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”