Photoelectron Imaging of Vibrational Autodetachment from Nitromethane Anions Chris L. Adams, Holger Schneider, J. Mathias Weber JILA, University of Colorado,

Photoelectron Imaging of Vibrational Autodetachment from Nitromethane Anions

Chris L. Adams, Holger Schneider, J. Mathias Weber

JILA, University of Colorado, Boulder, CO 80309-0440

OSU International Symposium on Molecular Spectroscopy

June 23, 2009

Novel Approach to studying intramolecular vibrational relaxation (IVR).

Motivation

What happens when a photon of hn interacts with an anion with EeBE < hn ?

Motivation

What happens when a photon of hn interacts with an anion with EeBE < hn ?

1. Direct photoemission of the excess electron.

A- + hn → A + e-

Motivation

What happens when a photon of hn interacts with an anion with EeBE < hn ?

1. Direct photoemission of the excess electron.

A- + hn → A + e-

2. Vibrational excitation followed by vibrational autodetachment (VAD) of the excess electron.

A- + hn → [A-]* → A + e-

First example: NH- (Lineberger and coworkers, 1985)

Motivation

• The excess electron is largely localized on the nitro group.

Nitroalkane Anions: A Model System

Nitroalkane Anions: A Model System

• The excess electron is largely localized on the nitro group.

Nitroalkane Anions: A Model System

• The excess electron is largely localized on the nitro group.

• The fundamental CH vibrational transitions have energies in excess of the adiabatic electronic affinity (AEA) ~200 meV (1600 cm-1).

ZOBS

Dark States

Intramolecular Vibrational Relaxation (IVR)

e-

MCP detector

Experimental Setup

2800 3000 3200

0.0

0.5

1.0

1.5

2.0

2.5

Ph

oto

ne

utr

al Y

ield

Photon Energy (cm-1)

IR Spectrum of MeNO2-

Autodetachment spectrum CH3NO2

- + hn CH3NO2 + e-

MCP detector

Experimental Setup

Ion Beam

Laser Beam Direction

Velocity Map Imaging Photoelectron Spectroscopy (VMIPES)

0 50 100 150 200 250

Ph

oto

ele

ctro

n Y

ield

[arb

. un

its]

Pixels

Raw Image Transformed ImageBASEXTransformed Image Integration over emission angles

Photoelectron Spectrum

Example: VMIPES of S- (532 nm)

V. Dribinski et al., RSI 73, 2634 2002.

2800 3000 3200

0.0

0.5

1.0

1.5

2.0

2.5

Ph

oto

ne

utr

al Y

ield

Photon Energy (cm-1)

IR Spectrum of MeNO2-

Autodetachment spectrum CH3NO2

- + hn CH3NO2 + e-

What do we expect from the direct photodetachment PES?

Ө = 14° Ө = 0°

Anion Neutral

What is the Geometry of the Anion and the Neutral?

•The wagging vibration of the neutral should give the most prominent vibrational progression in the PES.

Dominant FCF Active Modes

•The wagging vibration of the neutral should give the most prominent vibrational progression in the PES.

NO2 Wag ~ 655 cm-1 (81 meV )

Dominant FCF Active Modes

•The wagging vibration of the neutral should give the most prominent vibrational progression in the PES.

•Upon emission the methyl rotor goes from being hindered to a free rotor.

NO2 Wag ~ 655 cm-1 (81 meV )

Dominant FCF Active Modes

•The wagging vibration of the neutral should give the most prominent vibrational progression in the PES.

•Upon emission the methyl rotor goes from being hindered to a free rotor.

NO2 Wag ~ 655 cm-1 (81 meV )

Dominant FCF Active Modes

0 100 200 300 400

Phot

oele

ctro

n Yi

eld

Binding Energy [meV]

1MeNO2- at 3200 cm-1

0 50 100 150 200 250 300 350 400

Binding Energy [meV]

Pho

toel

ectron

Yie

ld

MeNO2-·Ar

3200 cm-1

MeNO2-

3200 cm-1

Peak Assignments – AEA determination

•Peaks are spaced by~ 645 cm-1 (80 meV), corresponding to the wagging motion of the neutral.

