6 . D oppler - Lim ite d A b s orption an d F lu ore s c en c e S pe ctro s c opy w ith L as ers

6 . D o pp le r - L im it e d A b s o rpt io n an d

F lu o re s c e n c e S p e c t ro s c o py w it h L a s e r s

6 .1 A dv an t ag e s of L as ers in S pe ctro s c opy

A b s orption S pe c tro s c opy :

- bro ad em i s s ion s ou rc e (l am p ) - tun able l as e r s ourc e

1 ) B ro ad em i s s ion s ou rc e ( I / I 10^{- 4} 10^{- 5}) - resolution : res olving pow er of spectrom et er

- sen sitivity : det ect or noise, int en sity n ois e of the sour ce 2 ) A dv an t ag e s of tu n ab le l as e r s ou rc e

- ( ) can be directly measured

- high spectr al pow er den sity => det ect or noise is negligible - good dir ection ality => multiple pas s (lon g ab s orption path )

=> sm all ab sorption coefficient , collision broadening reduction - frequency m arker

- frequency st abilization u sin g an ab sorption line

=> accur ate m easur em ent of a w av elength accur ate determin ation of the tr an sition line - r apid tun able source => tr an sient beh avior - accur at e m easurem ent of the line profile - int en se laser => ex cit ation spectroscopy

6 .2 H ig h - S e n s itiv ity M eth od of A b s orption S pe c tro s c opy

T r an snitted spectr al int en sity through an ab sorbing path length x ,

IT ( ) = I0 ex p [ - ( ) x ] (6.3)

F or sm all ab sorption x 1, IT ( ) I0 [ 1 - ( ) x ]

=> ( ) = I_R - I_T ( )

I_R x , w her e, IR : refer ence int en sity

** IR - IT ( ) 0 : not easy to distinguish => sen sitiv e detection !!

< s e n s itiv e de t e c tion m eth od >

- frequency m odulation - intr acavity ab sorption

6 .2 .1 F re qu e n c y M o du l ation (ph as e s en s itiv e de t e c tion )

d ( )

d = - 1

I_RL d IT

d (6.9)

If the laser fr equen cy is sinu s oidally m odulat ed at a frequency ,

IT ( L) = IT ( 0) +

n

aⁿ

n ! s in ⁿ t

(

)

F or L 1,

(

)

(

)

T he fir st thr ee deriv ativ es of the abs orption coefficient ( ),

6 .2 .2 Intrac av ity A b s orption

R¹=1 A～0 R²=(1- T²)

P _{in tr a} = q P _{ou t} , w here q = 1 / T₂

F or L 1, the pow er ab sorbed at the fr equen cy in the ab sorption cell w ith length of L is

P ( ) = ( ) L P _{i n t r a} = q ( ) L P _{o u t} (6/ 12)

ex )

P

P = g₀

g₀ - +

g₀

g₀ - (6.16)

s en sitivity enh an cem ent , Q

Q = g₀

( g₀ - ) (6.17)

If g₀ (lasing threshold), Q 1

=> laser output in st ability ! (in pr actice, Q 100)

ex ) ext ern al res on at or

A dv ant age

- abs orption cell cannot be placed directly in side the reson ator Dr aw b ack

- simult aneou s reson ator tuning w ith the laser w av elength - optical feedback

- m ode m at ching

ex ) Isotope selectiv e abs orption

6 .3 D ire c t D et e rm in ati on of A b s orb e d P h ot on s

F or the m olecule w ith v ery sm all ab sortion coefficient , it ' s v ery difficult t o m easur e the difference betw een the refer ence and tr an smitted int en sities => dir ect m easuring the abs orbed photon !!

6 .3 .1 F lu ore s c en c e Ex c it ation S pe c tro s c opy

T he number of photon s ab sorbed per secon d along the path len gth x,

n_a = N_i n_{L ik} x (6.20)

w here, nL : number of incident phot on s per second

ik : ab sorption cros s s ection

Ni : den sity of m olecules in the st at e |i>

T he number of fluorescence phot on s emitt ed per s econd from the ex cit ed lev el Ek,

n_{f l} = N_kA _k = n_{a k} (6.21)

w here, A _k =

m A _{k m} : t ot al spont an eou s emis sion prob ability

k = A _k / ( A _k + R_k) : r atio of the spont aneou s emis sion r ate (qu antum efficiency )

