Detection of molecular species (with lasers) Techniques Issues Direct absorption techniques Cavity Ring Down Cavity Enhanced Resolution Ionization detection (multi-photon) Pressure Laser-induced fluorescence Color (excitation level) Photo-acoustic technique Quantum state selectivity Nonlinear optical techniques Optogalvanic technique Masters Course: Experimental Techniques Spectroscopy vs quantitative Direct absorption Beer’s law, Beer-Lambert law I I0 exp nl Exponent n cross section in [cm2] density in [cm-3] l absorption length in [cm] nl column density in [cm-2] Problem: - Measuring against a non-zero baseline Masters Course: Experimental Techniques Photo-acoustic detection RT-VT relaxation Kinetic energy = acoustics Use of acoustic resonator Phase sensitive detection Acoustic modulation Masters Course: Experimental Techniques Optogalvanic detection Principle: change the resistance over a plasma discharge by resonantly exciting atoms/molecules Masters Course: Experimental Techniques Ionization detection Various production schemes for ions Ions/charged particles sensitive detection Time-of-flight mass spectrometry Masters Course: Experimental Techniques CARS = Coherent Anti-Stokes Raman CARS CSRS Production of a coherent beam of light Masters Course: Experimental Techniques BOXCARS Coherent beams in 3 dimensions Masters Course: Experimental Techniques Cavity Ring-Down spectroscopy Masters Course: Experimental Techniques Cavity Ring-Down spectroscopy Masters Course: Experimental Techniques CRD; the paradigm empty cavity What is it good for ? 0 L c1 R cavity filled with absorbing gas 0 L c1 R kL absorption coefficient; cross section 1 1 1 k n c 0 Masters Course: Experimental Techniques - 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” L=1 m R=99.99% = 30ms Path=10 km Weak transitions; in the benchmark CRD molecule: oxygen b Sg - X Sg (3,0) or d-band 1 lowest electronic states: O(3P) +O(3P) + 3 + f = 1.3 x 10-14 (104 times weaker than A-band) P 19 17 in electric dipole approximation: all transitions “forbidden” absorption factor (10-9 cm-1 ) 20.0 17 16 15 13 13 11 11 9 9 7 7 3 5 5 3 Q P P 1 O2 10.0 0.0 17165.0 Masters Course: Experimental Techniques 15 17190.0 17215.0 17240.0 cm-1 CRD in quantitative spectroscopy: Rayleigh Scattering single molecule cross section J.W. Strutt, 3rd Baron of Rayleigh 1899: full theory based on Electromagnetism 1918: Depolarization effects R.J. Strutt 1923: Depolarization and Cross section: King 1969: Full QM theory - Penney 2 2 n – 1 3 4 24 - ----------------------= -----------------F k 2 2 N n 2 + 2 King factor: polarization effect 6 3r n 3 6 r p Fk 1 2 6 7 r n 3 4 r p 3 2 1 rn, rp molecular depolarization, = 4+ No distinction of fine structure: Raman, Brillouin, Rayleigh wing, Cabannes Masters Course: Experimental Techniques Rayleigh scattering from the Index of refraction ! Rayleigh scattering: direct measurement Cavity Loss: b/c = |lnR|/L + N = 4+ SF6 for N2: th = 23.00 (0.23) 10-45 exp = 22.94 (0.12) 10-45 for Ar: th = 20.04 (0.05) 10-45 Ar exp = 19.89 (0.14) 10-45 for SF6: th = 183 (6) 10-45 exp = 180 (6) 10-45 Theory – QM - ab initio: Oddershede/Svendsen (N2, 500 nm): 6.2 x 10-27 cm2/molecule (10% off) method: Naus and Ubachs, Opt. Lett. 25 (2000) 347 Masters Course: Experimental Techniques To the liquid phase: The combination CRD – HPLC in our approach 100 ps, 10 Hz > mJ Philosophy: Liquid-Only cell First steps and goals effective volume 12 mL Detection, not spectroscopy Small volumes Universal wavelengths Masters Course: Experimental Techniques Demonstration of online HPLC detection in Liquid-Only cell liquid-only cavity (14 μl) R = 99.996 % 532 nm, 100 ps, 10 Hz, =70 ns CRD chromatogram Conventional chromatogram Detection limit: 15 nM for = 1 x 104 M-1cm-1 Baseline: 2.7 x 10-6 A.U. (1s) Masters Course: Experimental Techniques vd Sneppen et al, Anal. Chim. Acta 2006 Evanescent wave CRDS Ex, Ey, Ez fields Penetration depth, vs. Effective depth EW-CRDS principle h=~8 (high sensitivity for top layer) Target Biosensor Masters Course: Experimental Techniques Toward a biosensor (cytochrome c as test molecule) 70o prisms l = 538 nm Absorption signal of cyt c on various surfaces NH2-coated surface bare silica C18-coating Result: C18 surface yields irreversable binding NH2 surface (+ charged) releaves cyt c Masters Course: Experimental Techniques Quantum beat spectroscopy Excitation of a coherent superposition of quantum states t 0 ck k 0 c1 1 0 c2 2 0 k Limitation Natural lifetime (not laser pulse !!) Temporal evolution of the superposition t ck k 0ei k k / 2 t k Total fluorescence emitted I t t m m 2 So I t et A B cos21t with A c12 1 m m 2 c22 2 m m B 2c1c2 1 m m 2 m m Masters Course: Experimental Techniques 2 Direct frequency comb spectroscopy Excitation with phase controlled ultra-short pulses. Evolution atomic phase: Ee E g exp i t Interference between pulse contributions; Ramsey fringes in time Masters Course: Experimental Techniques Direct frequency comb spectroscopy Probe the oscillation of the wave function Find the right “mode” Masters Course: Experimental Techniques