masters course: experimental techniques detection of molecular species (with lasers) techniques...
TRANSCRIPT
Masters Course: Experimental Techniques
Detection of molecular species (with lasers)
Techniques
Direct absorption techniquesCavity Ring DownCavity 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
Masters Course: Experimental Techniques
Direct absorption
Beer’s law, Beer-Lambert law
nlII exp0
Exponent cross section in [cm2]
n 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 detectionAcoustic modulation
Masters Course: Experimental Techniques
Optogalvanic detection
Principle: change the resistance over a plasma dischargeby 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 beamof 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
cavity filled with absorbing gas
absorption coefficient; cross section
Rc
L
10
kLRc
L
10
111
0cnk
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 mR=99.99%
= 30sPath=10 km
Masters Course: Experimental Techniques
Weak transitions; in the benchmark CRD molecule: oxygen
17165.0 17190.0 17215.0 17240.0 cm-1
0.0
10.0
20.0
abso
rpti
on fa
ctor
(10
-9 c
m-1
)16O2
1357911131517PP
PQ35791113151719
b1g+ - X3g
+ (3,0) or -band
f = 1.3 x 10-14
(104 times weaker than A-band)
lowest electronic states: O(3P) +O(3P)
in electric dipole approximation:all transitions “forbidden”
Masters Course: Experimental Techniques
CRD in quantitative spectroscopy: Rayleigh Scattering
J.W. Strutt, 3rd Baron of Rayleigh
2434
N2
-------------------n2
1– 2
n2
2+ 2----------------------- Fk =
single molecule cross section
King factor: polarization effect
4 +=
n, p molecular depolarization,
1899: full theory based onElectromagnetism
No distinction of fine structure:Raman, Brillouin, Rayleigh wing, Cabannes
1918: Depolarization effectsR.J. Strutt
1923: Depolarization andCross section: King
1969: Full QM theory - Penney
1
321
43
63
76
362
p
p
n
nkF
Rayleigh scattering from theIndex of refraction !
Masters Course: Experimental Techniques
SF6
Ar
Cavity Loss:
/c = |lnR|/L + N
= 4+
for N2: 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 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)
Masters Course: Experimental Techniques
First steps and goals
Detection, not spectroscopySmall volumesUniversal wavelengths
100 ps,10 Hz> mJ
To the liquid phase: The combination CRD – HPLC in our approach
effective volume12 L
Philosophy: Liquid-Only cell
Masters Course: Experimental Techniques
liquid
flow
liquid-only cavity (14 μl)R = 99.996 %532 nm, 100 ps, 10 Hz, =70 ns
Demonstration of online HPLC detection in Liquid-Only cell
CRDchromatogram
Conventionalchromatogram
vd Sneppen et al, Anal. Chim. Acta 2006
Detection limit: 15 nM for = 1 x 104 M-1cm-1
Baseline: 2.7 x 10-6 A.U. (1s)
Masters Course: Experimental Techniques
Evanescent wave CRDS
Target
Biosensor
EW-CRDSprinciple
Ex, Ey, Ez fields Penetration depth, vs.Effective depth
=~8 (high sensitivity for top layer)
Masters Course: Experimental Techniques
70o prisms= 538 nm
Toward a biosensor (cytochrome c as test molecule)
bare silica
NH2-coated surface
C18-coating
Absorption signal of cyt con various surfaces
Result:C18 surface yields irreversable bindingNH2 surface (+ charged) releaves cyt c
Masters Course: Experimental Techniques
Quantum beat spectroscopy
0000 2211 ccct kk
k Excitation of a coherent superposition of quantum states
tik
kk
kkect 2/0 Temporal evolution of the superposition
2
mttI
Total fluorescence emitted So tBAetI t21cos
with2
222
2
121 mm ccA
mmccB 21212
LimitationNatural lifetime(not laser pulse !!)
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
t
EEi ge
exp
Masters Course: Experimental Techniques
Direct frequency comb spectroscopy
Probe the oscillation of the wave function
Find the right “mode”