prof. Otruba 1 Lasers - absorption methods 2010 Vítězslav Otruba Lasers in AAS Advantages against classical measuring radiation sources: 1. 2. 3. 4. 5. 6. Narrow spectral line (linear calibration) Continuous tuning of wavelength possible (scanning) High intensity (low noise, fast processes - spark, furnace) Low divergence (spatial profiles, mini atomizers) Short pulse time (time resolution) Any wavelength (AAS of excited and ionized atoms) AAS with low radiation intensity Low intensity = not changed significantly occupation of energy levels 1. 2. 3. 4. 5. 6. 7. Atomic absorption profiles (Δλ≈ 0,1 – 0,001 pm) Isotopic shifts (≈0,2 pm pro ΔM= 1) Splitting by nuclear spin Doppler broadening (temperature) Collision damping Stark broadening (electron density) Calibration graphs (linearity of up to 6 concentration orders) Absorption spectroscopy   dI = −αI 0 dx ⇒ I = I 0 e −αx Lambert's law: Lambert-Beer law : I log = −ε c x I0  General equation for light absorption dI c3 gm = − hν N n Pnm = − hν Bnm N n I = − Anm N n I 2 dx 8πν g n Nn depends on the concentration of atoms (molecules) in a given state n. Absorption Linear absorption Fine structure Hg I 404,7 nm Around 2/3 of the stable atomic nuclei has a resulting nuclear spin, causing a fine line structure. 199Hg and 201Hg have unpaired neutrons and show spin splitting. Resulting profile is then results of combination with the natural linewidth and Doppler broadening. Isotopes 198Hg; 200Hg; 202Hg and 204Hg are not splitted. AAS with graphite furnace Example of AAS spectrometer with laser diode and electrothermal atomizer (graphite furnace) AAS in a laser spark Spectral profile of the uranium absorption line. Distance to sample surface 5 mm, argon atmosphere 540 Pa. Molecular absorption spectroscopy  Identification of the composition of the sample is performed by monitoring the loss of light after passing through the sample Laser excels extraordinary monochromaticity emitted radiation, wavelength selection is thus ensured directly by the radiation source Laser radiation is extremely intense, so the signal to noise ratio is very high. Extremely low detection limits can be achieved when using a multireflective cuvette. Measurement speed using a laser is higher than with conventional spectrometers. There is no parasitic infrared radiation from the heat source The radiation source can be moved away from the measuring cuvette (using fiber optic) Considerable attention is paid to laser spectroscopy in the IR region. Laser (diode) IR spectrometer (semiconductor tunable laser) allows very precise identification of spectral lines in IR (structural analysis)      Absorption spectrometer with laser A – classical absorption spectrometer; B – spectrometer with laser source Methods of low absorbance measurement  intense source of monochromatic light, absorption is measured differentially multiple passage of absorbed light through the sample, increasing the absorption length absorption of radiation and measurement of fluorescence from excited states optoacoustic detection methods absorption of radiation and measurement of Raman emission spectra     Absorption spectroscopy in resonator cavity  Between the resonator mirror and the active laser medium is a space usable for spectroscopic purposes, in which a sample cuvette is inserted This is a special detection method that excels in extraordinary sensitivity Laser radiation passes through the cuvette located inside the resonator repeatedly The laser output parameters are strongly influenced by internal absorption Due to its sensitivity, the absorption method inside the resonator is best used to detect low concentrations of substances, especially gases Spectroscopy inside the resonator is particularly suitable for qualitative analysis      Absorption spectroscopy in resonator cavity Absorpti on cuvette Absorption environment: gas, flame, plasma, solution… External cuvette Internal cuvette Active medium Pumping Ф 100 10 1 10-9 10-7 Absorption enhancement ≈ 500 10-5 g.cm-2 I Differential measurement monochroma tor sourc e gray filter Before the actual measurement it is necessary to set the gain of both photomultipliers so that the resulting voltage is zero. In the presence of plasma we then record the differential voltage in the order of Multiple pass of light sourc e detect or basic disadvantages - we lose spatial resolution - the light beam is diverging Application of Fourier transform sourc e modulat or detect or The signal from the detector is modulated by the discharge. Fourier transform in a PC separates a signal with a different frequency than the modulation frequency; the result is noise reduction. It is also possible to realize direct modulation of the source (e.g. by lamp voltage). In absorption spectroscopy, it is suitable to use Fourier transform Cavity Ring Down Spectroscopy Cavity Ring Down Spectroscopy Cavity Ring Down Spectroscopy      Using memory oscilloscope and PC, a large number of pulses can be averaged and reducing noise. The concentration of absorbing particles can be determined from the exponential decrease in light intensity. The detection limit is of the order of A=10-6. Parabolic mirrors allow spatial localization of the detected area, so concentration profiles can be measured. Currently, this method is only used in the visible and near UV range. IR high resolution laser absorption spectrometry FP = Fabry Perot standard for wave number calibration (Δν=c/(2dn)) L = semiconductor