Laser induced fluorescence levels

The most widely employed optical method for the study of chemical reaction dynamics has been laser-induced fluorescence. This detection scheme is schematically illustrated in the left-hand side of figure B2.3.8. A tunable laser is scanned tlnough an electronic band system of the molecule, while the fluorescence emission is detected. This maps out an action spectrum that can be used to detemiine the relative concentrations of the various vibration-rotation levels of the molecule. 

It is now possible to design the experiments using molecular beams and laser techniques such that the initial vibrational, rotational, translational or electronic states of the reagent are selected or final states of products are specified. In contrast to the measurement of overall rate constants in a bulk kinetics experiment, state-to-state differential and integral cross sections can be measured for different initial states of reactants and final states of products in these sophisticated experiments. Molecular beam studies have become more common, lasers have been used to excite the reagent molecules and it has become possible to detect the product molecules by laser-induced fluorescence . These experimental studies have put forward a dramatic change in experimental study of chemical reactions at the molecular level and has culminated in what is now called state-to-state chemistry. 

Laser-induced fluorescence (LIF). Laser-induced fluorescence measurements have been applied to the atmosphere since the suggestion of Baardsen and Ter-hune in 1972 that this method should be feasible. Figure 11.43 shows the energy levels and transitions involved in LIF measurements. OH is excited from its ground X2n state into the first electronically excited A22 state. The v" = 0 to r = 0 transition is around 308 nm and the v" = 0 to v = 1 at 282 nm. Two schemes have been used excitation using 282 nm into v = 1 of the upper electronic state, or excitation using 308 nm into v = 0 of the upper state. Collisional quenching deactivates some of the v = 1 into u = 0 in competition with fluorescence, mainly in the (1,1) band of the electronic transition (that is, from v = 1 of the upper state into v" =1 of the lower state). Collisional deactivation of v = 0 then occurs in competition with fluorescence in the (0,0) band at 308 nm... 

Although site effects are not as prevalent in UV-vis absorption as they are in IR spectra, they do exist and manifest themselves sometimes very clearly in band systems that comprise sharp peaks. An example is the radical cation of all-trans-octatetraene whose first absorption band consists of multiple peaks that can be selectively bleached by highly monochromatic light. The site stmcture can become more evident in laser-induced fluorescence, where excitation of individual sites is possible down to the level of single molecules in favorable cases, but a discussion of this fascinating phenomenon is beyond the scope of this chapter. 

This behavior is exploited in SEP experiments [51] where the lowering of the population of level 2 for double-resonance conditions is probed by laser-induced fluorescence (LIF) or ion detection (ion dip experiments) by ionizing the molecules in level 2 with a third laser pulse. It is obvious from the rate equations that no dip depth larger than 50% of the maximum off-resonant signal can be obtained as long as no fast decays of the final levels must be considered. (However, for fast-decaying final levels deeper dips can be expected and the dip depth has been used for an estimate of the decay rate [53].)... 

Wang et al. (1058) have recently measured OH radical concentrations in a simulated smog chamber by the laser induced fluorescence of OH. The OH concentrations in the chamber range from 0.5 to 1.5 x I07 molec cm"3. In view of the difficulties involved in the absolute determination of OH radicals at such low levels, the uncertainty must be larger than 50%. Table VIII-1A summarizes the ambient concentrations of reactive species and their rate constants with hydrocarbons and NO in polluted air. 

Conventional analytical techniques generally operate at the part per million or higher levels. Some techniques such as laser photo acoustic spectroscopy are capable of measuring phenomena at the 10-8-10-6 mol/L level. The most sensitive conventional analytical techniques, time-resolved laser-induced fluorescence, and ICP-MS are capable of measuring concentrations at the part per trillion level, that is, 1 part in 1012, but rarely does one see detection sensitivities at the single atom level as routinely found in some radioanalytical techniques. While techniques such as ICP-MS are replacing the use of neutron activation analysis in the routine measurement of part per billion concentrations, there can be no doubt about the unique sensitivity associated with radioanalytical methods. 

Laser-induced fluorescence (LIF) has also been utilized as a highly sensitive detection principle for CE [48-51]. However, while the LIF detector is now able to achieve zeptomole (10 21) detection limits, conventional derivatization techniques are inefficient at these exceptional levels [52]. Also, CE has successfully been coupled with mass spectrometry (MS) [53], nuclear magnetic resonance (NMR) [54, 55], near-infrared fluorescence (NIRF) [56, 57], radiometric [58], flame photometric [59], absorption imaging [60], and electrochemical (conductivity, amperometric, and potentiometry) [61-63] detectors. A general overview of the main detection methods is shown is Table 1 [64]. 

Laser-induced fluorescence has been discussed in Chapter 2. Lasers excite molecules to excited levels from which they can lose energy by emission of radiation. This technique has been of major importance in the molecular beam experiments which force modification of the original collision theory outlined below. 

Giordano, B.C., Jin, L., Couch, A.J., Ferrance, J.P., Landers, J.P., Microchip laser-induced fluorescence detection of proteins at submicrogram per milliliter levels mediated by dynamic labeling under pseudonative conditions. Anal. Chem. 2004, 76, 4705—4714. 

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