LIF spectroscopy

Laser-induced fluorescence (LIF) is a preferred method over IR and Raman for detection of compounds in low concentrations due to its higher sensitivity. However, it requires access to relatively low lying excited states with sizeable oscillator strengths and Franck-Condon factors with respect to the ground state. 

In two recent publications we have tried to characterize the excited state properties of 1 and 3 in order to facilitate their detection by LIF-spectroscopy. Our main tool in this effort has been equation of motion coupled cluster theory (EOM-CC). The EOM-CCSD method, which is equivalent to linear response CCSD, has been shown to provide an accurate description of both valence and excited states even in systems where electron correlation effects play an important role [39]. Computed transition energies for excitations that are of mainly single substitution character are generally accurate to within 0.1 eV. We have found the EOM-CCSD method to perform particularly well in combination with the doubly-augmented cc-pVDZ (d-aug-cc-pVDZ) basis set. This basis seems to provide equally balanced descriptions of ground and excited states, 

EOM-CCSD/d-aug-cc-pVDZ vertical excitation energies (A Vexc) (in eV) and one photon oscillator strengths (/), and QR-CCSD/d-aug-cc-pVDZ two-photon transition probabilities (TPA) (in au) for (1).  

We have also computed the absorption energies for the lowest lying states of 3 [19]. There is a multitude of states below 10 eV, but the majority of the transitions are dipole-forbidden or have very low oscillator strengths. The most 

The 1 isomer is difficult to detect and identify by IR spectroscopy, since the IR spectrum only contains one weak line. In contrast, there are two transitions of intermediate strength in the Raman spectrum, and consequently Raman spectroscopy can be used for identification without the need of isotopic labeling. An alternative approach for detection is LIF spectroscopy. Excited state calculations, using linear and quadratic CCSD theory, indicates that the 

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Laser-induced multiphoton excitation, 160 Laser sputtering, 158 Li -bound diol complexes, 204 LIF spectroscopy... 

Figure 16.36. Experimental layout to perform analyses of laser-induced fluorescence (LIF) spectroscopy of opaque samples.

To simplify the measurements of LIF spectroscopy of whole soil, pellets were usually prepared. Soil sample aliquots of approximately 0.50g, after being further grinded to pass through a 250-pm mesh, were pressed into pellets of 1-cm diameter and 2-mm thickness, which were then inserted into an apparatus like the one... 

Figure 16.43. (a) Experimental apparatus to carry out LIF spectroscopy, (b) Details of the soil pellets. 

Figure 16.44. Correlation between the humification index determined by LIF spectroscopy (Huf) in whole soil samples and the humification index /t465 (Milori et al., 2002) determined by conventional fluorescence in the corresponding humic acid samples. The indexes are expressed as arbitrary units (a.u.) (Milori et al., 2006).

Figure 16.45. SOM humification degree obtained through LIF spectroscopy (//LIF) as affected by land use and soil management system in the experiment at Brazilian Cerrado (Milori et al., 2006).

Bruno, A.E., Briihlmann, U., and Huber, J.R. (1988). Photofragmentation LIF spectroscopy of NOCL at dissociation wavelengths > 450 nm. Parent electronic spectrum and spin state and A-doublet populations of nascent NO and CL fragments, Chem. Phys. 120, 155-167. 

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

Table 2. Electronic origin (in nm) of the excitation spectra (as obtained from the LIF spectroscopy and REMPl-TOF-MS) of 4-dialkylamino derivatives of benzonitrile (I, II, IV) and pyrimidine (VHX), methyl 4-dimethylaminobenzoate (XI) and their clusters with various polar solvent molecules [the displacement in cm (- denotes the red shift and + the blue shift) of the 0,0 transitions of the clusters with respect to the monomer origin are included]... 

There is another important difference between direct absorption and LIF spectroscopies. This difference reflects a fundamental distinction between direct and indirect detection schemes. In a direct absorption spectrum, the signal integrated over the absorption line is determined by a fundamental molecular property, the integrated absorption cross section, oA (see Eqs. 6.1.11 and 6.1.13), times the number density (molecules/cm3) of molecules in the lower level of the absorption transition. In an LIF spectrum, the detected signal is reduced by the product of the fluorescence quantum yield... 

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... 

FIGURE 2.2S Polarization of N2 drifting in He (at 0.5 Torr pressure) by coUisional alignment measured using LIF spectroscopy. (From Dressier, R.A., Meyer, H., Leone, S.R., J. Chem. Phys., 87, 6029, 1987.)... 

PH2(A2Ai) is a primary product of the photolysis (at least in the far-UV region) under collisionless conditions. However, its low quantum yield indicates that the main primary product is PH2(X2Bi) [19]. The direct observation of nascent PH2(X) by laser-induced fluorescence (LIF) spectroscopy was obstructed by the fluorescence of PH2(A) [14]. Thresholds of these and the other species which were investigated in detail are as follows ... 

To illustrate how LIF spectroscopy of free radicals in jets can be useful, let s consider several closely related alkoxy radicals, CH3O, C2H3O, C2H5O, and i C3H70. These species are shown schematically in Fig. 8. As can be seen from Fig. 8, each radical has a pair of tt molecular orbitals more or less localized on the 0 atom. In the ground state of the radical, there are three electrons available to populate... 

The advantages of LIF spectroscopy for the determination of molecular parameters may be summarized as follows ... 

All these methods represent modifications of absorption spectroscopy, whereas LIF spectroscopy is based on emission of fluorescence from selectively populated... 

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