6 . D oppler - Lim ite d A b s orpt ion an d

F lu ore s c en c e S pe ctro s c opy w ith L as ers

6 .1 A dv ant ag e s of Las ers in S pectro s copy

A bs orption S pectros copy :

- broad em is s ion s ource (lamp )

- tun able las er s ource

1) B road em is s ion s ource ( I / I 10 - 4 10 - 5 )

- resolution : resolving power of spectrometer

- sensitivity : detector noise, intensity noise of the source

2 ) A dv ant ag e s of tunable las er s ource

- ( ) can be directly measured

- high spectr al power density => detector noise is negligible

- good directionality => multiple pass (long absorption path )

=> small absorption coefficient , collision broadening reduction

- frequency marker

- frequency st abilization using an absorption line

=> accurate measurement of a w avelength

accurate determination of the transition line

- rapid tunable source => tr ansient behavior

- accur ate measurement of the line profile

- intense laser => excit ation spectroscopy

6 .2 Hig h - S en s itiv ity M ethod of A bs orption S pectros copy

T ransnitted spectral intensity through an absorbing path length x ,

I T ( ) = I 0 ex p [ - ( ) x ] (6.3)

For small absorption x 1, I T ( ) I 0 [ 1 - ( ) x ]

=> ( ) =I R - I T ( )

I R x, where, I R : reference intensity

** I R - I T ( ) 0 : not easy to distinguish => sensitive detection !!

< s en s itiv e detection m ethod>

- frequency modulation

- intracavity absorption

6 .2 .1 Frequency M odulation (phas e s en s itiv e detection )

d ( )d

= -1

I R Ld I T

d(6.9)

If the laser frequency is sinusoidally modulated at a frequency ,

I T ( L ) = I T ( 0 ) +n

a n

n !s in n t ( d nI T

d n )0

(6.10)

For L 1, ( d nI T

d n )0

= - I 0 x ( d n ( )d n )

0

T he first three derivatives of the absorption coefficient ( ) ,

6 .2 .2 Intrac av ity A bs orption

R 1=1 A∼0 R2 =(1- T 2 )

P in tr a = qP ou t , where q = 1 / T 2

For L 1, the power absorbed at the frequency in the absorption cell

with length of L is

P ( ) = ( ) L P i n t r a = q ( ) L P ou t (6/ 12)

ex )

PP

=g 0

g 0 - +g 0

g 0 -(6.16)

sensitivity enhancement , Q

Q =g 0

( g 0 - )(6.17)

If g 0 (lasing threshold), Q1

=> laser output instability ! (in practice, Q 100 )

ex ) external resonator

Advantage

- absorption cell cannot be placed directly inside the resonator

Drawback

- simultaneous resonator tuning with the laser w avelength

- optical feedback

- mode matching

ex ) Isotope selective absorption

6 .3 Direct D etermination of A b s orbed Photon s

For the molecule with very small absortion coefficient , it ' s very

difficult to measure the difference between the reference and tr ansmitted

intensities => direct measuring the absorbed photon !!

6 .3 .1 F luore s cence Ex cit ation S pectros copy

T he number of photons absorbed per second along the path length x ,

n a = N i n L ik x (6.20)

where, n L : number of incident photons per second

ik : absorption cross section

N i : density of molecules in the st ate |i>

T he number of fluorescence photons emitted per second from the excited

level Ek ,

n f l = N k A k = n a k (6.21)

where, A k =m

A k m : total spontaneous emission probability

k = A k / ( A k + R k ) : ratio of the spontaneous emission rate

(quantum efficiency )

T he number of photoelectrons counted per second,

n p e = n a k p h = ( N i ik n L x ) k p h (6.22)

where, : collection efficiency

ph = n pe / n ph : quantum efficiency of the photocathode

ex 6.5) = 500 nm ( n L = 1/ h = 3x 1018/ s ), n pe = 100, = 0.1, ph = 0.2,

k = 1, P = 1 W, n a = 5 x 10 - 3 / s

=> I / I 10 - 14

** k , ph , should be const ant over the whole spectral r ange

< collection optics >

<Applications>

- very small absorption medium, minute concentration of radicals,

short - lived intermediate products in chemical reactions

- appropriate to the spectroscopy in UV, vis , near IR region

< = k , ph , decrease with increasing w avelength

6 .3 .2 Photoacous tic S pectros copy

* minute concentrations in the presence of other components

at high pressure

* measurement of absorption by measuring the pressure w ave

principle

Laser excit ation - > (energy transfer by collision ) - > translational energy

of the collisional partner - > thermal energy (T emper ature/ pressure) - >

acoustic w ave

signal

W = N i ik x ( 1 - k ) P L t (6.23)

limitations

- k (radiative transiton ) => long lifetime (10- 2 ∼ 10- 5 s ) => molecule

- reflection/ scattering by window or aerosols

=> frequency modulation at acoustic resonance frequency of the cell

<Applications>

- vibrational spectra of molecule in the infrared region

- detection of polluting or poison gases (NO2 , SO2 ...)

- dissociation energy measurement

6 .4 Ionization S pectros copy

* Ion/ electron detection (efficient detection )

* Ionic energy state study / Isotope seper ation

6 .4 .1 B as ic T echniques

1) photoionization

- excitation of excited molecule (a)

M * ( E k ) + h 2 M + + e - + E k in

- excitation of Rydberg level to autoionization level M ** (efficient ) (b )

M * ( E k ) + h 2 M ** M + + e - + E k in

- nonresonant two photon process (c)

M * ( E k ) + 2h 2 M + + e - + E k in

2) collision - induced ionization (discharge)

M * ( E k ) + e - ( E k in , 0) M + + e - ( E '

k in )

3) field ionization

long- lived highly excited Rydberg st ate molecule

Ionization potential

IP =r

Z ef f e 2

4 0 r 2 dr =Z ef f e 2

4 0 r

where, e Z ef f : effective nuclear charge

When an external field, E ex t = - E 0 x is applied,

IP ef f = IP -Z ef f e 3E 0

0(6.32)

If E IP ef f , where the energy of the excited level => field ionization

6 .4 .2 S ens itiv ity of Ionization Spectros copy

signal

S I = N k P kI = n a

P kI

P kI + R k= N i n L ik x

P kI

P kI + R k(6.33)

where, N k : density of excited molecules in level E k

P kI : ionization probability of level E k

R k : total relax ation rate of level E k

** , - > 1 (direct measuring the ions)

P kI R k (intense laser beam )

=> S I n a : the most sensitive detection is possible !!

6 .5 Optog alv anic S pectros copy

* spectroscopy in gas discharge

principle

Laser excitation to various levels - >

(ionization probabilities from each levels are different ) - >

discharge current change ( I ) - > voltage change ( U = R I )

signal

U = R I = a [ n i IP ( E i) - n k ( E k ) ] (6.37)

# competing process (noise)

- A ( E i) + e - A + + 2e - (direct ionization )

- A ( E i) + A * A + + A + e - (collisional ionization by metast able atom A * )

- A ( E i) + h A + + e - (direct photoionization )