Fluorescence supersonic jets

Electronic transitions in molecules in supersonic jets may be investigated by intersecting the jet with a tunable dye laser in the region of molecular flow and observing the total fluorescence intensity. As the laser is tuned across the absorption band system a fluorescence excitation spectrum results which strongly resembles the absorption spectrum. The spectrum... 

Figure 9.46 shows an example of a fluorescence excitation spectmm of hydrogen bonded dimers of x-tetrazine (1,2,4,5-tetraazabenzene). The pressure of x-tetrazine seeded into helium carrier gas at 4 atm pressure was about 0.001 atm. Expansion was through a 100 pm diameter nozzle. A high-resolution (0.005 cm ) dye laser crossed the supersonic jet 5 mm downstream from the nozzle. 

In a skimmed supersonic jet, the parallel nature of the resulting beam opens up the possibility of observing spectra with sub-Doppler resolution in which the line width due to Doppler broadening (see Section 2.3.4) is reduced. This is achieved by observing the specttum in a direction perpendicular to that of the beam. The molecules in the beam have zero velocity in the direction of observation and the Doppler broadening is reduced substantially. Fluorescence excitation spectra can be obtained with sub-Doppler rotational line widths by directing the laser perpendicular to the beam. The Doppler broadening is not removed completely because both the laser beam and the supersonic beam are not quite parallel. 

The fluorescence excifafion spectra in Figures 9.47(a) and 9.47(b) show the Ojj bands of the A B2 X Ai system of aniline (see Figure 5.19) and the aniline Ar complex in a skimmed supersonic jet of aniline seeded into the argon carrier gas. 

Fluorescence excitation spectra of fairly large molecules in a supersonic jet are simplified, vibrationally, due to depopulation of low-lying vibrational levels in the ground electronic... 

Figure 9.48 Part of the fluorescence excitation spectrum of 1,2,4,5-tetrafluorobenzene in a supersonic jet. (Reproduced, with permission, from Okuyama, K., Kakinuma, T, Fujii, M., Mikami, N. and Ito, M., J. Phys. Chem., 90, 3948, f986)...

Although this book is devoted to molecular fluorescence in condensed phases, it is worth mentioning the relevance of fluorescence spectroscopy in supersonic jets (Ito et al., 1988). A gas expanded through an orifice from a high-pressure region into a vacuum is cooled by the well-known Joule-Thomson effect. During expansion, collisions between the gas molecules lead to a dramatic decrease in their translational velocities. Translational temperatures of 1 K or less can be attained in this way. The supersonic jet technique is an alternative low-temperature approach to the solid-phase methods described in Section 3.5.2 all of them have a common aim of improving the spectral resolution. 

In a supersonic jet, the fluorescence spectra are virtually free of environmental perturbations (in contrast to condensed-phase samples) and can thus provide information on isolated solute molecules. Moreover, van der Waals complexes with other solute molecules can be studied, which is of great fundamental interest. 

Fluorescence-excitation, hole-burning, and dispersed-fluorescence spectra of 2,2 -bithiophene in a supersonic jet were measured (94JPC12893). 

Fuke and Kaya [1989] studied the tautomerization of dimer (6.15) formed in a supersonic jet to investigate the vibrational selectivity of concerted two-proton transfer in excited electronic state. Tunneling in the NH N fragments leads to broadening of certain vibrational bands in the fluorescence excitation spectrum (Figure 6.16). Tunneling is promoted by the symmetric intermolecular vibration with frequency 120 cm-1. The widths of bands with n = 0, 1, and 2 are 5, 10, and... 

Other techniques that have been used to determine polycyclic aromatic hydrocarbons in soil extracts include ELISA field screening [86], micellar elec-tr okinetic capillary chromatography [ 87], supersonic jet laser-induced fluorescence [88,89], fluorescence quenching [90], thermal desorption gas chromatography-mass spectrometry [81,90,100], microwave-assisted extraction [91], thermal desorption [92], immunochemical methods [93,94], electrophoresis [96], thin layer chromatography [95], and pyrolysis gas chromatography [35]. 

Figure 9.49 Single vibronic level fluorescence spectrum of styrene, in a supersonic jet, with excitation in the Ojj band...

Several of these compounds have been studied without surrounding solvent in isolated gas phase conditions (supersonic jet studies) but evidence for TICT emission was found only in a few cases. More successful in terms of dual fluorescence in the jet were studies with respect to microsolvation and selfcluster formation. Both these processes enhance TICT formation. A beautiful example how pretwisting and microsolvation enhance the possibility for TICT formation is shown for a derivative of TriMAPYM in Fig. 4 [36],... 

Fig. 4. An ortho-methyl group in diethylamino-pyrimidin induces some ground state twist and hence energetically destabilizes the B state but not yet sufficiently to make the population of the A state a najor process in supersonic jet spectroscopy. Upper panel dispersed fluorescence spectra of the jet-cooled bare molecule [36]. In clusters with methanol, the TICT state is preferentially lowered, and the majority of the ob rved red-shifted fluorescence can be assign l to arise from the TICT state (lower panel). This does not occur for the compound without an ortho-methyl group.

Work on indole, tryptophan, etc. continues because of their relevance to the complex field of protein photophysics. Creed has produced reviews of the photophysics and photochemistry of near-u.v.-absorbing amino-acids, viz. tryptophan and its simple derivatives, tyrosine and its simple derivatives, and cysteine and its simple derivatives. The nature of the fluorescent state of methylated indole derivatives has been examined in detail by Meech et al. Another investigation on indole derivatives deals particularly with solvent and temperature effects. Fluorescence quenching of indole by dimethylfor-mamide has also been examined in detail. Fluorescence excitation spectra of indoles and van der Waals complexes by supersonic jets give microscopic solvent shifts of electronic origin and prominent vibrational excitation of L(, states. Conventional flash photolysis of 1-methylindole in water shows R, e p, and a triplet state to be formed. " Changes in the steady-state fluores-... 

Fluorescence excitation, fluorescence, and MPI spectra of tolane (PhC=CPh) in a supersonic jet expansion t One- and two-photon excitation of diphenylbutadiene. Isomerization and/or other non-radiative processes become important for vibrational energies in excess of 1050 cm- ... 

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-... 

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