Population LIFS, measuring

The dynamics of the O Dj) + H2S -> OH(t/) + HS reaction have recently been investigated. Time-resolved spectra at 0.4 cm-1 resolution were recorded at 40-/rs intervals, beginning at 20 fj.s and continuing until 540 fis after the laser pulse. The time-dependent OH vibrational populations recorded in this experiment are shown in Figure 15. The rotational distributions in all vibrational levels at all observation times could be fitted by near room-temperature Boltzmann distributions. The vibrational distribution obtained at the earliest time (corresponding to approximately two gas-kinetic collisions after the reaction) was strongly inverted [45]. The LIF measurements... 

Dornhofer, Hack, and Langel (180), in a detailed study of the fluorescence that is induced by an ArF laser, have been able to show that an intense ArF laser can distort the observed vibrational distribution by photodissociating CS radicals with v" > 5. The ArF laser absorption by CS will also produce electronically excited CS which, when it emits, will redistribute the vibrational populations. Probing the CS quantum state population under these conditions could distort the CS ground state populations. The LIF measurements will underestimate the amount of CS radicals that are produced, while the direct detection methods will overestimate the amount of S(3p) atoms because of the secondary photolysis of CS. The vibrational distribution of Lu et al. (178) will be less prone to this secondary photolysis because very low laser powers (< 1 mj) were used. Dornhofer, et al. concluded from their results that the S(3p)/S(J-D) ratio was 3, which is in reasonable agreement with the LIF measurements of Lu et al. 

Tunable laser spectroscopic techniques such as laser-induced fluorescence (LIF) or resonantly enhanced multi-photon ionization (REMPI) are well-established mature fields in gas-phase spectroscopy and dynamics, and their application to gas-surface dynamics parallels their use elsewhere. The advantage of these techniques is that they can provide exceedingly sensitive detection, perhaps more so than mass spectrometers. In addition, they are detectors of individual quantum states and hence can measure nascent internal state population distributions produced via the gas-surface dynamics. The disadvantage of these techniques is that they are not completely general. Only some interesting molecules have spectroscopy amenable to be detected sensitively in this fashion, e.g., H2, N2, NO, CO, etc. Other interesting molecules, e.g. 02, CH4, etc., do not have suitable spectroscopy. However, when applicable, the laser spectroscopic techniques are very powerful. 

Recent work (161) with a tunable VUV flash lamp has shown that the CN(A2II) can be detected directly using the LIF technique. Thus one is able, in principle, to determine the vibrational and rotational population of each of the fragments (CN(X2E), (A2n)). The tunable UV flash lamp allows one to measure these quantum state distributions as a function of the vibrational frequency of the upper electronic state. The results from these studies thus far are summarized in Table 8. 

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... 

Some of our recent studies of LIF on OH in flames demonstrate the close connection between current work in other areas of physical chemistry—in this case, state-to-state collisional energy transfer—and the development of diagnostic tools for combustion. In these experiments, measurements are made of the collisional redistribution of excited state population following laser excitation of OH to individual levels, in an atmospheric pressure flame. 

As discussed above, a LIFS signal is proportional to the excited state number density of the species being excited. This information is not itself normally useful. What is desired is a measure of the total population, or the temperature. Often one seeks the population of individual ground states. To be able to relate the observed signal to variables of interest one must be able to describe the dynamics of the excitation process. 

There is another approach which can be used in suitable circumstances. Developed by Kowalik and Kruger (31), it involves measuring the population of an excited atomic state by LIFS. If the ground state population is known to be uniform in the flow field, then information about temperature can be inferred. They have used the method to measure electron number density in MHD plasma flows. 

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. 

There are three main approaches to LIF thermometry. All of them are keyed to the measurement of the thermal (or Boltzmann) distribution of the molecular population in known rovibrational states. For this reason, a measurement can be considered reliable on the condition that the molecular target is well characterized in terms of its spectroscopy. The differences among the approaches are briefly summarized in the next sections. 

Like with vibrations (2.5.1), the rotational temperature (Tr) of polyatomic ions drifting in a gas increases at high E/N. That was shown for N2 in He by measuring the rotational state populations at various E/N using LIF spectroscopy (Figure 2.22). The dependence of Tr on E/N revealed by these data agrees with Equation 1.26, proving the thermal equilibration of rotations at T. The rotational... 

Another aspect of LIF concerns its application to the determination of the internal-state distribution in molecular reaction products of chemical reactions (Sects. 1.8.5 and 8.6). Under certain conditions the intensity /pi of LIF excited on the transition i) fe) is a direct measure of the population density iV, in the absorbing level />. 

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... 

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

To model these experiments, one usually identifies the count rate of the detector with the field-induced populations of the electronic or ionic states under consideration. To give an example for product detection through the measurement of the LIF, let us consider the pioneering Nal experiment of... 

Solid pellet fluorometry (or fluorimetry) is one of the classic older methods that were widely used to determine the uranium content in urine (Centanni et al. 1956). The urine sample is added to solid NaF or NaF/LiF that is fused by heating so that water and volatile organic and inorganic compounds are evaporated. The sample is then excited by UV radiation at 320-370 nm and the fluorescence at 530-570 nm is measured (perpendicular to the incident beam). The sensitivity is about 30 pg L for a 0.1 mL sample, but after preconcentration by ion exchange detection limits of 0.1 0.1 pg L for a 10 mL have been reported (Dupzyk and Dupzyk 1979). Even this improved MDL is insufficiently sensitive for monitoring unexposed populations where the expected concentration of uranium in urine is below 0.02 pg L (20 ng L )-... 

Principles and Characteristics The analytical capabilities of the conventional fluorescence (CF) technique (c/r. Chp. 1.4.2) are enhanced by the use of lasers as excitation sources. These allow precise activation of fluorophores with finely tuned laser-induced emission. The laser provides a very selective means of populating excited states and the study of the spectra of radiation emitted as these states decay is generally known as laser-induced fluorescence (LIF, either atomic or molecular fluorescence) [105] or laser-excited atomic fluorescence spectrometry (LEAFS). In LIF an absorption spectrum is obtained by measuring the excitation spectrum for creating fluorescing excited state... 

---