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Collision-induced relaxation of NO C2π(η′ = 0) and D2ϵ+(η′ = 0)
Volume 81, number 3
CHEMICAL PHYSICS LETTERS
1 August 1981
COLLISION-INDUCED RELAXATION OF NO C 2II(v' = O) AND D 2E+(v' = 0) F. LAHMANI, C. LARDEUX Laboratoire de Photophysique Moldculaire associ~ d l'Universit~ Paris-Sud, 91405 Orsay, France
and D. SOLGADI Laboratoire de Chimie Structurale Organique, and LURE, Universit~ de Paris-Sud, 91405 Orsay, France Received 7 July 1980; in fmal form 6 May 1981 The energy and time-resolved fluorescence of NO D 22;+(0' = 0) and C 2H(v' = 0) has been studied under selective excitation conditions using synchrotron radiation. Collisions with helium induce the transitions NO D(v' = 0) ~ NO C(v' = 0) -~
NO A(o' = 3) from which fluorescence is emitted. NO A(o' = 3) is also observed when NO D(v' = 1) and C(o' = 1) are excited in the presence of helium. Mechanisms for the collision-induced processes are discussed.
1. Introduction The electronic states of NO located close to or just above the ground-state dissociation limit (fig. 1) may decay by several competitive pathways: radiative deactivation, predissociation, as well as various nonradiative collisional processes. Recent progress in experimental techniques (selective excitation of single vibronic levels, fluorescence lifetime measurements in low-pressure conditions where collisional effects are negligible) has allowed us to obtain useful and direct information on the role of predissociation of low-lying states of NO [ 1 - 3 ] . It has been shown particularly that the v' = 0 - 3 levels of D 2N+ are not (or very weakly) predissociated with zero-pressure decay times of 1 5 - 2 0 ns, in good agreement with calculated radiative lifetimes. For the v' = 0 level of C 2II a drastic difference between fluorescence lifetimes from low rotational levels ( J < 9/2) (r = 1 5 - 3 0 ns) and higher ones (r = 1.4-3.0 ns) [1,4] suggests the predissociation threshold at 52400 cm -1. The u' = 1 level of the C state shows a very weak emission with an extremely short ( < 0.3 ns) decay time [5]. On the other hand, the vibronic levels of B 211(o' > 6) and A 2N+(o' > 3) are thought to be strongly predissociated [2,3]. However a weak fluorescence assigned
E(cm-1)
D2,Z +
.
t0~~/~ •
~
/_dis_~oci_a~_io_._ °//
VI-1~-6
-Z-:-/
50000
/.,0000
A :,Z + 1.0
I
7
0 1.5
r( N-O).~
Fig. 1. Potential energy curves of NO in the first dissociation
limit region (the b 41~-and a 4II are drawn according to ref. [18]).
to the emission from NO B 2H(v' = 9) has been obtained recently by excitation of NO with the 1849 A mercury line [6] and fluorescence from NO A 2Z+ (v' = 4) has been observed in the collisional relaxation of NO B 211(o' = 9) induced by helium.
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1 August 1981
CHEMICAL PHYSICS LETTERS
In this work, we are interested by the collisional relaxation of the C(v' = 0) and the D(v' = 0) states in the presence of helium. This process was previously studied by Callear and Pilling [7,8] with argon as collision partner. They showed that argon induces D(v' = 0) ~ C(v' = 0) transsitions and quenches partially the C(v' = 0) emission;however under broad-band excitation as used by Callear and Pilling, several excited states of NO were populated simultaneously and the analysis of the results was thus complex and sometimes ambiguous. In the present work a single vibronic level excitation was used and the fluorescence was time and energy resolved.
2. Experimental The synchrotron radiation of the Orsay collision ring (ACO) (a train of 1 ns pulses with a 13.7 MHz repetition rate) was used as the light source through a specially designed vacuum monochromator [9] with an ~.15 A band pass ensuring the global excitation of the rotational transition within the vibronic band under study. The exciting beam was focused in the center of a stainless-steel gas cell equipped with three MgF 2 windows. The fluorescence light was spectrally resolved by a second home-built vacuum monochromator [10] with a band pass o f ~ 2 0 A and monitored by means of a solar blind RTC 56, SBUVP photomultiplier. Conventional photon counting and correlated single-photon counting techniques were used for the study of fluorescence spectra and decay curves. In some cases, time-resolved spectra were recorded by counting the photons emitted between t and t + At after the exciting pulse by means of a single-channel analyzer receiving the output of the time-to-amplitude converter. The detection system was calibrated by comparing the intensity of the fluorescence bands from NO excited to different levels (NO D(v' = 1,0), C(v' = 0), A(v' = 3, 2,1)) with their relative emission probability (= qdo,,v 3 with qv'v" the Franck-Condon factors [11-13]). The response of the detection was found to be constant over the range 2100-2800 A.
