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Vibrational excitation of N2(C) and N2(B) by metastable argon atoms and the determination of the branching ratio
CHEMICAL
Volume 53. number 2
VIBRATIONAL
EXCITATION
AND THE DETERMINATION M. TOUZEAU
OF N,(C)
PHYSICS LETTERS
AND N2(B) BY METASTABLE
1.5 January
1978
ARGON ATOMS
OF THE BRANCHING RATIO
and D. PAGNON
Laboratoire de Physique des PIasmasf-, Universit6 Parissud. Centre d*Orsay. 91405 Orsay, France Received 15 September 1977 Revised manuscript received 27 September 1977
The reaction rates k&‘) for N,(C. u’) (v’ = 0, 1.2,3) and a,(~“) for Nz(B, u”) (0 4 0” < 11) of the reaction: AI<~P~,o) + Nz 4 N2(C, u’) + Ar(kC(u’));-f N2(B, u”) + Ar(kR(u”)) were measured in a Iow pressure flowing afterglow (0.1 to 2 torr). Ihe electronic branching ratio between N2 (B) and N,(C) was determined to be : kB/kC = 0.25 + 0.15 [kg = E,nfkB(v”), kC = Z,ekC(u’)]_ It is shown that it is necessary to take into account both the quenching and the vibrational relazzation of Nz(B). The vibrational relaxation rate k,(l) is determined to be 6.5 X lo-l2 cm3 s-I_
1. Introduction The excitation of nitrogen by argon metastable atoms has been studied by several different techniques during recent years. These techniques include flowing afterglows [ 1,2], afterglows [3--51 and molecular beams [6-g]. In the experiments, nitrogen is excited to two different electronic levels N2(C 3 II,) and N,(B 31Jg) by collision with argon metastables. Good agreement exists between the experiments for the total reaction rate and the vibrational distribution of N2(C 3 II,). However for the excitation of N,(B 3 llg) the results are quite different_ For the branching ratio between N,(B) and N2(C) Setser measured in a flowing afterglow k,/kC = 6 + 1 [l] _ In a similar experiment we found an upper limit of 1.3 [lo]. In an ArLN2 electron beam laser, Puech [ 1 1] observed for the first time a coherent emission of the N2(C, 0) + Nz(B, 0), 3371 A, line. He was forced to conclude from a kinetic analysis that almost all the excitation leads to the N2(C) state in the Ar*-N, transfer_ Krenos and Be1 Bruno [9] found that only 1.5% of the B state is created by direct excitation-for the Au = 2 band (2 < u” < 6), the remainder is formed by C-B radiative cascade. Finally a redetermination of the branching * J_aboratoire associd au C.N.R.S.
ratio by Setser’s group [12] using the ArCP2) + Kr reference reaction gave a value of kg/kc = 1 + O-2. This paper presents the determination of the values of the direct excitation rates kg(u”) of N2(B, u”) by metastable argon atoms. The measurements are made in a low pressure flowing afterglow.
2. Experimental The flowing afterglow appsratus which has been utilized has been described elsewhere [lo] _Metastable atoms are created k a low current positive column (1 mA) and are drawn along a 5 cm diameter flow tube with a speed typically of 50 m s-l. The argon pressure (0.1 to 2 torr) is proportional to argon flow rate. Nitrogen is added 20 cm downstream from the discharge where the metastable Ar(3P2) concentration is typically of 1010 cm -3_ The relative concentrations of the other metastable and resonant states are: [3P,]/[3P,] x0.15, [3P*]/[3P2]
Volume 53, number 2
CHEMICAL
15 January 1978
PHYSICS LETTERS
mediately after the injection of N2 with a 60 Wmrn monochlomator between 3 100 A and 10800 a. We have used two cooled photomultipliers, a “trialcali” for the spectral range, 3 100 _k to 8000 A and a “AgOCs photocathode” in the red and infrared region. The optical system was calibrated tith a standard tungsten lamp.
