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united nations educational, scientific and cultural organization JUUL the international centre for theoretical physics international atomic energy agency SMR 1302- 17 WINTER SCHOOL ON LASER SPECTROSCOPY AND APPLICATIONS 19 February - 2 March 2001 Photoelectron Spectroscopy with and without Photoeiectrons Martin COCKETT Department of Chemistry, University of York Heslington, York, U.K. These are preliminary lecture notes, intended only for distribution to participants. strada costiera, I I - 34014 trieste italy - tel. +39 04022401 I I fax +39 040224163 - [email protected] - www.ictp.trieste.it

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Page 1: Photoelectron Spectroscopy with and without Photoeiectronsindico.ictp.it/event/a0111/contribution/17/material/0/0.pdf · Outline 1) The origins of photoelectron spectroscopy Photoelectric

united nationseducational, scientific

and culturalorganization

JUUL the

international centre for theoretical physics

international atomicenergy agency

SMR 1302- 17

WINTER SCHOOL ON LASER SPECTROSCOPY AND APPLICATIONS

19 February - 2 March 2001

Photoelectron Spectroscopywith and without Photoeiectrons

Martin COCKETTDepartment of Chemistry, University of York

Heslington, York, U.K.

These are preliminary lecture notes, intended only for distribution to participants.

strada costiera, I I - 34014 trieste italy - tel. +39 04022401 I I fax +39 040224163 - [email protected] - www.ictp.trieste.it

Page 2: Photoelectron Spectroscopy with and without Photoeiectronsindico.ictp.it/event/a0111/contribution/17/material/0/0.pdf · Outline 1) The origins of photoelectron spectroscopy Photoelectric
Page 3: Photoelectron Spectroscopy with and without Photoeiectronsindico.ictp.it/event/a0111/contribution/17/material/0/0.pdf · Outline 1) The origins of photoelectron spectroscopy Photoelectric

THE UNIVERSITY

Photoelectron Spectroscopy withand without photoelectrons:

From the photoelectric effect to ZEKEspectroscopy

Martin Cockett

Department of ChemistryUniversity of YorkHeslington, York

UK.

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Outline

1) The origins of photoelectron spectroscopyPhotoelectric effectDevelopment of UVPES & XPSExperimental methodsFactors affecting Resolution

2) ApplicationsAtomsDiatomic MoleculesMolecular Orbital TheoryKoopmans' Theorem - closed and open shellAutoionisationPoly electronic molecules

3) The quest for improved resolutionRefinement of experimental conditionsLaser REMPhPES - state selection

4) Dispensing with energy analysisThreshold Photoelectron SpectroscopyAutoionisation revisited

5) The next step: ZEKE spectroscopyConceptsComparisons to PESRydberg states and Field lonisationExperimental detailsImproving resolution

6) Case Studies: Electronically excited states, autoionisation,Rydberg excited van der Waals complexes.

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Origins of Photoelectron Spectroscopy

(a) The Photoelectric Effect

• First observed onsurfaces of easilyionisable metals(e.g. Alkalimetals).

hv = ±

KJE. oc hv.

K.E.

Einstein1 developed theory in 1905

V

For alkali metal surfaces, O is of the order of afew eV which requires near UV radiation.

1 A. Einstein, Ann. Physik., 31 (1905) 132.

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(b) The development of Photoelectron spectroscopy

• 1956 Siegbahn2 invents X-ray photoelectronspectroscopy (XPS).• 1962 Turner et al3 and Vilesov et al4 developultraviolet phoioelecircm spectroscopy (iJVPES).

hv

# - # •

•m-m-

•#-#-

hv

Valence ionisationUVPES

Core ionisationXPS

2 K. Seibgahn and K. Everson, Nucl. Phys.y 1 (1956) 1373 M.I. ALJoboury and D.W. Turner, J. Chem. Phys., 37, (1962)3007.4 F.I. Vilesov, B.L. Kurbatov and A.N. Terenin, Dokl. Akad. NaukSSSR, 138 (1961) 1329.

