Helium Nanodroplet Isolation Spectroscopy of NO2 and van der

Waals ComplexesRobert Fehnel Kevin Lehmann

Department of ChemistryUniversity 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.

Nozzle Diameter = 10 μmSkimmer = 400 μmNozzle T ≥ 16 KBacking Pressure ≤ 60 Bar

LHe

Skimmer

Nozzle

Closed CircuitRefrigeratorsHe

Chopper

Pickup cell

Gas Cracker

Multipass Cell

1.5K bolometer N.E.P. ~ 2x10-14 W/Hz1/2

10 20 30 45 cm>5000 L/s 2500 L/s

IR OPO 2560 – 3125 cm-1

HMagnets with movement in and out

Machine Schematic

Bolometer noise ~ beam noise ~ 10-5 of chopped beam signal(1 Hz BW)

Acculight Argos OPO

S P I

WavemeterTo Spectrometer

OPO

Powermeter

150 MHz etalon7.5 GHz

etalon

Approximately 1.75 W of power measured entering the machine.

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

101.208○ 135.079○

Dipole = 0.6688 Dipole = 0.4239

O2 bond length = 1.18630 Å O2 bond length = 1.18611 Å

Doublet Quartet

NO2 spectrum in He~5000

NO2 v1 + v3 R(0)

.

NO2 spectrum Showing Baseline

NO2 v1 + v3 R(0) ->

NO2 + N2

NO2 + O2

P(2) ? R(2)?

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 Fit of v1 + v3 R(0) line for

R(0) = 2905.566 cm-1

0.035 cm-1 FWHM

Gas Phase:2906.9034 (J = 1.5)2906.9057 (J = 0.5)Dn = -1.34 cm-1

We lack a predictive theory forthe shape of ro-vibrational linesIn 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 spectrumNO2 R(0)

NO2 + H2O

Bleeding in H2O vapor through crackerAnd optimizing signal on Q branch

Q

R

P

NO2 + H2O Subtracting NO2 R(0)Q

RP

R branch modestly stronger thanP branch suggest this isa K = 0 -> 1 sub-branch

NO2 v1 + v3 R(0)

NO2 + H2O Subtracting NO2 R(0)Appears to be B-type dominated A,B hybrid band. Suggests Hydrogen bonding to lone pair on Nitrogen. Beff = 0.02 cm-1

K = 0 -> 1

K = 0 -> 0

2905.575 cm-1

FWHM 0.110 cm-1

2905.677 cm-1

FWHM 0.020 cm-1

2905.692cm-1

FWHM 0.0096 cm-1

2905.778 cm-1

FWHM 0.111 cm-1

NO2 + H2O Q Branch Splitting FitSplitting of Q branch likely dueto tunneling between different Hydrogen atoms H-bonding.Areas of the two peaks ~ 3:1

NO2+O2 spectrumNO2 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 tosecond pickup region hadno effect on intensity!

NO2+N2 spectrum

K = 1 → 0

K = 0 → 1

K = 1 → 2

NO2+N2 FitLine 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 trans-

asymmetric 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