into account quenching reactions for this model. The with argon pressure suggests an increase of N2(B, 9) density by vibrational relaxation. So we must consider the following reactions: increase of RuA(0)/RcB(O)
-kc@‘) * N2(C, u’) f Ar , kg(u”) N2(B, v”) + AI-, f N2C-W -
Ar* + N2W
(1)
k*
(2)
&.B#, N2(B, u”) + hv”.,,,. , N2(C, u’) =
3. Model We have shown [ 101 that it is possible to deduce the direct excitation rate ku(v”) from the measurement of the ratio RBA(u”)[RCB(~“).RCB(U”) is the ffux of photons cascading to the N&I, u”) level from the C state and RBA(u”) the flux of N2(13, u”) cascading to the A state. The decrease of the ratio RnA(u~)/RCB(u”) with the argon pressure (fig. I) shows that it is necessary to take
(3)
BA A “,,“,,,
NZ(A, u”‘) -r-?z~~~,u,.,,
N2(B, u”) -
(4)
4Ar(u”)
N2(B, u”) + Ar ~__f
N2(??) + Ar , 4@J”) N&3: u”) + N, ____+ N2(?) f N, , k,W’) N2(B, u”- 1) + Ar . N&3, u”) + Ar e
(6) (7)
Along with the equilibrium equation for N2(C, u’): i&G
~‘11M-J’) = k$~‘) Pr*I
&I
,
(8)
we calculate the vahre of the radiative flux: 3 &&“)
= gO A$$
[N2C
~‘11 ,
(9)
as a fictitious reaction with metastable atoms: QB (u”) = K(n”) LAr* I ]N2 I ,
(10)
with a fictitious reaction rate K(C), Iqu”) = c k&)A~~. u’
T(t.?‘).
(11)
7fu’) is the lifetime of N2(C, u’), T(u’) = 1/z&;.
(12)
.
U” The
steady state for N2(B, 0”):
R&u”) 0.5
1
15
2
PA,[Torr]
Fig. 1. Argon pressure dependence of the photon fiw ratio RBA(u”)/RCB(u”). RCB(u”) is defmed in eq. (9); a similar relation holds for RB*(LJ”)_ The reactant flow rate [N2] is Twed at [Nt] = 1 Qh-1 NTP. Argon pressure is monitored by argon flow rate ([Ar](Q h” NTP) = 700 pi (to&)_
356
+ kB (u”) [Ar* I lEJ2 I
+ k,(u"f l)[Ar] ]N2(B, u” ; l)] = R &u”) + @‘)
+ -&&“) I-W 1 IN,@,
k]
.+ 4N2(u”) [N, ]
u”N ,
produces the relation&i@:
(13)
Volume 53, number 2
CHEMICAL PHYSICS LETTERS Table 1
RBA(u”) (1 + [q&(u”)
+ k,(u”)]
RC&“)
+ qN2
@“iLN2i +“>,
=I+
kg@“) n + k&i’ + 1) Kg K(u )
X +n
f 1) RBA(“” R&u”+
= 1 +
Vibrational distribution of N2(C)
[Ar]@“)
‘;,‘)
+ I) 1) lAr’ ’
At zero pressure this expression RBA(U”)/&B(U”)
15 January 1978
(14)
11-z lb
0.21 0.23 0.16 0.25 0.25
0.05 O-05 0.05 0.06 0.018
IL-3/b 0.007 0.005
a) For 0.048 eV :ransIational energy. b, Ar(3P2) and Ar(3Po) are assumed equally reactive and present in a statistical distribution [3P2]/[3Po] = 5. In fact the reactivity of 3Po is less than that of 3P2, k(3Po) = 1.7 X lo-" cm3 s-’ (b CaG et al. [3]).
reduces to
kB(u”)/K(u”)
flowing afterglow [ 1] our results aftergIow [51 beam experiment [9] a) golden rule cahdation 19Jb)
klih
.
