NAT'L INST. OF STAND & TECH REFERENCE

AlllOS TbAflflT

«*•« •'

NBS TECHNICAL NOTE 1308

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

NO2 Heterodyne Frequency

Measurements With a Tunable

Diode Laser, a CO Laser Transfer

Oscillator, and CO2 Laser

Standards

L.R. Zink

M. Vanek

J.S. Wells

[00

AX?. /y?^
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*SoiTK division within the cena are located it Boulder, CO 80303. ^jocaaed m. Boulder. CO, with aoaie ekmeou at Gaithenburg, MD
NO2 Heterodyne Frequency

Measurements With a Tunable Diode

Laser, a CO Laser Transfer Oscillator,

and CO2 Laser Standards

L.R. Zink

M. Vanek

J.S. Wells

Time and Frequency Division

Center for Basic Standards

National Measurement Laboratory

National Bureau of Standards

Boulder, Colorado 80303-3328

Supported in part by

Upper Atmospheric Research Office

NASA

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1987
National Bureau of Standards Technical Note 1308

Natl. Bur. Stand. (U.S.), Tech Note 1308, 28 pages (Apr. 1987)

CODEN:NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1987

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
CONTENTS

ABSTRACT 1

INTRODUCTION 1

EXPERIMENTAL TECHNIQUES 4

CONCLUSION 6

ACKNOWLEDGMENT 6

REFERENCES 7

APPENDIX 10

111

N02 HETERODYNE FREQUENCY MEASUREMENTS WITH A TUNABLE DIODE LASER,

A CO LASER TRANSFER OSCILLATOR, AND CO2 LASER STANDARDS

L. R. Zink, M. Vanek, and J. S. Wells

Time and Frequency Division

National Bureau of Standards

Boulder, Colorado 80303

Heterodyne frequency measurements have been made on selected groups

of nitrogen dioxide rovibronic transitions between 1580.8 and 1650.7

cm" -^. The groups are separated by 7 to 10 cm" -^

, and the intent is

to provide a limited (and interim) calibration table for the region.

In addition to a table of measured frequencies in this region, we have included figures of spectra in the vicinity of the measured

lines to provide a map for identifying the transitions measured.

Key words: calibration frequencies at 6 . 2 /im, heterodyne frequency

measurements, nitrogen dioxide spectra.

INTRODUCTION

A program was established several years ago to provide reference frequencies for the use of spectroscopists for the calibration of spectra. The

objectives at the initiation of the program (with particular emphasis on the use of tunable diode lasers) were to select molecules meeting certain criterion such as

1

.

Absorption type standards

2. A marker every 0.5 cm" -"^ (to match TDL mode tunability)

3. A convenient molecule (one easily handled)

4. A simple Hamiltonian

5. Line positions should be good to + 3 MHz.

The last criterion requires heterodyne frequency measurements and that

has been a major task along with the selection and fitting of the best

available measurements from the literature. Some of the better candidates

were carbonyl sulfide (OCS) and nitrous oxide (N2O). A large body of work has

been completed on OCS [1-5]. OCS has a fairly high abundance of isotopic

species which help provide a definite signature (spectroscopic pattern) in the

vicinity of a particular calibration feature. A 6600 entry OCS atlas [6],
which gives the transition wavenumber, the transition frequency, the isotopic

species, the transition band, the J-value for the transition, the lower state

energy level and the intensity in a normal mixture for each line is available.

The coverage afforded by OCS shown in figure 1 below.

INFRARED FREQUENCY CALIBRATION STANDARDS

Key to accuracy

MMadequate accuracy ±10 MHz or better

Illlll||||±10 to ±60 MHz

588!±60 MHz or worse

NO

DBr

CO2

C2H2

OCS

N20

SUM

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500

I'll

1000

T—I—I—I—I—

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1500 2000

WAVENUMBER (cm-0

Fig. 1. Accuracy of frequency calibration tables for selected molecules in

the 0-2000 cm'^ region.
Another prominent molecule in figure 1 is nitrous oxide and a large

amount of work has been completed on this molecule [7-9] . An interim calibration table exists for the 8 /xm region but more work is needed and has been

planned for the future.

While these two molecules satisfy the five criteria above, there are regions of the spectrum where these two molecules leave gaps in the coverage.

