Tunable infrared diode laser spectroscopy

In our work on CO2-HX structures, tunable infrared diode laser spectroscopy was used to obtain geometries for X = F, Cl, and Br, using the CO2 asymmetric stretch chromophore [34], Both CO2-HF and CO2-HCI displayed the clear signatures of the linear equilibrium geometries that had been established previously [35, 36]. On the other hand, the C02-HBr spectrum showed that this complex is very asymmetric, which came as a surprise. We had originally assumed that C02-HBr was linear, by analogy with CO2-HF and COj-HCl. However, it is not. The observed C02-HBr spectra indicate a broadside inertial structure, with an u-axis moment of inertia close to that of uncomplexed CO2. Figures 11 and 12 show representative spectra that... 

Relative CO2-HCI and C02-HBr concentrations were measured under conditions similar to those used in the photoinitiated reactions, but in a completely separate apparatus that used pulsed slit nozzle expansions and tunable infrared diode laser spectroscopy. This was achieved by recording fully rotationally resolved CO2-HXIR absorption spectra using the CO2 asymmetric stretch chromophore, as described in Section III.A. The rotational distributions fit temperatures quite well, usually 3K. Rotational partition functions were calculated for the measured rotational temperatures and the areas under the absorption lines recorded for all transitions. From this, relative concentrations were determined. Details are available elsewhere [139, 140]. 

TUnable infrared diode laser spectroscopy Reid 1978 2 Method 404... 

Hanson, R. K., "High-Resolution Spectroscopy of Shock-Heated Gases Using a Tunable Infrared Diode Laser" in "Shock Tube and Wave Research" University of Washington Press, 1978,... 

Varghese PL and Hanson RK (1980) Tunable infrared diode laser measurements of line strengths and collision widths of 2C 0 at room temperature. Journal of Quantitative Spectroscopy Radiative Transfer 24 279. 

Other infrared absorption techniques are also used in ambient air measurements, including tunable diode laser spectroscopy (TDLS), nondispersive infrared (NDIR) spectroscopy, and matrix isolation spectroscopy. These are discussed in more detail later. 

Later chapters detail application of the present method to electron spectroscopy for chemical analysis (Chapter 5), high-resolution dispersive infrared spectroscopy (Chapter 6), and tunable-diode-laser spectroscopy (Chapter 7). Because the heart of the method is the repeated application of simple convolution, the method has been adapted to the processing of images (Kawata et al, 1978 Kawata and Ichioka, 1980a Saghri and Tescher, 1980 Maitre, 1981 Gindi, 1981). 

Airborne tunable laser absorption spectrometer infrared absorption by tunable diode laser spectroscopy 41... 

From the above considerations, it follows that C02-HI will most probably be inertially T-shaped, with at least as large a hydrogen zero point amplitude as for the case of C02-HBr. There is no fundamental reason why the structure of this complex has not yet been measured. We attempted this once by using high resolution infrared tunable diode laser spectroscopy and confirmed the inertially T-shaped character (Lin et al. n.d). However, before accurate rotational constants could be obtained, the HI ruined the vacuum pump, as it is prone to do. This lessened our appetite for a more precise structural determination. 

Among other new methods, tunable laser absorption spectroscopy using infrared diode lasers offers prospects for improved accuracy and specificity in concentration measurements, when a line-of-sight technique is appropriate. The present paper discusses diode laser techniques as applied to a flat flame burner and to a room temperature absorption cell. The cell experiments are used to determine the absorption band strength which is needed to properly interpret high temperature experiments. Preliminary results are reported for CO concentration measurements in a flame, the fundamental band strength of CO at STP, collision halfwidths of CO under flame conditions, and the temperature dependence of CO and NO collision halfwidths in combustion gases. 

The experimental setup applied is shown in Fig. 1. The experiments were performed in a 480 L Teflon-coated evacuable reaction chamber as shown by Fig. 1. The chamber is equipped with multiple reflection mirror systems for on-line Fourier Transform Infrared Spectroscopy (FlIR) and on-line Tunable Diode Laser Spectroscopy (TDL) as well as UVA IS spectroscopy. In addition, samples can be taken from the chamber and analysed by GC or by GC-MS. 

Successful concepts for "reaction-modulated" IR difference spectroscopy use the multiplex advantage of FTIR spectroscopy or the availability of high-intensity laser IR sources. A kinetic photometer using tunable IR diode lasers as sources for the mid-infrared has been developed in our laboratory and will be described elsewhere [6]. It covers the time-domain from approx. 500 nsec to some seconds. A second approach is time-resolved FTIR spectroscopy using a rapid-scanning interferometer, several scans can be recorded per second and the time-domain of slow reactions thus be covered [7]. The following schemes illustrate both concepts ... 

Tunable diode laser spectroscopy (TDLS) systems are tuned to access specific regions ofthe mid-infrared spectrum where most gases of interest such as UFe have strong absorption while common gases, such as oxygen and nitrogen, do not. TDLS systems have the potential to determine enrichment in UFs gas and to indicate the presence of HF gas, a by-product of enrichment activities. 

In Table 2 a list of the species detected by LMR to 1998 is presented. However, the pace of develop ments has slowed, since many of the better known free radicals accessible for LMR have now been ob served. As for lesser-known free radicals, LMR is a difficult method for obtaining initial knowledge (at least for polyatomics) because the Zeeman effect must be analysed as well as the zero Held spectrum. In the mid-infrared, the tunable semiconductor diode lasers are another factor in the development of infra red spectroscopy of high sensitivity and resolution, although LMR provides magnetic parameters inac cessible to the diode laser spectroscopy. In Figure 3 the rise and the fall of LMR are illustrated. 

