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C2 Radicals in a supersonic molecular beam. Radiative lifetime of the d 3Πg state measured by laser-induced fluorescence
Volume 143, number 5
CHEMICAL PHYSICS LETTERS
29 January 1988
C2 RADICALS IN A SUPERSONIC MOLECULAR BEAM. RADIATIVE LIFETIME OF THE d ‘l-I, STATE MEASURED BY LASER-INDUCED FLUORESCENCE
C. NAULIN, M. COSTES and G. DORTHE UA 348, Photophysique Photochimie Mokulaire,
Universitk de Bordeaux I, 33405 Talence Cedex, France
Received 16 November 1987
The combined techniques of pulsed supersonic molecular beams and radical generation by laser vaporisation of the corresponding solid target are used to produce C2 radicals seeded in a rare gas carrier beam. Extreme rotational cooling in the expansion, together with collision-free beam conditions, are shown to be particularly well suited for actual lifetime measurements, Analysis of laser-induced fluorescence decays of Cz (d ‘II,) species yields the following lifetime values for the first three vibrational levels ofdstate:~,.,~=101.8~4.2ns,r,.~,=96.7~5.2ns,andr~~~=104+17ns.
1. Introduction
other species than C2( a311,) can be pumped in the process and contribute to the decay curves.
The determination of the C,( d ‘II,) radiative lifetime is an old problem which has given rise to an abundant literature. An era of huge discrepancies, where too high values were reported, was closed in 1976 with the review of Tatarczyck et al. on the subject [ 11. Following their measurement yielding r,,o= 120? 10 ns, four other convergent determinations were reported for the v= 0 vibrational level: Curtis et al. in 1976: 123 f 6 ns [2], Cambell et al. in 1978: 120?20 ns [3], McDonald et al. in 1978: 119?6 ns [4], Erman in 1980: 120f4 ns [5]. Moreover, lifetimes deduced from the measurement of the band oscillator strength of the C,( d 311,-a311.) Swan system, namely Arnold in 1968: 124 ns [ 61, or Cooper and Nicholls in 1975: 125 ns [ 71, seemed to give great confidence for T,=~ at the 120 ns level. However two recent measurements lead to a further decrease: Stark and Davis in 1985: 92 + 5 ns [ 81, and Bauer et al. in 1986: 106+ 15 ns [9]. In this paper we report the tirst measurement performed with a supersonic beam source. The C2 radicals, produced in both low lying states, X ‘C: and a311u, are laser excited from aa, to d311, state (Swan system). Under such conditions, CZ radicals are free of any collisional relaxation. Furthermore, their rotational cooling greatly simplifies the excitation spectrum: it can thus be clearly seen that no 496
2. Experimental The experimental apparatus has been previously described [ 10,111. Briefly, a pulsed supersonic beam of carbon seeded in argon or helium is produced by laser ablation of a graphite rod at the exit of a pulsed nozzle, of the Gentry and Giese design [ 121, using a KrF* laser. The laser is spatially filtered to z 1.2 mrad, resulting in a fluence on the rod of z 5 x lo4 J m-‘. Such a high fluence is not optimal for C2 production, but dramatically reduces C, radical generation. Indeed the absorption spectrum of C3 (A ’ K-X ‘El ), which has an unusually low bending mode (YEW63 cm-‘) can easily plague the lifetime measurements (rC9(A,ny)~200 ns) [ 131. The C,(a311,) radicals are laser excited to the d311, state at a distance of 60 mm from the nozzle, using a Nd : YAG-pumped dye laser ( z 10 ns pulse duration). The latter is operated with Rh640 dye and the output is Raman shifted in Hz. The first anti-Stokes line is selected to excite Swan bands in the Av= 0 sequence. The fluorescence of the Av= - 1 sequence is collected withfll.2 optics, through a band-pass filter centred at 560 nm (10 nm fwhm, 50% transmission), and imaged onto the photocathode of a Ha-
0 009-2614/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
mamatsu R955 PMT (2.5 ns rise time, 50 R loaded). The PMT output is preamplitied (EG&G model Ml 15 wideband amplifier, 70 MHz, 5 ns rise time), and fed into either a boxcar averager (EG&G models Ml65 and M162) to record excitation spectra, or a transient digitizer (Tektronix 79 12, 7B80 time base, and 7A19 50 n vertical amplifier, 600 MHz) to measure radiative lifetimes. In a typical experiment, up to 64 individual fluorescence decay traces are accumulated, and then transferred to a computer for data treatment. Some single traces also have been recorded to ensure that no saturation of the electronics occurred.