4. Results
tional to the vibrational concentration N2(C, u’)_ The measurement of the vibrational distribution allows the comparison of K(u”) and kc(O):
4. I. Vibratiohal distribution of N2{C) From the second positive system, the vibrationai distribution of N2(C, u’) has been determined_ This distribution is independent of the argon and nitrogen flow rates and pressures. The results are tabulated and compared with other experiments in table 1. The different types of experiments give results in remarkable agreement_ The beam results [9] are for 0.048 eV ‘translational energy. With increasing energy the ratio, [N2(C, l)]/[N,(C, O)] also increases [13]. The lifetimes T(u’) of the different vibrational levels are approximately equal, thus the reaction rates kC(u’) are propor-
W-J”) = P(u”)~c(O)
,
(16)
with = (Aoum + 023 A Ium+ 0.05 Azu,,
fi”)
+ 0.005 f+)‘(U”)
_
(17)
These values oiP(v”) are listed in table 2. 4-L
Vibrational relaxation of N?(B) This effect is illustrated in fig. 2. The N(B, O)-
Table 2
Direct excitation rates of N2(B, u”) by me&table
argon atoms
PGJ”)
RBA~“)/RcB(V”)
102k,(u")/kc(0)
( ")3 iO=k Bu (cm3 S-I)
0
0.64
1.08
1 2 3 4 5 6 7 8 9
0.35 0.17 0.079 0.034 0.013 4.8 -3
1.14 1.40 1.58 2.1 2.85 4.25
5.1 5.1 6.9 4.6 3.7 2.5 1.55
l-6-3
7.40
1.01 0.77
1.4 f 1.4 f 1.9 5 1.3 f 1.0 i 0.7 + 0.43 0.28 0.21
0.80 0.73 0.80
0.22 0.20 0.22
U”
10 11
4.7 -4
l-l-4 3.6-5 7.3 -6
17.4
72 206 1100
2 1.2 0.7 0.3 0.2 0.1
lo2 kB(u")/kc(0)b)
2-l 4.8 5.8 5.3 3.8 2.4
1.4 0.8 0.4
0.2 0.05 0.04
a) The kg@") valuesarededuced from kg(v")/kc(0) values with a to+kdmeasured reaction Iate k = 3.6 X lo-” b, Golden rule calculation (see the text) with P(E%)/p
cm3 s-l.
357
Volume 53, number 2
CHEMICAL
PHYSICS LETTERS
0
1
-2
‘
3
Fig. 3. Determination of the viirational relaxation rate k,(l).
of the lbs of eq. (18) with [RBA(l)/ [Ar] _ From eq. (17), the extrapolation to zero pressure gives the value of kg(0)/kC(O) and the slope indicates the value of k,( I), in this case equal to 6.5 X lo-12 cm3 s-1 _This rapid vibrational relaxation could be expIained by a non-adiabatic mechanism proposed by Nikitin [ 171 as suggested by Gartner and Thrush [16] _ In this process, vibrational radiationless transition occurs between the components of the ll state which have been split apart in the collision. Such high cross sections of vibrational relaxation have been observed for N2(C 3iIu, u’ = 1 + u’ = 0) [18] _ the variation
RcB(l)]
85L3A (3-2)
8723 A 12-l)
10510A
8912 A 11-O)
to-01
Fig. 2. Spectrum of the emitted near infrared second positive system of Nz for an argon pressure of 0.15 torr and 2 torr
(Iinearscale).
N(A, 0) 10510 A line is more intense with increasing argon pressure. Detection of vibrational relaxation is very sensitive between v” = 1 and v” = 0 becailse the state N,(B, 0) can be depopulated only by a weak argon quenching: 4&(O) = 4 X lo-l3 cm3 s--I for the W 3Au and A3ZIi levels [IS] _ In consequence a vibrational depopulation of N2(B, u” = 1) produces an increase of N2(B, u” = 0). The evolution of RBA(0)/ R&O) with argon pressure is given by eq. (14)_
RBA(0) kB@) fl(l)RBA(l) = 1+kC(0)fl(O) + ‘?J(‘) fl(0)RCB( 1)
CAr1 -
u3
Because of the weak partial pressure and quenching rate of nitrogen (qN2 (0) = 2 X lo-l2 cm3 s-l) the quenching of N2(B, 0) has been neglected_ The values of /3(O) and p(1) are given in table 2. Fig. 3 presents 358
4.3. Direct vibrationalexcitation of lV2/.,
v”j
The extrapolation, to zero pressure of the curves RBA(u”)&B(v~) (fig. I) gives the value of kg($)/ kc(O) as:
k&J”) kc@)
C
=B(v”)
The results are presented in table 2 and displayed in fig. 3. The errors in k&v”) arecalculated from an estimate of the photon flux ratio A(RBA/R~B)/(RBA/R~) of 10% and A@(v”)/p(v”) between 5% and 10% for v” > 8_ (The error Aj3(v”)/P(v”) is calculated from the error in the determination of the vibrational distribution of the C state.) The direct excitation of Nz(B, v”) by argon (a few percent of the dir&t excitation of the C state) has little effect on the ratio of the radiative flues (.&&B * i) for low levek of Nz(B), but is important for higher levels (RBA >R,$
Volume 53, number 2
because the radiative cascade from N2(C) populates
principally the low levels of N2(B). Consequently the determinatiqn of kB(v”) is excellent for higher values of v”. 5. Discussion We have compared our results (fig_ 4) with the calculated values of the golden rule model proposed by Krenos and Be1 Bruno. In this model, l Jle transition probability Wii is proportional to the product of the Franck-Condon factor qii and the density of states pu. For the atom + diatom case pu is given by [lV] P”(E)
a?;’ 141 -f”)13’2 ,
15 January 1978
CHEMICAL PHYSICS LETTERS
cw
where B; is the rotational constant and f, is the fraction of the available energy channeled into a particular vibrational level. With this model, we have calculated the distribution among the vibrational level N,(B, u”):
q(v’), q(v”) are respectively the F&r&-Condon factors of the C and B transition [20]. The ratio of the rotational constants is Bt/Bz = 1.12. The available energy for the C and B states is respectively taken as ec = 0.63 eV (exoergicity f translational and rotational energy) and eB = 4.3 eV. The ratio of the electronic transition probabilities (which is unknown) is adjusted so the calculated distribution, kB(v”)/k&) [eq. (21)] fits with the measured values
Fig. 4. Values of the direct excitation transfer rates kg@“) by metastable argon atoms compared with the excitation rate of N2 (C, u’ = 0). Comparison with the golden rule calculated
(fig_ 4) The best fit is obtained for p(C)/P(B) = 116. These values are reported in table 2. For 8 < v” < 11 the experimental values of k&u”) are higher than the predicted values, suggesting a possible direct dissociation of N2 by metastable atoms with a recombination of the atomic nitrogen in high vibrational levels 8 < v” < 12.
