vibrational autodetachment in nitroalkane anions chris l. adams, j. mathias weber jila, university...
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Vibrational Autodetachment in Nitroalkane Anions
Chris L. Adams, J. Mathias Weber
JILA, University of Colorado, Boulder, CO 80309-0440
OSU International Symposium on Molecular
SpectroscopyJune 24, 2010
Nitroalkane Anions
Novel Approach to studying intramolecular vibrational relaxation (IVR).
Motivation: What happens when a photon of hn = Evib > EeBE ?
Conventional PES (off-resonance)
+ e-
Direct photoemission governed by Franck-Condon factors
Vibrational Autodetachment (VAD) PES (on-resonance)
+ e-
VAD governed by Intramolecular Vibrational Relaxation (IVR) prior to photoemission
Nitroalkane Anions
Model System
• The excess electron is largely localized on the nitro group.
• The fundamental CH vibrational transitions (>2750 cm-1) have energies in excess of the adiabatic electronic affinity (AEA) <200 meV (1600 cm-1).
Nitroalkane Anions
Ion Optics
Tunable IR (2000-4000 cm-1), 1064nm, or 532 nm light
Experimental Set-Up
e-
beam
Deflection and Focusing Optics
Microchannel plate assembly
Phosphor Screen
CCD Camera
Imaging Optics and photoelectron flight tube
Laser Beam
Neutral
Experimental Set-Up
Raw Image Transformed ImageBASEXTransformed Image Integration over emission angles
Photoelectron Spectrum
Experimental Set-Up
0 100 200 300 400
0
Inte
nsi
ty [a
rb. u
nits
]
Pixel
2700 2800 2900 3000 3100 3200
0
Ph
oto
ne
utr
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ield
[a
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nits]
Photon Energy [cm-1]
Nitromethane Anion
Autodetachment spectrum CH3NO2
- + hn CH3NO2 + e-
Ө ≈ 30°Ө ≈ 0°
Anion Neutral
Comparing geometry of neutral and anion
Nitroalkane Anions
•Expect wagging vibration of the neutral should give the most prominent vibrational progression in the PES.
NO2 Wag ~ 603 cm-1 (74 meV )
•Upon emission of excess electron hindered to free rotor
0 100 200 300
0
In
ten
sity
[arb
. un
its]
Binding Energy [meV]
Nitromethane Anion
Off-Resonance - 2740 cm-1
AEA = 172± 6 meV
74 meV
Adams et al., J. Chem. Phys. 130, 074307 (2009)
ZOBS
Dark States
Intramolecular Vibrational Relaxation (IVR)
e-
Nitroalkane Anions
J. M. Weber et al., JCP 115 (2001) 10718
2700 2800 2900 3000 3100 3200
0
Ph
oto
ne
utr
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ield
[a
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Photon Energy [cm-1]
Nitromethane Anion
Autodetachment spectrum CH3NO2
- + hn CH3NO2 + e-
Nitromethane Anion
Off-Resonance - 2740 cm-1
On-Resonance - 2775 cm-1
2725 2750 2775 2800 28250
Photon Energy [cm-1]
150 200 250 300 350
Inte
nsity
[arb
. uni
ts]
Binding Energy [meV]
150 200 250 300 350
Inte
nsity
[arb
. uni
ts]
Binding Energy [meV]
Nitromethane Anion
To extract the VAD photoelectron yield we subtract the off-resonance photoelectron spectrum from the VAD photoelectron spectrum. The baseline is then shifted by the AEA leaving us with the amount of energy remaining in the neutral molecule.
2725 2750 2775 2800 28250
Photon Energy [cm-1]
2700 2800 2900 3000 3100 32000
Photon Energy [cm-1]
0 50 100 150
VA
D y
ield
[arb
. uni
ts]
energy left in neutral [meV]150 200 250 300 350
Inte
nsity
[arb
. uni
ts]
Binding Energy [meV]
Nitromethane Anion
Where do we go from here?
Start with vibrational state (0,0,1,0,0,0,...,0,0,0)
System evolves to
(0,0,0,n4,n5,...,n13,n14,n15)When enough energies is pooled in NO2 wag we expect electron emission to occur. Based on this simple idea, we expect the population of vibrational states in the neutral will provide a rough map of how energy was distributed in the anion just prior to electron loss.
Model the VAD spectrum with final states of the neutral
Nitromethane Anion
Fourteen of the fifteen vibrational modes of the neutral have been experimentally determined.
