Download - 2 AB AB + + e AB* AB +* + e n h or n 1 h 1 + n 2 h 2 + : -absorption 1h n h -ionization Energy
2
AB
AB+ + e
AB*
AB+* + e
n h
or
n1 h1+n2 h2 +:
-absorption1hn h
-ionizationE
nergy
•Why multiphoton exitations(?); advantages/disadvantages•One color experiments / data
•Experimental methods: Multiphoton ionization (MPI & REMPI)•Data interpretations / theory: “What to see and what not to see(?)”•Results / examples:
- characterization of state properties / energies- (2+1) vs (3+1) REMPI- ”New” states observed- analysis of complicated spectra- state interactions - multi-photon absorption “mechanism”- energy distribution in molecules- polyatomic molecules
•Two color experiments / data
•Why multiphoton exitations(?); advantages/disadvantages•One color experiments / data
•Experimental methods: Multiphoton ionization (MPI & REMPI)•Data interpretations / theory: “What to see and what not to see(?)”•Results / examples:
- characterization of state properties / energies- (2+1) vs (3+1) REMPI- ”New” states observed- analysis of complicated spectra- state interactions - multi-photon absorption “mechanism”- energy distribution in molecules- polyatomic molecules
•Two color experiments / data
Few- photon absorption in atoms and molecules:
Excitation to high energy states of neutral species:
1xh2xh
3xh
nxh
A
A**
A+
AB
AB+ + e
AB**
2
Typically:
1xh 2xh 3xh
VUV UV
nxh
Visible Visible..IR
Technically more feasable to increase n
+
3
1xh2xh
3xh
nxh
A
A**
A+
AB
AB+ + e
AB**
4
•Increasing probability of resonance intermediate states.Hence spectra complications -
5
Spectroscopic characteristics:
I. Angular momentum quantum numbers (L) can change as:L = (0),.. n for nxh
Thus electronic angular momentum changes in atoms as L = 0,1 / l = 1 per “photon step”
1x 3x
6
II. Electronic angular momentum changes in molecules:
z
)1( L
2xh3xh
3xh
´=3() ´=2() ´=1() ´=0()
´´=0()
i 1xh 2xh 3xh
III. Total angular momentum changes:
z
L
NLJL
7
I, II, III =>More states can be excited as n increases
J = 0 ,1,.. ,nFor nxh
J:QJ-1;P J+1;RJ-3;N J-2;O J+2;S J+3;T
J
As n (in nxh) increases:
•Technically feasable•Larger number of excited states accessable.
Hence more spectroscopic information•Better consistancy / accuracy in determining
spectroscopic parameters due to larger number of transitions
•More complicated spectra / more overlap of features
++-
How to proceed?:
8
+
•Why multiphoton exitations(?); advantages/disadvantages•One color experiments / data
•Experimental methods: Multiphoton ionization (MPI & REMPI)•Data interpretations / theory: “What to see and what not to see(?)”•Results / examples:
- characterization of state properties / energies- (2+1) vs (3+1) REMPI- ”New” states observed- analysis of complicated spectra- state interactions - multi-photon absorption “mechanism”- energy distribution in molecules- polyatomic molecules
•Two color experiments / data
I. Experimentally:a) Simply and inexpensively / for gas samples:
+
-
LASER beam
9
AB
AB+ + e
AB**
3 photon absorption; 1 photon ionization
i.e.: (3+1)REMPI-Current
LASER beamLASER beamLASER beamLASER beam
REMPI-TOF
LASERS
REMPI
TOF
Voltagedivider
HV-2Kvsupply
HX Nozzle
TurboPump
TOF Tube
Focus lens
MCP Detector
Oscilloscope
ComputerEXT
Excimer Laser
One Shot
Cycle
Input
Output
Dye LaserSHG
Time Delay200-1200 S
Dye laser Control
G-Valve
External
Pellin Broca prism
SHG Control Box
In
Out
+HV
-30
-25
-20
-15
x10
3
403020100Mw (amu)
Mw=1 (H)
Mw=12 (C) Mw=28 (N2)
Mw=35(35
Cl)
Mw=36(H35
Cl)
Mw=37(37
Cl)
Mw=38(H37
Cl)