6. Conclusion The utilization of a low pressure flowing afterglow has verified that, in the collision with metastable ar8on atoms, nitrogen is principally excited to the C 3iI,, state. Low vibrational levels of the B state (where most of the radiative energy occurs) are populated by radiative cascade, but direct excitation from metastable argon atoms is dominant for v” > 5 levels. The measured vibrational distribution kg(v”) of the B state for direct excitation rate by metastable argon atoms is in good agreement with a golden rule model as for ‘the distribution kc(u’) of the C state.
References Ill D-W. Setser, D.H. Stedman and J.A. Coxon, J. Chem. Phys. 53 (1970)
1004.
121 L.G. Piper, I.E. Velazco and D-W_Setser, J. Chem. Phys.
59 (1973) 3323. r31 I. Ie Calve’ and M. Bourene. J. Chem. Phys. 58 (1973) 1446; J. le Cab&, R.A. Gutcheck and 0. Dutuit, Chem. Phys. Letters 47 (1977) 470. 141 J.M. Calo and R.C. Axtman, J. Cbem. Phys. 54 (1971) 4961; 55 (1972) 682. [51 0-P. Bochkova, N-V. Chernysheva and Yu.A. Tolmachev, Opt- Spectry. 36 (1974) 19. 161 D.H. Winicur and J.L. Fraites, J. Chem. Phys. 61 (1974) 1548. c71 W. Lee and R.M. Martin, I. Chem. Phys. 63 (1975) 962. PI Rk Sanders, AN. Schweid, M. Weiss and E.E. Muschtitz Jr., J. Chem. Phys. 65 (1976) 2700. 191 J. Krenos and J. Be1 Bruno, J. Chem. Phys. 65 (1976) 5017;Chem. Phys. Letters 49 (1977) 447. WI M. Touzeau, D. Pagnon and A. Ricard, J. Phys. (Paris) 38 (1977) 789. . 1111 V. Puech, F. Collier and P. Cottin, 1. Chem. Phys., to be published. WI J.H. Kolts, H.C. Brashears and D-W. Setser, to be published.
values. 359
Vohrme 53, number 2 [I?]
CHEMICAL
PHYSICS LETTERS,
AN. Schweid, M.A.D. Fluendy and E.E. Muschlitz Jr., Chem. Phys. Letters 42 (1976) 103. 1141 k Lofthus and P-H. Krupenie, J_ Phys. Chem. Ref. Data 6 (1977) 113. [lS] RF_ Heidner. D-G: Sutton and S.N. Suchard, Chem. Phys. Letters 37 (i975) 243. [16] EM. Gartner and B.A. Thrush, Proc. Roy. Sot. A 346 (1975) 121.
360
15 January 1978
1171 E-E. Nikitin and S.Ya. Umanski, F.&day Discussions Chem. Sot. 53 (1972).7. [ 18j J-M. caio and R-C Axtman; I. Chem. Phys. 54 (i97i) 1332. [19] J.L. Kinsey, J. Chem. Phys. 54 (1971) 1206. 1201 W. Benesch, J-T. Vanderslice, S-G. TiJford and P-G- Wil_ kinson, Astrophys. J. 143 (1966) 236; J-C. McCallum, W-R Jarmain and RW. Nicholls, CRESS Report No. 3, York University.
