1457
J. Phys. Chem. 1984, 88, 1457-1458
with
I
0 Figure 4. Plots of
1 2 ITiS2-l*CNa*l, I@ mol dm-' q-l
'0.5 3
and T [ I vs. the concentration terms in eq 2 and
3, respectively. where the superscript int denotes intrinsic, e is the elementary charge, kB is the Boltzmann constant, and T is the absolute temperature. The intrinsic value of P P P wps determined to be 4.1 X lo4 mol dm-3 by using the results shown in Figure 2. The static and kinetic parameters are listed in Table I. When the first step in mechanism I is slower than the second step, the fast and slow relaxation times are expressed as 7r1= k2 k-2([TiS2-] [Na+]) (2)
+
+
As mentioned in the previous paper, when the first step is faster than the second step only single relaxation for the second step should be observed. The plots of 7f1and 7c1vs. the concentration term in eq 2 and 3 are shown in Figure 4. These straight lines lead to the conclusion that double relaxation can be attributed to mechanism I. The four rate constants were evaluated from the slopes and the intercepts of the straight lines and are listed in Table I. The values of K 1 and KP calculated from the rate constants obtained are also listed in Table I. The values of (PPP)'"' estimated from eq 1 by using these values is in good agreement with that determined statically. This fact further supports the validity of mechanism I proposed here. Among the rate constants for cation intercalations in the TiS2(H) and TiS2(Na) systems listed in Table I, the intercalation and dissociation rate constants for the Na+ ion are larger than those for the proton. The former difference may be due to the difference in interlayer separation for the proton (1.5 A) and Na+ (4.5 A), while the latter may be related to the relatively weak interaction of S2-with Na+ compared with the proton. Further kinetic studies on other alkali metal intercalation in layered materials are in progress. Registry No. Na, 7440-23-5; Tis2, 12039-13-3.
Formatlon and Quenching of Metastable N, AS&+ in the Electronic Relaxation of NO C2nand D2Z+in an NWN, Mixture Kazuhiko Shibuya, Takashi Imajo, Kinichi Obi, and Ikuzo Tanaka* Department of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo 152, Japan (Received: December 13, 1983)
-
Time-resolved experiments are carried out to elucidate the electronic relaxation mechanism of NO C211 (or D2Z+) + N2 N O A2Z+ N2,following the selective SVL excitation by two-photon absorption. The fluorescence intensity-time profiles of NO A2Z+formed collisionally show that the relaxation involves metastable N2 A3ZUfas an intermediate in the energy transfer sequence of N O C211(or D2Z+) + N2 XIZg+ NO X211 + N2 AS&+ (1) and N, A3Z,+ + NO X211 N2 XIZg+ + N O A2Z+ (2). The second energy transfer (process 2) rate controls the N O A2Z+fluorescence decay measured in NO/N2 mixtures. The quenching rate constant of N2 A3Zu+by NO in the ground state has been determined to be (6.9 0.7) X lo-" cm3 molecule-' s-l at 298 f 3 K.
+
This Letter presents our results of time-resolved fluorescence experiments on the relaxation of an excited state of NO in the presence of N2. The lowest excited AZZ+state of NO is known to be efficiently produced by collisions with N2 in the ground state after selective preparation of a higher excited state of NO such as B211(v = 7),l B211(v = 9),2 C 2 n ( v = 0),3 D22+(v = 0): or (1) K. Shibuya and F. Stuhl, Chem. Phys., 79, 367 (1983). (2) T. Hikida, N. Washida, S. Nakajima, S. Yagi, T. Ichimura, and Y. Mori, J. Chem. Phys., 63,5470 (1975); T. Hikida and Y. Mori, ibid., 69,346 (1978). (3) A. B. Callear and I. W. M . Smith, Trans. Faraday SOC.,61, 2383 (1965). . ( 4 j A. B. Callear, M. J. Pilling, and I. W. M. Smith, Trans. Faraday SOC., 64,2296 (1968).
-
-
B'2A(~= 2).s In 1965 Callear and Smith3 proposed a relaxation mechanism composed of two step energy transfers involving formation of metastable N2A3Z,+ as an intermediate from an experiment based on the fluorescence intensity measurement in NO-CO/Nz mixtures. Their idea was that, when a small amount of CO was added, it quenched mainly the intermediate A32: state of N 2 but the kinetic treatment was still rather complicated. To the best of our knowledge, since then, no experiment has been carried out to confirm the proposed relaxation mechanism. We report here a straightforward experiment in NO/N2 mixtures to elucidate the relaxation mechanism aod also an improved mea( 5 ) L. C. Lee and T.G. Slanger, unpublished results quoted by T. G. Slanger, to be published.