1.2.03; Mass spectrumaccording to Mw=((0.04*n-0.11748)/2.2816)**2
REMPI-TOF:Mass spectrum of HCl, showing:H+,35Cl+,H35Cl+,37Cl+,H37Cl+
For = 477.795 nm LASERradiation
Mag
n jó
na
Flugtími jóna / massi jóna
20928 cm-1
20929 cm-1
20930 cm-1
20931 cm-1
20932 cm-1
H+
Cl+,HCl+
LASERRadiationwavenumbers
LASERRadiationfrequency
TOF/ion mass
Num
ber
of io
nsMass spectravs LASER radiation:
100
80
60
40
20
0
-20
x10
3
360355350345340335n (pnt)
35Cl+
H35Cl+
37Cl+
H37Cl+
83718 cm-1
83720 cm-1
Ion formationsvsLASER radiations 20930.0 cm-1
20929.5 cm-1
mass
35 37
-30
-20
-10
0
10
20
x103
120100806040200
H+
Ion formationsvsLASER radiations
mass
inte
nsit
y
20929 cm-1
20930 cm-1
1
REMPI-TOF (H+)spectrum
Mag
n C
l-hal
dand
i jón
a
20.932x103 20.93120.93020.929
Bylgjutala (1/) LASER geisla
37Cl+ og
H37
Cl+
37Cl+ og
H37
Cl+
35Cl+ og
H35
Cl+
35Cl+ og
H35
Cl+
..ANDREMPI –Currentspectrum:
LASER wavenumbers (1/ (cm-1))
I
nten
sity
HClREMPI-Current Spectrum
Mag
n C
l-hal
dand
i jón
a
20.932x103 20.93120.93020.929
Bylgjutala (1/) LASER geisla
37Cl+ og
H37
Cl+
37Cl+ og
H37
Cl+
35Cl+ og
H35
Cl+
35Cl+ og
H35
Cl+
20850 20900 20950 21000 21050/cm-1
Inte
nsit
y
•Why multiphoton exitations(?); advantages/disadvantages•One color experiments / data
•Experimental methods: Multiphoton ionization (MPI & REMPI)•Data interpretations/theory: “What to see and what not to see(?)”•Results / examples:
- characterization of state properties / energies- (2+1) vs (3+1) REMPI- ”New” states observed- analysis of complicated spectra- state interactions - multi-photon absorption “mechanism”- energy distribution in molecules- polyatomic molecules
•Two color experiments / data
Interpretation / Theory:
Determine:
1) Signal intensity transition probabilities population in ground state
E = Ej-Ei = photon energy x n
11
Ej
Ei
nxhn
1xh - absorption:
A/Ifl
Exp.
Calc.
E10 E20 E30
1xh
201 EM
Eh=E10=E1-E0
=(E1-E0)/h
E10Eh
2xh
2
0,
0EMEM1
i i
ii
=(E1-E0)/h
A
A**
A+
|i1>
|i4>|i3>
|i2>
:
AB
AB+ + e
AB**
2xh - absorption:
3xh
2
2,1 0,10,2 2
0EM11EM22EM1
ii ii
iiii
=(E1-E0)/h
AB
AB+ + e
AB**
|i1>
|i4>|i3>
|i2>
:
3xh - absorption:
1)
201 EM
:::
JeJev ssc 22
rot. contribution electr. Contrib.vibr. Contrib. ; cv = fasti
2)
2
0,
0EMEM1
i i
ii
|i1>
AB
AB+ + e
AB**
|i3>
|i2>
0,1i
0,2i
1) B-O approximation, 2) No resonance intermediate states,
Approximations:
....approximations:
...3) assume virtual intermediate states:
3xh
“i”
“i”
3)2
01 EM
2
0,
0EMEM1
i i
ii
2
2,1 0,10,2 2
0EM11EM22EM1
ii ii
iiii
.........................1
21 s
3231
21 ss
2220
20 ss .............
...
1,2,3), hence:
-for si(J,), i2 = e
2
121 s 3
231
21 ss
13
1xh 3xh2
2
20
2
0ss
2xh
EM1;EM
),(
fn
i2´s are treated as variables in simulation procedures
121 s
3231
21 ss
s1,s3 J and dependent;s1,s3 fyrir ´´= 0:
14
Selection rules:
= 0 ,1,.. ,n; n = 1,2,3,..J = 0 ,1,.. ,n; n = 1,2,3,..; 0
J = 1,.. ,n; n = odd; = 0 J = 0 ,2,.. ,n; n = even = 0
1) Intensity
2) Ei(B,D,0) E (nxh)
Intensity vs (nxh) / I() can be evaluated and compared with REMPI spectra:
Spectra simulations
15
•Why multiphoton exitations(?); advantages/disadvantages•One color experiments / data
•Experimental methods: Multiphoton ionization (MPI & REMPI)•Data interpretations / theory: “What to see and what not to see(?)”•Results / examples:
- characterization of state properties / energies- (2+1) vs (3+1) REMPI- ”New” states observed- analysis of complicated spectra- state interactions - multi-photon absorption “mechanism”- energy distribution in molecules- polyatomic molecules
•Two color experiments / data