Volume 53. number 2
VIBRATIONAL
EXCITATION
AND THE DETERMINATION M. TOUZEAU
OF N,(C)
PHYSICS LETTERS
AND N2(B) BY METASTABLE
1.5 January
1978
ARGON ATOMS
OF THE BRANCHING RATIO
and D. PAGNON
Laboratoire de Physique des PIasmasf-, Universit6 Parissud. Centre d*Orsay. 91405 Orsay, France Received 15 September 1977 Revised manuscript received 27 September 1977
The reaction rates k&‘) for N,(C. u’) (v’ = 0, 1.2,3) and a,(~“) for Nz(B, u”) (0 4 0” < 11) of the reaction: AI<~P~,o) + Nz 4 N2(C, u’) + Ar(kC(u’));-f N2(B, u”) + Ar(kR(u”)) were measured in a Iow pressure flowing afterglow (0.1 to 2 torr). Ihe electronic branching ratio between N2 (B) and N,(C) was determined to be : kB/kC = 0.25 + 0.15 [kg = E,nfkB(v”), kC = Z,ekC(u’)]_ It is shown that it is necessary to take into account both the quenching and the vibrational relazzation of Nz(B). The vibrational relaxation rate k,(l) is determined to be 6.5 X lo-l2 cm3 s-I_
1. Introduction The excitation of nitrogen by argon metastable atoms has been studied by several different techniques during recent years. These techniques include flowing afterglows [ 1,2], afterglows [3--51 and molecular beams [6-g]. In the experiments, nitrogen is excited to two different electronic levels N2(C 3 II,) and N,(B 31Jg) by collision with argon metastables. Good agreement exists between the experiments for the total reaction rate and the vibrational distribution of N2(C 3 II,). However for the excitation of N,(B 3 llg) the results are quite different_ For the branching ratio between N,(B) and N2(C) Setser measured in a flowing afterglow k,/kC = 6 + 1 [l] _ In a similar experiment we found an upper limit of 1.3 [lo]. In an ArLN2 electron beam laser, Puech [ 1 1] observed for the first time a coherent emission of the N2(C, 0) + Nz(B, 0), 3371 A, line. He was forced to conclude from a kinetic analysis that almost all the excitation leads to the N2(C) state in the Ar*-N, transfer_ Krenos and Be1 Bruno [9] found that only 1.5% of the B state is created by direct excitation-for the Au = 2 band (2 < u” < 6), the remainder is formed by C-B radiative cascade. Finally a redetermination of the branching * J_aboratoire associd au C.N.R.S.
ratio by Setser’s group [12] using the ArCP2) + Kr reference reaction gave a value of kg/kc = 1 + O-2. This paper presents the determination of the values of the direct excitation rates kg(u”) of N2(B, u”) by metastable argon atoms. The measurements are made in a low pressure flowing afterglow.
2. Experimental The flowing afterglow appsratus which has been utilized has been described elsewhere [lo] _Metastable atoms are created k a low current positive column (1 mA) and are drawn along a 5 cm diameter flow tube with a speed typically of 50 m s-l. The argon pressure (0.1 to 2 torr) is proportional to argon flow rate. Nitrogen is added 20 cm downstream from the discharge where the metastable Ar(3P2) concentration is typically of 1010 cm -3_ The relative concentrations of the other metastable and resonant states are: [3P,]/[3P,] x0.15, [3P*]/[3P2]
Volume 53, number 2
CHEMICAL
15 January 1978
PHYSICS LETTERS
mediately after the injection of N2 with a 60 Wmrn monochlomator between 3 100 A and 10800 a. We have used two cooled photomultipliers, a “trialcali” for the spectral range, 3 100 _k to 8000 A and a “AgOCs photocathode” in the red and infrared region. The optical system was calibrated tith a standard tungsten lamp.
