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Detection of molecular species (with lasers)

Detection of molecular species (with lasers). Issues Resolution Spectroscopy vs quantitative Pressure Color (excitation level) Quantum state selectivity. Techniques Direct absorption techniques Cavity Ring Down Cavity Enhanced Ionization detection (multi-photon)

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Detection of molecular species (with lasers)

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  1. Detection of molecular species (with lasers) Issues Resolution Spectroscopy vs quantitative Pressure Color (excitation level) Quantum state selectivity Techniques Direct absorption techniques Cavity Ring Down Cavity Enhanced Ionization detection (multi-photon) Laser-induced fluorescence Photo-acoustic technique Nonlinear optical techniques Optogalvanic technique

  2. Direct absorption Beer’s law, Beer-Lambert law cross section in [cm2] Exponent density in [cm-3] absorption length in [cm] column density in [cm-2] Problem: - Measuring against a non-zero baseline

  3. Photo-acoustic detection RT-VT relaxation Kinetic energy = acoustics Use of acoustic resonator Phase sensitive detection Acoustic modulation

  4. Optogalvanic detection Principle: change the resistance over a plasma discharge by resonantly exciting atoms/molecules

  5. Ionization detection Various production schemes for ions Ions/charged particles sensitive detection Time-of-flight mass spectrometry

  6. CARS = Coherent Anti-Stokes Raman CARS CSRS Production of a coherent beam of light

  7. BOXCARS Coherent beams in 3 dimensions

  8. Cavity Ring-Down spectroscopy

  9. Cavity Ring-Down spectroscopy

  10. CRD; the paradigm empty cavity • What is it good for ? • Spectroscopy (not extreme precision) • Sensitive detection (within limits) • Intensity – quantitative • (within severe limits/difficulty) • Gases, • to Solids, Surfaces, Liquids • Wavelength range • (limited – mirror quality) • Advantages: • No dependence on laser fluctuations • Extremely long effective path lengths • Easy to make “absolute” cavity filled with absorbing gas absorption coefficient; cross section L=1 m R=99.99% t = 30ms Path=10 km

  11. b1Sg+ - X3Sg+ (3,0) or d-band f = 1.3 x 10-14 (104 times weaker than A-band) Weak transitions; in the benchmark CRD molecule: oxygen lowest electronic states: O(3P) +O(3P) in electric dipole approximation: all transitions “forbidden”

  12. CRD in quantitative spectroscopy: Rayleigh Scattering single molecule cross section J.W. Strutt, 3rd Baron of Rayleigh King factor: polarization effect 1899: full theory based on Electromagnetism 1918: Depolarization effects R.J. Strutt 1923: Depolarization and Cross section: King 1969: Full QM theory - Penney rn, rpmolecular depolarization, No distinction of fine structure: Raman, Brillouin, Rayleigh wing, Cabannes • Rayleigh scattering from the • Index of refraction !

  13. SF6 Ar Rayleigh scattering: direct measurement Cavity Loss: bn/c = |lnR|/L + snN sn = sn4+e for N2: sth = 23.00 (0.23) 10-45 sexp = 22.94 (0.12) 10-45 for Ar: sth = 20.04 (0.05) 10-45 sexp = 19.89 (0.14) 10-45 for SF6: sth = 183 (6) 10-45 sexp = 180 (6) 10-45 Theory – QM - ab initio: Oddershede/Svendsen s(N2, 500 nm): 6.2 x 10-27 cm2/molecule (10% off) method: Naus and Ubachs, Opt. Lett. 25 (2000) 347

  14. To the liquid phase: The combination CRD – HPLC in our approach 100 ps, 10 Hz > mJ Philosophy: Liquid-Only cell effective volume 12 mL First steps and goals Detection, not spectroscopy Small volumes Universal wavelengths

  15. liquid flow Demonstration of online HPLC detection in Liquid-Only cell liquid-only cavity (14 μl) R = 99.996 % 532 nm, 100 ps, 10 Hz, t =70 ns CRD chromatogram Conventional chromatogram Detection limit: 15 nM for e = 1 x 104 M-1cm-1 Baseline: 2.7 x 10-6 A.U. (1s) vd Sneppen et al, Anal. Chim. Acta 2006

  16. Target Biosensor Evanescent wave CRDS Penetration depth, vs. Effective depth Ex, Ey, Ez fields EW-CRDS principle h=~8 (high sensitivity for top layer)

  17. Toward a biosensor (cytochrome c as test molecule) Absorption signal of cyt c on various surfaces 70o prisms l = 538 nm NH2-coated surface bare silica C18-coating Result: C18 surface yields irreversable binding NH2 surface (+ charged) releaves cyt c

  18. Quantum beat spectroscopy Excitation of a coherent superposition of quantum states • Limitation • Natural lifetime • (not laser pulse !!) Temporal evolution of the superposition Total fluorescence emitted So with

  19. Direct frequency comb spectroscopy Excitation with phase controlled ultra-short pulses. Evolution atomic phase: Interference between pulse contributions; Ramsey fringes in time

  20. Direct frequency comb spectroscopy Probe the oscillation of the wave function Find the right “mode”

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