3. Results 3.1. Excitation spectra Various spectra have been obtained, which show different degrees of cooling depending upon the operating conditions: nature of the carrier gas, gas load of the valve and delay between the opening of the valve and the tiring of the excimer laser. The rotationally supercooled Cz spectrum of fig. 1 exhibits
some unusual features, such as a prominent Q, branch and the absence of any bandhead. Only low J” levels of the lowest energy spin component (‘II z F ; term), remain strongly populated. Intermediate J” levels are not present, and no bandhead, which occurs at J” = 15, can be detected. However some excitation of high J” levels (J” > 40) remains, as the increasing rotational spacing with J” limits the efficiency of collision-induced rotational energy transfer [ 141 in the hydrodynamic region of the supersonic expansion where cooling occurs. Some vibrational excitation also survives this expansion: vibrational levels V”= 1 and 2 (not shown in fig. 1) are detected with the following approximate population ratios: N(u”=O):N(u”=l):N(u”=2)=100:10:1. The straightness of the baseline, which actually corresponds to the electric zero, indicates that no other species absorbs or emits in the excitation and fluorescence wavelength ranges. 3.2. Lifetime measurements Fluorescence decay has been recorded for several rotational levels of the u=O, 1 and 2 vibrational levels of CZd3TI,. The observed decays have been fitted using a least-squares method to an exponential function, y= exp(at+b)
I
29 January 1988
CHEMICAL PHYSICS LETTERS
Volume 143, number 5
I
I
516
515
.
(1)
However, such a method can introduce a deviance when y values become small compared to noise and uncertainties, as 1ny-t -co when y+O. This effect is very sensitive to any offset, even small. To avoid this problem, a blank has been recorded for each run under the same conditions. Furthermore, experimental measurements have been fitted over several time domains, corresponding to .the beginning (loo-50%), the central part (90-10%) and the trailing edge (50-2O/6) of the decay. The consistency of these determinations may then allow one to rule out any baseline problem, or non single-exponential decay. The lifetime derived from this analysis is then simply t=-l/a.
&SW (nm) Fig. 1. Excitation spectrum of Cl(a311U,v” ~0). Assignments of main branches originating from significantly populated terms, F; and F; , are only given; corresponding rotational temperatureis TzIO-15K.
Dispersion errors can easily be determined by an analysis of variance. Let AY be the difference between observed and calculated logarithmic values of the studied decay: 497
Volume 143, number 5
CHEMICAL PHYSICS LETTERS
AY=A(lny) =Ayly=Aat+Ab.
(2)
An upper limit of Au can be obtained assuming that measure dispersion is only due to the time-constant determination error, (AY)‘=(Aat)*
.
29 January 1988
have been controlled using a time calibrator (Ortec model 462): deviations observed did not exceed lo?& Convolution of PMT and electronics rise times (2.5 and 5 ns, respectively) results in an overestimation of the time constant:
(3)
l/2 T2t
Summing eq. (3) over the n points measured each St time sampling interval yields 1 (AY)2=(Aa&)2
ik’
.
(4)
I
Considering that n B 1 (i.e. Ck*e fn3, and n- 1x n), and noting that Aa=Ar/r*, allows one to express the upper limit of dispersion error, (Ar/r)*=3s*(~/nSt)*,
(5)
where s2= [X(AY)*]/(n- 1) is the variance estimate for the considered set of data. Experimental errors are mainly due to the transient digitizer time-base sweep error (2% specified). The sweep time accuracy, together with the linearity,
C(rise
times)*
.
(6)
>
However, this factor is almost negligible as (r’-~)/~=0.16%. On the other hand, underestimation of the time constant is possible as laser-excited radicals move out of the detector field of view with the beam velocity during the acquisition of the decay curves. In fact, such an effect is quite negligible for time constants in the 100 ns range. No difference could be pointed out in the results when seeding C2 in helium (beam velocity ~2140 m s-l) instead of argon (v=810 m s-l). The experimental uncertainty can therefore be set to an overestimated value of 3%.
