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Vibrational relaxation of N2(A3Σu+,υ = 1, 2,3) by CH4 and CF4
CHEMICAL
Volume 107, number I
VIBRATIONAL
RELAXATION
PHYSICS LETTERS
OF N2(A 3X;_ u = 1,2,3)
Joseph M. THOMAS. Jay B. JEFFRIES
* and Frederick
Department of Chentistq~, University of Pittsburgh, pirtsbu@l.
11 November 1983
BY CH4 AND CF4
KAUFMAN Pennsylvania 152260, USA
Received 16 .4ugust 1983
Nz(A. u = O-3) produced by the AI(~P~,~) + N2 reaction and detected by laser-induced fluorescence undergoes rapid, stcpwise vibrational relaxation but slow electronic quenching with added CH4 or CF4. Rate constants, k& of 1.5,3.1, and 5.0 X lo-l2 cm3 s-’ are measured for Q = CH4, u = 1-3, and O-47,1.8, and 5.5 X lo-r2 cm3 s-’ for Q = CF4, u = l-3, with =+20% accuracy (lo). Information is also obtained for the unrelaxed, relative u populations.
1. Introduction
Electronically excited. metastable N,(A 3Cz) is rapidly quenched by or reacts with most but not all molecular
collision
study the vibrational cient
electronic
partners
[l].
relaxation
quenchers
It is of interest
to
of N2(A, V) by ineffi-
for a variety
of reasons:
to prepare
N?(A) in its IJ = 0 state, to measure the vibrational relaxation rates of higher u levels and to deterrnine their u dependence_ aud to examine the nature of the relaxation process and the possible effect of electronic excitation of N2 on the rate of the purely vibrational energy transfer process_ In rhe work described here. two different N,-Ar compositions. 20% and 6% N2 are used in the fastflow. laser-induced fluorescence (LIF) measurement of N?(A. u) relaxation by added CH4 and CF4. At the lo\\er N, fracrion. the u = 3 state is detected in addition fo IJ = 0. 1, and 2, albeit with poorer sensitivity. thus extending rhe range of the relasation rate measuremems.
[2.3] _The 2.54 cm inner diameter flow tube was pumped by a Roots blower at an average linear flow velocity of 44 m s-l at a pressure of ==I -9 Torr, with a total flow of 50 cm3 s-l. STP, and with major contributions of either 20% N2> SO% Ar or 6% NZ, 94% Ar. Ar and N, were purified as before. Added CH4 (Matheson. UHP, 9997%) and CF4 (Matheson, 99.7%) were used without purification. They were added through the multiperforated, fast-mixing inlet placed 20 cm upstream of the LIF cell. Rapid mixing was aided by adding 4-8 cm3 s-l, STP, of the total Ar flow through the injector along with the CH4 or CF4 quencher_ The LIF measurements were done as before. The N2(A, u = 3) state was monitored by excitation at the P, bandhead of the (7,3) band of the first positive, B + A_ transition at 6013 nm. Both because of its lower concentration and the weaker laser output with rhodamine-B dye, the signal-to-noise ratio was poorer by about a factor of 3 than for the lower-v states.
3. Data analysis and results 2. Experimental The apparatus_ reactant purification. and LlF detection of N2(A, u) have been described previously * Present address: Molecular Physics Lab, SRI International, Menlo Park, California 94025, USA.
