The absorption, emission, and photodissociation yield spectra of C2N2 were measured in the 220 and 210 nm region near the 4(0)1 and 1(0)1 4(0)1 bands of the A 1 sigma + from the X 1 sigma + system. The emission spectrum showed very few lines which appeared in the absorption spectrum. Moreover, the emission had 660 ns lifetime and, at 210 nm a very large electronic emission quenching rate. Laser induced fluorescence was used to measure the relative yield of CN radicals as a function of photolysis wavelength. This spectrum seemed to follow the absorption spectrum below the dissociation threshold. Energy in the CN fragments appeared to be statistically distributed

Barts, Samuel A.

Halpern, Joshua B.

English

NASA Technical Reports Server

N91"280 4
--.e/c
THE UV PHOTOCHEMISTRY OF C2 N2
Joshua B. Halpern and Samuel A. Barts
Department of Chemistry
Howard University
Washington, DC
ABSTRACT
The absorption, emission and photodissociation yield spectra of C2 N2 have been measured in the
220 and 210 nm region near the 401 and 1o1401bands of the AlE - _ XIz + system. The emission spectrum
showed very few lines which appeared in the absorption spectrum. Moreover the emission had 660 ns
lifctime and, at 210 nm a very large electronic emission quenching rate. We used laser induced
fluorescence to measure the relative yield of CN radicals as a function of photolysis wavelength. This
spectrum seemed to follow the absorption spectrum below the dissociation threshold. Energy in the
CN fragments appeared to be statistically distributed.
231
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https://ntrs.nasa.gov/search.jsp?R=19910018780 2020-03-19T16:43:07+00:00Z
THE UV PHOTOCHEMISTRY OF C2 N2
Introduction
Cyanogen is a photochemically important molecule found in the atmosphere of Titan, and perhaps
in other astronomical systems. We have discovered that this molecule has a strong and distinctive
emission spectrum that can be used for remote detection.
Cyanogen is also an extremely simple tetra-atomic system in which predissociation can be studied.
Potential energy surfaces have been calculated for all of the excited states which means that models of
dissociation dynamics can be tested in detail.
Experimental
The apparatus was similar to that described in Reference [1]. C2 N2 from Matheson was purified
by freeze-thaw cycling. The gas flow from a 3 liter reservoir was controlled by a Granville-Phillips fine
metering valve and introduced into the cell through a hypodermic needle. The pressure was monitored
by a MKS Model 200 capacitance manometer of a cold cathode gauge.
The excitation source was a Spectra Physics PDL-2 dye laser pumped by a DCR-11 Nd-YAG
system. Light from the dye laser was frequency doubled in a KDP crystal and Raman shifted in about
10 atmospheres of hydrogen gas. For this experiment third andfourth anti-Stokes shifted beams were
used. Since the doubled light is 100% polarized, all of the Raman shifted beams will be fully polarized.
The intensity of the dyelaser beam ranged between 25 and 60/H depending on the dye gain curve and
dye used. The energy contained in each pulse of the photolysis beam ranged from a few to perhaps a
hundred microjoules.
Emission from C2 N2 was detected directly through an unfiltered 11 stage EMR VUV photomul-
tiplier. The signal was integrated by a PAR Model 160 boxcar analyzer.
CN fragments were detected by laser-induced fluorescence (LIF) excited by Molectron DL-II dye
laser pumped by UV400 Nitrogen laser. The roughly 200gJ beam was expanded, attenuated by neutral
density glass filters and passed through a thin film glass polarizer to eliminate any elliptical character.
Care was taken that the LIF signal was unsaturated. The electric vector of this dye laser beam was
parallelto that of the photolysis laser. The overlapping laser beams passed through the experimental
cell in opposite directions. The LIF signal was monitored about 100/_s after the photolysis pulse. This
delay eliminates the effect of-any changes in the nascent C-N distribution as the photolysis wavelength
was changed. The nitrogen pumped dye laser was tuned to resonance with the R(7) line of the CN
B2y. + *- X2_ + violet system. LIFwas monitored by an EM19558 photomultiplier tube looking through
a 388 nm bandpass filter. The photomultiplier was placed perpendicular to the plane of the laser beams
and the electric vector of the photolyzing light. The fluorescence signal was integrated in one channel
of a PAR Model 162 boxcar analyzer. The intensity of the dye laser beam was measured by picking off
a reflection and sending it into a photodiode.
The experiments were controlled by an IBM P_ microcomputer based data acquisition system
constructed in our laboratory. The firing of the lasers and the dye laser scanning was controlled by the
microcomputer.
232
Absorption spectra were measured directly on a Cary 2390 UV-VIS-NIR spectrophotometer or
by measuring the far UV light intensity before and after the cell using scintillators to convert the light
frequency into regions that can be detected by two Si photodiodes.
Results
Figure 1 shows an absorption spectra measured on the Cary 2390 spectrophotometer between 200
and 230 nm. The lower half of the diagram shows the difference between spectra measured at 20 and
70°C when the cell was heated by water from a thermostated bath. This identifies lines that originate
in excited vibrational states and aids in the assignment of the vibrational bands. Figure 2 shows an
absorption spectrum taken in the 220 nm region with a higher resolution instrument [2].
1.0
0.0
,0.10
0.00
-0.05
Cyano! en Absorplion Spectrum
I i I I
.o.  oO0,,,e,eoceSOeo.,om".o."BaoOs
old"_a_ds I l
48,000 46,000
FREQUENCY cm-1