0 50 100 150 200 250 300 350 400

Binding Energy [meV]

Pho

toel

ectron

Yie

ld

•Peaks are spaced by~ 645 cm-1 (80 meV), corresponding to the wagging motion of the neutral.

•The first prominent peak, located at (172±6) meV, is identified as the origin of the vibrational progression (vanion=0, vneutral=0).

Peak Assignments – AEA determination

MeNO2-·Ar

3200 cm-1

MeNO2-

3200 cm-1

0 50 100 150 200 250 300 350 400

Binding Energy [meV]

Pho

toel

ectron

Yie

ld

•Peaks are spaced by~ 645 cm-1 (80 meV), corresponding to the wagging motion of the neutral.

•The first prominent peak, located at (172±6) meV, is identified as the origin of the vibrational progression (vanion=0, vneutral=0).

• Argon solvation shifts the vibrational progression by ~63 meV (508 cm-1).

Peak Assignments – AEA determination

MeNO2-·Ar

3200 cm-1

MeNO2-

3200 cm-1

0 50 100 150 200 250 300 350 400

Binding Energy [meV]

Pho

toel

ectron

Yie

ld

Hot band

Peak Assignments – AEA determination

•Peaks observed at binding energies less than 172 meV are identified as hot bands.

MeNO2-·Ar

3200 cm-1

MeNO2-

3200 cm-1

0 50 100 150 200 250 300 350 400

Binding Energy [meV]

Pho

toel

ectron

Yie

ld

Peak Assignments – AEA determination

•Peaks observed at binding energies less than 172 meV are identified as hot bands.

•The difference in binding energies of the hot band and origin of the vibrational progression matches the energy of the anionic wag.MeNO2

-·Ar3200 cm-1

MeNO2-

3200 cm-1

0 50 100 150 200 250 300 350 400

Binding Energy [meV]

Pho

toel

ectron

Yie

ld

•Peaks observed at binding energies less than 172 meV are identified as hot bands.

•The difference in binding energies of the hot band and origin of the vibrational progression matches the energy of the anionic wag.

•The hot bands are suppressed upon Ar solvation.

Peak Assignments – AEA determination

MeNO2-·Ar

3200 cm-1

MeNO2-

3200 cm-1

Comparison of Experiment and Theory

0 100 200 300 400

electron binding energy [meV]

Franck-Condon Simulation (PESCAL)by Kent M. Ervin

• B3LYP/6-311++G(2df,2p) for anion and neutral geometries

• Independent Harmonic Oscillator Approximation with Duschinsky rotation

• 14 vibrational modes treated in simulation

• CH3 torsion treated separately

-400 -200 0 200 400

phot

oele

ctro

n yi

eld

[ar.

units

]

relative binding energy [cm-1]

•There exists a pronounced shoulder on all

of the dominant features of the PES

regardless of Ar solvation.

Contribution of Torsion to the PES

-400 -200 0 200 400

phot

oele

ctro

n yi

eld

[ar.

units

]

relative binding energy [cm-1]

•There exists a pronounced shoulder on all

of the dominant features of the PES

regardless of Ar solvation.

•The direct photodetachment involves a

transition from hindered-to-free methyl

rotor.

Contribution of Torsion to the PES

-400 -200 0 200 400

phot

oele

ctro

n yi

eld

[ar.

units

]

relative binding energy [cm-1]

•There exists a pronounced shoulder on all

of the dominant features of the PES

regardless of Ar solvation.

•The direct photodetachment involves a

transition from hindered-to-free methyl

rotor.

