Infrared Diode Laser Spectroscopy of the ν3 Fundamental Band of the PO2 Free Radical

Michael Lawson

PN

OCH2CF3

O

O2NO

ONO2

n

PPZ-E

• Low glass transition temperatures

• Easy to modify physical properties

• “High Energy”

Energetic Polyphosphazenes

Are transient phosphorus species generated by the pyrolysis of energetic polyphosphazenes??

PN PO PO2 PNO

PN

O

O

Lead-Salt Diode Laser Spectroscopy of Transient Phosphorus Species

Chemiluminescence

Observed: - PN, PH, PO, P2O, PO2,PNO, BrPO, ClPO, cis-HOPO

Sensitivity: ~1010 molecules cm-3

Temporal resolution: ~ 1 ms.

Resolution: ~10-4 cm-1

Discreet modes < ~1 cm-1

Modes spread over ~100 cm-1

Fiddly to use

Diode Laser Spectrometer.

Laser is modulated at 10.5 kHz.

Lock-in amplifiers detect signal at 2f.

Spectrum is recorded as the laser current is slowly ramped.

PN

OCH2CF3

O

O2NO

ONO2

n

PPZ-E

• Low glass transition temperatures

• Easy to modify physical properties

• “High Energy”

Energetic Polyphosphazenes

Are transient phosphorus species generated by the pyrolysis of energetic polyphosphazenes??

PN PO PO2 PNO

Do we have enough sensitivity?

Chemically Generating the PO2 Radical

P4 + He (500mt)

Pump (140 l s-1)

O2 (200mT) + discharge

S/N Limited by pumping capacity

ν3 fundamental

Extra Lines?

Exp.

Calc.

ν3

724 lines were measured and assigned, of which 504 were “new” observations.

75% coverage of the v3 band using 3 diode lasers.

ν3

1348.06 1348.07 1348.08 1348.09 1348.1 1348.11 1348.12 1348.13 1348.14 (cm-1)

Global Fitting of PO2 v3 band

Exp.

Calc.

• Microwave/LMR lines for the ground state were combined with our lines and lines from a previous work that filled gaps in our spectra to give a total of 887 lines.

• V3 = 1 hyperfine parameters were constrained to the ground-state.

• Unresolved spin rotation lines were appropriately weighted and blended together.

• Microwave/LMR lines for the ground state were combined with our lines and lines from a previous work that filled gaps in our spectra to give a total of 887 lines.

• V3 = 1 hyperfine parameters were constrained to the ground-state.

• Unresolved spin rotation lines were appropriately weighted and blended together.

1348.06 1348.07 1348.08 1348.09 1348.1 1348.11 1348.12 1348.13 1348.14 (cm-1)

Global Fitting of PO2 v3 band

Exp.

Calc.

Exp.

Global Fitting of PO2 v3 band

All fitting carried out using the SPFIT program.

Results:

Microwave/LMR RMS: 44 kHz

Infrared RMS: 0.00061 cm-1

Global RMS: 0.00057 cm-1

Calc.

Rotational Constants

Quartic centrifugal distortion constants

Sextic centrifugal distortion constants

Parameter Ground state v3 = 1

A 3.4859780(12) 3.408325(15)

B 0.28680773(12) 0.2859092(14)

C 0.26422996(13) 0.2630608(13)

Parameter Ground state v3 = 1

ΔN x 106 0.15534(33) 0.1583(10)

ΔNK x 104 -0.129613(90) -0.12975(23)

ΔK x 103 0.95353(17) 0.90803(36)

δN x 107 0.33126(63) 0.3376(36)

δK x 105 0.1507(18) 0.139(13)

Parameter Ground state v3 = 1

εaa 0.054590(13) 0.053384(53)

εbb 0.0007171(11) 0.000620(70)

εcc -0.0014810(12) -0.001504(66)

Electron spin-rotation constants

Parameter Ground state v3 = 1

HN x 1011 0.0035(28) 0.016(29)

HNK x 109 -0.0400(86) -0.044(11)

HKN x 108 -1.072(26) -1.006(28)

HK x 106 0.8833(36) 0.8222(46)

hN x 1012 0.073(73) Constrained

hNK x 109 0.046(21) Constrained

Parameter Ground state v3 = 1

LK x 108 -0.0918(32) -0.0854(33)

Octic centrifugal distortion

Global Fit Results

Exp.

Calc.

Unassigned Lines

1314.8 1315 1315.2 1315.4 1315.6 1315.8 1316

Energy cm-1

Species Responsible for Unassigned Lines?

• Metastable Excited state of PO2?

• Another phosphorus oxide, PO3 P4O?• Relatively sparse lines density –> light with high

symmetry.

DFT(B3LYP)/6-311++G(d,p)

0 Energy (cm-1) 0.6

Summary

• Excellent method of generating PO2 in large quantities in a discharge flow cell.

• Recorded the spectrum of the PO2 ν3 fundamental band at Doppler-limited resolution.

• Produced a definitive set of spectroscopic constants improving the v3 line predications by up to an order of magnitude.

• Ruled out the unassigned lines as arising from the ν3 fundamental of PO2.

AcknowledgementsPaul Davies – Supervisor.

Kristian Hoffman - Taught me how to use the spectrometer.

Mike CasfordPeter Kett - Group members.

Phillip Martin - Allowed us to use his helium cold head.

Adrian King - Useful discussions about SPFIT and Gaussian 03.

AWE, Aldermaston -Sponsorship.

Thank you!