3. Results
3.1. C state For pure NO at low pressure (0.050 < p < 0.20 Tort), the fluorescence spectrum recorded under selective excitation of the C(v' = 0) level is composed of the C,0 ~ X,v" bands and of the A,0 -* X,v" ones resulting from the C ~ A radiative cascade populating the A(o' = 0) level. From the intensity ratios extrapolated to zero pressure [7], we estimated the branching ratio kc_x/kc_ A to be 2.2 -+ 0.2, somewhat higher than previously determined by Groth et al. [14]. The fluorescence decay (fig. 2a) (recorded in the C,0 ~ X, 1 transition) may be well fitted by a sum of two exponentials with r 1 = 3 ns, T2 = 38 ns and the ratio of pre-exponential factors A 2/A 1 = 0.13, corresponding obviously to the emission from high (predissociated) and low (non-predissociated) rotational levels. In the presence of helium, fluorescence spectra and decays are deeply modified. (1) In the low-pressure range (PIle = 0--20 Torr) we observed the decrease in the C - X band intensity attaining 40% with the half-pressure of He Pl/2 = 2 Torr without a clear appearance of any new spectral
220
•c p s
no-¢-i Va-b
i
~
~
288 180 168 148 128 188 88 68 4~ 28 8 1888 1850
t988 ~s8 2~8~ 2~s8 21'88 21s8 2288 2zs8[~l
Fig. 2. Partial view of the fluorescence spectrum from
C 217(o'= 0) (t9 = 0.11 Torr) excited at 1910 A in the presence of helium (300 Torr): (a) global fluorescence; (b) short-lived fluorescence, gate 0--10 ns; and (c) long-lived fluorescence, gate 40-66 ns. 532
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CHEMICAL PHYSICS LETTERS
features. In this pressure range, the absorption band corresponding to the C(v' = 0) ~ X(v' = 0) transition (measured with the same apparatus under 2 A resolution) is not modified. Thus the observed quenching is not due to a pressure broadening effect on light absorption. However the excitation spectrum of the C --> X fluorescence recorded under the same resolution shows, besides an overall decrease in intensity of ~40% some distorsion in its contour when 5 Torr of helium is added to NO (0.2 Torr): the long-wavelength wing of the band corresponding probably to low J levels of C ~- X 2113/2 is clearly cut off while the shortwavelength part of C +- X 2111/2 (higher J levels) seems to be very little influenced by the presence of helium. These observations indicate a redistribution of emitting rotational levels of low J. This result is corroborated by lifetime measurements which show that in the presence of helium, the decay time and relative intensity of the long component is reduced by a factor o f ~ 2 (r 2 = 20 ns, A2/A 1 = 0.07) without any effect on the lifetime of short-lived levels (fig. 2b). (2) A further increase of the helium pressure has no effect on the intensity or decay time of the C - X emission but induces the appearance of new emission bands which may easily be assigned to the A(v' = 3) emission. The time-resolved fluorescence spectrum shows that for an early gate ( 0 - 1 0 ns with respect to the pumping pulse) the C - X emission predominates, while the spectrum recorded with a delayed ( 4 0 - 6 6 ns) gate is practically identical with that obtained under direct excitation of the A(v' = 3) level (fig. 3). The most important facts which can be deduced from fluorescence intensity measurements are that (i) the addition of helium above 20 Tort results in an enhancement of the total fluorescence because of A(v' = 3) emission increase superposed with the constant C(o' = 0) fluorescence, and (ii) the half-pressure for A -+ X fluorescence induction is much higher than that for the partial quenching of C -+ X fluorescence (Pl/2 = 500 Torr). Other information on the collisional relaxation mechanism is provided by monitoring the time evolution of A(v' = 3) fluorescence resulting from excitation of C(v' = 0). If it is assumed that A(v' = 3) is populated directly from C(v' = 0), a simple kinetic treatment for two species connected by collisions shows that the lowest of the two levels would decay according to the equation
1 August 1981
Log(N)8
7
6
:
:-.
":".%". ..
a
5
4
..
b
3
2
18
°
2~
.
..
38
48
~
68
78 ns
Fig. 3. Fluorescence decay of NO C(v' = 0), hex c = 1910 A: (a) hob s = 1920 A, pure NO (p = 0.11 Ton); (b) h o b s = 1910 A, NO (p = 0.11 Tort) + He (p = 100 Tort); and (c) exciting pulse.
