Intracavity laser absorption

Several species which are weak absorbers or which may not fluoresce readily, may be detected using intracavity laser absorption. This is a much more sensitive method of detection than the conventional absorption measurements because the output power of a laser is very sensitive to small variations in its gain which, in term, depends on the concentration of absorbing species within the cavity. Vibrational state populations for the CO produced in the reaction... 

Direct spectroscopic measurements of absorptions could provide substantial and much-needed complimentary information on the properties of BLMs. Difficulties of spectroscopic techniques lie in the extreme thinness of the BLM absorbances of relatively few molecules need to be determined. We have overcome this difficulty by Intracavity Laser Absorption Spectroscopic (ICLAS) measurements. Absorbances in ICLAS are determined as intracavity optical losses (2JI). Sensitivity enhancements originate in the multipass, threshold and mode competition effects. Enhancement factor as high as 106 has be en reported for species whose absorbances are narrow compared to spectral profile of the laser ( 10). The enhancement factor for broad-band absorbers, used in our work, is much smaller. Thus, for BLM-incorporated chlorophyll-a, we observed an enhancement factor of 10 and reported sensitivities for absorbances in the order of lO- (24). 

Figure 10 shows the schematics of the experimental setup used for intracavity laser absorption spectroscopy (ICLAS) of bilayer lipid membranes (BLMs). Simultaneous electrical and ICLAS measurements were carried out in a two-compartment container constructed from two 1 cm path lengths quartz cells (Figure 11). 

Figure 10. Schematics of the experimental setup for intracavity laser absorption spectroscopy (ICLAS). CD chopper driver PM power meter Mj, M2, M3, M4 spherical high reflection mirrors Mp = pump mirror MN monochromator PMT photomultiplier SP silicon photocell PC Pockels cell WF wedged filter LIA lock-in amplifier R recorder MS microscope OF optical fiber S sample (solution on BLM) IEM instruments for electrical measurements (see Figure 2).

In this section, we will review previous studies of the CH overtone spectroscopy of CD3H. There exist high-resolution, rotationally resolved, experimental data for the first three CH overtones ( uv,) with n = 2, 3, 4) in the infrared and near infrared region (91,94). In the visible region, there also exists data from photoacoustic laser spectroscopy (96,97) and from the intracavity laser absorption spectroscopy (ICLAS) technique, which provides absolute intensities (85,86). Compared to methane, analysis of the spectra is much easier for CD3H, because of the relatively isolated CH chromophore. 

The near-infrared electronic transition occurring in the region of 745 nm was originally assigned as connected to the ground state. O Brien et al. [OOOBr] have recorded and analyzed this transition using intracavity laser absorption spectroscopy and conclude that it occurs between two unknown excited states. 

J. Cheng et al.. Infrared intracavity laser absorption spectroscopy with a continuous-scan Fourier-transform interferometer. Appl. Opt. 39(13), 2221 (2000)... 

Handler KG, Harris RA, O Brien LC, O Brien JJ (2011) Intracavity laser absorption spectroscopy of platinum fluoride, PtF. J Mol Spectr 265 39- 6... 

Fig. 16.21 The absorption spectrum of PuF (g). Arrows indicate regions reported to show vibrational structure. Bars indicate regions examined by intracavity laser absorption I, 455 70 II, 550-574 III, 697-729 IV, 786-845 V, 918-971 nm. At the top is a densitometer trace of the high-resolution absorption spectrum of PuF in the 781-830 nm region obtained in multipass experiments. Data from ref. 88.

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... 

It is therefore better to pump the intracavity laser with a step-function pump laser, which starts pumping at r = 0 and then remains constant (Fig. 1.13). The intracavity absorption is then measured at times t with 0 < t which are... 

Fig. 8.2 Linewidth of the Lamb peak in the output power of a HeNe laser at X = 339 pm with an intracavity CH4 absorption cell and different beam waists of the expanded laser beam, causing a different transit-time broadening [976]...

Fig. 8.4 Line profiles of Lamb peaks of a HeNe laser at A = 3.39 pm with intracavity CH4 absorption cell (a) pure CH4 at 1.4 mbar (b) addition of 30 mbar He and (c) 79 mbar He [978]...

E.A. Sviridenko, M.P. Erolov, Possible investigations of absorption line profiles by intracavity laser spectroscopy. Sov. J. Quantum Electron. 7, 576 (1977)... 

Applications So far, intracavity laser spectroscopy has been applied primarily to the detection of absorption spectra of gaseous impurities such as NH3 and CH4 in the near-infrared region using a tunable broadband laser. Special DLs designed with an external cavity have also been investigated recently for this purpose. CRS has been applied successfully to trace element detection using the ICP as the atomization system. The detection limits observed are at sub-parts per billion level (e.g., 0.3 ng ml for lead) and comparable to the detection limits achieved with ICP-MS. 

All the intracavity laser flux cannot be extracted because the RGH laser mixture contains many absorbers at the laser wavelength. The percent contribution of the absorption channel is shown in Fig. 8. Main absorbers seem to be... 

The main part of the book presents various applications of lasers in spectroscopy and discusses the different methods that have been developed recently. Chapter 6 starts with Doppler-limited laser absorption spectroscopy with its various high-sensitivity detection techniques such as frequency modulation and intracavity spectroscopy, cavity ring-down techniques, excitation-fluorescence detection, ionization and optogalvanic spectroscopy, optoacoustic and optothermal spectroscopy, or laser-induced fluorescence. A comparison between the different techniques helps to critically judge their merits and limitations. 

The unsaturated gain Gq = exp[-2ANa(o))L] is determined by the pump power Pp which produces the inversion aN and by the absorption cross section a(v). The saturated gain G equals the total losses y. The intracavity laser intensity I is therefore... 

Fiq.l0.24a-c. Intracavity saturated absorption spectroscopy, (a) Experimental setup, (b) "Lamb peak" in the laser output, (c) derivative of a Lamb peak obtained by modulation of the laser frequency... 

Sensitivity can be improved by factors of 10 using intracavity absorption, placing an absorber inside a laser resonator cavity and detecting dips in the laser emission spectmm. The enhancement results from both the increased effective path length, and selective quenching of laser modes that suffer losses by being in resonance with an absorption feature. 

Goldman, A. et al.. Fiber laser intracavity absorption spectroscopy of ammonia and hydrogen cyanide in low pressure hydrocarbon flames, Chem. Phys. Lett., 423, 147, 2006. 

E. Unger and G. Patonay, Near-infrared laser diode intracavity absorption specttometry, Anal. Chem. 61, 1425-1427 (1989). 

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