into account quenching reactions for this model. The with argon pressure suggests an increase of N2(B, 9) density by vibrational relaxation. So we must consider the following reactions: increase of RuA(0)/RcB(O)
-kc@‘) * N2(C, u’) f Ar , kg(u”) N2(B, v”) + AI-, f N2C-W -
Ar* + N2W
(1)
k*
(2)
&.B#, N2(B, u”) + hv”.,,,. , N2(C, u’) =
3. Model We have shown [ 101 that it is possible to deduce the direct excitation rate ku(v”) from the measurement of the ratio RBA(u”)[RCB(~“).RCB(U”) is the ffux of photons cascading to the N&I, u”) level from the C state and RBA(u”) the flux of N2(13, u”) cascading to the A state. The decrease of the ratio RnA(u~)/RCB(u”) with the argon pressure (fig. I) shows that it is necessary to take
(3)
BA A “,,“,,,
NZ(A, u”‘) -r-?z~~~,u,.,,
N2(B, u”) -
(4)
4Ar(u”)
N2(B, u”) + Ar ~__f
N2(??) + Ar , 4@J”) N&3: u”) + N, ____+ N2(?) f N, , k,W’) N2(B, u”- 1) + Ar . N&3, u”) + Ar e
(6) (7)
Along with the equilibrium equation for N2(C, u’): i&G
~‘11M-J’) = k$~‘) Pr*I
&I
,
(8)
we calculate the vahre of the radiative flux: 3 &&“)
= gO A$$
[N2C
~‘11 ,
(9)
as a fictitious reaction with metastable atoms: QB (u”) = K(n”) LAr* I ]N2 I ,
(10)
with a fictitious reaction rate K(C), Iqu”) = c k&)A~~. u’
T(t.?‘).
(11)
7fu’) is the lifetime of N2(C, u’), T(u’) = 1/z&;.
(12)
.
U” The
steady state for N2(B, 0”):
R&u”) 0.5
1
15
2
PA,[Torr]
Fig. 1. Argon pressure dependence of the photon fiw ratio RBA(u”)/RCB(u”). RCB(u”) is defmed in eq. (9); a similar relation holds for RB*(LJ”)_ The reactant flow rate [N2] is Twed at [Nt] = 1 Qh-1 NTP. Argon pressure is monitored by argon flow rate ([Ar](Q h” NTP) = 700 pi (to&)_
356
+ kB (u”) [Ar* I lEJ2 I
+ k,(u"f l)[Ar] ]N2(B, u” ; l)] = R &u”) + @‘)
+ -&&“) I-W 1 IN,@,
k]
.+ 4N2(u”) [N, ]
u”N ,
produces the relation&i@:
(13)
Volume 53, number 2
CHEMICAL PHYSICS LETTERS Table 1
RBA(u”) (1 + [q&(u”)
+ k,(u”)]
RC&“)
+ qN2
@“iLN2i +“>,
=I+
kg@“) n + k&i’ + 1) Kg K(u )
X +n
f 1) RBA(“” R&u”+
= 1 +
Vibrational distribution of N2(C)
[Ar]@“)
‘;,‘)
+ I) 1) lAr’ ’
At zero pressure this expression RBA(U”)/&B(U”)
15 January 1978
(14)
11-z lb
0.21 0.23 0.16 0.25 0.25
0.05 O-05 0.05 0.06 0.018
IL-3/b 0.007 0.005
a) For 0.048 eV :ransIational energy. b, Ar(3P2) and Ar(3Po) are assumed equally reactive and present in a statistical distribution [3P2]/[3Po] = 5. In fact the reactivity of 3Po is less than that of 3P2, k(3Po) = 1.7 X lo-" cm3 s-’ (b CaG et al. [3]).
reduces to
kB(u”)/K(u”)
flowing afterglow [ 1] our results aftergIow [51 beam experiment [9] a) golden rule cahdation 19Jb)
klih
.