IF(A(v'= 3)) =
A [exp(-kct - exp(-kAat) ] k c _ kA a ,
where 1/k C is the lifetime of C(v' = 0) and 1/kAa the lifetime of A(v' = 3) for the helium pressure used in the experiment. In other words, the observed A(v' = 3) fluorescence decay would be reproduced (except for a scale factor) by the convolution of the C(v' = 0) decay curve by the directly measured lifetime of A(v' = 3) at the same helium pressure. Such an experiment has been performed for a typical NO(0.2 T o r r ) He(80 Torr) mixture excited at 1910 A (C, v' = 0) and monitored at 2030 A corresponding to the A(v' = 3) -+ X(v' = 1) emission band. The experimental decay curve lags behind a curve calculated from the convolution of the C(v' = 0) -+ X emission prot~fle with the A(v' = 3) lifetime. All these observations indicate that C(v' = 0) ~ A(v' = 3) electronic relaxation is not a direct process but involves an intermediate state. Under selective excitation of the C(v' = 1) level, no fluorescence could be detected under our experimental conditions. At increasing helium pressures we observed the appearance of a complex fluorescence spectrum with the most prominent features corre-
533
Volume 81, number 3
CHEMICAL PHYSICS LETTERS
160 cps
I40 120, 100 80
JL
60
20 0 1700
1750
180@
1850
2 00I
Fig. 4. Fluorescence excitation spectrum of NO A(o' = 3) hob s = 2030 A, p(NO) = 0.11 Torr, p(He) = 300 Tort (gate 30-60 ns). sponding to the emission from the A(v' = 3) state. Fig. 4 shows the excitation spectrum of the longlived emission from A(v' = 3) in the presence of helium where the C(o' = 1) level is clearly seen. 3. Z D state
Under selective excitation of the D(v' = 0) level in collision-free conditions, the fluorescence spectrum consists in D -+ X and A ~ X emission with the intensity distribution giving the branching ratio kD_X/kD_ A = 3.4 -+ 0.4. The fluorescence decay recorded in the D(v' = 0) -+ X(v' = 0) emission band is purely exponential with ~"= 16 ns. For low helium pressures (PIle = 0--30 Torr) we observed mainly a decrease of the strong D -+ X fluorescence intensity and the appearance of the C -* X emission. From the shortening of the D(v' = 0) decay time, one can determine the overall rate of the collisional depopulation of the D(v' = 0) state as equal to kq = 1.3 X 10 -10 cm 3 molecule -1 s-1. This value is in good agreement with the quenching rate constant of NO D(v = 0) by argon determined by Callear and Pilling (kq = 1.5 X 10 -10 cm 3 molecule -1 S-l).
For higher He pressures, the emission from A(v' = 3) builds up. At PIle = 300 Torr, the time-resolved spectra show that for an early gate ( 0 - 1 0 ns) the spectrum is composed of overlapping D ~ X and 534
1 August 1981
C -+ X bands, the relative intensity of the latter increasing with helium pressure. With a delayed gate ( 3 0 - 6 0 ns), emission from A(v' = 3) is predominant. In this pressure range, we observed a further shortening of the D(v' = 0) fluorescence but the decay curves become non-exponential and contain a long-lived component. This long decay is pressure dependent and a S t e r n - V o l m e r treatment gives a zero-pressure lifetime of 140 ns and a quenching rate constant by helium of (3.4 -+ 0.8) × 10 -12 cm 3 molecule -1 s-1. The contribution of this component to the total decay is only ~1%. A long-lived component superposed with the short D(v' = 0) -+ X decay has also been observed recently by Hikida et al. [15] and has been assigned to the A(v' = 4) -+ X emission. In fact the A(v' = 4) level appears at the same energy as the D(v' = 0) state but the A(v' = 4) +- X(v' = 0) transition probability is much weaker than D(v' = 0) +- X(v' = 0). In the presence of helium the D(v' = 0)-+ X fluorescence is quenched more rapidly than the A(o' = 4), leaving the long A(v' = 4) -* X emission easier to observe. The extrapolated zero-pressure lifetime obtained for this species (7-0 = 140 ns) is shorter than that obtained for lower vibrational levels of the A state and compares with that determined in ref. [ 15] (r 0 = 118 ns). As in the case of the C(v' = 0) -+ A(v' = 3) transition the time behaviour of the emission from C(v' = 0) populated by relaxation from D(o' = 0) has also been studied. The experimental decay curves are obviously different from those obtained by direct excitation of C(v' = 0) under the same conditions but can be fitted by a simulated curve calculated from the convolution of the observed D(v' = 0) -+ X(v' = 0) emission decay with the short lifetime of C(v' = 0) determined previously (r = 3 ns). This result shows that the D -+ C transition is a direct and rapid process. On the other hand, the time evolution of A(v' = 3) populated by excitation of D(v' = 0) cannot be monitored under our experimental conditions because of band overlap, but the half-pressure of A(v' = 3) -+ X fluorescence induction is higher than that for excitation of C(v' = 0) 601/2 = 1200 Torr). This indicates that the process populating A(v' = 3) takes place on a longer time scale than the depopulation of D(v' = O) and is consistent with a multistep mechanism. By excitation of D(v' = 1) at 1802 A at low NO pressure, a strong fluorescence is observed. In the
Volume 81, number 3
CHEMICAL PHYSICSLETTERS
presence of helium the spectrum is quenched and a complex spectrum of lower intensity appears. In this case also, the most intense bands correspond to the A(v' = 3) -+ X emission (fig. 4).