Fourier transform infrared spectroscopy, tunable diode laser spectroscopy, and differential optical absorption spectroscopy... 

For the visible and near-ultraviolet portions of the spectmm, tunable dye lasers have commonly been used as the light source, although they are being replaced in many appHcation by tunable soHd-state lasers, eg, titanium-doped sapphire. Optical parametric oscillators are also developing as useful spectroscopic sources. In the infrared, tunable laser semiconductor diodes have been employed. The tunable diode lasers which contain lead salts have been employed for remote monitoring of poUutant species. Needs for infrared spectroscopy provide an impetus for continued development of tunable infrared lasers (see Infrared technology and RAMAN spectroscopy). 

With respect to the considerations above, research is split into three parts. The first is connected to the kinetic description of the release of ammonia from the biomass as function of temperature. This research employs infrared spectroscopy using a tunable diode laser. Here very small biomass particles are used that are heated up very rapidly in a small reactor, which ensures that transport effects are virtually excluded from the kinetic release effects. Since ammonia is released in very small quantities it is quite hard to detect. Therefore, we first measure CO release, which is easier. In the second part we investigate the propagation of a conversion front in biomass layers. Here we perform experiments and try to establish a modeling approach for the propagation by analytical and numerical approaches. In the third part the gas-phase conversion processes are described in terms of... 

Tunable diode laser absorption spectroscopy (TDLAS) has been used to measure oxides of nitrogen during flight (71). By tuning the laser to specific infrared absorption bands, the technique can selectively measure each compound. Detection limits are higher (25-100 pptrv for a 3-min response time) than the best chemiluminescent methods, and the instrumentation is less amenable to aircraft operations than the chemiluminescence techniques because of weight and size. 

In regions of the spectrum where a tunable laser is available it may be possible to use it to obtain an absorption spectrum in the same way as a tunable klystron or backward wave oscillator is used in microwave or millimetre wave spectroscopy (see Section 3.4.1). Absorbance (Equation 2.16) is measured as a function of frequency or wavenumber. This technique can be used with a diode laser to produce an infrared absorption spectrum. When electronic transitions are being studied, greater sensitivity is usually achieved by monitoring secondary processes which follow, and are directly related to, the absorption which has occurred. Such processes include fluorescence, dissociation, or predissociation, and, following the absorption of one or more additional photons, ionization. The spectrum resulting from monitoring these processes usually resembles the absorption spectrum very closely. 

As mentioned in the introduction, a major advantage that Fourier transform spectroscopy has over laser spectroscopy is that it is straightforward to record the entire spectrum of a species at once. Diode lasers in the infrared are not continuously tunable and have mode gaps which can only be filled by switching diodes. Many ultraviolet lasers are not continuously tunable either. Tunable difference frequency methods and diode lasers involve much longer scan times than are necessary with a Fourier transform device. For example, the Bomem DA3.002 can scan a bandwidth of 100 or more wavenumbers in the mid-IR at a resolution of 0.005 cm-1 in less than 3 minutes. A diode laser which scans in 20 MHz steps may require more than a day to scan the same spectral region. 

Buker JF, Sample JD (1976) Tunable Diode Laser Instruments, ISA Reprint. Pittsburg, pp 76-613 Bulkin BJ (1976) Vibrational spectroscopy of liquid crystals. In Brown GH (ed) Advances in liquid crystals, vol 2. Academic, New York, p 199 Bulkin BJ (1981) Vibrational spectra of liquid crystals. In Clark RJH, Hester RE (eds) Advances in infrared and Raman spectroscopy, vol 8. Heyden, London, p 151 Bunker PR (1979) Molecular Symmetry and Spectroscopy. Academic Press, New York Bunow MR, Levin IW (1977a) Biochim Biophys Acta 464 202 Burch DE, Gryvnak DA (1967) J Chem Phys 47 4930... 

Niedziela, R.F., M.L. Norman, C.L. DeForest, R.E. Miller and D.R. Worsnop A temperature- and composition-dependent study of H2SO4 aerosol optical constants using Fourier transform and tunable diode laser infrared spectroscopy, J. Phys. Chem. A 103 (1999) 8030-8040. 

Optical-microwave double resonance (OMDR) can considerably improve the situation and extends the advantages of microwave spectroscopy to excited vibrational or electronic states, because selected levels in these states can be populated by optical pumping. Generally dye lasers or tunable diode lasers are used for optical pumping. However, even fixed frequency lasers can often be used. Many lines of intense infrared lasers (for example, CO2, N2O, CO, HF, and DF lasers) coincide with rotational-vibrational transitions of polyatomic molecules. Even for lines that are only close to molecular transitions the molecular lines may be tuned into resonance by external magnetic or electric fields (Sect. 1.6). The advantages of this OMDR may be summarized as follows ... 

Figure 9 (A) Spectral stripping of components from the tunable diode laser infrared spectrum of cigarette smoke. From bottom to top ethylene, acrolein, methanol, and comparison with hydrazine. The reference spectrum is the lower of the two traces in each case. (B) Expanded view of the residual spectrum compared to that of hydrazine. (Reprinted with permission from Plunkett S, Parrish ME, Shafer KH, Nelson D, Shorter J, and Zahniser M (2001) Time-resolved analysis of cigarette combustion gases using a dual infrared tunable diode infrared laser system. Vibrational Spectroscopy 27 53-63 Elsevier.)...

Key CL, chemiluminescence UV, ultraviolet IR, infrared FTIR, Fourier-transform infrared spectroscopy TOLAS, tunable diode laser absorption spectroscopy IDS, indigo-5,5 -disulfon-ate ASTM, American Society for Testing and Materials ERA, US Environmental Protection Agency approved methods JIS, Japanese Industrial Standard WHO, World Health Organization selected methods n.a., not available. 

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