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How does it work?

UVPES (gas phase) and XPS (solid state & gasphase) both involve the following process:

E3

hv +e

hv

M

hv =\mv+LE,

M

The kinetic energy of the photoelectrons depends onthe photon energy and the final state of the ion.

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Experimental Methods

(a) Photon Source (preferably monochromatic)

XPS AlKa (1487 eV) or MgKa (1253 eV)

5

x-rays

W filament Be or Al windowI I

Anode

Pump

r^r water

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UVPES

• Hel (21.22 eV)

Hell (40.8 eV)

To chamber

He out

•IH

He iin

1

Electrodes

®t

JL3T

Nel (16.67 and 16.85 eV)

1P1(2f)53s1)-> llSQ(2p6) 16.67 eV3P1(2p53s1)^lIS0(2f>6) 16.85 eV

-s-

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Summary

SynchrradHellHe!Nel

Core-

\falence-W l «V

Electronspectrum

Types of radiation used in PES correlated to theregions of electronic stucture probed.

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(b) Kinetic energy analysis

Deflection analysers: Electrons with differentenergies follow different paths when subject tomagnetic or electric fields

7(a)

Detector

Detector

(b)

Concentrichemisphericalplates

(0

Cylindricalplates

Detector

(a) Parallel plate, (b) 127 cylindrical and (c) hemispherical analysers

Electrons of a kinetic energy E will reach thedetector when

/ \

for plates of radii R{ and R2 (R2 >

'10

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(c) Resolution

(i) Radiation linewidth ~ 3-4 meV (He I)- 0.2-leV (AlKa)

(ii) Doppler broadening ~ 4 meV (m= 100,T-300 K, eKE = 10 eV)

(iii) T h e ana lyse r EXIT SLITS

V1 (R2-R,)

where

AR-S/2

is the variation in Rand S is the total slitwidth.

Insulating spacer

F • - — - / - J(electron / /

focus) w

{centre ofhemispheres) ^

ENTRANCESLITS

(electron r i - r)

source)

OUTERHEMISPHERE

+ve. voltage

-ve. voltage

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Other factors affecting resolution

V E ESAE = \e AR = - AR - - -

2 (R2-Rt) R R2

• The resolution improves as S is reduced (buttransmission suffers) •

• Resolution improves as the kinetic energy isreduced (but transmission suffers).

• AR depends on other factors such as strayelectric/magnetic fields, local point charges(dirt!).

PES always involves a compromise between resolutionand transmission.

• Typical resolution ~ 10-20 meV = 80-160 cm'1

Vibrations - YES (mostly)Rotations-NO!

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XPS vs UVPES

(a) XPS

KOO

«V UQ 535 40 30

BINDING ENERGY

hv «lOOOeV

AE « 1 eV

20 10

(b) UVPES ,b.HF

CEPA

2a,

" X .

ibi= 21.22eV

I. [ AE-20meV

EXP (eg.)

2000

0

ulA Ei

20 19 18 17 16 15 14Ionisation Energy / eV

"0.1

13 12

13'

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U VPES of Atoms

PES of atoms provides little additionalinformation over higher resolution techniques

= hv-1E

3,000

c13oO

16 0 15 9 15 8 157

lonization energy/eV

-if-

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UVPES of Diatomic Molecules

vib rot

where AEvib and AErot represent the internalvibrational and rotational energy of the ion.

How are molecular photoelectron spectradifferent from atomic PE spectra?

AEvlb and AErot

The Photoelectron Spectrum of Nitrogen3,000

Cf>

19-5 I9O 185 I&O 175 170 165 ISO 155 ISO

lonizatjon energy/eV

How can we explain the structure in a PE spectrum!

is-

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Molecular Orbital Theory

Molecular orbitals are constructed from linearcombinations of atomic orbitals (LCAO)

MO diagram for N2

-It

is <;' ">• is

"o2s

h :::' "> Is

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What is an electronic transition?