Table 1 Lifetime determination by least-squares regression V
Line
Domain *)
n b,
r (ns)
102S
0
Q,(2)
B C E
38 114 160
99.2 101.4 98.6
1.3 1.0 3.1
3.0 0.8 1.7
R,(2) + R,(2)
B C E
38 116 164
98.0 101.9 100.9
1.3 0.80 2.3
2.9 0.6 1.3
R, (4)
B C E
36 116 167
93.5 98.8 102.3
1.0 3.0 2.7
2.2 2.2 1.5
P, (44)
B C E
27 108 151
69.4 100.3 119.8
4.1 11.3 17
6.5 9.4 14.1
Q,(2) and R,(2)
B C* E
35 71 169
91.4 96.7 102.8
0.9 1.0 2.6
1.9 1.1
B C E
36 102 154
99.0 104.1 105.9
4.5 7.5 15
11 7.1 9.7
1
2
Q,(2) and R,(2)
AT (ns)
1.4
” B, C and E stand for the fitting domain, i.e. [ IOO-SO%], [90-IO%] and [ 50-2941 of the maximum signal amplitude; C*: for l-l lines, the best fit was obtained in the [ 80-20%] domain. b, n is the number of points sampled.
498
Volume 143, number 5
CHEMICAL PHYSICS LETTERS
600
800
time
(no)
Fig 2. Fluorescence decay of C,_(d3fI,) excited on the (O-O) Q, (2) line. Solid line: experimental; dashed line: calculated for r = 10 1.4 ns; deviation of experimental values from the exponential lit is displayed on a magnified scale.
Table 2 &(d’II,,
5
v=O, 1and 2) radiativelifetimes
“‘(ns)
( 1 ,,,.A,.,,.,,)- ’ h,
II=0
o=l
u=2
101.8k4.2 124.8
96.7 & 5.2 120.8
104+ 17 116.8
” 95% confidence interval ‘) Calculated from ref. [ 15 1.
Several measurements have been achieved for each line considered. Each one consists in the accumulation of 64 traces by the digitizer. Analysis has been performed separately for each one, to check data consistency. Final analysis has then been achieved, based upon the sum of all measurements (line by line for the v=O state, and all lines for v= 1 and 2). Results are summarized in table 1. For each case, r, s, and Ar values are given. The spread of 7 values obtained is indeed larger for the weak lines (especially the O-O, P,( 44) and 2-2 lines). However, as shown in fig. 2, a quite good signal/noise ratio can be obtained, thus allowing an accurate determination of Cz(d311,) lifetimes listed in table 2.
4. Discussion The interest of our method lies in the fact that the measured fluorescence decay time constant directly
29 January 1988
yields the lifetime value, with no need of any further extrapolation, Uncertainties therefore arise only from experimental measurement errors, which indeed results in a more accurate determination. Besides collisional energy transfer effects, inherent to bulk experiments, simultaneous absorption and/or emission of C3 radicals may be an important source of errors. As an example, Curtis et al. [ 21 found a fluorescence decay time constant of 120 ns at 0.008 nm resolution, but a value of 200 ns at 0.5 nm resolution. In the latter case, C3 emission, on the ‘II-lx (0~; O-l vi 2) transitions which occur in the Swan (O-O) bandhead region, probably contributed to the signal, thus resulting in a time constant value close to C,( ‘II,) lifetime [ 131. In our experiment, excitation spectra have clearly shown (see above) that C3 does not contribute to the laser-induced fluorescence signal. For v=O, our result lies within the uncertainty domain of Bauer et al. [ 91, but is higher than the Stark and Davis value [ 81. Lifetimes estimated from the sum of Einstein coefficients given by Cooper and Nicholls [ 151 (see table 2) are somewhat higher, lying in the 120 ns “plateau”. They are based upon a determination of the sum of squared electronic transition moments, 1 IR,I* = 3.52 f 0.50 au (atomic units). The latter factor can be re-estimated from our v=O lifetime measurement to a higher value, C IR,I*=4.3au, in good agreement with the theoretical values of 4.12 au [ 161 and 4.10 au [ 171.
Acknowledgement The authors would like to thank Dr. Roland Bonneau for the loan of his transient digitizer data acquisition system.