50
Figs. I and 2 show typical logILl, versus [Q] plots for Q = CH4 (fig. 1) and CF4 (fig. 2). They show the expected behavior: The u = 0 concentration increases with increasing quencher addition while those of u = 1-3 decrease_ The higher the u level, the steeper the slope, indicating a faster rate of vibrational rei.-ua0 009-2614/53/0000-0000/603.00
0 1983 North-Holland
cHEhficm_
Volume 102, number 1
the average flow velocity and d = 20 cm the injector-. to-detector distance. XDQwas used without further correction for u = 3 where there is ito observable
-I
0 -
I800 2
- 1600
11 November 1983
PHYSICS LETTERS
‘5
Fig. l_ Typical ln ILIF versus [Q] plots. Q = CH4.0, u = 0; o,v=l;~,v=2;~,u=3_T=300K,p=1.88Tou,I,=44m s-t ; t = d/G = 4.5 ms. u = 0, 1, and 2 experiments with 20% NZ, u = 3 with 6% Nz_
cascade from higher v. For u = 1 and 2, the cascade correction was made in several ways, most simply by graphical and/or computer least-squares fitting of the linear, large-[Q] parts of the semilog plots (e.g. figs. 1 and 2) where the cascade correction becomes small. Extrapolation to [Q] = 0 then gives the relative concentration in the particular u level that originates by relaxation of the u + 1 level. In more elaborate fitting methods (a) the coupled first-order differential equations were solved numerically to obtain the best fit for k6 with assumed input parameters of kc’ and u + 1 to u population ratios; and (b) the analytical solution of the above scheme, u + 1+ u, u + u - 1, was solved for at each [Q] for assumed input parameters as defined in (a)_ An example of method (a) is shown in fig. 3 for an experiment of N2(A, u = 1) relaxation by CF4. The two estimates of u = 2 to u = 1 population ratios came from our linear extrapolation (0.36) and an earlier qualitative estimate (020) [2], and the estimates of kh4 came from a forced linear fit of all points (0.40 X 10-12) and a fit of the high-[CFq] linear part of the plot (0.47 X lo-i2 cm3 s-1). The experimental points are in very good agreement with curve a which uses the higher population ratio and rate constant_ PQ
tion. Furthermore, there is clear curvature in the early, low-[Q] parts of the plots for u = 1 and, to a lesser estent, for u = 2 indicating a vibrational cascade process in which the u state is initially fed by the faster relaxation process from the next higher level. The pseudo-first-order rate constant for a single-
step process, kh = cr(G/#d In I,I,!d
[Q],takes ac-
count of the development of the laminar flow by the correction factor (Y= 1.34 as described earlier [2]_ i/d is the inverse plug-flow transit time, where V is
0
4 [CF,]
16 , :OI4 cm-’
Fig. 3. Typical in $1~ versus [Q] plots. Q = CF4.0, u = 0; 0, u=1.~,~=2;~,u=3.T=300K,p=1_89Torr,~=44ms~~, t = d/G = 4.5 ms. u = 0 experiments with 20% NZ, u = 1,2, and3 with6%NNZ_
[“‘43
3
10”
cmW3
Fig. 3. Computer fit of Ns(A, u = 1) + CF4 experiment. 0, experimental points; curve (a): k&F4 = 0.47 x lo-“, 2 to 1 population ratio, R21 = 0.36; curve (b): kcF4 = 0.47 x lo-‘*, R21 = 0.20; curve (c): “6~~ = 0.40 x 10-12, Rar = 0.36; curve(d): k&4 = 0.40 x 10-r*, R2r = 0.20.
51
Volume
CHEMICAL
102, number 1
Table 1 Vibrational relaxation rate constants k; Q
u=l
Cl14
1.5
CF4
0.47 = 0.08
k 0.1
(lo-”
cm3 s-t)
I?=2
u=3
3.1 * 0.3 1.8 i 0.3
5.0 + 0.8 5.5 2 0.3
The six relaxation rate constants are summarized in table I_ They represent 4.3. and 5 experiments. respectively, for CH,. u = 1.2. and 3. and 5.4. and 3 esperiments. respectively for CFq. u = 1,3, and 3. The uncertainties (lo) represent only the statistics of the results. The estimated accuracy of the “6 values is EO!G (1 a). The unrelaxed population ratios are shown in rable 2 for the 205%and 6% N2--Ar mixtures where they are compared with the calculated radiative cascade as discussed below.