4. Discussion
(1) Our data show that electronic relaxation from C(v' = 0), C(o' = 1), D(o' = 0) and D(o' = 1) levels, populating in its final step the A(v' = 3) state, is very efficient compared to the vibrational relaxation and electronic quenching in the A 2N+ state [16]. The D(v' = 0) + C(o' = 0) transition induced by argon was observed by Callear and Pilling [7] but they did not report the further transfer to t h e A(v' = 3) state. It is possible that argon and helium do not behave in the same way. It is equally possible that in the experimental conditions of ref. [7], where all levels were simultaneously populated, the collisional population of A(o' = 3) was balanced by its relaxation and could be overlooked. (2) In the case of the D(v' = 0) -+ C(o' = 0) transition, the time-resolved experiments show that the main part of the fluorescence decay from C(u' = 0) resulting from excitation at 1876 A of D(v' = 0) can be explained on the basis of a two-level system involving only D(o' = 0) and C(o' = 0) (predissociated levels). This is in agreement with the findings of Callear and Pilling [8] which have shown that argon induces the same process with unit efficiency. In contrast, the time behaviour and pressure dependence of C(o' = 0) and A(o' = 3) emission show that A(v' = 3) is not directly populated from the initially excited state but that an intermediate dark state is involved. The proposed mechanism is the following: the C(v' = 0) (J > 9/2) levels are coupled intramotecularly with another manifold Y(o). The collisions induce the Y(v) -+ A(u' = 3) transfer and the growth time of A(v' = 3) is not related to the population decay of C(v' = 0) but of the Y(o) levels. This hidden state Y may be identified with one of the states occurring in the relevant energy range - the valence B 21I or quartet a411 and b 4Z- states (fig. 1). It is known that C(v' = 0) is homogeneously perturbed by the B 211 o' = 7 level [17] and that potential energy curves of A 2N+ and B 21I states cross each other in the vicinity of the A(v' = 3) and B(o' = 7) levels. Both fac-
1 August 1981
tors may be favorable to the strong mixing and efficient electronic relaxation in collisionally perturbed molecules. Similar collisional transfer between B 211(o' = 9) and A 2Z+(o' = 4) has been observed by Hikida et al. [ 15] in the case of excitation of NO at 1849 A in the presence of helium. Other possible candidates are the quartet states a411 and b 4Z-. There is strong evidence that a4II is the state which causes predissociation of NO C(o' = 0) [4,7]. On the other hand, b 4N- has been postulated to be the immediate precursor of NO B produced in the radiative recombination o f N + O atoms [18]. (3) The initial effect of helium collisions on NO C(o' = 0) is the partial quenching of the emission from the lowest rotational levels without affecting the predissociated ones. The partial quenching of NO C(v' = 0) fluorescence intensity has also been observed by Callear and Pilling [7] for collisions with argon and has been explained by rotational relaxation which results in the transfer of low J levels to higher predissociated ones. The very large cross section (250 A 2) deduced for the crossing from non-predissociated levels to higher ones seems difficult to adopt, specially in view of the experimental value of 10 )k2 estimated by Melton and Klemperer [19] for rotational relaxation of NO A(o' = 1). Besides, the persisting 20 ns component observed iri the fluorescence decay from C(o' = 0) at pressures of helium above 20 Torr cannot be explained by this mechanism. Alternatively, it is known that the lowest rotational levels of C 2II(o' = 0) interact with the B 2113/2(o' = 7), yielding three sets of energy levels for each J value [17]. In the absence of collisions, the mean lifetime of these levels should be lengthened (r = 38 ns) relative to pure C levels because of the admixture of B character. The effect of collisions would be to depopulate the states which are substantially mixed via relaxation in the B manifold, leaving species of predominant C 211 character which decay radiatively with a 20 ns lifetime. It would be necessary to confirm this assumption and role out rotational relaxation being responsible for the low helium pressure behaviour by studying the fluorescence from NO C(v' = 0) in more selective conditions of excitation. The two-photon excitation of NO C(v' = 0), which achieves selective population of single low rotational levels, seems to be the best method to solve this problem [20]. 535
Volume 81, number 3
CHEMICAL PHYSICS LETTERS
Acknowledgement We are greatly indebted to the technical team of the Orsay Linear Accelerator Laboratory and o f L U R E whose work made this study possible.
References [ 1 ] O. Benoist d'Azy, R. L6pez-Delgado and A. Trainer, Chem. Phys. 9 (1975) 327. [2] J. Brzozowski, N. Elander and P. Erman, Physica Scripta 9 (1974) 99. [3] J. Brzozowski, P. Erman and M. Lyyra, Physica Scripta 14 (1978) 290. [4] S. Yagi, T. Hikida and Y. Mori, Chem. Phys. Letters 56 (1978) 113. [5] M. Assher and Y. Haas, Chem. Phys. Letters 59 (1978) 231. [6] T. Hikida, N. Washida, S. Nakajima, S. Yagi, T. lehimura and Y. Mori, J. Chem. Phys. 63 (1975) 5470.
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[7] A.B. Callear and M.J. Pilling, Trans. Faraday Soc. 66 (1970) 1618. [8] A.B. Callear and M.J. Pilling, Trans. Faraday Soc. 66 (1970) 1886. [9] M. LavoU~eand R. L6pez-Delgado, Rev. Sci. Instr. 48 (1977) 7. [10] M. Lavoll6e and A. Trainer, Chem. Phys. 44 (1979) 45. [11] G.R. Mohlmann, H.A. van Sprang, E. Bloeman and Z.J. de Heer, Chem. Phys. 32 (1978) 239. [12] D.C. Jain and R.C. Sahni, Trans. Faraday Soc. 62 (1966) 2997. [13] H.A. Ory, J. Chem. Phys. 40 (1964) 562. [14] W. Groth, D. Kley and V. Schurath, J. Quant. Spectry. Radiative Transfer 11 (1971) 1475. [15] T, Hikida, S. Yagi and Y. Mori, Chem. Phys. 52 (1980) 390. [16] E.H. Fink, Habilitationsschrift, Bonn Universitfft (1976). [17] A, Lagerqvist and E. Miescher, Helv. Phys. Acta 36 (1962) 257. [18] I.M. Campbell and R.S. Mason, J. Photochem. 8 (1978) 375. [19] L.A. Melton and W. Klemperer, J. Chem. Phys. 59 (1973) 1099. [20] P.A. Freedman, Can. J. Phys. 55 (1977) 1387.
