Molecular absorption, diode lasers

Recently, time-resolved experiments have been performed that employ molecular vibrations of the radicals to allow their detection.The concept is similar to the TROA technique, but instead uses strong IR absorptions in a radical to monitor its concentration dependence. To date this technique has been employed to examine RPs containing benzoyl radicals using the carbon-oxygen double bond stretching frequency close to 1800 cm This technique has the potential to extend the range and type of RPs available for study. The technique relies on the use of a solid-state IR diode laser and a fast mercury cadmium telluride (MCT) detector. 

Recently, very sensitive and selective measurements became possible by tunable diode laser absorption spectroscopy (TDLAS). Diode lasers that lase in the mid-infrared region give extremely high resolution (3 x 10 " cm ) and can tune the emission line to one of many vibration-rotation bands by changing the laser temperature and current (10-100K, 0.1-2.0 A). The TDLAS measurement is usually carried out at reduced pressure to avoid band broadening due to molecular collision. Practically interference-free measurements are possible with a typical detection limit of sub-ppbv levels (100 m path, at 25 Torr), although the TDLAS system is still expensive and under development. 

Diode laser based analytical devices can be used as element selective detectors for gas/liquid chromatography (GC/LC). The atomic constituents of different molecular species in a sample can be measured in an atomizer, such as a flame or low pressure plasmas, e.g.. an MIP or DCP. The GC-MIP/DCP in combination with DLAAS enables the detection of non-metals, such as H. O, S, noble gases, and halogens. Most of these elements have metastable levels, which are efficiently populated in a plasma, and which have strong absorption transitions in the red-NIR spectral region. 

Molecular Absorption Spectroscopy with Diode Lasers... 

Wormhoudt J. Radical and molecular product concentration measurements in CF and CH radio frequency plasmas by infrared tunable diode laser absorption. J Vae Sei Teehnol A. 1990 8 1722-5... 

The utility of laser diodes for spectroscopic applications has been demonstrated in molecular absorption spectrometry, molecular fluorescence spectrometry, atomic absorption spectrometry, and as light sources for detectors in various chromatographic methods. Recent advances in laser diode technology fueled by consumer demand for high-speed, high-capacity DVD players have resulted in the availability of blue laser diodes with output powers up to 50 mW at 473 nm. These light sources are appearing routinely in commercial spectrometric systems. 

Fig. 10.19, Set-up for difference-frequency generation using two diode lasers. Arrangements for simultaneous long-path absorption monitoring of water vapour, molecular oxygen and methane are shown. Included in the figure is also the dynamics of the evacuation of a methane release into the laboratory [10.81]...

Microwave measurements of uracil in a heated cell suggested the diketo form as the most abundant [54]. Brown et al. reported the first microwave measurements in a seeded molecular beam and also concluded that the diketo form was predominant [55]. Viant et al. reported the first rotationaUy resolved gas phase IR spectra of uracil [28]. This work employed a slit nozzle, an IR diode laser, and a multipass arrangement to obtain high resolution IR absorption spectra of the out-of-phase V(, C2—0, 4=0) stretching vibration. The rotational analysis unambiguously assigned the species to the diketo tautomer. Brown et al. also observed the diketo form of thymine in a seeded molecular beam, based on the hyperfine structure in the 14440-133,11 transition [56]. 

Channel (R3.10a) has been confirmed experimentally by Meads et al. [83], using pulsed photolysis and infra-red diode laser absorption, in a study of CN + ND3. Channel b has been suggested as a source of cyanamide, NCNH2, by Herbst et al. [84, 85], to explain the observed presence of low densities of this species in a molecular cloud. 

When the bleaching of the IR absorption band of many vibrational-rotational transitions was observed simultaneously, it was inferred that a great number of initial rotational states became depleted under the action of an intense IR pulse (Alimpiev et al. 1977). This experiment demonstrated that the fraction of molecules involved in the MPE process, g( ), at = 0.01-1 J/cm is much greater than the fraction / of molecules interacting linearly with the IR field, that is, q ) f = q 0). Thus, under typical conditions, the MPE process produces two molecular ensembles. The fraction q of molecules involved in MPE forms an ensemble of vibrationally hot, that is, highly excited, molecules. The rest of the molecules, 1 — q, remain in the lower vibrational levels and form an ensemble of vibrationally cold molecules (Fig. 11.8(a)). Figure 11.8(b) shows the dependence on the laser fluence of the fraction q of molecules excited into the vibrational QC for three different molecules. The conclusion as to the depletion of many rotational levels was later confirmed by probing the populations of individual rotational levels of the SFe molecule with a tunable diode laser (Apatin et al. 1983). 

At Waterloo, Gough, Miller, and Scoles have generated a molecular beam of N20 N20. A diode laser beam was then scanned over the V3 mode to produce N20(001) N20(000). [The notation (vx,V2,V3) labels the vibrational levels involving the chemical bonds of N2O.] They reported the lifetime of this vibrationally excited complex to be in the range 10 to 10 s. The upper limit was established from the time of flight from the place of irradiation to the detector, and the lower limit was obtained by assuming that the total width of the diffuse absorption bond was due to predissociation. 

In order to prepare successful NIR molecular probe dyes, NIR dyes must meet the following criteria adequate response to analytes, high lipophilicity and/or reactive functional groups, absorbance maxima compatible with available laser diodes, high fluorescence quantum yield, molar absorptivity, and high photostability. 

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