DEVELOPMENT OF BROAD RANGE SCAN CAPABILITIES WITH JET COOLED CAVITY RINGDOWN SPECTROSCOPY

Terrance Codd, Ming-Wei Chen, Terry A. Miller

The Ohio State University

Cavity Ringdown Spectroscopy

A =σNl

Photo-diode

τ0

cL )/(R1 -

= τabs

σ Nl+= cL )/(

R1-( ) tabs

Time

Inte

nsi

ty

0

A = L/(cτabs)- L/(cτ0)

Sensitivity of TechniqueIf R = 99.99% and L = 67cmthen τ0 = 22.3 μsLeff = 6.7 km ∼ 4.16 Miles l = 5 cmleff = 0.5 km

l

L

Comparison of CRDS Methods for A-X NO3

HR JC-CRDS Specs

Radiation 100-250 MHz FWHM

Rotational temps 15-30 K Benefits

Resolved transitions can be used to accurately determine rotational and spin rotational constants and orientation of TDM

Drawbacks Radiation difficult to

produce and scans very time consuming

RT CRDS Specs

~2 GHz FWHM Deev et al, 4.5 GHz Ambient temperatures

Benefits Can quickly scan very

broad ranges and radiation is easy to generate

Drawbacks Higher rotational

temperature leads to congestion of transitions making assignment difficult.

~~

Deev, J. Sommar, M. Okumura J. Chem. Phys. 122, 2243051 (2005)

Jacox, M; Thompson, W. E.; J. Chem. Phys.; 122, 224305 (2005)

Combining Mod Res/Jet Cooling

Want to combine some of the benefits of moderate resolution radiation sources (broad range scan capabilities and ease of use) with the benefits of the jet cooled system (rotationally cold samples)

Goal: couple moderate resolution radiation with JC-CRDS

Mod-Res Jet Cooled CRDS

Fiber Optic works very well. Roughly 90% efficient transmission of IR

Nd:YAG

20 Hz

Sirah

Dye Laser

H2 Raman CellFilters

1st or 2nd Stokes2-10 mJ/

650 - 700 mJ 75 - 115 mJ

20 m Fiber Optic

Collimator

~2 GHz FWHM

N2O5 in First Run Neon

NO2 + NO3

PD

Previous CRDS of NO3

Previous CRDS spectra of A-X transition of NO3 have been done in the Okumura lab under ambient conditions. High rotational temps make band assignments difficult.

No good assignment of the 3 band

401

201

30140

2

10140

1

10120

1

403

10140

2

A. Deev, J. Sommar, M. Okumura J. Chem. Phys. 122, 2243051 (2005)

~ ~

Comparison To Room Temp

7500 8000 8500-100000

-80000

-60000

-40000

-20000

0

20000

40000

60000

80000

100000

120000

a.u

.

wavenumber

8500 9000 9500-100000

-80000

-60000

-40000

-20000

0

20000

40000

60000

80000

100000

120000

a.u

.

wavenumber

Ambient NO3 data courtesy of M. Okumura

Comparison To Room Temp

9500 10000 10500-100000

-80000

-60000

-40000

-20000

0

20000

40000

60000

80000

100000

120000

a.u

.

wavenumber

Ambient NO3 data courtesy of M. Okumura

NO3 Radical 401 Band

See intensity cut by ~2/3 compared to HR Can resolve rotational structure but not individual

transitions.

7590 7600 7610 7620

-3

-2

-1

0

1

2

3

4

ModRes (30 K) HiRes*0.3 -3ppm (15 K)

pp

m

wavenumber

TD07

Characterizing Method

Have lower signal to noise ratio than with high-resolution radiation

Reduction in signal probably due to the fact that the transitions being observed have a much narrower linewidth than the radiation Signal intensity is limited by both intensity and density

of transitions Increase in noise probably caused by using radiation

which is much broader than FSR of cavity Means we span a number of longitudinal cavity modes Consistent with our experience on RT-CRDS

Provides sufficient sensitivity for weak transitions

7500 8000 8500 9000 9500 10000 10500

0

1

2

3

4

5

ppm

wavenumber

Making Assignments

We used these assignments as a starting point to get good fundamental frequencies for the vibrational modes.

401

201

30140

2

10140

1

10120

1

403

10140

2

A. Deev, J. Sommar, M. Okumura J. Chem. Phys. 122, 2243051 (2005)

401

201

402

10140

1

10120

140

3

10140

2

Predicting Transitions

To aid in the assignment of other transitions a set of predicted transitions were calculated.

Used pure harmonic approximations as a first guess based on fundamental frequency.

Made assignments based on that and then fit anharmonic constants and frequencies.

Done with all possible modes combinations NOT including any quanta of excitation in 3.

Predicted Assignments

7600 7800 8000 8200 8400

0

1

2

3

4

5

ppm

wavenumber

401 40

2 20140

1

10140

1

201 10

1 202

8600 8800 9000 9200 9400

0

1

2

3

4

5

ppm

wavenumber

Predicted Assignments

403 20

1402

20140

3

10140

2

10140

320

3

10140

1 20240

1

10120

1401

10240

1

102

10120

2 103

10220

1

404

Predicted Assignments

9600 9800 10000 10200 10400

0

1

2

3

4

5

ppm

wavenumber

20340

1

10240

2 20140

410

2403

20340

2

3 Assignment

Most prominent unassigned transition in the lower frequency range is the weak parallel band at 8753 cm-1. This is 1689 cm-1

from the origin. Eisfeld and Morokuma*1 have calculated the frequency of the 3

band to be 1602 cm-1. Previous assignment by Deev*2 for 3 was 300-400 cm-1 away from the predicted frequency and did not have parallel band contour.

Based on the contour and the predicted frequency we assign this to the 30

1 band.

1. W. Eisfeld, K. Morokuma, J. Chem. Phys. 114, 9430 (2001)

2. A. Deev, J. Sommar, M. Okumura J. Chem. Phys. 122, 2243051 (2005)

8600 8800 9000 9200 9400

0

1

2

3

4

5

ppm

wavenumber

403

20140

2

20140

3

10140

2

10140

320

3

10140

1 20240

1

10120

1401

10240

1

102 10

1202 10

3

10220

1404

Other Work

Wanted to use this apparatus to investigate other molecules as well

Hydroxy Propyl Peroxy radical is being studied on the RT-CRDS system

A jet cooled spectrum could aid in the assignment of some of the low frequency transitions observed

Hydroxy Propyl Peroxy

Entire Jet-Cooled spectrum was done in ~2 hours

WJ11

000

Conclusion

We have developed a moderate resolution jet cooled cavity ringdown spectrometer capable of quickly obtaining broad range scans with good sensitivity.

We have obtained spectra of the A-X transition of NO3 Almost all of the cold vibronic spectrum has been

assigned including the 301 band.

Additional broadband structure has been revealed which is coincident with vibronically forbidden transitions

Splitting in υ1υ4 combination bands have been observed

Ongoing analysis should provide more insight into the Jahn-Teller effect and vibronic coupling in the A state of NO3.

~~

~

Acknowledgments

Dr. Terry Miller Miller Group

Members Ming-Wei Chen Gabriel Just* Rabi Chhantyal-Pun Neal Kline Jin-Jun Liu Phillip Thomasǂ

NSF-$$$$$

You for your attention!

*Currently at Lawrence Berkeley National Lab, Berkeley CaliforniaǂCurrently at Leiden Institute of Chemistry, Netherlands