CHEMICAL PHYSICS LETTERS
1 August 1981
COLLISION-INDUCED RELAXATION OF NO C 2II(v' = O) AND D 2E+(v' = 0) F. LAHMANI, C. LARDEUX Laboratoire de Photophysique Moldculaire associ~ d l'Universit~ Paris-Sud, 91405 Orsay, France
and D. SOLGADI Laboratoire de Chimie Structurale Organique, and LURE, Universit~ de Paris-Sud, 91405 Orsay, France Received 7 July 1980; in fmal form 6 May 1981 The energy and time-resolved fluorescence of NO D 22;+(0' = 0) and C 2H(v' = 0) has been studied under selective excitation conditions using synchrotron radiation. Collisions with helium induce the transitions NO D(v' = 0) ~ NO C(v' = 0) -~
NO A(o' = 3) from which fluorescence is emitted. NO A(o' = 3) is also observed when NO D(v' = 1) and C(o' = 1) are excited in the presence of helium. Mechanisms for the collision-induced processes are discussed.
1. Introduction The electronic states of NO located close to or just above the ground-state dissociation limit (fig. 1) may decay by several competitive pathways: radiative deactivation, predissociation, as well as various nonradiative collisional processes. Recent progress in experimental techniques (selective excitation of single vibronic levels, fluorescence lifetime measurements in low-pressure conditions where collisional effects are negligible) has allowed us to obtain useful and direct information on the role of predissociation of low-lying states of NO [ 1 - 3 ] . It has been shown particularly that the v' = 0 - 3 levels of D 2N+ are not (or very weakly) predissociated with zero-pressure decay times of 1 5 - 2 0 ns, in good agreement with calculated radiative lifetimes. For the v' = 0 level of C 2II a drastic difference between fluorescence lifetimes from low rotational levels ( J < 9/2) (r = 1 5 - 3 0 ns) and higher ones (r = 1.4-3.0 ns) [1,4] suggests the predissociation threshold at 52400 cm -1. The u' = 1 level of the C state shows a very weak emission with an extremely short ( < 0.3 ns) decay time [5]. On the other hand, the vibronic levels of B 211(o' > 6) and A 2N+(o' > 3) are thought to be strongly predissociated [2,3]. However a weak fluorescence assigned
E(cm-1)
D2,Z +
.
t0~~/~ •
~
/_dis_~oci_a~_io_._ °//
VI-1~-6
-Z-:-/
50000
/.,0000
A :,Z + 1.0
I
7
0 1.5
r( N-O).~
Fig. 1. Potential energy curves of NO in the first dissociation
limit region (the b 41~-and a 4II are drawn according to ref. [18]).
to the emission from NO B 2H(v' = 9) has been obtained recently by excitation of NO with the 1849 A mercury line [6] and fluorescence from NO A 2Z+ (v' = 4) has been observed in the collisional relaxation of NO B 211(o' = 9) induced by helium.
0 0 0 9 - 2 6 1 4 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 02.50 © North-Holland Publishing Company
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CHEMICAL PHYSICS LETTERS
In this work, we are interested by the collisional relaxation of the C(v' = 0) and the D(v' = 0) states in the presence of helium. This process was previously studied by Callear and Pilling [7,8] with argon as collision partner. They showed that argon induces D(v' = 0) ~ C(v' = 0) transsitions and quenches partially the C(v' = 0) emission;however under broad-band excitation as used by Callear and Pilling, several excited states of NO were populated simultaneously and the analysis of the results was thus complex and sometimes ambiguous. In the present work a single vibronic level excitation was used and the fluorescence was time and energy resolved.
2. Experimental The synchrotron radiation of the Orsay collision ring (ACO) (a train of 1 ns pulses with a 13.7 MHz repetition rate) was used as the light source through a specially designed vacuum monochromator [9] with an ~.15 A band pass ensuring the global excitation of the rotational transition within the vibronic band under study. The exciting beam was focused in the center of a stainless-steel gas cell equipped with three MgF 2 windows. The fluorescence light was spectrally resolved by a second home-built vacuum monochromator [10] with a band pass o f ~ 2 0 A and monitored by means of a solar blind RTC 56, SBUVP photomultiplier. Conventional photon counting and correlated single-photon counting techniques were used for the study of fluorescence spectra and decay curves. In some cases, time-resolved spectra were recorded by counting the photons emitted between t and t + At after the exciting pulse by means of a single-channel analyzer receiving the output of the time-to-amplitude converter. The detection system was calibrated by comparing the intensity of the fluorescence bands from NO excited to different levels (NO D(v' = 1,0), C(v' = 0), A(v' = 3, 2,1)) with their relative emission probability (= qdo,,v 3 with qv'v" the Franck-Condon factors [11-13]). The response of the detection was found to be constant over the range 2100-2800 A.
