OCR Text |
Show ./ parlance it is referred to as Laser-Induced Fluorescence (LIF). As noted in Fig. 1, the fluorescence is usually red-shifted (i.e. to longer wavelengths) from the exciting laser light and it is thus possible to use filters or a monochromator to separate the fluorescence emission from scattered laser light. LIF is a powerful technique that is quite sensitive for the detection of small molecules and radicals and is used extensively for diagnostics in flames and engines.5 It is also applicable to some large molecules, such as dyes, in which the quantum yield of fluorescence is large (quantum yield is essentially the fraction of excited molecules that fluoresce). However in most large aromatics (especially halogenated ones) there is a nonradiative process known as intersystem crossing that competes with fluorescence. In this process the molecule crosses from the first excited singlet state to one of the lower lying triplet states (TI in Fig. I) in which the electron in the excited 1t * orbital is no longer spin paired with the electron in the 1t orbital. This intersystem crossing can occur very rapidly in chlorinated aromatics because the CI substituents enhance the coupling of the two spin manifolds. If this crossing is fast enough the quantum yield of fluorescence can be essentially zero because all of the excited molecules convert to TI before they fluoresce. Once in the T 1 state the molecules can phosphoresce back to SO. Because the T I-SO transition is spin-forbidden, it is very weak and the phosphorescence lifetimes are very long (sometimes seconds). Both factors make phosphorescence detection difficult and usually insensitive. The second technique that we shall examine is also shown in Fig. 1. In this method the excited S 1 molecules absorb a second photon, either from the same laser (A. 1 ) or another laser (A.2), that ionizes the molecule. In general this technique is referred to as Resonantly Enhanced Multiphoton Ionization (REMPI) since it corresponds to ionization via absorption of two or more photons that is strongly enhanced by a resonant absorption step. The scheme shown in Fig. 1 corresponds to the specific case of 1+1 REMPI (i.e. one photon resonant and one more to ionize); this is also commonly referred to as Resonant Two-Photon Ionization (R2PI). Since REMPI is a multiphoton process in which the ionization transition is typically weak it requires relatively high laser intensities that are typically achieved through the use of focused pulsed lasers. Under low intensity conditions only the parent ion, ArCl+ in our example, is produced. However at higher laser intensities either the excited neutral molecule or the parent ion absorb more laser photons and fragment into daughter ions that may subsequently absorb even more photons to produce fragments of lower mass. REMPI signals are detected by collecting the ions that are produced. The most useful way of doing this is to perform REMPI in the ionization source region of a mass spectrometer. Several different types of mass spectrometers have been used for this purpose including magnetic sector types, quadrupoles, and time-of-flight instruments.6 Mass-selective REMPI has the decided advantage of providing a mass spectrum simultaneously with an optical spectrum and this "twodimensional" capability provides enhanced selectivity over purely optical techniques such as LIF. In our experiments we employ a high resolution time-of-flight mass spectrometer (fOF MS) because it allows us to obtain an entire mass spectrum in a single laser shot. 3 |