T he number of photoelectron s count ed per second,

np e = na k p h = ( Ni iknL x ) k p h (6.22)

w here, : collection efficiency

ph = np e/ nph : qu antum efficien cy of the phot ocathode

ex 6.5) = 500 nm ( n_L = 1/ h = 3x 10¹⁸/ s ), n_{p e} = 100, = 0.1, _{p h} = 0.2,

k = 1, P = 1 W , n_a = 5 x 10 ^{- 3} / s

=> I / I 10^{- 14}

** _k, _{p h}, should be con st ant ov er the w hole spectr al r ang e

< collection optics >

< Application s >

- v ery sm all ab sorption m edium , minute concentr ation of r adicals , short - liv ed interm ediat e product s in chemical reaction s

- appropriat e t o the spectros copy in UV , vis , near IR region

< = _k, _ph, decrease w ith increasing w av elength

6 .3 .2 P h ot o ac ou s tic S pe c tro s c opy

* minute con centr ation s in the presence of other component s at high pr es sure

* measurem ent of ab sorption by measuring the pres sure w av e

principle

Las er ex cit ation - > (ener gy tr an sfer by collision ) - > tr an slation al en ergy of the collision al partner - > therm al en ergy (T emper ature/ pres sure) - >

acou stic w av e

sign al

W = Ni ik x ( 1 - k) PL t (6.23)

limit ation s

- _k (r adiativ e tr an sit on ) => long lifetim e (10^{- 2} ～ 10^{- 5} s ) => m olecule - reflection/ s cattering by w indow or aerosols

=> fr equen cy modulation at acou stic res on ance frequ ency of the cell

< Application s >

- vibr ation al spectr a of m olecule in the infr ar ed region - det ection of polluting or poison g ases (NO², SO² ...) - dis s ociation energy m easur em ent

6 .4 Ioni z ation S pe c tro s c opy

* Ion/ electron det ection (efficient det ection )

* Ionic energy st ate study / Isotope seper ation

6 .4 .1 B as ic T e c hn iqu e s 1) photoionization

- ex cit ation of ex cited m olecule (a )

M^*( Ek) + h 2 M⁺ + e^- + Ek in

- ex cit ation of Ry dberg lev el t o aut oionization lev el M^{* *} (efficient ) (b )

M^*( Ek) + h 2 M^** M⁺ + e^- + Ek in

- nonr eson ant t w o phot on pr oces s (c )

M^*( E_k) + 2 h ₂ M⁺ + e^- + E_{k in}

2) collision - indu ced ionization (dis ch ar ge)

M^*( E_k) + e^- ( E_{k in ,0}) M⁺ + e^- ( E^'_{k in})

3) field ionization

long - liv ed highly ex cit ed Ry dberg st at e m olecule

Ionization pot ential

IP = r

Z_{ef f} e²

4 ₀r² d r = Z_{ef f} e² 4 ₀r

w here, e Z_{ef f} : effectiv e nu clear ch ar ge

W hen an ex tern al field, E_{ex t} = - E₀x is applied,

IP_{ef f} = IP - Z_{ef f} e³E₀

0

(6.32)

If E IP_{ef f}, w here the energy of the ex cited lev el => field ionization

6 .4 .2 S e n s itiv ity of Ion iz ati on S p e c tro s c opy sign al

SI = NkPkI = na

P_kI PkI + Rk

= NinL ik x P_kI PkI + Rk

(6.33) w here, N_k : den sity of ex cit ed m olecules in lev el E_k

P_kI : ionization prob ability of lev el E_k R_k : tot al relax ation r at e of lev el E_k

** , - > 1 (direct m easuring the ion s )

P_kI R_k (inten se las er beam )

=> SI na : the m ost s en sitiv e det ection is pos sible !!

6 .5 Opt og alv an ic S pe c tro s c opy

* spectroscopy in g as disch arg e

principle

Las er ex cit ation t o v ariou s lev els - >

(ionization prob abilities fr om each lev els are different ) - >

dis ch arge curr ent ch ang e ( I ) - > v olt age ch ange ( U = R I )

sign al

U = R I = a [ n_iIP ( E_i) - n_k( E_k) ] (6.37)

# competing pr oces s (n ois e)

- A ( Ei) + e^- A ⁺ + 2 e^- (dir ect ionization )

- A ( E_i) + A ^* A ⁺ + A + e^- (collision al ionization by m et ast able at om A ^*) - A ( Ei) + h A ⁺ + e^- (direct photoionization )