Laboratory investigations of CN radical formation by photodissociation of parent molecules have suggested the possibility of observing emission lines in cometary spectra from newly formed CN radicals. These laboratory studies have shown that high initial internal excitation of CN is the rule with excitation of rotational levels N up to 70. In the collisionless environment of the cometary atmosphere this initial excitation would yield a corresponding distribution for the lowest vibrational level of the ground X(2) Sigma (+) state. Our calculations show that it is feasible with present observational techniques to detect photochemically excited lines with N approx. equal to 30 in the 0-0 band of the violet system

Cody, R. J.

Donn, B.

English

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https://ntrs.nasa.gov/search.jsp?R=19770025320 2020-03-22T08:20:34+00:00Z
X-691-77-188
ON THE DETECTION OF NEWLY
CREATED CN RADICALS AND COMETS
(NASI-TM-Y- 713891 CN ZEiE DETECTION OF NEWLY
N77-32264
CREAZED CN FALICAIS AID CCMEIS (NASA)	 15
15
HC ACi/MF A01	 Unclas
G3/25 48571
4. -
BERTRAM DONN
REGINA J. CODY
JULY 1977
GODDARD SPACE FLIGHT CENTER
GREENBELT, MARYLAND
(C-eleoa
9 s6 j
^
oe
Fi
0
ON THE DETECTION OF NEWLY CREATED
CN RADICALS AND COMETS
Bertram Donn and Regina J. Cody
NASA/Goddard Space Flight Center
Laboratory for Extraterrestrial Physics
Astrochemistry Branch
Greenbelt, Maryland 20771
t2
ABSTRACT
Laboratory investigations of CN radical formation by photodissocia-
tion of parent molecules have suggested the possibility of observing
emission lines in cometary spectra from newly formed CN radicals. These
laboratory studies have shown that high initial internal excitation of
CN is the rule with excitation of rotational levels N up to 70. In
•	 the collisionless environment of the cometary atmosphere this initial
excitation would yield a corresponding distribution for the lowest
vibrational level of the ground X2 E+ state. Our calculations show
that it is feasible with present observational techniques to detect
photochemically excited lines with N 30 in the 0-0 band of the violet
system.
3The mechanism(s) of formation of cometary radicals remains uncertain,
although several sources, particularly, photodissociation of parent
molecules, have been suggested (Whipple and Huebner, 1976). Recent
laboratory investigations (Cody et al., 1977; Sabety-Dzvonik et al., 1976,
1977) on the formation of CN radicals by photodissociation indicate new
spectroscopic cometary observations which could be used as a probe for
the formation mechanism of radicals. This laboratory work measured the
amount and the distribution of energy released into the newly formed
radicals by the photodissociation process. A summary of the experimental
method and results follows.
The experimental method consisted of flash-photolysis of the parent
compound followed by laser induced fluorescence detection of the CN
fragment (Cody et al., 1977). Two low energy (• l joule) flashlamps
with cut-off filters covered the photolysis region from 135 - 270 nm.
A variable time delay of a few microseconds was introduced between the
firing of the flashlamp and the triggering of an N 2-laser pumped dye
laser. The flashlamp and the laser were pulsed at a rate of 50 Hz, while
the dye laser was slowly scanned over the 8u = 0 band sequence of the
B 
2 
E + 4- X2 E+ system of the CN radical. The resulting fluorescence from
the excited radicals in the B state was detected photoelectrically. The
spectral resolution, (which was determined by the parameters of the dye
laser output) was about 0.02 nm and allowed detection of individual
rotational lines. By this method the relative populations of the
vibrational-rotational levels of the X state were directly monitored.
CN formation in the A( 29) electronically excited state was detected
indirectly. In the u' = 0 vibrational level of the A state there are
vibrational levels of the A state can be detected by crossover into
higher levels of the X state u > 4.
The photodissociative process was studied for a number of CN parent
molecules: hydrogen cyanide (HCN), methylcyanide (CH 3CN), cyanoactylene
-
	
	
(HC C-CN), dicyanoacetylene (NC-C = C-CN), cyanogen (NC-CN), and the
halogen cyanides (XCN where X = Cl, Br, I). In order to observe the
rotational and vibrational energy of the newly formed radicals in the
X state, experiments were done under low collision conditions, pressures
_-
	
	 P .-.10-80 V and short time delays 2 us. Fig. 1 shows a spectrum of CN
formed from cyanoacetylene under these conditions. For all parent
molecules which produced CN(X), the radicals were highly rotationally
and vibrationally excited. Therefore, rotational and vibrational ex-
=
	
	
citation of the product CN appears W,, be the rule. It would be expected
that CN formed in the A state would also be rotationally excited, since
there is ample energy left over from the photon after the dissociation
and electronic excitation have occurred.
For cometary CN radicals, the parent molecules HO and CH 3CH are
particularly interesting, since both were observed in Comet Kohoutek
(Huebner et al., 1974; Mich and Conklin, 1974). Our preliminary
4
laboratory studies as well as the work of others (West and Berry, 1974)
-
show that CN from these parents is formed predominately in the A state
in the two lowest vibrational levels.	 In the collisionless environment
-	 _	 - of the cometary atmosphere the rotationally excited CN(A) radicals would
i
impose through Red System (A-X) radiation a corresponding excitation
upon the ground state rotational levels.
	