(a) The One-electron description

The excitation of an electron from an occupiedMO to an unoccupied MO.

e.g. N 2

o*2p o*2p

n*2p _ nu*2p

o2p alp

- ( } •

nu2p mw mm nu2p

a *2s . _ a.. *2s

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(b) Electronic transitions really occur between states!

The ground electronic state configuration of N2:{ah)\a:h)\G2s)\au2s)2{nu2p)\a2p)

1 y +ONE state

The excited configuration of N:

TWO states

n*2p

nu2p

a*2s

-#-•-

-#-•-CT*2S

TL

+

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(c) The one-electron description provides a goodrepresentation of ionisation in most closed shell molecules

(1) (2) (3)N 2

+

-•-#-olp

#-# a*2s

3,000

c

K*lP

i

i

) E,(2)

B2S.,

E,(3)

N,

19-5 1*0 185 ISO 175 YfO 165 ISO 156cV

lonization energy/eV

(V

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Koopmans' Theorem

'For a closed shell molecule, the ionisation energy foran electron in a particular orbital is approximatelyequal to the negative of the orbital energy'

JL • JL> • '"w O •i

(1) Orbitals are an entirely theoretical concept.

(2) Their energies can only be obtained bycalculation.

(3) Experimental ionisation energies are themeasureable quantity which most closelycorresponds to the orbital energy.

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Understanding the origin of Vibrational StructureThe Franck'Condon Principle

110

100 -

60 -

4 ?

R /A

I.

Energy

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Limitations ofKoopmans theorem:

(a) Neglect of electron reorganisation

Orbital relaxation, or reorganisation, mayoccur following ionisation.

I.E. ~ —£i + ER

(b) Neglect of electron correlation

Electronic motion is correlated - electrons donot see an 'averaged out' charge of all theother electrons.

Both effects can be small, and fortuitously, inmany cases cancel out!

(c) Fails to account for open shell molecules

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(c) Koopmans* theorem fails for open shell moleculesOxygen

(i)

-•-•-

-#-•-

(2)=•=•=

-#-#-

(3)n *2p

• I I I 7Iu2p

#—a*2s -r

E«(2) E,

x2n

0 2 Oxygen

x2ng

1 I 1 1 1

J L ' 1 L12 14 16 18

Ionization Energy (eV)20

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Autoionisation in PES

O2 He I PES

O7 Ne I PES

16 15 U 13

lonizQtion energytfor 16.85 eV radiation)/eV

12

Figure 7.19 Autoionization in the Ot(X2/7?) «- O2 (X3Zg) band system using a Nel source. The bandsmarked with arrows are due to the 16.67eV radiation and do not show autoionization[Reproduced, with permission, from Eland, J. H. D. (1974). Photoelectron Spectroscopy, p. 63, Butterworths,London]

<J

o

Photoionisation

Nel 16.85 eV

Rydberg state

Autoionisation

\X

Z TT / Z T1 Z \n (a TC K )

R /A

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Polyelectronic MoleculesBenzene

• Partially resolved vibrational structuree.g. Vi (ring breathing mode) in 3alg band,

• Bands arise from ionisation from a and n MO's,

16 15 14BINDING ENERGY

13 12 If 10 9

•AS-

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The quest for higher resolution

• In the 1960's and 70's typical resolution nobetter than 10 meV (80 cm'1).

• 1992 Balzer, Karlsson and Wannberg5 improvedto 4.5 meV (36 cm'1) using mw electron-cyclotron-resonance source (linewidth 1.2 meV)

.d

PhotoelectronspectrumhV=21.218eVX 2 r f l (v=0)

Ne2ph*«23.087eV A

18.78 1876 1874 18.72

BINDING ENERGY (eV)

FIG. 1. The upper part shows the He I excited photoelectronspectrum corresponding to the X l2^"(N2, v=0)-+X 2 2 / ( N 2

+ , v=0) transition. The lower part shows the He Iexcited photoelectron spectrum corresponding to theX 'S+CN^Nj, I > = 0 ) - * J B ^ ( N 2

+ , »=0> transition. In theupper right part the Ne 2/?J/2 line excited with the He I £ radia-tion at 23.087 eV is inserted. The lines connecting the experi-mental points are drawn as guides to the eye.

i

ID)

tozIDg

HJB^I-O)

7 \/ \/ !

