Classes of Spectroscopic Experiment

For lmW of detected radiation at 500 nm, (Abackground)1 2 2 x 109 in a 1 second detection period (1 Hz bandwidth), which would be an astonishingly large SBR (corresponding to a minimum detectable fractional absorption of 

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 

LIF (Ezekiel and Weiss, 1968 Cruse, et al., 1973 Zare and Dagdigian, 1974 Kinsey, 1977) is an example of an indirect technique for the detection of a one-photon resonant upward transition. There are many other indirect detection techniques (optogalvanic, optothermal, photoacoustic, cavity ringdown), but Multi-Photon Ionization (MPI) is a special type of indirect technique uniquely well suited for combining absorption detection with other useful functionalities (see Section 1.2.1.1). In MPI, photo-ion detection replaces photon detection. The one-color, singly-resonant-enhanced (n + m) REMPI f process consists of an n-photon resonant e, v, J — e ,v ,J excitation, followed by a further nonresonant m-photon excitation into the ionization continuum 

Nevertheless, the resonance enhanced production of AB+ and e at the t = 0 arrival time of the excitation pulse, combined with the high efficiency 

Emission spectra axe usually much more complicated than Direct Absorption spectra, because an absorption spectrum contains all possible upward transitions from a single initial vibration-electronic state, typically the ground state, whereas an emission spectrum contains all possible downward transitions from 

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