In Fig. 1 rotational lines up
- to N ... 50 in the B-X violet system appear.
	
In spectra of CN formed from
= some of the other parent molecules, rotational levels with maximum
N	 60-80 were measured.
In high dispersion cometary spectra the maximum CN rotational
_- line has N .r 25 as shown in the spectrum of Comet Bennett in Fig. 2.
Detailed resonance fluorescence calculations of the CN(B-X) cycle in
Jcometary spectra have been carried out by Arpigny (1965).
	
Table I,
abstracted from the calculations of Arpigny (1965) shows the relative
populations of the higher rotational levels as a function of heliocentric
distance r.
	
The first row lists, in parenthesis, the level with the
maximum population and the fraction in that level. 	 The remaining rows
show the population of the indicated level.
	
We see from Table I that solar in-
duced fluorescence significantly populates levels beyond N = 25 within
about 0.5 AU.	 At 1 AU level N - 25 has << 1% of the peak population
_e
and near 1.5 AU level	 N = 20 has < 1% of the peak population. 	 High
_=
levels not affected by solar induced fluorescence can be used to detect
and study newly formed radicals.
We have calculated the expected relative intensities of the lines
N .r 30 from newly formed radicals as compared to the observed intensities
of lines N .r 20 from resonance fluorescence. 	 The ratio, over the entire
5
(coma, of the intensity of a high rotational line with N = 30 to that of
a line N = 20 is given by
= N=34) _ n3o 	 Q T30 /B
=N - 2O	 n.o r Q am f20
where the n's are the respective rotational populations in the X state,
u = 0 level; Q is the CN production rate from the parent compound with
lifetime T p ; T30 is the radiative lifetime of the level N - 30 and is
N 4 sec according to Arpigny (1965); ICN is the lifetime of the CN
radical against dissociation or ionization; 0 is the fraction of newly
created radicals which are formed in levels N 1 , 30; f20 is the fraction
of fluorescently excited 
C--^N^^ 
in level N = 20. The quantity f 20 was
evaluated from the ratio 1.p/f 1 , for the R-branch of the spectrum
N
of Aikman et al. (1974) and has a value of — 0.01. To obtain the
fraction of CN in level N - 30 we assume that only a radiative cascade
occurs for the newly formed CN fragments. This will be true throughout
the region of the coma where the dissociating ultraviolet radiation
penetrates. The dissociation process has a production rate q  into a
Nax
rotational level N and ? N = ^. Under the assumption that the
n=o
upper levels N z 30 are in a steady state and that a level with maximum
N(=M) exisrs. the following kinetic analysis can be performed:
ftM - q
M - nM TM-1 0	 (2)
nM TM- 1MgM	 (3)
i,M-1 
_ 
qM-1 + WHIM	 nM_l IM-1 = 0	 (4)
r
6
.d
-1
nM-1 TM-1 ° qM-1 + qM	 (5)
1	 1
nj qj + nj+1
	
	
(6)Tj+1 " nj Tj - 0
n - T F, q	 j =30
	 (7)
M
The fraction
	
OJ 
=16 4 is produced into levels N > J. The value of
.J v
was obtained by taking the ratiot-Tm j .
	