0022-3654/84/2088- 1457%01.50/0 0 1984 American Chemical Society
1458 The Journal of Physical Chemistry, Vol. 88, No. 8, 1984
surement of the quenching rate constant of metastable N2A3Z,+(u = 0 and 1) by a ground state N O X211(u = 0) in a simple NO/N2 system. A commercial nitrogen laser pumped dye laser (Molectron UV24 and DL14) beam was focussed into a fluorescence cell to induce a two-photon absorption to the excited states N O CzII(u = 0 and' 1) and D22+(u = 0 and 1) in a lasing wavelength of 358-383 nm. The pressures of NO (>99.0%) and N2 (>99.995%) were in the ranges of 0.1-2.0 and 100-760 torr, respectively. The fluorescence was dispersed by a monochromator (Nikon G.500) and the intensity was measured by a solar blind photomultiplier (Hamamatsu R166UH). The fluorescence-time profiles were recorded with a digital waveformer (Iwatsu DM-901, 10 ns/ channel) which was connected to a microcomputer (Sharp M Z 80B). For N O and N z mixtures, the fluorescence spectra recorded under selective excitation to N O C211(u = 0 and 1) and D22+(u = 0 and 1) could be assigned to spectra originating from three excited levels of N O namely an initially prepared level and A2Z+(u = 0 and 1). Except for C211(v = 1) excitation, the dominant fluorescence comes from A2Z+ which is formed by collision. Among the four vibronic levels prepared, the C211(u= 0) excitation is found to give the strongest A state fluorescence. Although a collisional electronic/vibrational relaxation cannot be neglected under the conditions of 1.5 torr of N O and 760 torr of N,, the fluorescence intensity ratio of A2Z+(v = 1)/A2Z+(v = 0) is around 0.17 which agrees with the vibrational population ratio of 0.15 measured in the N2A3Z,+ N O A%+ energy transfer.6 Since the lifetime of C211(u = 1) is very short (10.3 ns),' only half of the C211(u = 1) fluorescence intensity was quenched by an addition of 760 torr of Nz. Figure 1 shows a time profile of collisionally formed AZZ+(u = 0) fluorescence when NO is excited to C211(u = 1). The time profiles were independent of the initial excited state of NO, although the intensity was the weakest in the C211(v = l ) excitation. We assume the N O C 2 n , D2Z+ N O AZZ+relaxation takes place via two-step energy transfers.
-
Letters
w I-
z w ~
U
z W
U
104 4
N2 A3Z,+
N O A22+
k3
NO
+ N2 A3Z,+
+ N O -% N2 + N O AZZ+
-
I
TIME (NS)
-
5 234 1
'0
_jl'l
0." 0
(1) LL
0
(2a)
!z
(2b)
a
fluorescence and collisional quenching (3)
Then the time profile of N O A2Z+ fluorescence is in a form, Z ( t ) exp(-k2[NO]t) - exp(-k3t), because process 1 proceeds a t a rate of lo9 s-' which is much faster than the rates of processes 2 and 3 (-lo6 and -10' s-l, respectively) under the pressure conditions employed. In fact, the observed time profile of Figure l a presents a fast rise and a slow decay (rdecay = 0.71 ps). The reported values of kz differ widely but Clark and Setser recommend kz = (10 & 5) X lo-" cm3 molecule-' s-' as the present best estimate.6 This rate constant gives a metastable N 2 lifetime of 0.3-0.9 ps at 0.70 torr of NO, which agrees with the measured decay (0.71 ps) in Figure 1. The two-step energy transfer mechanism (processes 1 and 2) has thus been confirmed as a major N O CzII, D2Z+ NO AZZ+relaxation by N2 in the present time-resolved experiment. The decay rates of metastable NzA3Zu+determined from the experiment described above are plotted in Figure 2 for the excitation of the four different vibronic levels of NO. The decay rate was independent of N 2 pressures in the range 100-760 torr, cm3 molecule-' reflecting slow self-quenching by N, (13.7 X s-').* The slopes of the plots in Figure 2 give a quenching rate
G
U W
-
-
(6) W. G. Clark and D. W. Setser, J . Phys. Chem., 84,2225 (1980), and the references therein. (7) 0. Benoist d'Azy, R. LBpez-Delgano, and A. Tramer, Chem. Phys.,
2
1
Figure 1. Time profile of collisionally formed NO AZZ+(u = 0) fluorescence in an NO(0.70 torr)/N,(760 torr) mixture. The C211(u= 1) level of N O was initially excited and the fluorescence intensity of an A2Z+(u = 0) X211(v = 1) band was measured. The plots are linear (a) and semilogarithmic (b). The dwell time per channel was 10 ns. The decay time is 0.71 ps.
W
other processes
IO 0
-
NO CzII(or D2Z+) + N2
io5
v, w
0
1.0
2.0
10
20
.M 3
c8.do
2
1
0
0
10
2.0
NO PRESSURE ( T o r r 1 Figure 2. Decay rate of metastable N2 A%,+ as a function of NO pressure. The N2 pressure was constant at 760 torr. The slopes give the quenching rate constant (in torr-' ps-') of N, AS&+ by NO as (a) 2.17 for C211(u = 0) excitation, (b) 2.16 for C211(u= 1) excitation, (c) 2.16 for DZZ+(u= 0) excitation, and (d) 2.39 for DZZ+(u = 1) excitation.
constant of N2A3ZU+by ground-state NO of k, = (6.9 k 0.7) X lo-*' cm3 molecule-' s-' at 298 f 3 K. The value is independent of the initially excited levels of NO, although the maximum vibrational levels are u = 1 and 3 and u = 2 and 4 in N2A3Zu+ formed by energy transfers from N O C211(u = 0 and 1) and DZZ+(u= 0 and l), respectively. The slower vibrational levels(u = 0 and 1) of N2A3Z,+ are likely to be dominantly populated if process 1 gives a similar vibrational distribution as prodess 2. Our k2 value fits the value of 8.0 X lo-" cm3 molecule-' s-' measur& by Callear and Wood,g although many values of k2 have been reported6 ranging from 2.8 X lo-" to 1.5 X cm3 molecule-' S-1.
9, 327 (1975).
( 8 ) J. W. Dreyer and D. Perner, J. Chem. Phys., 58, 1195 (1973).
(9) A. B. Callear and P. M. Wood, Trans. Faraday SOC.,67,272 (1971).
