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The recombination of nitrogen atoms
Volume 4. number 1
CHEMICAL
THE
PHYSICS
RECOMBINATION
OF
LETTERS
NITROGEN
15 September
ATOMS
1969
*
EDWARD C. SHANE and W. BRENNEN Department of Chmisb-y, Universib of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
Received 21 July 1969
Preliminary results of a study of nitrogen atom recombination in a static system are summarized. The third-order homogeneous recombination rate coefficient is found to be pressure dependent at low pressures. Real and apparent wall effects are evaluated quantitatively.
studied in a nitrogen
We have
atoms
the recombination of nitrogen buffer in the pressure range
0.03 to 5.3 Torr by recording the decay of the intensity of the (11, 7) first positive band of N2 in a static sample of active nitrogen in a 3.1 liter Pyrex bulb, using a narrow-band interference filter and an uncooled photomultiplier tube. Since our early pressure-dependence measurements [l], we have abandoned the traditional use of syrupy phosphoric acid wall poisoning because more consistent results and longer decays could be had by washing with fuming red nitric acid and rinsing with distilled water, 10% aqueous HF, and again distilled water. Typically, the times required for the afterglow signal to decay to the room temperature dark current level at 1 Torr, 0.5 Torr, and 0.1 Torr were 1 hr, 1.3 hr, and 2.3 hr. respectively. We established the relationship between observed afterglow intensity, Z, and the volume density of nitrogen atoms in the bulb, [N], by means of the NO titration at pressures of 1 or 2 Torr, and then used our results on the pressure dependence of Z to relate Z to [N] at any pressure by means of the formula
I=
K[N12 [Nz]@T&j]
+ 1)-1 ,
where K is a constant evaluated from NO Qtrations and krg = (2.8*0.4) ): lo-18 CC molec-1 set-1, which is a refinement of our earlier value PI.
The afterglow decays were analyzed in terms of the differential equation
* This work is supported by the Advanced Research Projects Agency, Contract SD-69.
- q=
(kl[N2]+k2)[N]2
+ k3LNI-
0)
The solution of eq. (1) when combined with the proportionality of Z to [N]2 gives the following equation governing the decay of light intensity ” ’ = (1 +A) exp(k3t) - A 2= ( T- >
(2)
where Zo is the intensity at f = 0 and A = k$[NJo(kl[N ]+k ), IN&, being the value at t = 0. Values of K3 ani A were obtained graphitally by approximating d.z/dt by AZ “AI over one minute intervals and plotting ln(Az,‘At) against t. The slope of such a graph gives k3 and the intercept gives ln(k3A + kg). Knowing [N]o from Z,, the coefficients of both [N]2 and [N] on the right side of eq. (1) can be obtained According to eq. (1) and the definition of A, a graph of k3A/[N], versus [N2] should be a straight line with slope kl and intercept k2. We find instead that such a graph of our data for 37 runs Is distinctly curved concave to the abscissa and, within our experimental error, passes through the origin. The zero intercept means, conservatively, that k2 c 1 x 10-15 cc molec-1 see-1. The curva ure of th graph means that ! -‘i +k2) is not constant but dokl = [N21- &&NJ, creases with increasing nitrogen pressure as shown in fig. l**. In calculating ordinates of the points in fig. 1 we took k2 = 0, since we cannot experimentally distinguish it from zero***. ** Measurements over a limited pressure range could
easily lead to too low values of kl and non-zero va,Lues of k2 if fitted with a strai&t line on a graph of kgA/[N], versus [Nz]. *** Footnote see next page.
31
CHEMICAL PHYSICS
Volume 4, number 1
LETTERS
efficient,
15
September 1969
should have the form
where kG, k , a, and a’ are constants. The form of kl in eq. 3 can be made to fit the experimen-
PI
L
OQ
I
I
1
1
3 2 PRESSURE (torrk
I
4
5
fal: results of fig_ 1 fairly well with reasonably chosen constants which are consistent with the pressure dependence of I. The results in fig. 1 are iudepender,f evidence that the afterglow pathway accounts for a substantial fraction of recombfnatioll eve&s. We found that RQ, the apparent first order wall decay coefficient, increased with increasing pressure. Experiments with small amounts of 02 added te the static afterglow bulb confirmed quantitatively that this effect is due to pseudo-first order homogeneous destruction of nitrogen atoms by the slow reaction l!X4. c$ -) ?I%&-L.Q W& ;_i;s%%%Y.q~fiW. T&5= Ya&e @.f45%G?x.t%%?U&ts, %%Y&-$WW~~ ?impzies that y = (tLOfL4j x lo-8 for our bulb. This value
is an upper
6-14~‘Liwrior
lhit
for
our
of our apparatus
areas which either could nat
The assumption
that nitrogen atoms recombine
homogeneouslsly eifCulr iatJ3 f;lrp,~XYJ.R& sQ~k8 VL QGg directly or into the A%: state by a mecknism like that proposed barCampbell and Thrush f!Z]
leads to the prediction that kl, the effective third order homogeneous recombination rate co**+ A vsfue of k2 arising from rz~dtdiative ~c~ornbi~at~on is iikefy to be of order 10-19 cc moiec-1 SW-1. To see this it would be necessary to do expcri;nents at pressures two orders of magnitude lower than ours.
likely to be especially
surface
contains
because
SrnalL
be cleaned or are
active in destroying nitrogen atoms. The equivalent of’ c&y 5 mn22 of SWface WK~Iy = IINt for wamp&, erm’id account for all our observed first order waXI decay. REFERENCES [If W. Brennen and E C.Sbane; Chem. Phys. Letters 2 (1968) 143. [2] I. M. Campbell and B.A.Thrush, Proc. Roy.Soc.