4. Comparison
and discussion
Our observation of very slow electronic quenching of N,(A. u = 0) by CH, is in full agreement with Clark and Setser [I] who reported an upper limit of I X IO-l4cd s-1 and with other investigators. particularly Slanger et al. [4] who studied the temperature dependence
of the reaction
and reported
k =
3.2
X IO-l5 cm3 s-1. For CF4. no information is a\aildble_ but our experiments show negligibly slow quenchiltg. Furtltennore, Clark and Setser reported an upper litnit of 1 X IO-l4 cm3 s-1 for C2F6. The only reporred value for vibrational relaxation. “6, is that of 1.1 X 1O-12 cm3 s-1 by Clark and Setser [ 1 ]
Table 2 I‘ractionJ
population in N&i.
1
2 3 4 2
52
for Q = CH4, N2(A, u = 1). in moderate agreement with our (1 S i 0.3) X IO-l?, particularly since Clark and Setser did not correct for laminar flow development. whereas we use (Y= 1.34. The u dependence of our “6 values, in power law form, i.e. “6 a fl gives II = 1.0 for CH4 and =2.2 for CFq, both n being larger for going from u = 2 to 3 than from 1 to 2. This is fairly common behavior in HX(u) relaxation (X = Cl, F) [5,6] even though its physical basis is unclear, especially for JI > 2. The vibration frequency of N2(A) is 1433 cm-l for its (1,O) transition, 1406 cm-l for (2,1), and 1379 cm-l for (3,2), much lower than the 2330 cm-l (1.0) frequency of N,(X). Available acceptor frequencies in CH4 and CF4 are the triply degenerate C-H bend. v4 = 1306 cm-l for CH4 and the triply degenerate C-F stretch, v3 = 1283 cm-l for CF4_ Other fundamental frequencies are too high for CH, and too low for CF4. Collisional energy transfer probabiiities,PE kg@. where X-6” IS _ the hard-sphere collision frequency, are 4.5,S.l,and 13.7X 1O-3 forCHqand 1.5,5.6,and 1S-6 X 10d3 for CFq. Lambert-Salter plots [7], In(uP-I) versus M (cm-l), the rotationless energy gap, are not very meaningful. The range of AE is only 54 cm-l in both cases, and AE is small, 73-127 cm-l for CH+ and 96-l 50 cm-l for CF4_ The slope is near zero for CH, and too large for CF4. ~2.6 X 1O-2 cm compared with S.3 X 10m3 cm for H-containing and 1.7 X 10m2 cm for non-H-containing molecules [7] in V-T transfer correlations. The reported trends are thus qualitatively but not quantitatively consistent with the larger “6 dependence for non-H-containing quenchers [7 J. The comparison with published experimental data
V) hek
Experimental
0
11 November 19S3
PHYSICS LETI-ERS
20% N-,
65 NZ
0.61 I 0.05 0.29 z 0.05 0.11 i 0.02
0.59 I 0.05 0.26 2 0.06 0.095 + 0.03 0.035 + 0.02
Calculated radiative cascade only
Radiilive cascade plus N?(A) + 20% Nz
Radiative cascade plus N2(A) + 6% N2
0.45
0.50
0.47
0.31
0.38
0.36
0.12 0.06 0.03
0.11
0.14 0.027
0.008 0.014
Volume 102, number 1
CHEMICAL
PHYSICS
on other processes is confined to CO, NO, and 02(X, u = 1) relaxation by CH4 and CO relaxation by CFq_ For CO(X, u = 1) + CHq, laser studies have given rate constants of 9.0 X lo-l5 cm3 s-l [8] and 9-2 X lo-l5 [9] ; for NO(X, u = 1) + CHd, 1.89 X lo-l3 cm3 s-l [lo]; and for 0,(X, u = 1) + CH+ 73 X IO-l3 cm3 s-l [ 111, the latter by an indirect method. CF4 quenches CO(X, u = 1) considerably faster than CH,, k = 1.9 X lo-l3 cm3 s-l [8]. In spite of some scatter, these data give a fairly normal Lambert-Salter plot for Q = CH4 when the v4 = 1306 cm-l frequency is assumed to be the V-V acceptor_ The slope is ~7 X 10-S cm, and our N2(A) + CH4 data fall well on the line. For Q = CF4, only the CO relasation is available for comparison_ As Green and Hancock [8] have suggested the Y + v3 combination frequency of CF4 at 2192 