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

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

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

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

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

6. Masters Course: Experimental Techniques CARS = Coherent Anti-Stokes Raman CARSCSRS Production of a coherent beam of light

7. Masters Course: Experimental Techniques BOXCARS Coherent beams in 3 dimensions

8. Masters Course: Experimental Techniques Cavity Ring-Down spectroscopy

9. Masters Course: Experimental Techniques Cavity Ring-Down spectroscopy

10. Masters Course: Experimental Techniques CRD; the paradigm empty cavity cavity filled with absorbing gas absorption coefficient; cross section 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” L=1 m R=99.99%  = 30  s Path=10 km

11. Masters Course: Experimental Techniques Weak transitions; in the benchmark CRD molecule: oxygen b 1  g + - X 3  g + (3,0) or  -band f = 1.3 x 10 -14 (10 4 times weaker than A-band) lowest electronic states: O( 3 P) +O( 3 P) in electric dipole approximation: all transitions “forbidden”

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

13. Masters Course: Experimental Techniques SF 6 Ar Cavity Loss:  /c = |lnR|/L +  N  =  4+  for N 2 :  th = 23.00 (0.23) 10 -45  exp = 22.94 (0.12) 10 -45 Rayleigh scattering: direct measurement for Ar:  th = 20.04 (0.05) 10 -45  exp = 19.89 (0.14) 10 -45 method: Naus and Ubachs, Opt. Lett. 25 (2000) 347 for SF 6 :  th = 183 (6) 10 -45  exp = 180 (6) 10 -45 Theory – QM - ab initio: Oddershede/Svendsen s(N 2, 500 nm): 6.2 x 10 -27 cm 2 /molecule (10% off)

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

15. Masters Course: Experimental Techniques liquid flow liquid-only cavity (14 μl) R = 99.996 % 532 nm, 100 ps, 10 Hz, t =70 ns Demonstration of online HPLC detection in Liquid-Only cell CRD chromatogram Conventional chromatogram vd Sneppen et al, Anal. Chim. Acta 2006 Detection limit: 15 nM for e = 1 x 10 4 M -1 cm -1 Baseline: 2.7 x 10 -6 A.U. (1s)

16. Masters Course: Experimental Techniques Evanescent wave CRDS Target Biosensor EW-CRDS principle E x, E y, E z fields Penetration depth, vs. Effective depth  =~8 (high sensitivity for top layer)

17. Masters Course: Experimental Techniques 70 o prisms  = 538 nm Toward a biosensor (cytochrome c as test molecule) bare silica NH 2 -coated surface C 18 -coating Absorption signal of cyt c on various surfaces Result: C18 surface yields irreversable binding NH2 surface (+ charged) releaves cyt c

18. Masters Course: Experimental Techniques Quantum beat spectroscopy Excitation of a coherent superposition of quantum states Temporal evolution of the superposition Total fluorescence emittedSo with Limitation  Natural lifetime (not laser pulse !!)

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

20. Masters Course: Experimental Techniques Direct frequency comb spectroscopy Probe the oscillation of the wave function Find the right “mode”