For example DBr covers some of the gap at 1800 cm" •"• [10], but it does not meet

criterion #2. In DBr the spacing between calibration points is about 10 cm"

and then only two markers appear (separated by about 0.2 cm" ^) before another

10 cm"-"- gap. Nonetheless, users have reported this to be quite useful.

The emphasis of the current work is to address the gap at 1500 cm" ^ in

figure 1 and to provide references similar to that afforded by DBr. In

particular, we chose to provide markers every 7 to 10 cm" ^ and also provide a tunable diode laser (TDL) spectrum of the other lines in the vicinity to serve

as a signature.

Nitrogen dioxide does not meet the fourth criterion in that it is an

asjnmnetric rotor with spin split states and its Hamiltonian requires about 25

constants. Further, more than 3000 lines exist in the current Air Force

Geophysical Laba (AFGL) tables, and only one or two percent of these are candidates for a heterodyne frequency measurement. Thus, our approach will

serve two purposes. First, it will provide frequency measurement based

calibration points with strongly absorbing lines in the gap region and serve

a!s an interim calibration table. Second, our past experience with data from

different techniques indicates that while Fourier transform spectroscopic

(FTS) data are quite satisfactory for some requirements, there can sometimes

be a small calibration error. We suggest that the results from our study

either will be useful for the calibration of NO2 in a FTS study, or serve as an independent check of that work. We think that a measurement every 10 cm"

or so will be satisfactory for this purpose. Figure 2 on the next page shows

a compressed FTS spectrum of nitrogen dioxide (the intense lines outside the

familiar P and R branch structure are H2O lines) and the position of CO laser

lines which were used to make the heterodyne frequency measurements.
15000 17000 CM-1

Positions of CO

laser lines used

for measurements

1500, 1550. 1^00 1650,

^ h

17 00cCM-l

Fig. 2. Overlap between nitrogen dioxide spectra and selected CO laser

lines

.

EXPERIMENTAL TECHNIQUES

The measurement technique used for these measurements requires the use of

a CO laser to act as a transfer oscillator. Its role is to transfer the

frequency synthesized from CO2 laser frequency standards to the frequency of

the molecular feature of interest. Each of the frequency measurements of a
nitrogen dioxide transition listed in Table I is a combination of two nearly

simultaneous measurements. The first one is a determination of the frequency

of a CO laser transition relative to the CO2 laser standards. By the use of

two stabilized CO2 lasers, a metal- insulator-metal diode, and a microwave

oscillator, we generate a frequency, i/^ , given by

where i/^ and 1^2 ^^^ ^^^ frequencies of the CO2 laser standards, and i/„ is a microwave frequency. The quantities i, m, and n are integers which are allowed both positive and negative values. For these experiments, the value i was always 3, and m was -1. The microwave frequencies are listed in Table I,

and the n value may be inferred since an X-band klystron was used. An rf beat

note j/g ^ (the difference between the frequency i/g and the CO laser frequency

j/qq) was taken from the MIM diode, amplified and displayed on an rf spectrum

analyzer where its center frequency was marked by an rf synthesizer. The

frequency of the transfer oscillator was then

I'CO = '^xfer = ^s ± ^B 1

• (2)

The second part of the measurement was a difference frequency measurement

between the CO laser transfer oscillator and a tunable diode laser whose

frequency was locked to the NO2 absorption feature of interest. This difference frequency beat note 1/^2 ^^^ its center frequency marked by a marker

oscillator whose frequency was counted and the frequency of the NO2 transition

was then

1/ + i^Rp. (3) NO ""xfer ^"6 2

2

The measurements are given in Table I, which gives the CO2 transitions

and the microwave frequency (in some cases a second harmonic is the value

shown) involved in each measurement, the frequency of the NO2 transition along

with our estimated uncertainty and finally the quantum numbers assigned are those taken from the AFGL tapes

.
Tunable diode laser spectra in the vicinity of the each of the CO laser

lines used for the measurements are given in the appendix. These were

observed at a pressure of 113 Pa and a path length of 1.7 m. Differences

between the calculated frequency values in the AFGL tables and our heterodyne

measurements range from a few megahertz up to hundreds of megahertz for some transitions

.

For the heterodyne frequency measurements in Table I, the path length was also 1.7 m. The pressure was reduced so that for each transition measured the

absorption was in the linear regime. Typically, these pressures were in the

50 to 100 Pa range.