PARAMETIRS IH^ THE CALCULATION

/ / J U A H — 1

7T/ \.' \\

AN•1

-1•3

- i

Wtlght

1.21

too0.22072

=^—t40 30 20 10 0 -tO -20 -30

BINDING ENERGY RELATIVE TO THE 0-0 TRANSITION (meV)

FIG. 3. The experimental (full line) and fitted theoretical(dashed line) Xl2+m2, u=0)-+J?2:£+(N2

+ , t>=0) spectrashown together. The spectra are very similar and an obviousdifference can be seen only at -12 meV from the zero level,,where the intensity of the experimental spectrum is somewhathigher. For clarity, the subbands corresponding to transitionswith AJV"=±1 and ± 3 are included. Also the weights used inthe fitting of the theoretical spectrum are shown in the figurenormalized to 1.00 for A # = — 1 subband. The energy scalerefers to zero at the (unobserved) 0-0 transition.

5 P. Baltzer, L. Karlsson and B. Wannberg, Phys. Rev A, 46 (1992)315

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Laser Photoelectron Spectroscopy (1980's)

Employs resonance enhanced multiphotonionisation (REMPI) - a two step process.

State selection

Supersonic jets and molecular beams(Ytrans « 1 K, T r o t « 10 K, Tvlb « 100 K)

Narrow laser line width (< 1

Moleculespecificity

A

CD

[ ' 1 ' I ' I r T ' I ' 1 ' I ' I ' 1 '

Laser Photoelectron Spectroscopy

M

M

1 , 1 , 1 , 1 , 1 . 1 , 1

2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

•/A

- 2?

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REMPI spectroscopy characterises the intermediatestate

M

co 2 (fixed) J

M

! (scanned)

M

Page 30: Photoelectron Spectroscopy with and without Photoeiectronsindico.ictp.it/event/a0111/contribution/17/material/0/0.pdf · Outline 1) The origins of photoelectron spectroscopy Photoelectric

A Case Study: The AivNO van der Waals Complex

Ar-NO*(X)

Ar-NO*(C)

Ar-NO (X)

KNO*(X)

NO"(C)

NO (X)

FIG. 5. Schematic energy level diagrams of NO and Ar-NO, relevant to(2+1) resonant multiphoton ionization through their Rydberg C states.

385 384 383 382 381 380Laser Wavelength(nm)

FIG. 1. An MPI ion-current spectrum obtained with a free jet of a mixtureof 5% NO in Ar in the laser wavelength region 380-385 nm. The stagnationpressure was 1 atm.

NO(e)38i.87nm

Ar-NO(d)383-Unm

(3-0)

(c)383*7nm(2*0)

(b)383.85nm0-0)

04(170.6 05 (UPhot electron Energy (#V)

FIG. 4. Photoelectron spectra obtained by the (2 -f 1) resonant multiphotonionization of a free jet of the Ar-NO (5%) mixture at the ion-current peaksof the Ar-NO van der Waals complex [spectra (aHd)] and of the free NOmolecule [spectrum (e)].

0 - 0 1-0 2 -0 3*0 Resolution of 20 meV, butmolecule specificity!

K. Sato, Y. Achiba and K. Kimura,J. Chem. Phys., 81 (1984) 57

384.5 384.0 383.5 383.0Laser Wavelength (nm)

FIG. 2. An MPI ion-current spectrum attributed to the Ar-NO van derWaals complex, shown in an extended scale. The experimental conditionsare the same as in Fig. 1.

-si-

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Magnetic bottles, supersonic jets and lasers

• Late 1990's de Lange et al obtained a resolutionof 8 meV (64 cm'1) using magnetic bottleelectron energy analysis.

