Photoelectron Photoion Coincidence Spectroscopy:

Trimethylphosphine

András Bődi

Málstofa í efnafræðiRaunvísindastofnun Háskólans

Reykjavík, 18/02/2005

Acknowledgements

• Baer Group, University of North Carolina– Tomas Baer, Jim Kercher

• Photoelectron Spectroscopy Group,Eötvös University, Búdapest– Bálint Sztáray, Zsolt Gengeliczki,

László Szepes

http://www.chem.unc.edu/people/faculty/baert/tbgroup/PEPICO_Home_Page.htmlhttp://www.chem.elte.hu/departments/altkem/sztaray/

Outline

• Introduction to TPEPICO– Why detect photoelectron and photoions?– Why the coincidence?– Experimental setup

• The measurement of P(CH3)3

• Data analysis and modeling • Ab initio calculations• Thermochemistry

Dissociative Photoionization

• Neutral thermal energy distribution

• h→ photoionization• Dissociation• Consecutive and parallel

recations– k, k1, k2

h

dissociation

AB

A + B

A+ + B

AB+

Photoelectrons and Photoions

• Photoionization Mass Spectrometry• M + hν M+ + e– • Information: dissociation of the ion

• Ultraviolet Photoelectron Spectroscopy• M + hν M+ + e–

• Information: ionization energies (MO energies)

• Photoelectron Photoion Coincidence Spectroscopy

• M + hν M+ + e–

Coincidence

• Start signal – e–

• Stop signal – ion

Mass Spectrum at h

e– optics Ion optics

Detection of Zero Kinetic Energy Electrons

• Threshold Photoelectron Photoion Coincidence

• Energetics

h = IEad + Eintion + KEion + KEe

Conservation of momentum

Detection of zero kinetic E e–

Apparatus I

Tunable h source (H2 lamp)

Grating monochromator

Sample chamber

Reflectron

e– optics

Sample inlet

Apparatus IIh

e–ion

P(CH3)3 – Photodissociation Products

?

CH3 loss

CH4 lossH loss

P(CH3)3 Data – TOF Distributions

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96Ion time-of-flight / ms

PE

PIC

O s

igna

l / a

rb. u

nits

10.76 eV

10.87 eV

11.09 eV

12.07 eV

P(CH3)3 Data – Breakdown Curves

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

10.5 10.7 10.9 11.1 11.3 11.5 11.7 11.9 12.1 12.3 12.5 12.7

Photon energy / eV

Rel

ativ

e ab

unda

nce

H2CP(CH3)2+

H2CPCH3+

HP(CH2)(CH3)+

P(CH3)3+

Simulation Overview

P(CH3)3+ freq. & rot.

const.

P(CH3)3 vibrational frequencies & rotational constants

Ion optics parameters

P(CH3)3 internal energy distribution

IEad

P(CH3)3+

internal energy distribution

RRKM + TOF calculation

varie

d to

acq

uire

the

best fi

t

Transition state frequenciesBond energiesTunneling params.

Ab initio input

Ab initio Input: Bond Energies

E0 / (kJ/mol)P(CH3)3

+ CCSD(T)/ cc-pVTZ G3 Expt.

P(CH3)2+ + CH3 304 308 242.3

HP(CH2)CH3+ + CH3

269 267 242.3 P(CH2)(CH3)2

+ + H 252 248 241.6 P(CH2)CH3

+ + CH4 239 249 155 – 250 TS (CH3)2P

…H…CH2+ 199 –

HP(CH2)(CH3)2+ 40 34

Potential Energy Curves

E

(a)

(TSab) (CH3)2P…H…CH2+ (c) HP(CH2)CH3

+ + CH3

(e) P(CH2)CH3+ + CH4

P(CH3)3+

P(CH3)2+ + CH3

(d) P(CH2)(CH3)2+ + H

(d) P(CH2)(CH3)2+ + H

(b) HP(CH2)(CH3)2+

(TSbe) (H2C)(H3C)P…H…CH3+

Gas Phase ThermochemistryΔ

fH°

P(CH3)3

P(CH3)3+

P(CH2)(CH3)+

P(CH2)(CH3)2+ HP(CH2)(CH3)+

IE

AE3 AE2

AE1

Analogous Parent

IE + AEn

Recapitulation

• TPEPICO – Photoionization followed by the detection of photoions and zero kinetic energy photoelectrons in coincidence

• Measurement – TOF spectra vs h• Known ion internal energy – Kinetics model for

photodissociation with ab initio input• Bond energies from kinetics model –

Thermochemical cycles Heats of formation

End

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