•This leads to progressions of the free

internal rotor states superimposed on all

transitions

Contribution of Torsion to the PES

2700 2800 2900 3000

ph

oto

ne

utr

al y

ield

[a

rb. u

nits

]

photon energy [cm-1]

CH Stretching Vibrations

ν13 = 2775 cm-1

ν14= 2922 cm-1

ν15= 2965 cm-1

n14 n15

n13

Autodetachment spectrum CH3NO2

- + hn CH3NO2 + e-

IR Spectrum of MeNO2-

Vibrational Autodetachement Direct Photoelectron Emission

Comparison of Off and On Resonance Images

0 100 200 300 400

P

ho

toe

lectr

on

Yie

ld [a

rb. u

nits]

Binding Energy [meV]

Vibrational Autodetachement

Direct Photoelectron emission

Comparison of Off and On Resonance Images

0 100 200 300 400

P

ho

toe

lectr

on

Yie

ld [a

rb. u

nits]

Binding Energy [meV]

Vibrational Autodetachement

Direct Photoelectron emission

Comparison of Off and On Resonance Images

0 40 80 120 160

po

pu

latio

n [

arb

. u

nits]

energy left in neutral [meV]

On-Resonance Interpretation

Both on-resonant and direct detachment contributions

® subtract contribution of direct photodetachment

0 100 200 300 400

Ph

oto

ele

ctr

on

Yie

ld

Binding Energy [meV]

0 100 200 300 400

Pho

toel

ectr

on Y

ield

Binding Energy [meV]

Compare with vibrational states of the neutral, neglecting torsion

Without Torsion

On-Resonance Interpretation

0 40 80 120 160

po

pu

latio

n [

arb

. u

nits]

energy left in neutral [meV]

0 40 80 120 160

po

pu

latio

n [

arb

. u

nits]

energy left in neutral [meV]

With Torsion

On-Resonance Interpretation

Compare with vibrational states of the neutral, including torsion

0 40 80 120 160

popula

tion [arb

. units]

energy left in neutral [meV]

Inconsistencies with purely statistical argument.• Some states preferentially occupied• Nonstatistical population

With Torsion

On-Resonance Interpretation

• Considerable differences between direct detachment and

vibrational autodetachment

Summary

• Considerable differences between direct detachment and

vibrational autodetachment

• Redistribution of vibrational energy before electron emission

Summary

• Considerable differences between direct detachment and

vibrational autodetachment

• Redistribution of vibrational energy before electron emission

• Retention of vibrational energy in the molecule, leading to

emission

of low-energy electrons.

Summary

• Considerable differences between direct detachment and

vibrational autodetachment

• Redistribution of vibrational energy before electron emission

• Retention of vibrational energy in the molecule, leading to

emission

of low-energy electrons.

• Methyl torsion very important for IVR

Summary

Continue the study with the larger nitroalkane chains:

Summary

Continue the study with the larger nitroalkane chains:

• Determine AEA and assign the vibrational features in the direct photodetachment spectra

Summary

Continue the study with the larger nitroalkane chains:

• Determine AEA and assign the vibrational features in the direct photodetachment spectra

• Monitor the evolution of the VAD PES as the site of initial excitation is moved further away from the nitro group

Summary

Acknowledgements

Mathias Weber

Holger Schneider

Jesse Marcum

Kent Ervin (UN Reno)

Carl Lineberger

and the Lineberger Lab

0 25 50 75 100 125

0

20

40

60

Inte

nsi

ty [a

rb. u

nits]

Energy Left in Neutral Molecule [meV]

On-Resonance Interpretation

NO2 rocking (475 cm-1)

NO2 Wag (603 cm-1)

NO2 Scissor (657 cm-1)

CN stretch (918 cm-1)

2 quanta NO2 rocking (475X2 cm-1)

1 quanta NO2 rocking (475 cm-1) and 1 quanta NO2 Wag (603 cm-1)

CH3 rocking (1096 cm-1)

1 quanta NO2 rocking (475 cm-1) and 1 quanta NO2 Scissor (657 cm-1)

Averaging in the Lab Frame along the Transition Dipole of the CH Stretch Vibration (2775 cm-1)