Line strength, measurement

In the ideal case for REMPI, the efficiency of ion production is proportional to the line strength factors for 2-photon excitation [M], since the ionization step can be taken to have a wavelength- and state-mdependent efficiency. In actual practice, fragment ions can be produced upon absorption of a fouitli photon, or the ionization efficiency can be reduced tinough predissociation of the electronically excited state. It is advisable to employ experimentally measured ionization efficiency line strengdi factors to calibrate the detection sensitivity. With sufficient knowledge of the excited molecular electronic states, it is possible to understand the state dependence of these intensity factors [65]. 

The peak absorption (scattering) cross sections are thus useful comparative measures of detectivity because the latter is a product of the line strength and the practical line resolution. 

By measuring the relative intensities of satellite and main lines, the population ratio is obtained, if it can be assumed that the dipole moment and line strength is not appreciably different in the two cases. From the population ratio R, the energy interval AE is obtained from the Boltzmann law i.e.,... 

We have selected several unblended CH lines located in the near-UV between 3145-3190 A modified their oscillator strengths (gf-values) by fitting the solar high-resolution spectrum and assuming solar abundance of carbon 8.56 from Anders and Grevesse (1989). These lines are measurable in dwarfs down to the metallicities —3. Our results are shown in the Fig. 1. We confirm the metallicity dependence of the C/O ratio (Tomkin et al. 1992, Akerman et al. 2004). On the other hand, our plot for C/O shows a steep rise at [0/H]< —1. It is not clear if this effect is real. The work is in progress to address this issue using other abundance indicators such as CH 4300 A and better quality spectra. 

Fig. 3.25. Trends of nebular line strengths in H n regions with oxygen abundance. This figure shows oxygen abundance in H n regions of the Milky Way and spiral and irregular galaxies (determined using measured electron temperatures) vs. log R23, after Pilyugin (2003) the p parameter is the line ratio [O iii]/([0 11] + [O hi]).

Figure 6.2 displays experimental data concerning tensile strengths measured on dry samples (0% RH) and on samples stored in 50% RH air.The continuous lines are modelled tensile strengths. 

The intensity of an electric dipole transition in absorption or emission depends, on one hand, on factors particular to the experiment measuring the intensity, e.g., the number density of molecules in the initial state of the transition and, for absorption experiments, the absorption path length and the intensity of the incident light. On the other hand, the intensity involves a factor independent of the experimental parameters. This factor, the line strength 5(f <— i), determines the probability that a molecule in the initial state i of the transition f <— i will end up in the final state f within unit time. 

J.C. Edwards and P.J. Giammatteo, On-line acid strength measurement and sulfuric acid alkylation process control using process NMR, ISA Tech/Expo Technol. Update, 2, 63-67 (1998). 

Figure 24.9a shows a plot of measured total carbon (CO plus CO2, mole percent) versus equivalence ratio. The solid line was calculated assuming chemical equilibrium at the measured temperatures. The data points represent the measured CO and CO2 mole fractions (dry basis) using the fast extractive-sampling system. Horizontal bars represent the uncertainty in (f> due to reading and calibration errors vertical bars represent the uncertainty in the CO and CO2 mole-fraction sum due to line strength and absorption measurement uncertainty. The data are consistent to within 4% of the equilibrium predictions at all values of (p, indicating reliable operation of the system. 

When pressure broadening dominates, the situation is more complicated because the resulting Lorentzian profile contributes significant area far from the line center. A further complication in this case is that the Lorentzian half-width cannot be accurately calculated and must be measured in other experiments. If both Doppler and pressure broadening are present, however, and if the Lorentzian to Doppler half-width ratio is small, the correction necessitated by pressure broadening is small. In this situation an accurate value of the Lorentzian half-width may not be needed. Line strength in the case of combined Doppler and pressure broadening may be obtained from the equivalent width by the use of tables (Jansson and Korb, 1968). 

The relatively weak line strengths for hydrogen peroxide limits the detection of hydrogen peroxide to 100 pptv for 5 min integration times. Measurements made with this system showed that the HbOa mixing ratios in rural and ambient air is generally above this detection limit. 

Here pt and pj denote the fractional populations of states i and j (p( = exp —Ei/kT /q in thermal equilibrium, where q is the partition function) pm and pn denote the corresponding fractional populations of the energy levels, and dm and dn the degeneracies (pf = pm/dm, etc.). The absorption intensity Gji9 and the Einstein coefficients and Bji9 are fundamental measures of the line strength between the individual states i and j they are related to each other by the general equations... 

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