Field coil Magnetic fieldlines

MCPs

Pumping Pumping

Fig. 1. Schematic of 'magnetic bottle' spectrometer showing the key functional elements.

A.M. Rijs et al. I Journal of Electron Spectroscopy and Related Phenomena 112 (2000) 151-162 161

87 86 85 84 83 82 N+

L95 2.00 2.05 2.10 2.15 2.20 2.25 2.30

Electron Kinetic Energy (eV)

Fig. 4. Rotationally resolved photoelectron spectrum obtained by a one-photon ionisation from the Q(84) level of the intermediate(t/ =0) state of rotationally excited CO.

2 0

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Laser limited Photoelectron Spectroscopy?

Weinkauf et al6 achieve 4 - 8 cm'1 resolution

• Minimising excess energy (<300 meV)• Careful reduction of point charges in ionisation

region• REMPI state selection & time-of-flight energy

analysis

(a)

=— Ne

samplepulsed nozzle

(c)

Fig. 2 Experimental setup of the MPI high-resolution photoelectron spectrometer: (a) inlet chamber, (b)field-free photoelectron spectrometer, (c) pulsed time-of-flight mass spectrometer. Note that MPI, MPI-PE andmass spectra can be recorded in the same instrument.

6 R. Weinkauf, F. Lehrer, E.W. Schlag and A. Metsala, FaradayDisscuss. 115 (2000)363

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Photoelectron Spectrum of n-propylbenzene(a) REMPI spectrum

I ' ' ' ' I • ' ' ' I ' ' ' ' I ' ' ' ' I

37500 37550 37600 37650 37700energy/cm'1

Ia=70420cm-

IQ=70278cm-1/'D / 293 cm"1

37534cm"1

37583cm"1

"$151 cm"1

200cm"1

T r a n s Gauche

(b) REMPI-PE spectrum

865 8.75,energy /eV

8.85

Fig. 6 The MPI-PE spectra of n-propylbenzene: (a) gauche and (b) ami conformer. The very different vibron-ic transition pattern allows a clear conformer assignment. The transition intensities show a relative smallstructural change upon ionization. The resolution of the MPI-PE spectrum is better than 8 cm"1.

Faraday Discuss., 2000, 115, 363-381

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Dispensing with kinetic energy analysisThreshold Photoelectron Spectroscopy (TPES)

Collects thresholdphotoelectrons ('zero'energy) emitted when aresonant transition isexcited.

Synchrotron radiation

Penetrating field electron lens

M

hv

M

Trajectories of 0-OOleV electrons emitted from a point source are focused and collimatedby the weak electric field from an 'extractor' electrode that penetrates through the 0 Voltaperture.

-33

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A Case Study: Molecular Iodine

I t > t nu5p

3/2,l/2u

•3/2,l/2g

L

2Each IT state is split into two spin-orbit states

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TPES of Molecular Iodine7

• Resolution ~ 5 meV (40 cm'1)

.3

I I I I

Photon Energy / eV

PES

J L J_ J L9.0 9.2 9.4 9.6 9.8 10.0 10.2 10.4 10.6

Energy (eV)

7 A.]. Yencha, M.C.R. Cockett, J.G. Goode, RJ. Donovan, A.Hopkirk and G.C. King, Chem. Phys. Lett., 229 (1994) 347.

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Autoionisation Revisited

90

o 85T—I

X

80

ui «a^ o

75

I(n=9tol4) VT^"-

3/2,g

' Autoionisation region

/FC region [ATL,2u]c 7p

\ [B2Z+] 6s

2.0 2.5 3.0 3.5

R (A)4.0

?

V = 1

AIIVZ,u

I

+ QV —0

+V•

= 16H !

HV

• " = 17

7b

Weak

4.5

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The next step: Zero Electron Kinetic Energy(ZEKE) (photoelectron) spectroscopyLasers, delay, long life and Rydberg states

REMPI+TPES=ZEKE

M

co2 (scanned)

M*

J

1

co! (fixed)

M

-37

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How do we separate 'ZEKP electrons from 'fast'electrons?