CONCLUSION

Since NO2 is an asymmetric rotor with spin split states and the majority

of the more than 3000 lines given in the AFGL tables are not close enough to

CO laser lines to permit heterodyne frequency measurements on them, we believe

the better approach is to measure only enough lines for the calibration and

leave the detailed spectra for a FTS study. This will permit us to use our resources to look at some N2O hot band features in the 1600 cm" ^ region. This

represents something of a trade-off; a long path cell will be required for the

N2O spectra, but a marker every 0.5 cm" ^ should be available. The greater

absorption afforded by NO2 will permit its use by all, but until a more

detailed study with a high resolution FTS instrument such as that accessible

to the JPL is complete, only a limited number of reference points will be

available

.

ACKNOWLEDGMENT

We would like to thank Dr. J. Burkholder for taking some preliminary FTS

data on the local Bomen instrument of NOAA at our request. We also thank Dr.

D. A. Jennings for the use of his frequency synthesis programs. We are grateful to Dr. A. Maki for his continuing cooperation and advice regarding

our frequency standards program. This work was supported in part by the Upper

Atmospheric Research Office of NASA.
REFERENCES

[1] Heterodyne Frequency Measurements with a Tunable Diode Laser CO2 Laser

Spectrometer: Spectroscopic Reference Frequencies in the 9 . 5 /xm Band of

Carbonyl Sulfide, J.S. Wells, F.R. Petersen and A.G. Maki , Appl. Opt. 18,

3567-3573 (1979).

[2] Heterodyne Frequency Measurements on the 11.6 /zm Band of Carbonyl Sulfide

(OCS) and New Frequency/Wavelength Calibration Tables for both the 11.6

and 5.8 /zm OCS Bands, Appl. Opt. 20, 1676-1684 (1980). Erratum: Appl.

Opt. 20, 2874 (1981).

[3] Heterodyne Frequency Measurements (at 11.6 /xm) on Isotopic Species of

Carbonyl Sulfide, OC^'-S, O^^^g, OC^^S, ^^OCS, and O^^^sas^ j 5. Wells,

F.R. Petersen, a.G. Maki, and D.J. Sukle, J. Mol. Spectrosc. 89, 421-429

(1981).

[4] Heterodyne Frequency Measurements of Carbonyl Sulfide Transitions at 26

and 51 THz ; Improved OCS, O^^CS, and OC^^'S Molecular Constants, J.S.

Wells, F.R. Petersen and A.G. Maki, J. Mol. Spectrosc. 98, 404-412

(1983).

[5] Heterodyne Frequency Measurements on the 12° 0-00° Band of OCS, A.G.

Maki, J.S. Wells, and A. Hinz , Int'l J. IR & MM Waves, 7, 909-917 (1986).

[6] Tables of Carbonyl Sulfide (OCS) Absorption Line Frequencies and Wavenumbers for the Calibration of Tunable Infrared Lasers, A.G. Maki, J.S.

Wells, F.R. Petersen, W.J. Lafferty, W.B. Olson, A. Fayt, and J. P. Sattler, J. Phys . Chem. Ref. Data (in process).

[7] Absolute Frequency Measurements of the 00°2-00°0, 20°l-00°0, and 12° 1-00°

Bands of N2O by Heterodyne Spectroscopy, C.R. Pollock, F.R. Petersen,

D.A. Jennings, J.S. Wells, and A.G. Maki, J. Mol. Spectrosc. 107, 62-71

(1984).

[8] Heterodyne Frequency Measurements on N2O at 5.3 and 9.0 /im, J.S. Wells,

D.A. Jennings, A. Hinz, J.S. Murray, and A.G. Maki, J. Opt. Soc. Am. B2.

857-861 (1985).

[9] Heterodyne Frequency Measurements on the 10°0-00°0 Band of N2O Near 8 ^m,

J.S. Wells, A. Hinz, and A.G. Maki, J. Mol. Spectrosc. 114, 84-96 (1985).

[10] Heterodyne Measurements of Hot Bands and Isotopic Transitions of N2O near
1300 cm" ^

, A. Hinz , J.S. Wells, and A.G. Maki , Atoms, Molecules, and

Clusters, (in press).

11] Improved Deuterium Bromide 1-0 Band Molecular Constants from Heterodyne

Frequency Measurements, J.S. Wells, D.A. Jennings, and A.G. Maki, J. Mol

Spectrosc. 107, 48-61 (1984).
APPENDIX

This appendix contains ten figures of nitrogen dioxide spectra in the

vicinity of the CO laser lines which were used in the heterodyne frequency

measurements. We have included them because we found the lower resolution

AFGL data to be somewhat difficult to reconcile with the higher resolution

TDL spectra. In some cases, this may have been due to less accurate line

position data (calculated values in error due to poor constants) for example.

