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" MAR

NBS MONOGRAPH 24

A Spectrophotometric Atlas of the

Spectrum of CH from 3000A to 5000A

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, SecretuTy NATIONAL BUREAU OF STANDARDS • A. V. Astin, Dirtctor

A Spectrophotometric Atlas of the

Spectrum of CH from 3000 A to 5000 A

Arnold M. Bass and H. P. Broida

National Bureau of Standards Monograph 24

Issued February 14, 1961

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington 25, D.C. Price 20 cents

A Spectrophotometric Atlas of the

Spectrum of CH From 3000 A to 5000 A

Arnold M. Bass and H. P. Broida

The near ultraviolet and visible emission spectrum of CH was recorded and presented

in the form of a spectrophotometric atlas. The spectrum was recorded photoelectrically

from an acetylene-oxygen flame in the region 4900 to 3000 A by use of a high-resolution

grating monochromator. Each of the lines in the CH spectrum is identified.

1. Introduction

In 1953 we published a spectrophotometric

tlas of the A^Z+—X.^U transition of OH/ with

le expectation that other atlases would be preared as time and general interest dictated. At

lat time we recorded, also, the spectrum of CH

1 the region from 3000 to 5000 A in order to preare a second atlas. However, the work had to

e discontinued, and the preparation of the atlas

as been completed only recently.

A number of valuable suggestions were received

after the publication of the OH atlas and, where

possible, these suggestions have been incorporated

into the present atlas. We would, however, be

interested in receiving any comments or suggestions which the users of this work think would be

of value in the preparation of other atlases of this

type.

2. Arrangement of the Atlas

The spectrum of CH in the visible and near

Itraviolet regions is characterized by the three

ectronic transitions : A^A—X^n, B^S"—X^II, and

^2"*"—X^n. The notation, assignments, and

avelengths used in this atlas are those given in

le report by Moore and Broida.^ As stated in

!iat paper, the source of data for A^A—X^II

't,0; 1,1; 2,2) and B^S"—X^n (0,0; 1,0; 1,1) is ero;3 for A^A—X^n (0,1; 1,2), Kiess and

roida;" for ^2+—X^H (0,0; 1,1), Heimer.^

The spectrum charts are arranged so that an

•erall view of the entire region from 3000 to

•GO A is given in figures 1 and 2. The main

atures are labeled, including the bands of C2

i^ich occur in the flame source. However, the

1 1 and 1, 2 bands of A^A—X-n are not included

the overall scan. Following this, the spectrum

presented on a larger scale in figures 3 to 18.

ae spectrum is subdivided into five spectral

.?ions: 4940 to 4740 A (A^A—X'H, 0,1; 1,2)

I'ures 3 and 4; 4420 to 4140 A (A^A—X^H, 0,0;

.;2,2) figures 5 to 10; 4130 to 3880 A (6^2-—

in, 0,0; 1,1) figures 11 to 14; 3720 to 3630 A

''2-—X^n, 1,0) figure 15; and 3230 to 3070 A

X^n, 0,0; 1,1), figures 16 to 18. In

\. Bass and H. P. Broida, NBS Cire. 541 (1953). E. Moore and H. P. Broida. J. Research NBS 6JA, 19 (1959).

^. Qero, Z. Physik. 11^, 27 (1941).

•J. H. Kiess and H. P. Broida, Astrophys. J. 1Z3, 166 (1956).

". Heimer, Z. Physik. 78, 771 (1932).

addition, the very closely spaced, intense, heads

of the Q-branches of the A^A—X^H, 2,2 band and

the C^2+—X-n, 0,0 band are shown in figures 7 and 17 in very slow scan to provide the maximum

resolution of the rotational structure possible

with the instrumentation used.

The rotation-vibration assignments of each of

the lines in the spectrum, as described in the refer- ences mentioned above, are marked above each

spectrum. The leadmg lines indicate the positions

of the lines, but no attempt was made to indicate

the relative intensities of the individual lines. On the charts containing the region from 3000 to 3500 A many Imes of the A^X+~X^Il transition of

OH appear, as these are readily excited in hydrocarbon flames. These lines are labeled as to P,

Q, or E, branch designation, but are not identified

in detail. For specific identifications one should

refer to the OH atlas.

Each graph has an intensity scale which indi- cates the relative intensities of the lines as compared with the very intense line near 4303.9 A

(R.eal, Ridcl, Qicl2, Q,dl2 of A^A—X2n(0, 0))

which was arbitrarily set at intensity 500. As

mentioned in the OH atlas, the relative intensities

of the various lines and bands are dependent on

temperature and emissivity in the particular source used, and so it is necessary to use with

care an atlas such as this to identify lines of the same spectrum produced in dift'erent sources.

1
3. Experimental Arrangement

The spectrum of CH was recorded by using a

high-resolution gratmg monochromator,^'^ with

a grating having 1200 groov^es/mm. The region

from 4700 to 5000 A was recorded in the first order at a scan rate of 1 A per min. The remaining large scale charts were recorded in the second

order at a scan rate of 0.5 A per min. The slow

scans of figures 7 and 17 were made at 0.1 A per

min. For the most part the resolution is sufficient to separate lines which are 0.1 A apart. The

spectra were recorded on blank paper, and the

figures are photographs of the origmal records.

The spectra were excited in an acetyleneoxygen flame burnmg on a water-cooled slot burner of dimensions 50 mm X 0.076 mm. Since

the excitation of the different transitions m the

* This instrument was constructed by the Research Department of Leeds and Northrup Co., and loaned to the Heat Division of the National Bureau

of Standards on a field-trial arrangement.

' W. G. Fastie, J. Opt. Soc. Am. 42, 641 (1952).

CH molecule, relative to the other features ol served in the spectrum, depends strongly on tl characteristics of the source, it was not possib

to keep the flame conditions constant in recordii

the entire spectrum. Generally speakmg, tl region between 4500 and 3600 A (that is part

the A^A—X^n transition, and the B^S"—

transition) were obtained from a stoichiometi

mixture of acetylene and oxygen. .In order

bring out the C^S+—X^II system relative to overlapping OH lines it was necessary to use

fuel-rich mixture (about 2.5 times stoichiometric

The long wave end of the A^A—^X^n system (4 to 5000 A) is best excited, relative to the

Swan bands in the same region, in very le mixtures (about 0.3 times stoichiometric).*

Washington, D.C, August 8, 1960.
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U.S. GOVERNMENT PRINTING OFFICE: 1961


THE NATIONAL BUREAU OF STANDARDS

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