We do nothing!

>

//////.i

n_r

detector

electronsignal

2

I time-of—flight

Jet-Valve

Reflectron

Electrons

•31

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1984 The first ZEKE spectrum8

• Resolution better than ~ 2 cm4

• Rotational structure

PE signal

20.0

- i£/cm

Figure 2. The first VUV photoelectron spectrum of NO'41 (top) with avibrational progression from v = 0 to v+ = 4 of the l2+ electronic groundstate of NO' and comparison of the v̂ — 0 peak with the first ZEKEspectrum obtained.1141 The ZEKE spectrum shows the lowest rotationalstates in the vibrational state v" = 0 with N+ = 0-2.

8 K. MulleivDethlefs, M. Sander and E.W. Schlag, Chem. Phys. Lett,112(1984)291.

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Is ZEKE spectroscopy 'real' photoelectronspectroscopy?

Photoelectron Spectrum ofn-propylbenzene6

jL^A^ujL^/uk^mA^0kM

ZEKE Spectrum ofn-propylbenzene9

0 200 400

ION INTERNAL ENERGY (cm"1)

6R. Weinkauf, F. Lehrer, E.W. Schlag and A. Metsala, FaradayDisscuss. 115 (2000)363

9M. Takahashi and K. Kimura, J. Chem. Phys., 97 (1992) 2920.

jo

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It looks like a duck, and walks like a duck but • • •

• Late 1980's: ZEKE spectra are not equivalent toPE spectra.

Each band is shifted by a few cm to lower energy

Are we really detecting 'ZEKE' electrons?

ZEKE spectra frequently exhibit autoionisation.

Band intensities cannot be trusted!

1i

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ZEKE electrons are not really photoelectronsThe Role of Rydberg States and Field lonisation

What are Rydberg States?

• Rydberg states are electronic states of highprincipal quantum number n.

The electron occupiesan orbital whose sizeis large relative to thedimensions of theionic core.

Atoms:Sodium Is22s22p6 P 3 / 2 , 1 / 2 < Is22s22p6 3s1 2< - Is22s22p6 3s1 2S 1/2

6p5p

4p

330 nm

3 / 7 - ,

589 nm

3s

285 nm

Fig. 2. Absorption Spectrum of the Na Atom [Kuhn (42)]. The spectro-gram gives only the short wave-length part, starting with the fifth line of theprincipal series. The linos appear as bright. Hues on a dark continuous back-ground, just as <>n the photographic plate.

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Molecular Rydberg StatesWhen is a molecule an atom?

• At sufficiently high levels of electronic excitation, allmolecules eventually resemble the hydrogen atom.

DABCO (1+T) double resonance spectrum via v20 Ax (3s+)

10n i ' i • r11 12 13 14 15

IP,

i • r20 25

I

i I I I I I I i i i i i 1111 mum

i r 1 1 1—! I S M I !

1 I I I I I I i i i i 111 Minimi

1.436

eP/243

,054

IP,, (calculated)

57800 58000 58200 58400 58600 58800

Two-Photon Energy / cm"

59000 59200

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Lifetimes

Rydberg states are potentially HUGE,n2h24ne

r = 0

Ze2m

Non-Penetrating n r1 0.53 A10 53 A100 5300 A200 21200A

Fia. 7.3.—Comparison of the quantum-mechanical with the classical model of theneutral sodium atom. Three of the lowest possible states* for the single valence electronare also shown.

