Half-pressure

Any fluorescent species may be characterized by its quenching half density, defined by [M1/2] = k(/kqi where kq represents a weighted average of the HO-quenching rate coefficients of the gases present. ([M1/2] is termed a half pressure here Pl/2 = RT[Ml/2 ), where R and T are the gas constant and temperature. At the half pressure, half of the excited molecules are quenched and half of them fluoresce. Substitution of [M1/2] into equation 18a gives a simplified expression for the fluorescence signal Sf ... 

Fig. 3-2. Variation of COz quenching half-pressure with NO pressure for the y(0,2) band (from Callear and Smith78 with permission of the Faraday Society).

By plotting Py2(COa) versus [NO] and extrapolating to zero NO and Ar pressure, the true half-pressure of C02 can be determined (Fig. 3-2). The quenching efficiency of NO is marked that for Ar is small but measurable. The true half-pressure for C02 was found to be 0.31 torr. This number is k22jk. With this value and the data presented in Figure 3-2, k2ljk22 and k /k22 can be computed from eq. (e). The rate constants were obtained and are listed in Table 3-3. 

Callear and Smith monitored the emission from all three spectroscopic levels. When the 8 bands were monitored, NO pressures from 2 to 12 torr were used. The variation in pressure had little effect as the half-pressure for the self-quenching is >15 torr. When the y bands... 

Table V-6. Quenching Half-Pressures in torr of Various States of NO (175, 185)...

Quenching half-pressure is equal lo (, t) 1 where kt is the rate constant for quenching reaction and r is the mean lifetime (radiative and predissociative) of excited NO. 

Emission bands from the 42Z, B2n, C2n, and D2X states have been observed and decay rates of fluorescence have been measured extensively [Callear et al. (167-171, 174, 175)]. Various spontaneous processes of electronically excited NO are given in Table V-5. These states are quenched to a different degree by various gases. Quenching half pressures, p1/2, in torr defined as P112 — where is the quenching rate constant in sec-1 torr-1... 

This is the form chosen for implementation in Fig. 10.17. The control variable is mouth half-pressure hm, and h = h m - p+b is computed from the incoming bore pressure using only a single subtraction. The table is indexed by h, and the result of the lookup is then multiplied by h Finally, the result of the multiplication is subtracted from h m to give the outgoing pressure wave into the bore. The cost of the reed simulation is only two subtractions, one multiply, and one table lookup per sample. 

Figure 20. Evolution of compound droplet impacting on a flat surface (left half pressure contours and right half pressure distribution on the surface of the cell). Time evolves from left to right and from top to bottom.

The half-pressuFes quoted here for both reactions are for the strong collision calculation as explained in Chapter 7, the methyl isocyanide reaction does not conform exactly to the strong collision properties and the observed values of the half-pressure are actually some 20 Torr higher, see Figure 7.3. 

Figure 4.8 Comparison of the WER-440 V-213 (left half) and 1000 (right half) pressure vessels...

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