Figure 1
233
1
4 0
0 2
222 220
WAVELENGTH (nm)
Figure 2
Figure 3 shows the spectrum of total emission following laser excitation in the 220 nm region.
Attempts to detect CN-t'ragrnents from phot_issociation Were unsuccessful although we were able to
detect fragments if we used the much weaker 200 nm fourth anti-Stokes beam. This confirms the bond
energy measurements of Eres and McDonald [3] which predicted that the dissociation threshold would
be 212.76 nm. The emission spectrum does not match the absorption spectrum in all detail. The ratio
of intensities between the 401 and 412 bands is somewhat different from that seen in absorption as is the
shape of the bands. The emission lifetime obtained at the peak of the 401 band is 660 ns. Figure 4 shows
the fluorescence spectrum obtained when the dye laser is f'Lxed on the Q bandhead of the 401 line.
_1_1 ABSORPTION
3x10 -19 cm 2
ZtQWj
O
L____I .... I .... I .... ,I
219.0 219.5 220.0 220.5
WAVELENGTH (nm)
Figure 3
234
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¢..
n
tic
v
Iii
O
Z
1.1.1
O
CO
III
O
..A
LI_
15
I0
5
0
I I I
0 1
1n4ol I I
0 1
ln4_ I I
- 03041 i i
] 4U,-'_,_,A |
t 1___ !
I I
I I
I I
1 I
210 23O 290
1. 1 I
250 270
Figure 4
Figure 5 shows the absorption spectrum measured between 206.4 and 211 nm. Figure 6 shows the
yield of CN fragments in the same region. The relative yield of CN was measured as a function of
photolysis wavelength by (LIF). The photolysis yield generally follows the absorption spectrum as would
be expected in any predissociation. Figure 7 shows the emission spectrum measured in the same region.
Only a few peaks are observed. Note that the signal from the 10'401 band extends far above the top of
the scale. The insert in Figure 7 shows the ..... band attenuated by a factor of 50. Structure can be
seen on the red side of the emission at higher gain. We have measured the lifetimes of the emission
excited in this band as a function of pressure. A Stern-Volmer plot of the measured decay rates is shown
in Figure 8.
28
20
10
0
206 208 210 212
Figure 5
235
mnc
V
...J
<
Z
O9
Ii
m
_J
Z
o
28
20
10
B
m
0
206 208 210 212
WAVELENGTH (nm)
28
m
m
20 -
10
0
206
Figure 6
I _ 1
208 210
WAVELENGTH (nm)
212
Figure 7
236
8o_ 7
"-'I
,," 6
5
!.1.1
4
n
i--- 3
LI.i
It.
-i 2
"- 1
0 , I L I ___l , I j I L____ I I I ,
0 20 40 60 80 100 120 140 160 180
CYANOGEN PRESSURE mtorr
Figure 8
Figure 9 shows the fragment yield spectrum between 200 and 202 nm corresponding to excitation
of the 102401transition. The right hand half of Figure 10 shows the emission spectrum in the 102401 region
as compared to that from the 401 region. There is a factor of several thousand difference in the intensities
of the emissions.
12
II
in
C yanogen Fragment Yield
IJF on Rt7)
Figure 9
237
3rd Anti-Sto_kes 4th Anti-Stokes3O
c
O
2O
.nE
I.IJ
nt-
10
0
1
40
1000 X Gain
Scan ! Scan2 Scan3 Scan4 Scan5
Figure 10
Discussion
Figure 11 is a linear correlation diagram from Reference 4. These observations can be qualitatively
explained with the following model: C2 N2 is excited from the ground XI_: + state into the AI_ - state.
At excitation energies below the dissociation limit the molecule simply fluoresces and returns to the
ground state. This is shown conclusively by the fluorescence spectrum as explained in Reference 1.
After excitation above the dissociation limit there is a competition between fluorescence, dissociation
(where energetically possible) and crossing over to a lower triplet system. Emission can also occur from
this triplet system back down to the ground state. The correlation diagram indicates that the AI_ - state
cannot directly dissociate to ground state CN fragments in the linear configuration, _ile this restriction
would be relaxed in a bent configuration it is also possible that dissociation occurs through a crossing
to the a3 Eta+ system which correlates with ground state fragments. The barrier in the linear configura-
tion can be avoided if the molecule bends.
Future experiments will attempt to elucidate this mechanism and add quantitative detail to this
qualitative model. Experiments to be done include measurement of the fluorescence spectra below the
dissociation limit. This will allow us tO elucidate the-me_chanism of dlssoc_iation. Second, we will compare
the intensity of emission with that from known pressures Of NO, allowing us to calculate quantum yields
for all processes. This will allow us to map the state density of the emitting triplet state and provide a
clock for measuring photodissociation rates.
238
- (
8-
7-
¢_ _, x..'_.-- A'n
i "l
ORIGINAL PAGE IS
OF POOR QUALITY
C2N i - 2 CN
Figure 11
Conclusion
The study of cyanogen C2 N2 photolysis has produced several important results. First, significant
emission was observed from C2 N2 following excitation to both bound and unbound states. This emission
has not previously been reported. If, as has been suggested, cyanogen is a minor constituent of
astronomical objects the intensity of this emission would be the measure of the cyanogen concentration.
Second, we made initial measurements of the competition between emission and dissociation.
Acknowledgments
Parts of this work were supported by NASA under Grant NAG-5071 and a grant of money from
the Howard University Research Development program. S.A. Barts was supported by a Danforth
Foundation Fellowship.
1. J.A. Russell, I.A. McClaren, W.M.Jackson and J.B. Halpern, J. Phys. Chem., 91 (1987)
3248.
2. Taken with the assistance of Dr. Alan Laufer at NIST.
3. D. Eres, M. Gurnick and J.D. McDonald, J. Chem. Phys., 81 (1984) 5552.
4. Dateo, C.; Thesis, University of California, Berkeley, 1987.
239


The UV photochemistry of C2N2

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