LifetimesFor low-1, Toe nVox high-1, Toe n

In sodiumn — 30* / = O T— 32 us

n = 30; £ = n — 1 (circular): T= 2.3 ms

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ZEKE spectroscopy relies on the existence ofmetastablehigh-n molecular Rydberg states,

• ZEKE Rydberg states are high^n, high^, high-m

F=0

magic region

F>0

The signal comes from below threshold.

irb.

unit

y/a

c•4—>

-1 1 1

n=62

_ ,7=72 n=90

A0.2 cm J ^ 0.08 cm"jf\

JL1 \

/ v

• u I 1 t i l l I

i i 1 1 1 1 1 1 1 1

70

i l lnilJP

1 1 I l l l l l l l

80

in I1If

i

iii i i i i i i iniii i i i i i i i

90

i

NIf

iii1 v+

f l1\}

1

= 1

IV

1'

via

=57555

ITNnBl TnWi

i

S^v-l)

.72 cm"'

1

57510 57520 57530 57540 57550

Two photon energy / cm

57560 57570

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The position of a ZEKE band depends on field strength

7.00 V/cm

4.984 V/cm

3.684 V/cn[i

2.35 V/cm

Field Free lonisation Energy

80470 80475 80480 80485 80490 80495

lonisation Energy / cm

80500 80505

The theoretical red-shifts for pulsed fieldionisation are:

- 3. W V cm'1 for diabatic ionization measuredat the band maximum

- 4-6V V for diabatic ionization measured at thelow wave number on-set of the band.

-n-

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A typical ZEKE experiment

Excimer XeCl

Dye Laser

Dye Laser

(a)

molecularbeam

(b)

» Electron^

(c) -2V

Ions

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Improving the resolution• Slow Rise-time and Stepped Pulses (< 0.25 cm'1)10

Rydberg molecule fast-rising pulse

(a) 0.2 0.4 / (|is)

UuuuGates: 1 2 3 4

Rydberg molecule slow-rising pulse

(b) 0.4 t(\is)

UuUuGates: 1 2 3 4

Figure 2.15 Shape of the multistcp extraction pulse and resulting electron time-of-flightpattern. Each step changes die field strength by 46 mV/cm. The first laser populated the Si 61

E l u if = 4, K* - 4, + /) intermediate rovibronic level; the second laser was tuned to anenergy of 35 948 cm"1 (after frequency doubling). Each peak in the electron time-of-flightsignal corresponds to a step of the extraction pulse. (Redrawn from Lindner et al., 1994)

Figure 2.13 Pulsed field ionization of long-lived Rydberg states using a fast-rising (a) ana aslowly rising (b) extraction pulse and collection within a certain time gate

N

10 R. Lindner, H.J. Dietrichand K.Muller-Dethlefs,Chem. Phys. Lett., 228(1994) 417.

35945 35950 35955 35960

Ionising laser energy ( c m 1 )

Figure 2.14 Spectral resolution of ZEKE detection as a function of the rise time of theextraction pulse. The spectrum of the Do

2Eig*-Si 61 lE)u (f = 2, K! = 2t + /) transition ofbenzene was recorded using 1 + 1 '-two-colour-REMPI. The rise time was varied from 5 to600 ns but the final field strength was kept at 600 mV/cm. The width of the detection timegate was kept constant (20 ns). (Reproduced from Lindner et aly 1994)

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Double inverted pulses (< 0.5 cm )

ZEKE-PFI Pulse Sequence

Effective Pulse Sequence

CO

4—>

I ' I ' I

1730 mV cm'1

137 mV cm

i . i . i . i

58870 58875 58880 58885 58890 58895 58900 58905

Two Photon Energy / cm

ZEKE spectra of DABCO11 recorded at fieldstrengths ranging from 137 mV cm'1 to 1730 cm'1.

UM.J. Watkins and M.C.R. CockettJ. Chem. Phys., 113 (2000) 10560.