3. Results
3.1. C state For pure NO at low pressure (0.050 < p < 0.20 Tort), the fluorescence spectrum recorded under selective excitation of the C(v' = 0) level is composed of the C,0 ~ X,v" bands and of the A,0 -* X,v" ones resulting from the C ~ A radiative cascade populating the A(o' = 0) level. From the intensity ratios extrapolated to zero pressure [7], we estimated the branching ratio kc_x/kc_ A to be 2.2 -+ 0.2, somewhat higher than previously determined by Groth et al. [14]. The fluorescence decay (fig. 2a) (recorded in the C,0 ~ X, 1 transition) may be well fitted by a sum of two exponentials with r 1 = 3 ns, T2 = 38 ns and the ratio of pre-exponential factors A 2/A 1 = 0.13, corresponding obviously to the emission from high (predissociated) and low (non-predissociated) rotational levels. In the presence of helium, fluorescence spectra and decays are deeply modified. (1) In the low-pressure range (PIle = 0--20 Torr) we observed the decrease in the C - X band intensity attaining 40% with the half-pressure of He Pl/2 = 2 Torr without a clear appearance of any new spectral
220
•c p s
no-¢-i Va-b
i
~
~
288 180 168 148 128 188 88 68 4~ 28 8 1888 1850
t988 ~s8 2~8~ 2~s8 21'88 21s8 2288 2zs8[~l
Fig. 2. Partial view of the fluorescence spectrum from
C 217(o'= 0) (t9 = 0.11 Torr) excited at 1910 A in the presence of helium (300 Torr): (a) global fluorescence; (b) short-lived fluorescence, gate 0--10 ns; and (c) long-lived fluorescence, gate 40-66 ns. 532
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features. In this pressure range, the absorption band corresponding to the C(v' = 0) ~ X(v' = 0) transition (measured with the same apparatus under 2 A resolution) is not modified. Thus the observed quenching is not due to a pressure broadening effect on light absorption. However the excitation spectrum of the C --> X fluorescence recorded under the same resolution shows, besides an overall decrease in intensity of ~40% some distorsion in its contour when 5 Torr of helium is added to NO (0.2 Torr): the long-wavelength wing of the band corresponding probably to low J levels of C ~- X 2113/2 is clearly cut off while the shortwavelength part of C +- X 2111/2 (higher J levels) seems to be very little influenced by the presence of helium. These observations indicate a redistribution of emitting rotational levels of low J. This result is corroborated by lifetime measurements which show that in the presence of helium, the decay time and relative intensity of the long component is reduced by a factor o f ~ 2 (r 2 = 20 ns, A2/A 1 = 0.07) without any effect on the lifetime of short-lived levels (fig. 2b). (2) A further increase of the helium pressure has no effect on the intensity or decay time of the C - X emission but induces the appearance of new emission bands which may easily be assigned to the A(v' = 3) emission. The time-resolved fluorescence spectrum shows that for an early gate ( 0 - 1 0 ns with respect to the pumping pulse) the C - X emission predominates, while the spectrum recorded with a delayed ( 4 0 - 6 6 ns) gate is practically identical with that obtained under direct excitation of the A(v' = 3) level (fig. 3). The most important facts which can be deduced from fluorescence intensity measurements are that (i) the addition of helium above 20 Tort results in an enhancement of the total fluorescence because of A(v' = 3) emission increase superposed with the constant C(o' = 0) fluorescence, and (ii) the half-pressure for A -+ X fluorescence induction is much higher than that for the partial quenching of C -+ X fluorescence (Pl/2 = 500 Torr). Other information on the collisional relaxation mechanism is provided by monitoring the time evolution of A(v' = 3) fluorescence resulting from excitation of C(v' = 0). If it is assumed that A(v' = 3) is populated directly from C(v' = 0), a simple kinetic treatment for two species connected by collisions shows that the lowest of the two levels would decay according to the equation
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Log(N)8
7
6
:
:-.
":".%". ..
a
5
4
..
b
3
2
18
°
2~
.
..
38
48
~
68
78 ns
Fig. 3. Fluorescence decay of NO C(v' = 0), hex c = 1910 A: (a) hob s = 1920 A, pure NO (p = 0.11 Ton); (b) h o b s = 1910 A, NO (p = 0.11 Tort) + He (p = 100 Tort); and (c) exciting pulse.
