-dependent spectrum). Then, these assumed values for C and also < 2200 were adJusted by trial and error procedures to produce reasonable agreement with experimentally determined tranmutation reactions. Table I shows values presently in use for the parameters 02200 and for both capture and fission for the transuranic nuclides considered in this program. 

Loh and Capelli (1997) have measured the CH3 density in a supersonic arcjet during the growth of diamond films. The arcjet used CH4 diluted in H2 for the feed gas. The spectroscopy system used was similar to that used by Childs et al. (1992) except that a CCD array rather than a diode array was used for the detector. They measured the transmittance near 216.3 nm and obtained the column density by dividing the logarithm of the transmittance by the peak absorption cross section. The column density was obtained as a function of the CH4 fraction in the feed gas, and as a function of the process pressure. At 2% CH4 in the feed gas, they found that the CH3 column density was about 4 X 10 cm . The CH3 densities were used to obtain a sticking coefficient for CH3 that was on the order of 10 under diamond growth conditions. 

F7/2(1) F5/2(1), with a larger peak absorption cross section that could lead to low quantum defect laser emission, is veiy narrow, especially at low temperatures, so that diode laser pumping is difficult However, the recent development of stabilized diode lasers has enabled the pumping in this transition in various Yb-doped ceramic lasers at room temperature. As a consequence, the small absorption cross sections require high concentrations of Yb or longer path of pump radiation inside the laser materials. According to spectroscopic studies, this absorption line can be... 

To understand what controls the value of Up, we recall that for molecules with weak electron-phonon coupling with appreciable oscillator strength in the lowest purely electronic transition, the optical homogeneous linewidth yn becomes very small at low temperatures. The key point to remember is that due to well-known sum rules on optical transitions, the peak cross section depends inversely or yu, so that the narrow finewidth of a ZPL translates into a very large peak absorption cross section. For allowed transitions of rigid molecules, the value of Up becomes extremely large, approaching the ultimate limit of... 

Although the importance of the peak absorption cross section has been recognized since the first SMS experiment [1], it is useful to review how Up can be estimated using sum-rule techniques. The standard integrated absorption sum rule [50-54] may be written... 

Assiuning the collection efficiency D is maximized, Eq. 4 shows that there are several physical parameters which must be chosen carefully in order to maximize the SNR. First, as stated above, the values of and [Pg.16]

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