Detection system, multiphoton

Figure 1. Multiphoton UV-photolysis-laser fluorescence detection system...

Multiphoton ionization spectroscopy has been reviewed in two recent articles. An apparatus has been described for constant intensity multiphoton ionization spectroscopy. A sensitive molecular vapour detection system utilizing resonance-enhanced two-photon ionization has been used to monitor naphthalene to a limit of 5 x 10 molecules cm Excitation was achieved... 

Since TIRF produces an evanescent wave of typically 80 nm depth and several tens of microns width, detection of TIRF-induced fluorescence requires a camera-based (imaging) detector. Hence, implementing TIRF on scanning FLIM systems or multiphoton FLIM systems is generally not possible. To combine it with FLIM, a nanosecond-gated or high-frequency-modulated imaging detector is required in addition to a pulsed or modulated laser source. In this chapter, the implementation with of TIRF into a frequency-domain wide-field FLIM system is described. 

A number of other laser spectroscopic techniques are of interest but space does not permit their discussion. A few specialized methods of detecting laser absorption worthy of mention include multiphoton ionization/mass spectrometry (28), which is extremely sensitive as well as mass selective for gas-phase systems optically detected magnetic resonance (29) laser intracavity absorption, which can be extremely sensitive and is applicable to gases or solutions (30) thermal blooming, which is also applicable to very weak absorbances in gases or liquids (31) and... 

A technique that has been used in laboratory studies for oxides of nitrogen and shows promise for field measurements is resonance-enhanced multiphoton ionization (REMPI) (Guizard et al., 1989 Lemire et al., 1993 Simeonsson et al., 1994). For example, Akimoto and co-workers (Lee et al., 1997) have reported a REMPI system in which a (1 + 1) two-photon absorption of light at 226 nm by NO results in ionization (vide supra). They report a detection limit of 16 ppt in their laboratory studies. Other oxides of nitrogen such as NOz and HN03 can also photodissociate in the... 

In addition to laser fluorescence excitation, several other laser spectroscopic methods have been found to be useful for the state-selective and sensitive detection of products of reactive collisions resonance-enhanced multiphoton ionization [58], coherent anti-Stokes Raman scattering [M], bolometric detection with laser excitation [30], and direct infrared absorption [7]. Several additional laser techniques have been developed for use in spectroscopic studies or for diagnostics in reacting systems. Of these, four-wave mixing [ ] is applicable to studies of reaction dynamics although it does have a somewhat lower sensitivity than the techniques mentioned above. 

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