4. Results
tional to the vibrational concentration N2(C, u’)_ The measurement of the vibrational distribution allows the comparison of K(u”) and kc(O):
4. I. Vibratiohal distribution of N2{C) From the second positive system, the vibrationai distribution of N2(C, u’) has been determined_ This distribution is independent of the argon and nitrogen flow rates and pressures. The results are tabulated and compared with other experiments in table 1. The different types of experiments give results in remarkable agreement_ The beam results [9] are for 0.048 eV ‘translational energy. With increasing energy the ratio, [N2(C, l)]/[N,(C, O)] also increases [13]. The lifetimes T(u’) of the different vibrational levels are approximately equal, thus the reaction rates kC(u’) are propor-
W-J”) = P(u”)~c(O)
,
(16)
with = (Aoum + 023 A Ium+ 0.05 Azu,,
fi”)
+ 0.005 f+)‘(U”)
_
(17)
These values oiP(v”) are listed in table 2. 4-L
Vibrational relaxation of N?(B) This effect is illustrated in fig. 2. The N(B, O)-
Table 2
Direct excitation rates of N2(B, u”) by me&table
argon atoms
PGJ”)
RBA~“)/RcB(V”)
102k,(u")/kc(0)
( ")3 iO=k Bu (cm3 S-I)
0
0.64
1.08
1 2 3 4 5 6 7 8 9
0.35 0.17 0.079 0.034 0.013 4.8 -3
1.14 1.40 1.58 2.1 2.85 4.25
5.1 5.1 6.9 4.6 3.7 2.5 1.55
l-6-3
7.40
1.01 0.77
1.4 f 1.4 f 1.9 5 1.3 f 1.0 i 0.7 + 0.43 0.28 0.21
0.80 0.73 0.80
0.22 0.20 0.22
U”
10 11
4.7 -4
l-l-4 3.6-5 7.3 -6
17.4
72 206 1100
2 1.2 0.7 0.3 0.2 0.1
lo2 kB(u")/kc(0)b)
2-l 4.8 5.8 5.3 3.8 2.4
1.4 0.8 0.4
0.2 0.05 0.04
a) The kg@") valuesarededuced from kg(v")/kc(0) values with a to+kdmeasured reaction Iate k = 3.6 X lo-” b, Golden rule calculation (see the text) with P(E%)/p
cm3 s-l.
357
Volume 53, number 2
CHEMICAL
PHYSICS LETTERS
0
1
-2
‘
3
Fig. 3. Determination of the viirational relaxation rate k,(l).
of the lbs of eq. (18) with [RBA(l)/ [Ar] _ From eq. (17), the extrapolation to zero pressure gives the value of kg(0)/kC(O) and the slope indicates the value of k,( I), in this case equal to 6.5 X lo-12 cm3 s-1 _This rapid vibrational relaxation could be expIained by a non-adiabatic mechanism proposed by Nikitin [ 171 as suggested by Gartner and Thrush [16] _ In this process, vibrational radiationless transition occurs between the components of the ll state which have been split apart in the collision. Such high cross sections of vibrational relaxation have been observed for N2(C 3iIu, u’ = 1 + u’ = 0) [18] _ the variation
RcB(l)]
85L3A (3-2)
8723 A 12-l)
10510A
8912 A 11-O)
to-01
Fig. 2. Spectrum of the emitted near infrared second positive system of Nz for an argon pressure of 0.15 torr and 2 torr
(Iinearscale).
N(A, 0) 10510 A line is more intense with increasing argon pressure. Detection of vibrational relaxation is very sensitive between v” = 1 and v” = 0 becailse the state N,(B, 0) can be depopulated only by a weak argon quenching: 4&(O) = 4 X lo-l3 cm3 s--I for the W 3Au and A3ZIi levels [IS] _ In consequence a vibrational depopulation of N2(B, u” = 1) produces an increase of N2(B, u” = 0). The evolution of RBA(0)/ R&O) with argon pressure is given by eq. (14)_
RBA(0) kB@) fl(l)RBA(l) = 1+kC(0)fl(O) + ‘?J(‘) fl(0)RCB( 1)
CAr1 -
u3
Because of the weak partial pressure and quenching rate of nitrogen (qN2 (0) = 2 X lo-l2 cm3 s-l) the quenching of N2(B, 0) has been neglected_ The values of /3(O) and p(1) are given in table 2. Fig. 3 presents 358
4.3. Direct vibrationalexcitation of lV2/.,
v”j
The extrapolation, to zero pressure of the curves RBA(u”)&B(v~) (fig. I) gives the value of kg($)/ kc(O) as:
k&J”) kc@)
C
=B(v”)
The results are presented in table 2 and displayed in fig. 3. The errors in k&v”) arecalculated from an estimate of the photon flux ratio A(RBA/R~B)/(RBA/R~) of 10% and A@(v”)/p(v”) between 5% and 10% for v” > 8_ (The error Aj3(v”)/P(v”) is calculated from the error in the determination of the vibrational distribution of the C state.) The direct excitation of Nz(B, v”) by argon (a few percent of the dir&t excitation of the C state) has little effect on the ratio of the radiative flues (.&&B * i) for low levek of Nz(B), but is important for higher levels (RBA >R,$
Volume 53, number 2
because the radiative cascade from N2(C) populates
principally the low levels of N2(B). Consequently the determinatiqn of kB(v”) is excellent for higher values of v”. 5. Discussion We have compared our results (fig_ 4) with the calculated values of the golden rule model proposed by Krenos and Be1 Bruno. In this model, l Jle transition probability Wii is proportional to the product of the Franck-Condon factor qii and the density of states pu. For the atom + diatom case pu is given by [lV] P”(E)
a?;’ 141 -f”)13’2 ,
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CHEMICAL PHYSICS LETTERS
cw
where B; is the rotational constant and f, is the fraction of the available energy channeled into a particular vibrational level. With this model, we have calculated the distribution among the vibrational level N,(B, u”):
q(v’), q(v”) are respectively the F&r&-Condon factors of the C and B transition [20]. The ratio of the rotational constants is Bt/Bz = 1.12. The available energy for the C and B states is respectively taken as ec = 0.63 eV (exoergicity f translational and rotational energy) and eB = 4.3 eV. The ratio of the electronic transition probabilities (which is unknown) is adjusted so the calculated distribution, kB(v”)/k&) [eq. (21)] fits with the measured values
Fig. 4. Values of the direct excitation transfer rates kg@“) by metastable argon atoms compared with the excitation rate of N2 (C, u’ = 0). Comparison with the golden rule calculated
(fig_ 4) The best fit is obtained for p(C)/P(B) = 116. These values are reported in table 2. For 8 < v” < 11 the experimental values of k&u”) are higher than the predicted values, suggesting a possible direct dissociation of N2 by metastable atoms with a recombination of the atomic nitrogen in high vibrational levels 8 < v” < 12.