References [ 11 T. Tatarczyk, E.H. Fink and K.H. Becker, Chem. Phys. Lettern40 (1976) 126. [2] L. Curtis, B. Engman and P. Erman, Physica Scripta 13 (1976) 270. [ 31J.D. Campbell, M.H. Yu and C. Wittig, Appl. Phys. Letters 32 (1978) 413. [4] JR. McDonald, A.P. Baronavski and V.M. Donnelly, Chem. Phys. 33 (1978) 161.
499
Volume 143, number 5
CHEMICAL PHYSICS LETTERS
[ 5 ] P. Em-ran, Physica Scripta 22 (1980) tO8. [ 61 J.O. Arnold, J. Quant. Spectry. Radiative Transfer 8 (1968) 1781. [7] D.M. Cooper and R.W. Nicholls, J. Quant. Spectry. RadiativeTransfer 15 (1975) 139. [8] G. Stark and S.P. Davis, Z. Physik A321 (1985) 75. [ 91 W. Bauer, K.H. Becker, M. Bielefeld and R. Meuser, Chem. Phys. Letters 123 (1986) 33. [lo] M. Costes, C. Naulin, G. Dot-the, C. Vaucamps and G. Nouchi, Faraday Discussions Chem. Sot. 84 (1987)paper 6. [ 1I] G. Dorthe, M. Costes, C. Naulin, J. Joussot-Dubien, C. Vaucamps and G. Nouchi, J. Chem. Phys. 83 (1985) 3 171.
500
29 January 1988
[ 121 W.R. Gentry and C.F. Giese, Rev. Sci. Instr. 49 (1978) 595.
[ 131 K.H. Becker, T. Tatarczyk and J. Radic-Peric, Chem. Phys. Letters 60 (1979) 502.
[ 141 S. Hay, F. Shokoohi, S. Callister and C. Wittig, Chem. Phys. Letters 118 (1985) 6.
[ 151 D.M. Cooper and R.W. Nicholls, Spectry. Letters 9 (1976) 139.
[ 161 J.O. Arnold and S.R. Langhoff, J. Quant. Spectry. RadiativeTransfer 19 (1978) 461. [ 171 D.M. Cooper, J. Quant. Spectry. Radiative Transfer 26 (1981) 113.
CHEMICAL PHYSICS LETTERS
29 January 1988
C2 RADICALS IN A SUPERSONIC MOLECULAR BEAM. RADIATIVE LIFETIME OF THE d ‘l-I, STATE MEASURED BY LASER-INDUCED FLUORESCENCE
C. NAULIN, M. COSTES and G. DORTHE UA 348, Photophysique Photochimie Mokulaire,
Universitk de Bordeaux I, 33405 Talence Cedex, France
Received 16 November 1987
The combined techniques of pulsed supersonic molecular beams and radical generation by laser vaporisation of the corresponding solid target are used to produce C2 radicals seeded in a rare gas carrier beam. Extreme rotational cooling in the expansion, together with collision-free beam conditions, are shown to be particularly well suited for actual lifetime measurements, Analysis of laser-induced fluorescence decays of Cz (d ‘II,) species yields the following lifetime values for the first three vibrational levels ofdstate:~,.,~=101.8~4.2ns,r,.~,=96.7~5.2ns,andr~~~=104+17ns.
1. Introduction
other species than C2( a311,) can be pumped in the process and contribute to the decay curves.