cm- I! or v; + u4 at 19 14 cm-l are more likely acceptors in that fast relaxation_ Our NZ(A) + CF4 results correlate well with the latter choice, giving a Lambert-Salter slope of 1 .l X lo-* cm. The questions posed at the outset can thus be answered: The N2(A, u) relaxation processes are V-V energy transfer steps; and the electronic excitation of the N2 molecule does not greatly accelerate the rate of the process_ Lastly, our measurements have provided information on the unrelaxed v-level population ratios for two N2-Ar mixtures. The purely radiative distribution in table 2 was calculated as described earlier [3] on the basis of an initial N2(C) distribution of 75% u = 0,20% u = 1, and 5% u = 2 [ 121 followed by the radiative transition rates of the second and first positive systems of N2 [ 13]_ Since surface deactivation of N*(A) is a very efficient process, it is unlikely to introduce substantial vibrational discrimination in the ~7 ms transit time between the Ar* + N2 mixing region and the LIF cell. Vibrational relaxation by ground-state N2 is therefore the major source of N2(A, u) removal in the absence of added quenchers. Dreyer and Perner [ 141 have measured I& for u =G7 and provided evidence that Au = 2 for the process_ For N2(A, IJ = 3),X- = 3.8 X IO-14 cm3 s-l, i.e. the effective first-order rate constant is 470 s-1 for our 20% N, mixture and 140 s-l for the 6% mixture. In 7 ms, this would reduce the u = 3 population by a factor of ~25 for the 20% and by a factor of 2.7 for the 6% mixture. This may explain why the u = 3 state was detected in 6% but not in 20% N2-Ar flows. For the
LETTERS
11 November 1983
u = 2 to 0 relaxation of N2(A) by N*(X), the corresponding reductions would be factors of 1 S for 20% and 1.14 for 6%, i.e. quite minor. These relaxation rates are used to calculate the populations in the fourth and fifth columns of table 2. The radiative cascade also neglects the fast collisional relaxation processes in the B state of N, which compete successfully with the relatively slow, radiative first positive transition [3,15] and further increase the population of the lower u levels of N*(B). With these changes and with the rapid relaxation of N2(A, u > 3) the observed relative populations are in satisfactory agreement with prediction_ Acknowledgement This work was supported by the Air Force Geophysics Laboratory and Defense Nuclear Agency under Task S99 QAXHD. References [l]
W.G. Clark and D-IV_ Setser, J. Phys. Chem. 84 (1980) 2225. [2] hlP. lannuzzi and F. Kaufman, J. Phys. Chem. S5 (1981) 2163. [3] BIP. lannuzzi, J-B. Jeff&s and F. Kaufman, Chem. Phys. Letters 87 (1982) 570. [4] T-G. Slanger, B J. Wood and G. Black, J. Photochem. 2 (1973) 63. 1.51 B-M_ Berquist, L-S. Dzelzkalns and F_ Kaufman, J. Chem. Phys. 76 (1982) 2984. [6] L-S. Dzelzkalns and F. Kaufman, J. Chem. Phys., to be published. [7] J.D. Iambert, Vibrational and rotational relaxation in gases (Clarendon Press, O_tiord, 1977) pp_ 102,109. [S] W-H. Green and JX. Hancock, J. Chem. Phys. 59 (1973) 4326. [9] J-C. Stephenson and E-R. hlosburg Jr.. J. Chem. Phys 60 (1974) 3562. [lo] J-C. Stephenson, J. Chem. Phys. 60 (1974) 4289. [ll] J-T. Yardley and C-B. hloore, J_ Chem. Phys. 48 (1968) 14. [12] E-A. Gislason, A.W. Kleyn and J. Los, Chem. Phys. Letters 67 (1979) 252. [ 131 A. Loftus and P-H. Krupenie, J. Phys. Chem. Ref. Data 6 (1977) 113. [14] J-W. Dreyer and D. Pemer, J. Chem. Phys. 58 (1973) 1195. [15] N. Sadeghi and D.W. Setser, Chem. Phys. Letters 77 (1981) 304; A. Rotem, I. Nadler and S. Rosenwaks, Chem. Phys. Letters 83 (1981) 281.
