detection of molecular species (with lasers)
DESCRIPTION
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) - PowerPoint PPT PresentationTRANSCRIPT
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
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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”