A296 (1967) 201.
CHEMICAL
THE
PHYSICS
RECOMBINATION
OF
LETTERS
NITROGEN
15 September
ATOMS
1969
*
EDWARD C. SHANE and W. BRENNEN Department of Chmisb-y, Universib of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
Received 21 July 1969
Preliminary results of a study of nitrogen atom recombination in a static system are summarized. The third-order homogeneous recombination rate coefficient is found to be pressure dependent at low pressures. Real and apparent wall effects are evaluated quantitatively.
studied in a nitrogen
We have
atoms
the recombination of nitrogen buffer in the pressure range
0.03 to 5.3 Torr by recording the decay of the intensity of the (11, 7) first positive band of N2 in a static sample of active nitrogen in a 3.1 liter Pyrex bulb, using a narrow-band interference filter and an uncooled photomultiplier tube. Since our early pressure-dependence measurements [l], we have abandoned the traditional use of syrupy phosphoric acid wall poisoning because more consistent results and longer decays could be had by washing with fuming red nitric acid and rinsing with distilled water, 10% aqueous HF, and again distilled water. Typically, the times required for the afterglow signal to decay to the room temperature dark current level at 1 Torr, 0.5 Torr, and 0.1 Torr were 1 hr, 1.3 hr, and 2.3 hr. respectively. We established the relationship between observed afterglow intensity, Z, and the volume density of nitrogen atoms in the bulb, [N], by means of the NO titration at pressures of 1 or 2 Torr, and then used our results on the pressure dependence of Z to relate Z to [N] at any pressure by means of the formula
I=
K[N12 [Nz]@T&j]
+ 1)-1 ,
where K is a constant evaluated from NO Qtrations and krg = (2.8*0.4) ): lo-18 CC molec-1 set-1, which is a refinement of our earlier value PI.
The afterglow decays were analyzed in terms of the differential equation
* This work is supported by the Advanced Research Projects Agency, Contract SD-69.
- q=
(kl[N2]+k2)[N]2
+ k3LNI-
0)
The solution of eq. (1) when combined with the proportionality of Z to [N]2 gives the following equation governing the decay of light intensity ” ’ = (1 +A) exp(k3t) - A 2= ( T- >
(2)
where Zo is the intensity at f = 0 and A = k$[NJo(kl[N ]+k ), IN&, being the value at t = 0. Values of K3 ani A were obtained graphitally by approximating d.z/dt by AZ “AI over one minute intervals and plotting ln(Az,‘At) against t. The slope of such a graph gives k3 and the intercept gives ln(k3A + kg). Knowing [N]o from Z,, the coefficients of both [N]2 and [N] on the right side of eq. (1) can be obtained According to eq. (1) and the definition of A, a graph of k3A/[N], versus [N2] should be a straight line with slope kl and intercept k2. We find instead that such a graph of our data for 37 runs Is distinctly curved concave to the abscissa and, within our experimental error, passes through the origin. The zero intercept means, conservatively, that k2 c 1 x 10-15 cc molec-1 see-1. The curva ure of th graph means that ! -‘i +k2) is not constant but dokl = [N21- &&NJ, creases with increasing nitrogen pressure as shown in fig. l**. In calculating ordinates of the points in fig. 1 we took k2 = 0, since we cannot experimentally distinguish it from zero***. ** Measurements over a limited pressure range could
easily lead to too low values of kl and non-zero va,Lues of k2 if fitted with a strai&t line on a graph of kgA/[N], versus [Nz]. *** Footnote see next page.
31
CHEMICAL PHYSICS
Volume 4, number 1
LETTERS
efficient,
15
September 1969
should have the form
where kG, k , a, and a’ are constants. The form of kl in eq. 3 can be made to fit the experimen-
PI
L
OQ
I
I
1
1
3 2 PRESSURE (torrk
I
4
5
fal: results of fig_ 1 fairly well with reasonably chosen constants which are consistent with the pressure dependence of I. The results in fig. 1 are iudepender,f evidence that the afterglow pathway accounts for a substantial fraction of recombfnatioll eve&s. We found that RQ, the apparent first order wall decay coefficient, increased with increasing pressure. Experiments with small amounts of 02 added te the static afterglow bulb confirmed quantitatively that this effect is due to pseudo-first order homogeneous destruction of nitrogen atoms by the slow reaction l!X4. c$ -) ?I%&-L.Q W& ;_i;s%%%Y.q~fiW. T&5= Ya&e @.f45%G?x.t%%?U&ts, %%Y&-$WW~~ ?impzies that y = (tLOfL4j x lo-8 for our bulb. This value
is an upper
6-14~‘Liwrior
lhit
for
our
of our apparatus
areas which either could nat
The assumption
that nitrogen atoms recombine
homogeneouslsly eifCulr iatJ3 f;lrp,~XYJ.R& sQ~k8 VL QGg directly or into the A%: state by a mecknism like that proposed barCampbell and Thrush f!Z]
leads to the prediction that kl, the effective third order homogeneous recombination rate co**+ A vsfue of k2 arising from rz~dtdiative ~c~ornbi~at~on is iikefy to be of order 10-19 cc moiec-1 SW-1. To see this it would be necessary to do expcri;nents at pressures two orders of magnitude lower than ours.
likely to be especially
surface
contains
because
SrnalL
be cleaned or are
active in destroying nitrogen atoms. The equivalent of’ c&y 5 mn22 of SWface WK~Iy = IINt for wamp&, erm’id account for all our observed first order waXI decay. REFERENCES [If W. Brennen and E C.Sbane; Chem. Phys. Letters 2 (1968) 143. [2] I. M. Campbell and B.A.Thrush, Proc. Roy.Soc.
A296 (1967) 201.