Laser-induced fluorescence, 49

Several detectors are now available that provide a choice of conductivity, fluorescence, laser-induced fluorescence, photodiode array, and mass spectrometry. A number of separations of peptides and proteins have been reported with mPC-CE-MS and mPC-CE-MS-MS, and some are listed next. 

Laser-Induced Fluorescence. Laser-induced fluorescence (Lif) provides, much as does ir spectroscopy, fingerprints of different organic molecules, which can be quantified by measuring fluorescence intensities. Selectivity is excellent, as both pump and fluorescence frequencies can be individually chosen for optimum performance, and it can be improved with measurements of fluorescence lifetimes and polarization behavior. The enhanced null-background sensitivity can achieve single-atom or single-molecule detection (256—258). Lif has important applications in gas analysis (259) and combustion and plasma diagnostics (260). 

A variety of detectors have been used for CE, including UV absorbance, fluorescence, laser-induced fluorescence, and mass spectrometry. The most widely used is the UV absorbance detector. In some cases, two or more detectors are connected in series. A more detailed the description of the detection system is given in Section 5.I.2.4. 

LIF Laser-induced fluorescence Incident laser beam excites Excited-state processes ... 

Greene C H and Zare R N 1983 Determination of product population and alignment using laser-induced fluorescence J. Chem. Rhys. 78 6741 -53... 

Crosley D R 1981 Collisional effects on laser-induced fluorescence Opt. Eng. 20 511-21... 

Altkorn R and Zare R N 1984 Effects of saturation on laser-induced fluorescence measurements of population and polarization Annual Review of Physical Chemistry ed B S Rabinovitch, J M Schurr and H L Strauss (Palo Alto, CA Annual Reviews)... 

Hamilton C E, Bierbaum V M and Leone S R 1985 Product vibrational state distributions of thermal energy charge transfer reactions determined by laser-induced fluorescence in a flowing afterglow Ar" + CC -> CC (v= 0-6) + Ar J. Chem. Rhys. 83 2284-92... 

Sonnenfroh D M and Leone S R 1989 A laser-induced fluorescence study of product rotational state distributions in the charge transfer reaction Ar <-i. i, ) + Ni Ar + MfXjat 0.28 and 0.40 eV J. them. Phys. 90 1677-85... 

Duncan M A, Bierbaum V M, Ellison G B and Leone S R 1983 Laser-induced fluorescence studies of ion collisional excitation in a drift field rotational excitation of N in He J. Chem. Phys. 79 5448-56... 

However, with the advent of lasers, the teclmique of laser-induced fluorescence (LIF) has probably become the single most popular means of detennining product-state distributions an early example is the work by Zare and co-workers on Ba + FLT (X= F, Cl, Br, I) reactions [25]. Here, a tunable laser excites an electronic transition of one of the products (the BaX product in this example), and the total fluorescence is detected as a... 

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. 

Irvine AML, Smith I W M, Tuckett R P and Yang X-F 1990 A laser-induced fluorescence determination of the complete internal state distribution of CH produced in the reaction H + NG2 CH + NC J. Chem. Phys. 93 3177-86... 

Tunable visible and ultraviolet lasers were available well before tunable infrared and far-infrared lasers. There are many complexes that contain monomers with visible and near-UV spectra. The earliest experiments to give detailed dynamical infonnation on complexes were in fact those of Smalley et al [22], who observed laser-induced fluorescence (LIF) spectra of He-l2 complexes. They excited the complex in the I2 B <—A band, and were able to produce excited-state complexes containing 5-state I2 in a wide range of vibrational states. From line w idths and dispersed fluorescence spectra, they were able to study the rates and pathways of dissociation. Such work was subsequently extended to many other systems, including the rare gas-Cl2 systems, and has given quite detailed infonnation on potential energy surfaces [231. 

Goodwin P M, Ambrose W P and Keller R A 1996 Single-molecule detection in liquids by laser-induced fluorescence Acc. Chem. Res. 29 607-13... 

Wilkerson C W Jr, Goodwin P M, Ambrose W P, Martin J C and Keller R A 1993 Detection and lifetime measurement of single molecules in flowing sample streams by laser-induced fluorescence Appl. Phys. Lett. 62 2030-2... 

Figure 9.31 Detection of products of multiphoton dissociation by laser-induced fluorescence...

New to the fourth edition are the topics of laser detection and ranging (LIDAR), cavity ring-down spectroscopy, femtosecond lasers and femtosecond spectroscopy, and the use of laser-induced fluorescence excitation for stmctural investigations of much larger molecules than had been possible previously. This latter technique takes advantage of two experimental quantum leaps the development of very high resolution lasers in the visible and ultraviolet regions and of the supersonic molecular beam. 

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