Determination of Population Distributions by LIF

An interesting application of LIF is the measurement of relative population densities N v , 7 ) and their distribution over the different vibrational-rotational levels (v , J ) under situations that differ from thermal equilibrium. Examples are chemical reactions of the type AB - - C AC -I- B, where a reaction product AC with 

The absorption rate of the laser tuned to the transition (/ (A is directly proportional to the population distribution N v , J- ). Therefore the intensity ratio of the total LIT induced by the laser tuned to the different absorption transitions yields the relative population densities of the absorbing levels. The population density Nk(vk, Jk) in the excited state ) can be determined from the measurement of the fluorescence rate 

In order to obtain the population densities Ni (u/, 7/) in the electronic ground state, the laser is tuned to the absorbing transitions /) k) starting from levels v l, 7/0 of the reaction products under investigation, and the total fluorescence rate (1.59) is measured for different upper levels k). Under stationary conditions these rates are obtained from the rate equation 

1 Doppler-Limited Absorption and Fluorescence Spectroscopy with Lasers 

Under stationary conditions the population densities Ni are determined by the rate equations 

An interesting application of LIF is the measurement of relative population densities // ) and their distribution over the different vibrational- 

The population density A K(r K, 7k) in the excited state k) can be determined from the measurement of the fluorescence rate 

Rj is the total deactivation rate of the level i) whereas E N Rmi is the sum of all transition rates from other levels m) into the level i). 

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To determine the nascent distribution, we examined 4 produced by reaction (10) under low pressure conditions. Pulsed photolysis of I2 at 496 nm was used to generate I. As only a fraction of the I2 was dissociated, 4 then appeared due to I + I2 collisions. The excited vibrational levels were probed using LIF of the D X transition with full rotational resolution. The nascent vibrational population distribution extracted from this spectrum is shown in Fig. 6, which reveals a strong preference for near resonant E-V transfer. 

Butler et al. (175) measured the LIF spectra of the ground state of the CS radical, and found that it was produced vibra-tionally excited. Their vibrational distribution curve peaks at v" = 3 and extends to v" = 6 (see Figure 10). Their high resolution studies indicated that the rotational population could be described with a "temperature" of about 700 K. Addison et al. (176) directly measured the S(4i) concentration change in time using resonance fluorescence detection. From the time dependence they extrapolated the concentration back to zero time and determined the nascent atom concentration for the 4). The yield of the S(3p)/S(4)) ratio was obtained by measuring the... 

In addition to the experiments in which CO internal excitations were determined using time-resolved IR absorption spectroscopy, a recent paper by Rice et at. [124] reports nascent CO v = 0 and 1 rotational distributions deriving from reaction (1), obtained using A H - X Z VUV LIF detection of CO. As in our measurements, 193 nm HjS photolysis and single-collision conditions were used, but in addition to using room temperature CO2/H2S mixtures, the use of expansion-cooled but uncomplexed samples enabled two additional studies to be carried out with low CO2 and H2S rotational temperatures, i.e., near 70 and 40 K. With room temperature samples, a CO(i = 0) rotational temperature of 800K was observed, and the CO(i = 1) rotational distribution was nonstatistical, although an actual distribution was not reported. As in both our work [123] and a report by Weston and co-workers [136], i = 2 was absent. Rice et al. concluded that the E = 1 and 0 populations are comparable, in contrast to our results and those of Weston and co-workers. 

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