53
Volume 107, number I
VIBRATIONAL
RELAXATION
PHYSICS LETTERS
OF N2(A 3X;_ u = 1,2,3)
Joseph M. THOMAS. Jay B. JEFFRIES
* and Frederick
Department of Chentistq~, University of Pittsburgh, pirtsbu@l.
11 November 1983
BY CH4 AND CF4
KAUFMAN Pennsylvania 152260, USA
Received 16 .4ugust 1983
Nz(A. u = O-3) produced by the AI(~P~,~) + N2 reaction and detected by laser-induced fluorescence undergoes rapid, stcpwise vibrational relaxation but slow electronic quenching with added CH4 or CF4. Rate constants, k& of 1.5,3.1, and 5.0 X lo-l2 cm3 s-’ are measured for Q = CH4, u = 1-3, and O-47,1.8, and 5.5 X lo-r2 cm3 s-’ for Q = CF4, u = l-3, with =+20% accuracy (lo). Information is also obtained for the unrelaxed, relative u populations.
1. Introduction
Electronically excited. metastable N,(A 3Cz) is rapidly quenched by or reacts with most but not all molecular
collision
study the vibrational cient
electronic
partners
[l].
relaxation
quenchers
It is of interest
to
of N2(A, V) by ineffi-
for a variety
of reasons:
to prepare
N?(A) in its IJ = 0 state, to measure the vibrational relaxation rates of higher u levels and to deterrnine their u dependence_ aud to examine the nature of the relaxation process and the possible effect of electronic excitation of N2 on the rate of the purely vibrational energy transfer process_ In rhe work described here. two different N,-Ar compositions. 20% and 6% N2 are used in the fastflow. laser-induced fluorescence (LIF) measurement of N?(A. u) relaxation by added CH4 and CF4. At the lo\\er N, fracrion. the u = 3 state is detected in addition fo IJ = 0. 1, and 2, albeit with poorer sensitivity. thus extending rhe range of the relasation rate measuremems.
[2.3] _The 2.54 cm inner diameter flow tube was pumped by a Roots blower at an average linear flow velocity of 44 m s-l at a pressure of ==I -9 Torr, with a total flow of 50 cm3 s-l. STP, and with major contributions of either 20% N2> SO% Ar or 6% NZ, 94% Ar. Ar and N, were purified as before. Added CH4 (Matheson. UHP, 9997%) and CF4 (Matheson, 99.7%) were used without purification. They were added through the multiperforated, fast-mixing inlet placed 20 cm upstream of the LIF cell. Rapid mixing was aided by adding 4-8 cm3 s-l, STP, of the total Ar flow through the injector along with the CH4 or CF4 quencher_ The LIF measurements were done as before. The N2(A, u = 3) state was monitored by excitation at the P, bandhead of the (7,3) band of the first positive, B + A_ transition at 6013 nm. Both because of its lower concentration and the weaker laser output with rhodamine-B dye, the signal-to-noise ratio was poorer by about a factor of 3 than for the lower-v states.
3. Data analysis and results 2. Experimental The apparatus_ reactant purification. and LlF detection of N2(A, u) have been described previously * Present address: Molecular Physics Lab, SRI International, Menlo Park, California 94025, USA.