The determination of the C,( d ‘II,) radiative lifetime is an old problem which has given rise to an abundant literature. An era of huge discrepancies, where too high values were reported, was closed in 1976 with the review of Tatarczyck et al. on the subject [ 11. Following their measurement yielding r,,o= 120? 10 ns, four other convergent determinations were reported for the v= 0 vibrational level: Curtis et al. in 1976: 123 f 6 ns [2], Cambell et al. in 1978: 120?20 ns [3], McDonald et al. in 1978: 119?6 ns [4], Erman in 1980: 120f4 ns [5]. Moreover, lifetimes deduced from the measurement of the band oscillator strength of the C,( d 311,-a311.) Swan system, namely Arnold in 1968: 124 ns [ 61, or Cooper and Nicholls in 1975: 125 ns [ 71, seemed to give great confidence for T,=~ at the 120 ns level. However two recent measurements lead to a further decrease: Stark and Davis in 1985: 92 + 5 ns [ 81, and Bauer et al. in 1986: 106+ 15 ns [9]. In this paper we report the tirst measurement performed with a supersonic beam source. The C2 radicals, produced in both low lying states, X ‘C: and a311u, are laser excited from aa, to d311, state (Swan system). Under such conditions, CZ radicals are free of any collisional relaxation. Furthermore, their rotational cooling greatly simplifies the excitation spectrum: it can thus be clearly seen that no 496
2. Experimental The experimental apparatus has been previously described [ 10,111. Briefly, a pulsed supersonic beam of carbon seeded in argon or helium is produced by laser ablation of a graphite rod at the exit of a pulsed nozzle, of the Gentry and Giese design [ 121, using a KrF* laser. The laser is spatially filtered to z 1.2 mrad, resulting in a fluence on the rod of z 5 x lo4 J m-‘. Such a high fluence is not optimal for C2 production, but dramatically reduces C, radical generation. Indeed the absorption spectrum of C3 (A ’ K-X ‘El ), which has an unusually low bending mode (YEW63 cm-‘) can easily plague the lifetime measurements (rC9(A,ny)~200 ns) [ 131. The C,(a311,) radicals are laser excited to the d311, state at a distance of 60 mm from the nozzle, using a Nd : YAG-pumped dye laser ( z 10 ns pulse duration). The latter is operated with Rh640 dye and the output is Raman shifted in Hz. The first anti-Stokes line is selected to excite Swan bands in the Av= 0 sequence. The fluorescence of the Av= - 1 sequence is collected withfll.2 optics, through a band-pass filter centred at 560 nm (10 nm fwhm, 50% transmission), and imaged onto the photocathode of a Ha-
0 009-2614/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
mamatsu R955 PMT (2.5 ns rise time, 50 R loaded). The PMT output is preamplitied (EG&G model Ml 15 wideband amplifier, 70 MHz, 5 ns rise time), and fed into either a boxcar averager (EG&G models Ml65 and M162) to record excitation spectra, or a transient digitizer (Tektronix 79 12, 7B80 time base, and 7A19 50 n vertical amplifier, 600 MHz) to measure radiative lifetimes. In a typical experiment, up to 64 individual fluorescence decay traces are accumulated, and then transferred to a computer for data treatment. Some single traces also have been recorded to ensure that no saturation of the electronics occurred.
3. Results 3.1. Excitation spectra Various spectra have been obtained, which show different degrees of cooling depending upon the operating conditions: nature of the carrier gas, gas load of the valve and delay between the opening of the valve and the tiring of the excimer laser. The rotationally supercooled Cz spectrum of fig. 1 exhibits
some unusual features, such as a prominent Q, branch and the absence of any bandhead. Only low J” levels of the lowest energy spin component (‘II z F ; term), remain strongly populated. Intermediate J” levels are not present, and no bandhead, which occurs at J” = 15, can be detected. However some excitation of high J” levels (J” > 40) remains, as the increasing rotational spacing with J” limits the efficiency of collision-induced rotational energy transfer [ 141 in the hydrodynamic region of the supersonic expansion where cooling occurs. Some vibrational excitation also survives this expansion: vibrational levels V”= 1 and 2 (not shown in fig. 1) are detected with the following approximate population ratios: N(u”=O):N(u”=l):N(u”=2)=100:10:1. The straightness of the baseline, which actually corresponds to the electric zero, indicates that no other species absorbs or emits in the excitation and fluorescence wavelength ranges. 3.2. Lifetime measurements Fluorescence decay has been recorded for several rotational levels of the u=O, 1 and 2 vibrational levels of CZd3TI,. The observed decays have been fitted using a least-squares method to an exponential function, y= exp(at+b)
I
29 January 1988
CHEMICAL PHYSICS LETTERS
Volume 143, number 5
I
I
516
515
.
(1)
However, such a method can introduce a deviance when y values become small compared to noise and uncertainties, as 1ny-t -co when y+O. This effect is very sensitive to any offset, even small. To avoid this problem, a blank has been recorded for each run under the same conditions. Furthermore, experimental measurements have been fitted over several time domains, corresponding to .the beginning (loo-50%), the central part (90-10%) and the trailing edge (50-2O/6) of the decay. The consistency of these determinations may then allow one to rule out any baseline problem, or non single-exponential decay. The lifetime derived from this analysis is then simply t=-l/a.