IF(A(v'= 3)) =
A [exp(-kct - exp(-kAat) ] k c _ kA a ,
where 1/k C is the lifetime of C(v' = 0) and 1/kAa the lifetime of A(v' = 3) for the helium pressure used in the experiment. In other words, the observed A(v' = 3) fluorescence decay would be reproduced (except for a scale factor) by the convolution of the C(v' = 0) decay curve by the directly measured lifetime of A(v' = 3) at the same helium pressure. Such an experiment has been performed for a typical NO(0.2 T o r r ) He(80 Torr) mixture excited at 1910 A (C, v' = 0) and monitored at 2030 A corresponding to the A(v' = 3) -+ X(v' = 1) emission band. The experimental decay curve lags behind a curve calculated from the convolution of the C(v' = 0) -+ X emission prot~fle with the A(v' = 3) lifetime. All these observations indicate that C(v' = 0) ~ A(v' = 3) electronic relaxation is not a direct process but involves an intermediate state. Under selective excitation of the C(v' = 1) level, no fluorescence could be detected under our experimental conditions. At increasing helium pressures we observed the appearance of a complex fluorescence spectrum with the most prominent features corre-
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160 cps
I40 120, 100 80
JL
60
20 0 1700
1750
180@
1850
2 00I
Fig. 4. Fluorescence excitation spectrum of NO A(o' = 3) hob s = 2030 A, p(NO) = 0.11 Torr, p(He) = 300 Tort (gate 30-60 ns). sponding to the emission from the A(v' = 3) state. Fig. 4 shows the excitation spectrum of the longlived emission from A(v' = 3) in the presence of helium where the C(o' = 1) level is clearly seen. 3. Z D state
Under selective excitation of the D(v' = 0) level in collision-free conditions, the fluorescence spectrum consists in D -+ X and A ~ X emission with the intensity distribution giving the branching ratio kD_X/kD_ A = 3.4 -+ 0.4. The fluorescence decay recorded in the D(v' = 0) -+ X(v' = 0) emission band is purely exponential with ~"= 16 ns. For low helium pressures (PIle = 0--30 Torr) we observed mainly a decrease of the strong D -+ X fluorescence intensity and the appearance of the C -* X emission. From the shortening of the D(v' = 0) decay time, one can determine the overall rate of the collisional depopulation of the D(v' = 0) state as equal to kq = 1.3 X 10 -10 cm 3 molecule -1 s-1. This value is in good agreement with the quenching rate constant of NO D(v = 0) by argon determined by Callear and Pilling (kq = 1.5 X 10 -10 cm 3 molecule -1 S-l).
For higher He pressures, the emission from A(v' = 3) builds up. At PIle = 300 Torr, the time-resolved spectra show that for an early gate ( 0 - 1 0 ns) the spectrum is composed of overlapping D ~ X and 534
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C -+ X bands, the relative intensity of the latter increasing with helium pressure. With a delayed gate ( 3 0 - 6 0 ns), emission from A(v' = 3) is predominant. In this pressure range, we observed a further shortening of the D(v' = 0) fluorescence but the decay curves become non-exponential and contain a long-lived component. This long decay is pressure dependent and a S t e r n - V o l m e r treatment gives a zero-pressure lifetime of 140 ns and a quenching rate constant by helium of (3.4 -+ 0.8) × 10 -12 cm 3 molecule -1 s-1. The contribution of this component to the total decay is only ~1%. A long-lived component superposed with the short D(v' = 0) -+ X decay has also been observed recently by Hikida et al. [15] and has been assigned to the A(v' = 4) -+ X emission. In fact the A(v' = 4) level appears at the same energy as the D(v' = 0) state but the A(v' = 4) +- X(v' = 0) transition probability is much weaker than D(v' = 0) +- X(v' = 0). In the presence of helium the D(v' = 0)-+ X fluorescence is quenched more rapidly than the A(o' = 4), leaving the long A(v' = 4) -* X emission easier to observe. The extrapolated zero-pressure lifetime obtained for this species (7-0 = 140 ns) is shorter than that obtained for lower vibrational levels of the A state and compares with that determined in ref. [ 15] (r 0 = 118 ns). As in the case of the C(v' = 0) -+ A(v' = 3) transition the time behaviour of the emission from C(v' = 0) populated by relaxation from D(o' = 0) has also been studied. The experimental decay curves are obviously different from those obtained by direct excitation of C(v' = 0) under the same conditions but can be fitted by a simulated curve calculated from the convolution of the observed D(v' = 0) -+ X(v' = 0) emission decay with the short lifetime of C(v' = 0) determined previously (r = 3 ns). This result shows that the D -+ C transition is a direct and rapid process. On the other hand, the time evolution of A(v' = 3) populated by excitation of D(v' = 0) cannot be monitored under our experimental conditions because of band overlap, but the half-pressure of A(v' = 3) -+ X fluorescence induction is higher than that for excitation of C(v' = 0) 601/2 = 1200 Torr). This indicates that the process populating A(v' = 3) takes place on a longer time scale than the depopulation of D(v' = O) and is consistent with a multistep mechanism. By excitation of D(v' = 1) at 1802 A at low NO pressure, a strong fluorescence is observed. In the
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presence of helium the spectrum is quenched and a complex spectrum of lower intensity appears. In this case also, the most intense bands correspond to the A(v' = 3) -+ X emission (fig. 4).