In other cases, perhaps it was simply a resolution problem. In any case, we have found it preferable to have a signature at the resolution attainable by

the instrument at hand and, given the current status of the data (numerical

data not available at accuracy reported here) , we feel spectral samples are

the interim answer.

Most of the spectra were recorded at a pressure of 113 Pa (0.85 torr) in an absorption cell 1.7m long. For the more intense regions of the spectra,

the pressure was reduced for the duration of a particular heterodyne frequency

measurement. The pressure was in general reduced to the linear absorption

regime and this afforded lines to be resolved that do not appear so in the

spectra shown (see Figs. A-4 and A-8 for example).

Three sets of numbers (given in cm" -^ ) appear in the figures. The heterodyne frequency measurements are given in square brackets with the uncertainty

in the last digits for the measurement given in parentheses within the

bracket, i.e., [1580.73450(33)]. These numbers are positioned on the page so that the decimal point lines up above the center of the line that was measured, and in most cases they are placed above the spectra. A second set of

numbers comes from newly taken FTS data; in this case only the numbers to the

right of the decimal point are listed, except for a few obvious instances.

The FTS numbers are meant to supplement the frequency measurements and are listed in parentheses. In some instances, these have been corrected where a disagreement with heterodyne measurement seems to warrant it. Uncertainties

are not given for these measurements, but we believe that the values we list are good to within 0.0005 cm" ^ . The third set of numbers was taken from the

AFGL tables; these are positioned near the NO2 spectra baseline and consist of

only the three significant digits listed in the tables.
Fringes from a 7.5 cm germanium etalon were recorded below the NO2 spectra,

Since the etalon was not temperature stabilized, these should not be relied

upon for interpolation. We have used them only for line identification

purposes

10
Table I. Heterodyne Frequency Mesisurenients on Nitrogen Dioxide with the Aid of a CO Laser

Transfer Oscillator.

Synthesis Scheme

COg trans .| COg trans .| W freq

for 3v for -Vy I MHz

1636^ 22 II626 P 32 24 200

1636 P 18 II626 P 10 11 600

1636 P 24 II626 P 20 7 500

1636 P 24 II626 P 12

1636 P 18 II626 R 16 -11 000

1636 P 30 II626 P 10 22 000

1636 P 22 II626 R 28 -10 000

1636 P 36 II626 P 14 10 000

1636 P 28 II626 R 24 - 8 800

1636 P 30 II626 R 26 -20 200

CO laser NC 2 Transition

[. trans. Frequency^ Quantum Numbers

Pv..(J") MHz J' V Kc' J" Ka" Kc" s-st

Pl7(15) 49488134.4(90) 49 49 48 48 +

Piedi) 49190236.8(70) 37 4 33 36 4 32 _ XV 49190087.6(70) 37 4 33 36 4 32 +

Pl8(13) 48973387.0(70) 22 1 22 21 1 21 _ 48971356.7(90) 24 3 22 23 3 21 - 48971185.8(90) 24 3 22 23 3 21 + 48967881.8(70) 26 4 23 25 4 22 - 48967623.7(70) 26 4 23 25 4 22 +

Pieds) 48758099.8(70) 19 5 14 18 5 13 _

XV 48752991.2(70) 22 6 17 21 6 16 - 48749412.1(70) 14 3 12 13 3 11 - 48748404.3(70) 16 4 13 15 4 12 +

Pig (10) 48555023.3(70) 21 8 13 20 8 12 +

A^

48551625.6(70) 10 5 6 9 5 5 - 48550704.5(100) 13 6 7 12 6 6 -

PjgdS) 48231914.6(50) 6 6 1 6 6 - 48231205.7(50) 6 3 4 7 3 5 + 48229667.9(50) 7 6 1 7 6 2 +

48229360.7(50) 8 6 3 8 6 2 - 48228499.0(100) 8 6 3 8 6 2 + 48227145.8(100) 9 6 3 9 6 4 + 48226313.4(50) 10 6 5 10 6 4 - 48225637.6(50) 10 6 5 10 6 4 +