&SW (nm) Fig. 1. Excitation spectrum of Cl(a311U,v” ~0). Assignments of main branches originating from significantly populated terms, F; and F; , are only given; corresponding rotational temperatureis TzIO-15K.
Dispersion errors can easily be determined by an analysis of variance. Let AY be the difference between observed and calculated logarithmic values of the studied decay: 497
Volume 143, number 5
CHEMICAL PHYSICS LETTERS
AY=A(lny) =Ayly=Aat+Ab.
(2)
An upper limit of Au can be obtained assuming that measure dispersion is only due to the time-constant determination error, (AY)‘=(Aat)*
.
29 January 1988
have been controlled using a time calibrator (Ortec model 462): deviations observed did not exceed lo?& Convolution of PMT and electronics rise times (2.5 and 5 ns, respectively) results in an overestimation of the time constant:
(3)
l/2 T2t
Summing eq. (3) over the n points measured each St time sampling interval yields 1 (AY)2=(Aa&)2
ik’
.
(4)
I
Considering that n B 1 (i.e. Ck*e fn3, and n- 1x n), and noting that Aa=Ar/r*, allows one to express the upper limit of dispersion error, (Ar/r)*=3s*(~/nSt)*,
(5)
where s2= [X(AY)*]/(n- 1) is the variance estimate for the considered set of data. Experimental errors are mainly due to the transient digitizer time-base sweep error (2% specified). The sweep time accuracy, together with the linearity,
C(rise
times)*
.
(6)
>
However, this factor is almost negligible as (r’-~)/~=0.16%. On the other hand, underestimation of the time constant is possible as laser-excited radicals move out of the detector field of view with the beam velocity during the acquisition of the decay curves. In fact, such an effect is quite negligible for time constants in the 100 ns range. No difference could be pointed out in the results when seeding C2 in helium (beam velocity ~2140 m s-l) instead of argon (v=810 m s-l). The experimental uncertainty can therefore be set to an overestimated value of 3%.
Table 1 Lifetime determination by least-squares regression V
Line
Domain *)
n b,
r (ns)
102S
0
Q,(2)
B C E
38 114 160
99.2 101.4 98.6
1.3 1.0 3.1
3.0 0.8 1.7
R,(2) + R,(2)
B C E
38 116 164
98.0 101.9 100.9
1.3 0.80 2.3
2.9 0.6 1.3
R, (4)
B C E
36 116 167
93.5 98.8 102.3
1.0 3.0 2.7
2.2 2.2 1.5
P, (44)
B C E
27 108 151
69.4 100.3 119.8
4.1 11.3 17
6.5 9.4 14.1
Q,(2) and R,(2)
B C* E
35 71 169
91.4 96.7 102.8
0.9 1.0 2.6
1.9 1.1
B C E
36 102 154
99.0 104.1 105.9
4.5 7.5 15
11 7.1 9.7
1
2
Q,(2) and R,(2)
AT (ns)
1.4
” B, C and E stand for the fitting domain, i.e. [ IOO-SO%], [90-IO%] and [ 50-2941 of the maximum signal amplitude; C*: for l-l lines, the best fit was obtained in the [ 80-20%] domain. b, n is the number of points sampled.
498
Volume 143, number 5
CHEMICAL PHYSICS LETTERS
600
800
time
(no)
Fig 2. Fluorescence decay of C,_(d3fI,) excited on the (O-O) Q, (2) line. Solid line: experimental; dashed line: calculated for r = 10 1.4 ns; deviation of experimental values from the exponential lit is displayed on a magnified scale.
Table 2 &(d’II,,
5
v=O, 1and 2) radiativelifetimes
“‘(ns)
( 1 ,,,.A,.,,.,,)- ’ h,
II=0
o=l
u=2
101.8k4.2 124.8
96.7 & 5.2 120.8
104+ 17 116.8
” 95% confidence interval ‘) Calculated from ref. [ 15 1.