The last degree of freedom corresponds to the free internal rotor. These internal rotor states were described using a particle-on-a-ring model where
Counting States
,...3,2,1,0;2
22
2 JCJI
JE TPOR
where J is the quantum number of the free internal rotor and I is the reduced moment of inertia for the torsional motion
23
23
NOCH
NOCH
II
III
390 vibrational and torsional states of neutral CH3NO2 within the first 200 meV of the ground state
First Model: CH stretching modes couple to other vibrations, but not to torsional motion, because of large energy mismatch
→ all a v,J are zero for |J| > 0, adjust the av,0 for best fit
Nitromethane Anion
Modeling
Intensity distribution, IVAD(E)
written as Jv
JvJvthrVAD EEIaEfEI,
,0, )()()(
• v and J are the vibrational and free internal rotor quantum states
• fthr(E) is an energy dependent emission probability
• a v,J gives the intensities of the states at energies E v,J
•I0(E - E v,J) is the experimental response functions,
represented by a gauss function corresponding to the experimental resolution
Nitromethane Anion
0 50 100 150
VA
D y
ield
[arb
. un
its]
energy left in neutral [meV]
0 50 100 150
VA
D y
ield
[arb
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energy left in neutral [meV]
First Model:
•The high-energy states are well represented.
•The low-energy region is not recovered at all
•Peak at 100 meV is completely missing
•The width of the peaks is too narrow
Nitromethane Anion
Intensity distribution, IVAD(E)
written as Jv
JvJvthrVAD EEIaEfEI,
,0, )()()(
Second Model (DOS): Energy is completely randomized before VAD occurs, so the density of states describes the population of final states
→ all coefficients av,J are given equal weight.
• v and J are the vibrational and free internal rotor quantum states
• fthr(E) is an energy dependent emission probability
• a v,J gives the intensities of the states at energies E v,J
•I0(E - E v,J) is the experimental response functions,
represented by a gauss function corresponding to the experimental resolution
Modeling
Nitromethane Anion
Second Model:
•The DOS closely resembles the spectrum at low energies
•At high energies, the two curves deviate quickly
•Peak at 100 meV is missing
0 50 100 150
VA
D y
ield
[arb
. un
its]
energy left in neutral [meV]
Nitromethane Anion
Second Model:
•The feature at low energy is due to free internal rotor excitations without contributions from vibrational modes
•The contribution of ΙJΙ = 7 and ΙJΙ = 8 overestimate the experimental curve.
®Ignore all states with ΙJΙ > 8 and keep the weight of ΙJΙ < 8 constant 0 50 100 150
VA
D y
ield
[arb
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its]
energy left in neutral [meV]
-10 0 10 20 30 40 50
VA
D y
ield
[arb
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energy left in neutral [meV]
J = 0-3 4 5 6 7 8
Nitromethane Anion
Intensity distribution, IVAD(E)
written as Jv
JvJvthrVAD EEIaEfEI,
,0, )()()(
Third Model (Partial Randomization Model): Energy is only partially randomized among the vibrations before VAD occurs. Model Two indicates randomization holds for the low-energy internal rotor states.
→ keep internal rotor contour for vibrations constant, adjust vibrational intensities
• v and J are the vibrational and free internal rotor quantum states
• fthr(E) is an energy dependent emission probability
• a v,J gives the intensities of the states at energies E v,J
•I0(E - E v,J) is the experimental response functions,
represented by a gauss function corresponding to the experimental resolution
Modeling
Nitromethane Anion
Third Model:
•Agreement is excellent with the exception of the peak at 100 meV
•There are two potential candidates:
• w(NO2) + |J|=6
• d(NO2) + |J|=5
0 50 100 150
VA
D y
ield
[arb
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energy left in neutral [meV]
2700 2800 2900 3000 3100 3200
0
Ph
oto
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utr
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ield
[a
rb. u
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Photon Energy [cm-1]
Nitromethane Anion
Higher Energy CH Stretches
0 50 100 150
a(CH3)
VA
D y
ield
[arb
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energy left in neutral [meV]
0 50 100 150
VA
D y
ield
[arb
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energy left in neutral [meV]
s'(CH3)
Higher Energy Stretches
Nitromethane Anion
Conclusions and Future Directions
• Methyl torsion plays important role in IVR
•Modeling recovers PES remarkably well, with the exception of the feature at 100 meV.
•Determine AEA of nitroethane and larger nitroalkanes and extend analysis to larger molecular systems.
Acknowledgements
Mathias Weber
Holger Schneider
Jesse Marcum
and the rest of the cast
Carl Lineberger
and the Lineberger Lab