P2o(8) 48023593.6(70) 12 4 9 13 4 10 +

Aw

48023320.2(70) 12 4 9 13 4 10 - 48020088.1(70) 15 2 13 16 2 14 + 48019774.1(70) 16 1 16 17 1 17 + 48018936.9(70) 10 5 6 11 5 7 + 48018294.6(70) 10 5 6 11 5 7 —

P20(10) 47817726.7(100) 21 3 18 22 3 19 +

P2o(12) 47605993.0(100) 16 8 9 17 8 10 + 47605568.1(90) 16 8 9 17 8 10 —

P20(14) 47394158.7(100) not assigned

47389778.9(100)*= 32 5 28 33 5 29 + 47389452.2(100) 20 9 12 21 9 13 + 47389228.2(100) 20 9 12 21 9 13 -

a) The uncertainty in the last digits is given in parentheses.

b) The notation 626 and 636 refers to the ^^cIGq^ and ^^clGoZ isotopic lasers

respectively.

c) We estimate that we locate the maximum point of this line to within 10 MHz,

however the line is blended, possibly with a water vapor line.

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BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS/TN-1308

2. Performing Organ. Report No 3, Publication Date April 1987

4. TITLE AND SUBTITLE

N0„ Heterodyne Frequency Measurements With a Tunable Diode Laser,

A CO Laser Transfer Oscillator, and C0„ Laser Standards

5. AUTHOR(S)

L. R. Zink, M. Vanek, and J. S. Wells

6. PERFORMING ORGANIZATION (If joint or other thon N6S. see /nstruct/onsj

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Supported in part by Upper Atmospheric Research Officer NASA

10. SUPPLEMENTARY NOTES

QU Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Heterodyne frequency measurements have been made on selected groups of

nitrogen dioxide rovibronic transitions between 1580.8 and 1650.7 cm

The groups are separated by 7 to 10 cm , and the intent is to

provide a limited (and interim) calibration table for the region. In

addition to a table of measured frequencies in this region, we have

included figures of spectra in the vicinity of the measured lines to

provide a map for identifying the transitions measured.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only prober names; and separate key words by semicolons)

calibration frequencies at 6.2 ym; heterodyne frequency measurements;

nitrogen dioxide spectra

13. AVAILABILITY

£k] Unl i mi ted

I I For Official Distribution. Do Not Release to NTIS

^x] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

I I Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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NBSTechnical Publications

Periodical

Journal of Research—^The Journal of Research of the National Bureau of Standards reports NBS research and development in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad

range of subjects, with major emphasis on measurement methodology and the basic technology underlying

standardization. Also included from time to time are survey articles on topics closely related to the Bureau's

technical and scientific programs. Issued six times a year.

Nonperiodicals

Monographs—Major contributions to the technical literature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice Cmcluding safety codes) developed in cooperation with interested industries, professional organizations, and regulatory bodies.

Special Publications—Include Fwroceedings of conferences sponsored by NBS, NBS zuuiual rerorts, and other

special publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists,

engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and

technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties

of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide pro- gram coordinated by NBS under the authority of the National Standiird Data Act (Public Law 90-396).

NOTE: The Journal of Physical and Chemical Reference Data (JPCRD) is published quarterly for NBS by

the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints,

and supplements are available from ACS, 1155 Sixteenth St., NW, Washington, E>C 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and perfor- mance criteria related to the structural and environmental functions and the durability and safety

characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a

subject. Amdogous to monographs but not so comprehensive in scope or defmitive in treatment of the subject

area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce in Part 10, Tide 15, of the Code of Federal Regulations. The standards establish nationally recognized re- quirements for products, and provide aD concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private

sector standardizing organizations.

Consumer information Series—Practical information, based on NBS research and experience, covering areas of interest to the consumer. Easily understandable language and illustrations provide useful background

knowledge for shopping in today's technological marketplace.

Order the above NBS publications from: Superintendent of Documents, Government Printing Office. Washington. DC 20402. Order the following NBS publications—FIPS and NBSIR's—from the National Technical Information Service. Springfield. VA 22161.

Federal information Processing Standards Publications (FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register serves as the official source of

information in the Federal Government regarding standards issued by NBS pursuant to the Federal Property

and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 1 1717 (38 FR 12315, dated May 11, 1973) and Part 6 of Tide 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSiR)—A special series of interim or final reports on work performed by NBS

for outside sponsors (both government cuid non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Service, Springfield, VA 22161, in paper copy or microfiche form.
U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300