for the R-branch frompNa0
an unpublished investigation of cyanoacetylene and has a value of about
0.3.
The lifetime of the Ch radical TCN is taken as 105 sec. (Delsemme,
1976). Substituting the above numbers into equation 1 yields an intensity
ratio for the entire coma of I30/I20 - 10 3 . However, the photolytic
fragments are produced within a time -- T  - 10 4 sec and are contained
within a volume of • 104 km radius with the usual expansion velocity
of 1 km/s. For a characteristic projected slit length of — 104 km only
the CN radicals within this distance of the nucleus contribute to the
spectrum. A more accurate estimate of the CN molecules in a fluorescent
steady state which contribute to the spectra is obtained using 7 C - 104
sec. Therefore, for a spectrum the ratio I30/I20 becomes — 0.01. When
spectra are obtained at a heliocentric distance z 1 AU, lines with N - 25
may be used for the newly created radicals according to Table I. This
will increase the relative intensity ratio. We conclude that the re-
lative intensity ratio of lines from newly farmed CN to fluorescently
excited lines will be in the range 0.1 - 0.01. It is possible that
existing plates may reveal these high rotational lines, and a search
is planned using computer image processing techniques.
WIt appears likely that observations of these lines could be made
in a future bright comet by concentrating on the high rotational lines
rather than the entire CN violet system. Because of the greatly dif-
ferent lifetimes of photochemically and fluorescently excited CN radicals,
the spatial distribution of the two species will differ considerably.
	 i
It is also likely that other mechanisms suggested for CN production,
such as ion-molecule reactions (Oppenheimer, 1975; Shimizu, 1975) or
electrca currents (Ip and Mendis, 1977) will produce rotationally
excited CN. The spatial distribution of newly formed, rotationally
excited CN can be expected to vary with the production mechanism. Hence
the detection and intensity variation along the slit of high rotational
lines of CN may make an important contribution to the study of radical
production. This problem will be discussed in a later paper. A
similar analysis is expected to be applicable to other radicals e.g.
OH, NE and C 2 . A laboratory and theoretical study is planned for them.
i
W-, x	 m
B	 I
Flinnimm
TABLE
Mom
I
RELATIVE POPULATIONS OF ROTATIONAL LEVELS AS
FUNCTIONS OF HELIOCENTRIC DISTANCE
N/r (AU) 0.4	 0.6 0.8 1.0 1.4
N (15)	 .055	 (13)	 .066 (11)	 .076	 (10) .084 (8)	 .099max
15 .055	 .061 .056 .044 .021
20 .046	 .031 .015 .006 .0007
25 .023	 .006 .001 .0002 0
29 .006	 .0006 0 0 0
- 
&J,
REFERENCES
Aikman, G.C.L., Balfour, W.J., and Tatum, J.B. (1974). The cyanogen
bands of Comet Bennett 1970 II. Icarus 21, 303-316.
Arpigny, C. (1965). A study of molecular and physical processes in
comets. Acad. R. Belg. Mem. Cl. Sci. Collect. 8 0
 35, 1-157.
Cody, R.J., Sabety-Dzvonik, M.J., and Jackson, W.M. (1977). Laser-
induced fluorescence of CN(X2 E+) produced by photolysis of C 2 N2
at 160 nm. J. Chem. Phys. 66, 2145-2152.
Delsemme, A.H. (1976). The neutral coma of comets: a review. In
"The Study of Comets" (B. Donn, M. Mumma, W. Jackson, M. A'Hearn,
R. Harrington, eds.), pp. 711-732. NASA SP-393, U.S. Govt.
Printing Office, Washington, D.C.
Huebner, W.F., Snyder, L.E. and Buhl. D. (1974). HCN radio emission
from Comet Kohoutek (1973f). Icarus 23, 580-584.
Ip, W.-H., and Mendis, D.A. (1977). Proc. of IAU Colloquium No. 52,
"Interrelations among comets, asteroids, and meteorites"
(A.H. Delsemme, ed.), Univ. of Toledo Press, in preparation.
Oppenheimer, M. (1975). Gas phase chemistry in comets. Aetrophys. J.
196, 251-259.
Sabety-Dzvonik, M.J., Cody, R.J., and Jackson, W.M. (1976). Product
state distributions of CN(X) produced in the photodissociation of
C 4 N 2 at 160 rim. Chem. Phys. Lett. 44, 131-134.
Sabety-Dzvonik, M.J. and Cody, R.J. (1977). The internal state
distribution of CN free radicals produced in the photodissociation
of ICN. J. Chem. Phys. 66, 125-135.
a ^
Shimizu, M. (1975). Ion Chemistry in the Cometary Atmosphere.
Astrophys. Space Sci. 36, 353-361.
Mich, B.L., and !:nnki-._, E.K. (1974). Detection of methyl cyanide
in Comet Kohoutek. Nature, 248, 121-122.
West, G.A., and Berry, M.J. (1974). CN photodissociation and predis-
sociation chemical lasers: molecular electronic and vibrational
laser emissions. J. Chem. Phys. 61, 4700-4715.
Whipple, F . L., and Huebner, W.F. (1976). Physical processes in comets.
In "Annual Review of Astronomy and Astrophysics" (G.R. Burbidge,
D. Layzer, and J.G. Phillips, eds.), pp. 143-172. Annual Reviews
Inc., Palo Alto, Ca.
i
FfGURE CAPTIONS
Figure 1. Laser induced fluorescence spectrum (Violet System) of
2 +
CN(X E ) formed by photodissociation of cyanoacetylene
under low collision conditions.
Figure 2. The CN Violet (0,0) band in Comet Bennett (1970 II)
(Mount Wilson Observatory; original dispersion: 4.5 9/mm).
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On the detection of newly created CN radicals and comets

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