Transcript ohiostateno2talk.pptx

Helium Nanodroplet Isolation
Spectroscopy of NO2 and van
der Waals Complexes
Robert Fehnel
Kevin Lehmann
Department of Chemistry
University of Virginia
Why study Nitrogen Oxides?
• Reactions between “NOx” and related compounds are
important in atmospheric and combustion chemistries.
– Include NO, NO2, NO3, N2O3, N2O4, N2O5, HNO, HNO2,
HNO3. (with multiple isomers).
– Reactions between these and with On (n = 1-3) often have
low or no barrier.
• Several are free radicals (NO, NO2, NO3).
– Association reactions should occur on multiple spin
surfaces but higher spin compounds or complexes are not
yet known. -- May trap these in Helium.
– Potential to use Spin orientation in Magnetic field to
control chemistry -- Requires fast spin relaxation
– Potential to do optical detected ESR studies in helium
droplets. -- Extend to molecules what Callegari & Ernst
have done for alkali atoms on droplets.
Previous Work Done by Conjusteau
• Studied the photodissociation of NO2 in He
nanodroplets
– Used 1-photon process at 398nm
– Did not find an LIF signal below threshold showing
that the relaxation time is less than 90ns as to
suppress the signal
– Did not see dissociation and came up with 4 reasons
for that
• Vibrational relaxation in ground state is faster than
dissociation
• It recombined in three different ways either exactly as it was,
van der Waals complex, and or a complex is made mixing the
original doublet state with a new quartet state
Conjustea, A. Thesis. (2002)
No Dissociation
Previous Work Done by Wittig
• A mass spec depletion signal for 17,700 – 18,300
cm-1 was observed
• In this region the intramolecular dynamics are
believed to be chaotic
• Found that the spectra could be fitted well by
shifting the R0 line positions by 7 cm-1, adding 7
cm-1 widths to all lines and adjusting intensities
– The increased line widths are due to rapid vibrational
relaxation in the He
E. Polyakova et al. Chem. Phys. Lett. 375 (2003) 253.
Machine Schematic
He
IR OPO
2560 – 3125 cm-1
Closed Circuit
Refrigerators
Gas Cracker
Multipass Cell
Nozzle
Chopper
Skimmer
Nozzle Diameter = 10 μm
Skimmer = 400 μm
Nozzle T ≥ 16 K
Backing Pressure ≤ 60 Bar
>5000 L/s
LHe
Pickup cell
H
Magnets with 1.5K bolometer
movement in N.E.P.
~ 2x10-14 W/Hz1/2
and out
2500 L/s
10
20
30
45 cm
Bolometer noise ~ beam noise ~ 10-5 of chopped beam signal(1 Hz BW)
Acculight Argos OPO
Approximately 1.75
W of power
measured entering
the machine.
S
P
To Spectrometer
Wavemeter
150 MHz
I
7.5 GHz
etalon
OPO
etalon
Power
meter
Produces over 2 W of CW over the tunable range of 3.2 – 3.9 μm. Continuous scans
of 45 GHz. Also produces 2 - 5 W of 1.5 μm light.
NO2 O2 spin states
Doublet
101.208○
O2 bond length = 1.18630 Å
Dipole = 0.6688
Quartet
135.079○
O2 bond length = 1.18611 Å
Dipole = 0.4239
NO2 spectrum in He~5000
NO2 v1 + v3 R(0)
.
NO2 spectrum Showing Baseline
NO2 v1 + v3 R(0) ->
Based upon tentative P(2), R(2) assignments,
B(NO2) = 0.24 cm-1 (60% gas phase)
Compared to:
B(CO2) = 0.154 cm-1 (40% of gas phase)
B(N2O) = 0.073 cm-1 (17% of gas phase)
NO2 + N2
NO2 + O2
P(2) ?
R(2)?
NO2 Fit of v1 + v3 R(0) line for
R(0) = 2905.566 cm-1
Gas Phase:
2906.9034 (J = 1.5)
2906.9057 (J = 0.5)
Dn = -1.34 cm-1
0.035 cm-1 FWHM
We lack a predictive theory for
the shape of ro-vibrational lines
In helium.
NO2 R(0) Showing Wings & Lorentzian Fit
These “steps” are reproducible.
Similar step seen on
red wing of CO2 R(0) transition.
NO2 + H2O spectrum
NO2 R(0)
Bleeding in H2O vapor through cracker
And optimizing signal on Q branch
NO2 + H2O
Q
P
R
NO2 + H2O Subtracting NO2 R(0)
Q
R branch modestly stronger than
P branch suggest this is
a K = 0 -> 1 sub-branch
R
P
NO2 v1 + v3 R(0)
NO2 + H2O Subtracting NO2 R(0)
K = 0 -> 1
2905.677 cm-1
FWHM 0.020 cm-1
2905.575 cm-1
FWHM 0.110 cm-1
K = 0 -> 0
2905.692cm-1
FWHM 0.0096 cm-1
Appears to be B-type
dominated A,B hybrid band.
Suggests Hydrogen bonding
to lone pair on Nitrogen.
Beff = 0.02 cm-1
2905.778 cm-1
FWHM 0.111 cm-1
NO2 + H2O Q Branch Splitting Fit
Splitting of Q branch likely due
to tunneling between different
Hydrogen atoms H-bonding.
Areas of the two peaks ~ 3:1
NO2+O2 spectrum
NO2 v1 + v3 R(0)
NO2 + O2
NO2 + H2O
NO2+O2 Fit for K = 0 → 1
Strongly B-type band,
Suggesting C2v symmetry.
B ~ 0.02 cm-1
Moving ~1T magnet up to
second pickup region had
no effect on intensity!
NO2+N2 spectrum
K=0→1
K=1→0
K=1→2
NO2+N2 Fit
Line spacing of Approximately
0.043 cm-1 in each transition.
B-type transition
(B+C)/2 = 0.017 cm-1
A = 0.17 cm-1
K = 1 branches =>
Not T shaped!
Things that have not worked (yet?)
• We could not find the symmetric N2O4 dimer band
(2973 cm-1 in gas phase).
– Did see a weak, broad transition that likely is transasymmetric dimer.
• We could not find any evidence of NO2 formed by
sequential pickup of NO and then O2 passed through
the cracker (which is expected to produce ~80%
dissociation of O2).
• Could not detect HNO (n1 @ 2681 cm-1 in gas phase)
formed by pickup of H atoms (from cracker) after NO
pickup.
Near Term plans
• Make N16O18O to firmly determine (B+C)/2 value.
• Look for NO3 formed from either NO + O2 or from NO2
+ O.
– NO3 has strong electronic band at 662 nm.
• Look for IR spectrum of N2O3 formed from NO + NO2.
• Study microwave spectrum (Brooks Pate) and do ab
initio calculations for NO2 complexes.
• Use Mass Spec to characterize products from various
inputs to the cracker.
Acknowledgements
• Dr. Ozgur Birer who help construct the
HENDI machine at UVa.
Funding:
• National Science Foundation, UVa