50
Figs. I and 2 show typical logILl, versus [Q] plots for Q = CH4 (fig. 1) and CF4 (fig. 2). They show the expected behavior: The u = 0 concentration increases with increasing quencher addition while those of u = 1-3 decrease_ The higher the u level, the steeper the slope, indicating a faster rate of vibrational rei.-ua0 009-2614/53/0000-0000/603.00
0 1983 North-Holland
cHEhficm_
Volume 102, number 1
the average flow velocity and d = 20 cm the injector-. to-detector distance. XDQwas used without further correction for u = 3 where there is ito observable
-I
0 -
I800 2
- 1600
11 November 1983
PHYSICS LETTERS
‘5
Fig. l_ Typical ln ILIF versus [Q] plots. Q = CH4.0, u = 0; o,v=l;~,v=2;~,u=3_T=300K,p=1.88Tou,I,=44m s-t ; t = d/G = 4.5 ms. u = 0, 1, and 2 experiments with 20% NZ, u = 3 with 6% Nz_
cascade from higher v. For u = 1 and 2, the cascade correction was made in several ways, most simply by graphical and/or computer least-squares fitting of the linear, large-[Q] parts of the semilog plots (e.g. figs. 1 and 2) where the cascade correction becomes small. Extrapolation to [Q] = 0 then gives the relative concentration in the particular u level that originates by relaxation of the u + 1 level. In more elaborate fitting methods (a) the coupled first-order differential equations were solved numerically to obtain the best fit for k6 with assumed input parameters of kc’ and u + 1 to u population ratios; and (b) the analytical solution of the above scheme, u + 1+ u, u + u - 1, was solved for at each [Q] for assumed input parameters as defined in (a)_ An example of method (a) is shown in fig. 3 for an experiment of N2(A, u = 1) relaxation by CF4. The two estimates of u = 2 to u = 1 population ratios came from our linear extrapolation (0.36) and an earlier qualitative estimate (020) [2], and the estimates of kh4 came from a forced linear fit of all points (0.40 X 10-12) and a fit of the high-[CFq] linear part of the plot (0.47 X lo-i2 cm3 s-1). The experimental points are in very good agreement with curve a which uses the higher population ratio and rate constant_ PQ
tion. Furthermore, there is clear curvature in the early, low-[Q] parts of the plots for u = 1 and, to a lesser estent, for u = 2 indicating a vibrational cascade process in which the u state is initially fed by the faster relaxation process from the next higher level. The pseudo-first-order rate constant for a single-
step process, kh = cr(G/#d In I,I,!d
[Q],takes ac-
count of the development of the laminar flow by the correction factor (Y= 1.34 as described earlier [2]_ i/d is the inverse plug-flow transit time, where V is
0
4 [CF,]
16 , :OI4 cm-’
Fig. 3. Typical in $1~ versus [Q] plots. Q = CF4.0, u = 0; 0, u=1.~,~=2;~,u=3.T=300K,p=1_89Torr,~=44ms~~, t = d/G = 4.5 ms. u = 0 experiments with 20% NZ, u = 1,2, and3 with6%NNZ_
[“‘43
3
10”
cmW3
Fig. 3. Computer fit of Ns(A, u = 1) + CF4 experiment. 0, experimental points; curve (a): k&F4 = 0.47 x lo-“, 2 to 1 population ratio, R21 = 0.36; curve (b): kcF4 = 0.47 x lo-‘*, R21 = 0.20; curve (c): “6~~ = 0.40 x 10-12, Rar = 0.36; curve(d): k&4 = 0.40 x 10-r*, R2r = 0.20.
51
Volume
CHEMICAL
102, number 1
Table 1 Vibrational relaxation rate constants k; Q
u=l
Cl14
1.5
CF4
0.47 = 0.08
k 0.1
(lo-”
cm3 s-t)
I?=2
u=3
3.1 * 0.3 1.8 i 0.3
5.0 + 0.8 5.5 2 0.3
The six relaxation rate constants are summarized in table I_ They represent 4.3. and 5 experiments. respectively, for CH,. u = 1.2. and 3. and 5.4. and 3 esperiments. respectively for CFq. u = 1,3, and 3. The uncertainties (lo) represent only the statistics of the results. The estimated accuracy of the “6 values is EO!G (1 a). The unrelaxed population ratios are shown in rable 2 for the 205%and 6% N2--Ar mixtures where they are compared with the calculated radiative cascade as discussed below.
4. Comparison
and discussion
Our observation of very slow electronic quenching of N,(A. u = 0) by CH, is in full agreement with Clark and Setser [I] who reported an upper limit of I X IO-l4cd s-1 and with other investigators. particularly Slanger et al. [4] who studied the temperature dependence
of the reaction
and reported
k =
3.2
X IO-l5 cm3 s-1. For CF4. no information is a\aildble_ but our experiments show negligibly slow quenchiltg. Furtltennore, Clark and Setser reported an upper litnit of 1 X IO-l4 cm3 s-1 for C2F6. The only reporred value for vibrational relaxation. “6, is that of 1.1 X 1O-12 cm3 s-1 by Clark and Setser [ 1 ]
Table 2 I‘ractionJ
population in N&i.