Several measurements have been achieved for each line considered. Each one consists in the accumulation of 64 traces by the digitizer. Analysis has been performed separately for each one, to check data consistency. Final analysis has then been achieved, based upon the sum of all measurements (line by line for the v=O state, and all lines for v= 1 and 2). Results are summarized in table 1. For each case, r, s, and Ar values are given. The spread of 7 values obtained is indeed larger for the weak lines (especially the O-O, P,( 44) and 2-2 lines). However, as shown in fig. 2, a quite good signal/noise ratio can be obtained, thus allowing an accurate determination of Cz(d311,) lifetimes listed in table 2.
4. Discussion The interest of our method lies in the fact that the measured fluorescence decay time constant directly
29 January 1988
yields the lifetime value, with no need of any further extrapolation, Uncertainties therefore arise only from experimental measurement errors, which indeed results in a more accurate determination. Besides collisional energy transfer effects, inherent to bulk experiments, simultaneous absorption and/or emission of C3 radicals may be an important source of errors. As an example, Curtis et al. [ 21 found a fluorescence decay time constant of 120 ns at 0.008 nm resolution, but a value of 200 ns at 0.5 nm resolution. In the latter case, C3 emission, on the ‘II-lx (0~; O-l vi 2) transitions which occur in the Swan (O-O) bandhead region, probably contributed to the signal, thus resulting in a time constant value close to C,( ‘II,) lifetime [ 131. In our experiment, excitation spectra have clearly shown (see above) that C3 does not contribute to the laser-induced fluorescence signal. For v=O, our result lies within the uncertainty domain of Bauer et al. [ 91, but is higher than the Stark and Davis value [ 81. Lifetimes estimated from the sum of Einstein coefficients given by Cooper and Nicholls [ 151 (see table 2) are somewhat higher, lying in the 120 ns “plateau”. They are based upon a determination of the sum of squared electronic transition moments, 1 IR,I* = 3.52 f 0.50 au (atomic units). The latter factor can be re-estimated from our v=O lifetime measurement to a higher value, C IR,I*=4.3au, in good agreement with the theoretical values of 4.12 au [ 161 and 4.10 au [ 171.
Acknowledgement The authors would like to thank Dr. Roland Bonneau for the loan of his transient digitizer data acquisition system.
References [ 11 T. Tatarczyk, E.H. Fink and K.H. Becker, Chem. Phys. Lettern40 (1976) 126. [2] L. Curtis, B. Engman and P. Erman, Physica Scripta 13 (1976) 270. [ 31J.D. Campbell, M.H. Yu and C. Wittig, Appl. Phys. Letters 32 (1978) 413. [4] JR. McDonald, A.P. Baronavski and V.M. Donnelly, Chem. Phys. 33 (1978) 161.
499
Volume 143, number 5
CHEMICAL PHYSICS LETTERS
[ 5 ] P. Em-ran, Physica Scripta 22 (1980) tO8. [ 61 J.O. Arnold, J. Quant. Spectry. Radiative Transfer 8 (1968) 1781. [7] D.M. Cooper and R.W. Nicholls, J. Quant. Spectry. RadiativeTransfer 15 (1975) 139. [8] G. Stark and S.P. Davis, Z. Physik A321 (1985) 75. [ 91 W. Bauer, K.H. Becker, M. Bielefeld and R. Meuser, Chem. Phys. Letters 123 (1986) 33. [lo] M. Costes, C. Naulin, G. Dot-the, C. Vaucamps and G. Nouchi, Faraday Discussions Chem. Sot. 84 (1987)paper 6. [ 1I] G. Dorthe, M. Costes, C. Naulin, J. Joussot-Dubien, C. Vaucamps and G. Nouchi, J. Chem. Phys. 83 (1985) 3 171.
500
29 January 1988
[ 121 W.R. Gentry and C.F. Giese, Rev. Sci. Instr. 49 (1978) 595.
[ 131 K.H. Becker, T. Tatarczyk and J. Radic-Peric, Chem. Phys. Letters 60 (1979) 502.
[ 141 S. Hay, F. Shokoohi, S. Callister and C. Wittig, Chem. Phys. Letters 118 (1985) 6.
[ 151 D.M. Cooper and R.W. Nicholls, Spectry. Letters 9 (1976) 139.
[ 161 J.O. Arnold and S.R. Langhoff, J. Quant. Spectry. RadiativeTransfer 19 (1978) 461. [ 171 D.M. Cooper, J. Quant. Spectry. Radiative Transfer 26 (1981) 113.