4. Discussion
(1) Our data show that electronic relaxation from C(v' = 0), C(o' = 1), D(o' = 0) and D(o' = 1) levels, populating in its final step the A(v' = 3) state, is very efficient compared to the vibrational relaxation and electronic quenching in the A 2N+ state [16]. The D(v' = 0) + C(o' = 0) transition induced by argon was observed by Callear and Pilling [7] but they did not report the further transfer to t h e A(v' = 3) state. It is possible that argon and helium do not behave in the same way. It is equally possible that in the experimental conditions of ref. [7], where all levels were simultaneously populated, the collisional population of A(o' = 3) was balanced by its relaxation and could be overlooked. (2) In the case of the D(v' = 0) -+ C(o' = 0) transition, the time-resolved experiments show that the main part of the fluorescence decay from C(u' = 0) resulting from excitation at 1876 A of D(v' = 0) can be explained on the basis of a two-level system involving only D(o' = 0) and C(o' = 0) (predissociated levels). This is in agreement with the findings of Callear and Pilling [8] which have shown that argon induces the same process with unit efficiency. In contrast, the time behaviour and pressure dependence of C(o' = 0) and A(o' = 3) emission show that A(v' = 3) is not directly populated from the initially excited state but that an intermediate dark state is involved. The proposed mechanism is the following: the C(v' = 0) (J > 9/2) levels are coupled intramotecularly with another manifold Y(o). The collisions induce the Y(v) -+ A(u' = 3) transfer and the growth time of A(v' = 3) is not related to the population decay of C(v' = 0) but of the Y(o) levels. This hidden state Y may be identified with one of the states occurring in the relevant energy range - the valence B 21I or quartet a411 and b 4Z- states (fig. 1). It is known that C(v' = 0) is homogeneously perturbed by the B 211 o' = 7 level [17] and that potential energy curves of A 2N+ and B 21I states cross each other in the vicinity of the A(v' = 3) and B(o' = 7) levels. Both fac-
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tors may be favorable to the strong mixing and efficient electronic relaxation in collisionally perturbed molecules. Similar collisional transfer between B 211(o' = 9) and A 2Z+(o' = 4) has been observed by Hikida et al. [ 15] in the case of excitation of NO at 1849 A in the presence of helium. Other possible candidates are the quartet states a411 and b 4Z-. There is strong evidence that a4II is the state which causes predissociation of NO C(o' = 0) [4,7]. On the other hand, b 4N- has been postulated to be the immediate precursor of NO B produced in the radiative recombination o f N + O atoms [18]. (3) The initial effect of helium collisions on NO C(o' = 0) is the partial quenching of the emission from the lowest rotational levels without affecting the predissociated ones. The partial quenching of NO C(v' = 0) fluorescence intensity has also been observed by Callear and Pilling [7] for collisions with argon and has been explained by rotational relaxation which results in the transfer of low J levels to higher predissociated ones. The very large cross section (250 A 2) deduced for the crossing from non-predissociated levels to higher ones seems difficult to adopt, specially in view of the experimental value of 10 )k2 estimated by Melton and Klemperer [19] for rotational relaxation of NO A(o' = 1). Besides, the persisting 20 ns component observed iri the fluorescence decay from C(o' = 0) at pressures of helium above 20 Torr cannot be explained by this mechanism. Alternatively, it is known that the lowest rotational levels of C 2II(o' = 0) interact with the B 2113/2(o' = 7), yielding three sets of energy levels for each J value [17]. In the absence of collisions, the mean lifetime of these levels should be lengthened (r = 38 ns) relative to pure C levels because of the admixture of B character. The effect of collisions would be to depopulate the states which are substantially mixed via relaxation in the B manifold, leaving species of predominant C 211 character which decay radiatively with a 20 ns lifetime. It would be necessary to confirm this assumption and role out rotational relaxation being responsible for the low helium pressure behaviour by studying the fluorescence from NO C(v' = 0) in more selective conditions of excitation. The two-photon excitation of NO C(v' = 0), which achieves selective population of single low rotational levels, seems to be the best method to solve this problem [20]. 535
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Acknowledgement We are greatly indebted to the technical team of the Orsay Linear Accelerator Laboratory and o f L U R E whose work made this study possible.
References [ 1 ] O. Benoist d'Azy, R. L6pez-Delgado and A. Trainer, Chem. Phys. 9 (1975) 327. [2] J. Brzozowski, N. Elander and P. Erman, Physica Scripta 9 (1974) 99. [3] J. Brzozowski, P. Erman and M. Lyyra, Physica Scripta 14 (1978) 290. [4] S. Yagi, T. Hikida and Y. Mori, Chem. Phys. Letters 56 (1978) 113. [5] M. Assher and Y. Haas, Chem. Phys. Letters 59 (1978) 231. [6] T. Hikida, N. Washida, S. Nakajima, S. Yagi, T. lehimura and Y. Mori, J. Chem. Phys. 63 (1975) 5470.
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[7] A.B. Callear and M.J. Pilling, Trans. Faraday Soc. 66 (1970) 1618. [8] A.B. Callear and M.J. Pilling, Trans. Faraday Soc. 66 (1970) 1886. [9] M. LavoU~eand R. L6pez-Delgado, Rev. Sci. Instr. 48 (1977) 7. [10] M. Lavoll6e and A. Trainer, Chem. Phys. 44 (1979) 45. [11] G.R. Mohlmann, H.A. van Sprang, E. Bloeman and Z.J. de Heer, Chem. Phys. 32 (1978) 239. [12] D.C. Jain and R.C. Sahni, Trans. Faraday Soc. 62 (1966) 2997. [13] H.A. Ory, J. Chem. Phys. 40 (1964) 562. [14] W. Groth, D. Kley and V. Schurath, J. Quant. Spectry. Radiative Transfer 11 (1971) 1475. [15] T, Hikida, S. Yagi and Y. Mori, Chem. Phys. 52 (1980) 390. [16] E.H. Fink, Habilitationsschrift, Bonn Universitfft (1976). [17] A, Lagerqvist and E. Miescher, Helv. Phys. Acta 36 (1962) 257. [18] I.M. Campbell and R.S. Mason, J. Photochem. 8 (1978) 375. [19] L.A. Melton and W. Klemperer, J. Chem. Phys. 59 (1973) 1099. [20] P.A. Freedman, Can. J. Phys. 55 (1977) 1387.