1
2 3 4 2
52
for Q = CH4, N2(A, u = 1). in moderate agreement with our (1 S i 0.3) X IO-l?, particularly since Clark and Setser did not correct for laminar flow development. whereas we use (Y= 1.34. The u dependence of our “6 values, in power law form, i.e. “6 a fl gives II = 1.0 for CH4 and =2.2 for CFq, both n being larger for going from u = 2 to 3 than from 1 to 2. This is fairly common behavior in HX(u) relaxation (X = Cl, F) [5,6] even though its physical basis is unclear, especially for JI > 2. The vibration frequency of N2(A) is 1433 cm-l for its (1,O) transition, 1406 cm-l for (2,1), and 1379 cm-l for (3,2), much lower than the 2330 cm-l (1.0) frequency of N,(X). Available acceptor frequencies in CH4 and CF4 are the triply degenerate C-H bend. v4 = 1306 cm-l for CH4 and the triply degenerate C-F stretch, v3 = 1283 cm-l for CF4_ Other fundamental frequencies are too high for CH, and too low for CF4. Collisional energy transfer probabiiities,PE kg@. where X-6” IS _ the hard-sphere collision frequency, are 4.5,S.l,and 13.7X 1O-3 forCHqand 1.5,5.6,and 1S-6 X 10d3 for CFq. Lambert-Salter plots [7], In(uP-I) versus M (cm-l), the rotationless energy gap, are not very meaningful. The range of AE is only 54 cm-l in both cases, and AE is small, 73-127 cm-l for CH+ and 96-l 50 cm-l for CF4_ The slope is near zero for CH, and too large for CF4. ~2.6 X 1O-2 cm compared with S.3 X 10m3 cm for H-containing and 1.7 X 10m2 cm for non-H-containing molecules [7] in V-T transfer correlations. The reported trends are thus qualitatively but not quantitatively consistent with the larger “6 dependence for non-H-containing quenchers [7 J. The comparison with published experimental data
V) hek
Experimental
0
11 November 19S3
PHYSICS LETI-ERS
20% N-,
65 NZ
0.61 I 0.05 0.29 z 0.05 0.11 i 0.02
0.59 I 0.05 0.26 2 0.06 0.095 + 0.03 0.035 + 0.02
Calculated radiative cascade only
Radiilive cascade plus N?(A) + 20% Nz
Radiative cascade plus N2(A) + 6% N2
0.45
0.50
0.47
0.31
0.38
0.36
0.12 0.06 0.03
0.11
0.14 0.027
0.008 0.014
Volume 102, number 1
CHEMICAL
PHYSICS
on other processes is confined to CO, NO, and 02(X, u = 1) relaxation by CH4 and CO relaxation by CFq_ For CO(X, u = 1) + CHq, laser studies have given rate constants of 9.0 X lo-l5 cm3 s-l [8] and 9-2 X lo-l5 [9] ; for NO(X, u = 1) + CHd, 1.89 X lo-l3 cm3 s-l [lo]; and for 0,(X, u = 1) + CH+ 73 X IO-l3 cm3 s-l [ 111, the latter by an indirect method. CF4 quenches CO(X, u = 1) considerably faster than CH,, k = 1.9 X lo-l3 cm3 s-l [8]. In spite of some scatter, these data give a fairly normal Lambert-Salter plot for Q = CH4 when the v4 = 1306 cm-l frequency is assumed to be the V-V acceptor_ The slope is ~7 X 10-S cm, and our N2(A) + CH4 data fall well on the line. For Q = CF4, only the CO relasation is available for comparison_ As Green and Hancock [8] have suggested the Y + v3 combination frequency of CF4 at 2192 cm- I! or v; + u4 at 19 14 cm-l are more likely acceptors in that fast relaxation_ Our NZ(A) + CF4 results correlate well with the latter choice, giving a Lambert-Salter slope of 1 .l X lo-* cm. The questions posed at the outset can thus be answered: The N2(A, u) relaxation processes are V-V energy transfer steps; and the electronic excitation of the N2 molecule does not greatly accelerate