laser PbSnTe; PbCdS; Pb1-xSnxSe; λ≈2 – 30 μm; R≈107; UV – VIS: dye lasers, R≈5.107 Double resonance methods 3 ω2,3 = rf, mw 2 (virtual level) 3 ω2,3 2 ω3,4 4 1 ω1,2 1 ω1,2 = optical ω1,2 = constant frequency laser, strong population level 2 ω2,3 = tuned radio frequency, microwave or optical radiati ω3,4 = fluorescence radiation indicating resonance Optical-microwave double resonance Indication: Absorption of auxiliary beam Pω1,2 At resonance 2 → 3, depopulation of level 2 occurs and its occupancy decreases, thereby increasing absorption 1 → 2 laser active medium Pω1 ,2 3 ω2,3 2 ω1,2 Pω1,2 A(Pω1,2) ∼ detector mw dete ctor cuvette 1 prof. Otruba f≈ω2,3 f Tuning of absorption levels  Zeeman effect can be used for molecules with permanent magnetic dipole moment. The magnetic field causes splitting of degenerate levels: ΔE = -μBgm, where g=Landé factor; μB=Bohr magneton; B= magnetic induction; m = magnetic quantum number. Sensitivity is particularly high for radicals with unpaired electrons.  Stark effect causes the splitting of the molecules in a permanent electric dipole moment. Necessary intensities of homogeneous el. fields are of order 1000V/mm. Especially in the IR region, lasers HF, DF, CO, CO2, N2O, H2O, D2O, HCN  Tuning of absorption levels– Zeeman S dete ctor → B cuvette J E ΔM=1 ΔM=1 ΔM=0 ΔM=-1 -1/2 B lasers active medium +3/2 +1/2 -1/2 +1/2 -3/2 Nonlinear spectroscopic methods  Methods based on simultaneous absorption of multiple photons by the same sample particle The absorption coefficient value changes when more photons are absorbed by the particles at the same time Interaction of the sample with a large amount of photons increases the occupancy of the upper energy level and reduces the absorption of the sample due to saturation of the absorption transition If non-linear effects occur, Lambert-Beer's law cannot be used for absorption    Non-linear absorption Saturation spectrometry  Optically thick absorption - absorption coefficient regulation Spectroscopy without Doppler Broadening e.g.:    Lamb-dip spectrometry Two-photon Doppler-free spectrometry   Laser frequency stabilization Multiphoton methods Saturation Doppler-free Spectroscopy (Lamb-Dip Spectroscopy)  The saturation sub Doppler spectroscopy method finds its main application in determining the exact values of absorption lines and in stabilizing of lasers The principle of saturation sub Doppler spectroscopy is based on the Doppler effect Saturation sub Doppler spectroscopy is a method used for the study of gaseous substances Gas particles that move chaotically appear in radiation interaction with frequency shift according to the speed of movement relative to the viewing direction The particles interact with the frequency shift radiation given by the instantaneous velocity component     Natural linewidth Heisenberg uncertainty principle Doppler broadening Homogeneous and inhomogeneous broadening Movement of particles in gas  Gas particles that move chaotically appear in interaction with radiation by frequency shift according to the speed of movement relative to the viewing direction hν v0-vD v0 v0+vD Absorption saturation on inhomogeneously broadened line Both the saturation and probe beams are parallel λ0; λ0 ± λD λs λm D D Doppler Lorentz λ0 λs=λm=λ0+λD Bennet (Lamb) hole Saturation and probing beam in opposite λ0; λ0 ± λD λs D D Doppler λm Lorentz λm ≈ λ0-λD λ0 λs Final solution λ0; λ0 ± λD λs D D Doppler Lorentz λs=λm=λ0 Saturated absorption spectroscopy Doppler-free spectroscopy Experimental arrangement Experiment results: Na(g),p=40μPa, t=110°C, λ=589 nm ΔλDOP ≅1,7 pm (1500 MHz) ΔλLAS ≅0,008 pm (7 MHz) ΔλEXP≅0,068 pm (40 MHz) ΔλNAT ≅0,01 pm (≈τ=16 ns) Apparatus for Doppler-free saturated absorption spectroscopy of I2 and Na Doppler-Free Saturated Absorption Spectroscopy of Iodine and Sodium Using a Tunable Ring Dye Laser Absorption spectrum of iodine Ordinary, Doppler broadened, (dashed line) and (b) Doppler-free (solid line) absorption spectra of the 5682 Å, P(117), 21-1, X --> B transition of 127I2. c) 300 MHz interferometer transmission peaks for frequency calibration of laser Laser stabilization aser active medium, Z – mirrors, K = cuvette, talon, PP = piezoelectric interface, LO – tuning circuit, etector, R – feedback control. mple: He-Ne laser 3390 nm, methane cuvette, stabilization ± Doppler-free spectroscopy - summary  If the frequency of the intense laser beam is tuned off the centre of the absorption line profile, a dip is formed in the absorption line indicating a decrease in the absorption coefficient (non-linear effect) In the experiment the laser beam is divided into two beams, intensive saturation and weaker - test beams Both beams pass through the cuvette against each other - the effects of the two beams, when aligned from the center of the absorption line, are positioned symmetrically with respect to the center of the line When tuning the laser to the center of the line, the test beam is less absorbed due to the decrease in absorption in the dip and the laser beam intensity is significantly increased A narrow resonance peak is obtained which represents the absorption line of the sample without Doppler broadening Doppler-free spectroscopy allows experimentally removing the line broadening by selecting particles of zero velocity from the sample      Experimental results Two-photon Doppler-free spectroscopy 1 Δω=ωL(v/c) ωL Δω=-ωL(v/c) V ωL 0 τv ≈10-12 s ωL =½ E01 VIS: Δλ≈1 GHz ФL