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Non-resonant multiphoton ionisation:The ZEKE spectrum of Molecular Iodine

100 -

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

0 -

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2Exptl.4)

Jv(eV)

9.3569.983

11.0111.8112.94

10 12 14

Ionization Energy (eV)

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(1+1) non-resonant ZEKE-PFI spectrum of L

75000 76000 77000 78000 79000 80000 81000 82000

-1Ionisation Energy / cm"

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•e

KtV*+~Kj*f

Threshold Photoelectron spectrum of I* X EL'2 * 3/2,g

ZEKE-PFI photoelectron spectrum of I* X II2 * *3/2,g

^W^A» KA,4W+A*^^

9.30 9.35 9.40 9.45

Ionisation Energy / eV

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u

• rn

] IT TTT"5 10

x2n3/2,g

x2n60

l/2,g

30

u

I T ! ! I II TTTTT720 25 30 31

65 70 75 80 85

35 40i

90

86000 86500 87000 87500 88000 88500 89000 89500 90000 90500 91000 91500

-1Ionisation Energy / cm'

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(a) (2+11) OODR Spectrum

(b) (1+1) ZEKE-PFI Spectrum

[A2n3/2 u]c 5d Rydberg state

75200 75600

Total Transition Energy /cm

76000-i

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a

a _

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DABCO (RHF cc-pvdz)

HOMO

LUMO

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Eco

CD <r—

t

a. COCO

Xc

CO1

oinri-ll

Ec

CO

CMII

3

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One and two photon REMPI spectra of DABCO A. (3s+)1 I ' I ' I

- (a) (1+1) REMPI spectrum of DABCO

(b) (2+1) REMPI spectrum of DABCO

ol

25' (e')

(+825 cm1)

>-^vn^*>r^w*«*AvAvurv<w»

24'

(+1011 cm1)

35600 35800 36000 36200 36400

One photon energy / cm

36600-l

36800 37000

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<N

s

OOOO

OS-H

oOO

<N

<N

s^iun 'qxv / ^ T

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22281.0

22280.5

22280.0

22279.5

22279.0

22278.5

22278.0o

5-122281.0

22280.0 -

22279.0 -

22278.0 -

22277.0

22276.0 -

22275.0 -

-4.04 V l/2

-3.1 V 22260 22275 22290

-1.43 Vl/2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

(Field Strength/V cm1)172

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

1.4

- G l -

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CM<N

O

cs

O

o

oo

S

c/3

o

sixiin *qjB /

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The van der Waals Interaction

The Neutral

O

The Ion

Rydberg States

The dominant attractive force betweenneutral species is the DispersionInteraction

32 +IR 'gy

In addition to the dispersioninteraction, charge induceddipole forces assert themselves

2 A(4ns0) r

Depends on n, I

HOW?

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I

I

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QUESTIONS

• What is the relationship between n and £ of the Rydbergelectron and the binding energy of the complex?

• Can the formation of different isomers help us to learnmore about the interaction?

• How stable are very high-n Rydberg states to solvation?

EXAMPLES

(a) I2-Ar

.oor

(b) DABCO-N2

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DABCO-N, (1+10 REMPI spectrum

•e

i i i i rxO

+67 cm-i

K/TLI *MU .r

S.+750 cm-i

Av = +129 cm-i

i r i r

+847 cm-i

Liu_L I I

35800 36000 36200 36400 36600

One photon energy / cm-i36800 37000

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"sor-

IIo

o o

H ii

o

ooII

o

oin

o

CO

CO

II

sjiun *qj^ / X t̂

uCO

(N

CO

"S8 °00 £?

§ |CO Q

8 OoCO

CO

ooO N

CO

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1 t

DABCO-N zeke spectra via A (3s+)

ALE. = 57492 ± 5 cm -i

AIE = -526 cm-i

via v=0

via v=l

via v=2

via v=3

57400 57500 57600 57700 57800 57900 58000

Two Photon Energy / cm"

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CO

II'ao

00t o

IIo

>

'Bo

II--

•gO

II(N

"go

IICO

o00

II

ooON

r

oo00

oo

Oo

oo

O

oo

sjran qjB /

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V(x)=kx2/1

V(x)=kx /I

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THE

Acknowledgements

YorkMark Watkins

Klaus Muller-Dethlefs

EdinburghJon Goode

Robert DonovanKenneth Lawley

and

££The Royal Society

The University of YorkThe EC Science Programme

EPSRC