the rate of the process_ Lastly, our measurements have provided information on the unrelaxed v-level population ratios for two N2-Ar mixtures. The purely radiative distribution in table 2 was calculated as described earlier [3] on the basis of an initial N2(C) distribution of 75% u = 0,20% u = 1, and 5% u = 2 [ 121 followed by the radiative transition rates of the second and first positive systems of N2 [ 13]_ Since surface deactivation of N*(A) is a very efficient process, it is unlikely to introduce substantial vibrational discrimination in the ~7 ms transit time between the Ar* + N2 mixing region and the LIF cell. Vibrational relaxation by ground-state N2 is therefore the major source of N2(A, u) removal in the absence of added quenchers. Dreyer and Perner [ 141 have measured I& for u =G7 and provided evidence that Au = 2 for the process_ For N2(A, IJ = 3),X- = 3.8 X IO-14 cm3 s-l, i.e. the effective first-order rate constant is 470 s-1 for our 20% N, mixture and 140 s-l for the 6% mixture. In 7 ms, this would reduce the u = 3 population by a factor of ~25 for the 20% and by a factor of 2.7 for the 6% mixture. This may explain why the u = 3 state was detected in 6% but not in 20% N2-Ar flows. For the
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11 November 1983
u = 2 to 0 relaxation of N2(A) by N*(X), the corresponding reductions would be factors of 1 S for 20% and 1.14 for 6%, i.e. quite minor. These relaxation rates are used to calculate the populations in the fourth and fifth columns of table 2. The radiative cascade also neglects the fast collisional relaxation processes in the B state of N, which compete successfully with the relatively slow, radiative first positive transition [3,15] and further increase the population of the lower u levels of N*(B). With these changes and with the rapid relaxation of N2(A, u > 3) the observed relative populations are in satisfactory agreement with prediction_ Acknowledgement This work was supported by the Air Force Geophysics Laboratory and Defense Nuclear Agency under Task S99 QAXHD. References [l]
W.G. Clark and D-IV_ Setser, J. Phys. Chem. 84 (1980) 2225. [2] hlP. lannuzzi and F. Kaufman, J. Phys. Chem. S5 (1981) 2163. [3] BIP. lannuzzi, J-B. Jeff&s and F. Kaufman, Chem. Phys. Letters 87 (1982) 570. [4] T-G. Slanger, B J. Wood and G. Black, J. Photochem. 2 (1973) 63. 1.51 B-M_ Berquist, L-S. Dzelzkalns and F_ Kaufman, J. Chem. Phys. 76 (1982) 2984. [6] L-S. Dzelzkalns and F. Kaufman, J. Chem. Phys., to be published. [7] J.D. Iambert, Vibrational and rotational relaxation in gases (Clarendon Press, O_tiord, 1977) pp_ 102,109. [S] W-H. Green and JX. Hancock, J. Chem. Phys. 59 (1973) 4326. [9] J-C. Stephenson and E-R. hlosburg Jr.. J. Chem. Phys 60 (1974) 3562. [lo] J-C. Stephenson, J. Chem. Phys. 60 (1974) 4289. [ll] J-T. Yardley and C-B. hloore, J_ Chem. Phys. 48 (1968) 14. [12] E-A. Gislason, A.W. Kleyn and J. Los, Chem. Phys. Letters 67 (1979) 252. [ 131 A. Loftus and P-H. Krupenie, J. Phys. Chem. Ref. Data 6 (1977) 113. [14] J-W. Dreyer and D. Pemer, J. Chem. Phys. 58 (1973) 1195. [15] N. Sadeghi and D.W. Setser, Chem. Phys. Letters 77 (1981) 304; A. Rotem, I. Nadler and S. Rosenwaks, Chem. Phys. Letters 83 (1981) 281.
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