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