6. Conclusion The utilization of a low pressure flowing afterglow has verified that, in the collision with metastable ar8on atoms, nitrogen is principally excited to the C 3iI,, state. Low vibrational levels of the B state (where most of the radiative energy occurs) are populated by radiative cascade, but direct excitation from metastable argon atoms is dominant for v” > 5 levels. The measured vibrational distribution kg(v”) of the B state for direct excitation rate by metastable argon atoms is in good agreement with a golden rule model as for ‘the distribution kc(u’) of the C state.
References Ill D-W. Setser, D.H. Stedman and J.A. Coxon, J. Chem. Phys. 53 (1970)
1004.
121 L.G. Piper, I.E. Velazco and D-W_Setser, J. Chem. Phys.
59 (1973) 3323. r31 I. Ie Calve’ and M. Bourene. J. Chem. Phys. 58 (1973) 1446; J. le Cab&, R.A. Gutcheck and 0. Dutuit, Chem. Phys. Letters 47 (1977) 470. 141 J.M. Calo and R.C. Axtman, J. Cbem. Phys. 54 (1971) 4961; 55 (1972) 682. [51 0-P. Bochkova, N-V. Chernysheva and Yu.A. Tolmachev, Opt- Spectry. 36 (1974) 19. 161 D.H. Winicur and J.L. Fraites, J. Chem. Phys. 61 (1974) 1548. c71 W. Lee and R.M. Martin, I. Chem. Phys. 63 (1975) 962. PI Rk Sanders, AN. Schweid, M. Weiss and E.E. Muschtitz Jr., J. Chem. Phys. 65 (1976) 2700. 191 J. Krenos and J. Be1 Bruno, J. Chem. Phys. 65 (1976) 5017;Chem. Phys. Letters 49 (1977) 447. WI M. Touzeau, D. Pagnon and A. Ricard, J. Phys. (Paris) 38 (1977) 789. . 1111 V. Puech, F. Collier and P. Cottin, 1. Chem. Phys., to be published. WI J.H. Kolts, H.C. Brashears and D-W. Setser, to be published.
values. 359
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AN. Schweid, M.A.D. Fluendy and E.E. Muschlitz Jr., Chem. Phys. Letters 42 (1976) 103. 1141 k Lofthus and P-H. Krupenie, J_ Phys. Chem. Ref. Data 6 (1977) 113. [lS] RF_ Heidner. D-G: Sutton and S.N. Suchard, Chem. Phys. Letters 37 (i975) 243. [16] EM. Gartner and B.A. Thrush, Proc. Roy. Sot. A 346 (1975) 121.
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1171 E-E. Nikitin and S.Ya. Umanski, F.&day Discussions Chem. Sot. 53 (1972).7. [ 18j J-M. caio and R-C Axtman; I. Chem. Phys. 54 (i97i) 1332. [19] J.L. Kinsey, J. Chem. Phys. 54 (1971) 1206. 1201 W. Benesch, J-T. Vanderslice, S-G. TiJford and P-G- Wil_ kinson, Astrophys. J. 143 (1966) 236; J-C. McCallum, W-R Jarmain and RW. Nicholls, CRESS Report No. 3, York University.