- Email: [email protected]
Excited state dynamics of the11BO2 free radical
EXCITED
STATE
LlJ Cheng-zai ~parfment of
27 January 1984
CHEMICAL PHYSICS LETTERS
Volume 104, number 1
DYNAMICS
Physics,
OF THE llBO,
FREE
RADICAL
Fudnn University. Shanghai, China
and Michael A.A. CLYNE* and Ian P. LLEWELLYN Deportment of Chemistry. Queen Mary College, Mile End Road, London El 4NS. UK Received 21 July 1983;
in fiil
form
3 November 1983
The excitation spectrum of the double-headed OO’O-00’0 band of B02(AZrIn-X 2~g) wasrecorded by LIF. Special attention was paid to determine the dependence of the radiative lifetime of (OO”O) A 2~ state with J and the quenching by bath gases N2. Ar, 02. The determinations of fluorescence decay were made in real time. The mean radiative lifetime 7, of the A2~3,2(0000) state of l1 BOz was determined to be 91 * 4 ns (1~).
2. Experimental
1. Introduction B02 is a linear symmetric triatomic radical, analogous to the isoelectronic molecule CO;. Its electronic spectra have been studied by a number of workers. The absorption spectrum of B02 has been analysed systematically by Johns [ 11, showing that the bands arise from two electronic transitions, B 2Zi-X 211g and A 2fIu-X211,. Both 211 states show the Renner xffect. In this work, resolved ro-vibrational states of excited B02 have been produced using pulses from a Doppler-limited scanning dye laser, by absorption in the A 211u-X2fI, transition_ The excitation spectrum of the double-headed OO”O-OO”O band was recorded (fig. 1) and assigned up to J = 5 7.5_ Determinations of fluorescence decay were made in real time. The mean radiative lifetime 7R of the A2113/2(OOO) state of B02 91 * 4 ns (10).
was determined
Experiments
performed
in both
a flow cell
(300-l 200 mTorr) and in a lowpressure chamber (C500 mTorr)_ The flow cell has been described previously [2] _Most of the experiments were carried out at low pressure in a cyliidrical stainless steel chamber. A flow tube was attached to the top of the chamber, and was used to generate BO, by the reaction of BC13 with O(3P) atoms from a microwave discharge in 02_ The BO, radicals were sampled through a pinhole 0.9 mm in diameter, and expanded into the fluorescence cell. Pressure was monitored with a monel capacitance manometer (M-KS. Baratron model 3 IO). A Q-switched frequency-doubled YAG laser was used for pumping a pressure-tuned dye laser. Coumarin 495 in ethanol was used, both in the oscillator and the amplifier, covering the 540-550 nm wavelength range. The laser spectral width was 1 pm with a 20 Hz repetition rate. For obtaining excitation spectra, the fluorescence signal was detected with a cut-off filter and a photomultiplier tube (520 photocathode, 2 ns rise time), and input to a boxcar averager (Brookdeal). Determinations of fluorescence decay were made in real time using a fast transient digitizer (Biomation 6500,2 ns/channel) and a minicomputer_
to be
* Deceased_
0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
were
at high pressure
B.V.
97
V&me
CHEMICAL PHYSICS LETTERS
104,number 1
wavelength
Laser
Fig. 1. Part of the laser excitation spectrum of BOz A *II~,~-X
3. Results and discussion 3.1. Laser excitation
Ln
and relaxation of 30,
A 21&
Typical logarithmic decay curves of fluorescence were shown in fig;2. Fluorescence decay rates showed 98
nn
‘13~~ at 1 pm kser bandwidth.
I
spectrum of BUz(A-X)
The high-resolution laser excitation spectrum of B02(A-X) from 540 to 550 mn was obtained by recording the undispersed fluorescence intensity as a function of laser waveleng~. Fig. 1 shows part of the laser excitation spectrum of B02(A-X). like spectrum was assigned as a double-headed band, namely, the (OO*O--OO”O) 211band of the BO (A 2IIu-X 211g) systerri. The two sub-bands, 211S,2- 3 II,,, and each have a strong P and R branch. 2q2-2nl/2 The branches diverge slowly since the values of the rotational constants B,v (0.3 106) in the upper state and B,.* (0.3292) in the lower state are very similar. The spectra were tialysed up to at least J = 57.5. It was apparent that the rotational structure shows a distinct staggering caused by the absence of alternate rotational levels, as would be expected for a linear symmetric molecule with a nucfeti spin of zero. Thus, odd and even J-value lines are dud to different A doublets and the spacing between these lines is not exactly equal. 3.2. &&ztion
27 January 1984
Time
MS
Fig. 2. Fluorescencedecay of BOz A *n following excitation of the (ODD) vibronic state. Total pressure of BO2 + Oz. 50 mTorr. Plot is a logarithmic decay curve.
single-exponential decays up to at least 5 lifetimes. For the A2113,2 sub-state (000) of BO,, no significant variation of lifetime could be found with rotational energy over the range 48.5 > J’> 6.5 (table 1). Since predissociation in the Iower part of the excited-state manifold is energetically ruled out, this result is not unexpected. The result also confms the I absence of perturbation in the A 211~,2(000) state. We note that the variation of Franck-Condon densities with J’ would be small over the range of rotational energies examined; consequently, little effect upon lifetime is expected.
Volume 104, number 1 Table 1 Lifetime determinations ues of J Line
CHEMICAL
fluorescence of BOz A2Q12
7 (ns)
with different val-
Correlation coefficient of log fit
87 f 2
0.974
82F2
0.965
p13.5
93 + 2
0388
h7.5
8552
0.990
p20.5
90+2
0.987
P6.5 P10.5
p24.5 P28.5 p30.5 p34.5 p42.5
PHYSICS LETTERS
9012 851r2 9222 89k2 10322
0.983 0.978 0.986 0.991 0.968
p45.s
99+2
0.962
P48.5
92+2
0.966
Detailed quenching rate determinations were not feasible, in view of the limited range of total pressures accessible; the emphasis of the present work was upon lifetime determinations under collision-free conditions. However, the variation of lifetime with quenching gas pressure was examined for 0, (O-760 m Torr), N2 (0400 m Torrj and Ar (O-220 m Torr). No significant quenching was found for N2 and Ar as bath gases, giving an upper limit for the rate con-
17 January decay,
probably
due to trace
1984
impurities
such as N02_ Correct weighted averaging of the decay curves and appropriate truncation minimized the contribution of the long-lived transient to the measured lifetimes of BO2. However, the more reliable BO2 data obtained below 10 mTorr was used to give the best estimate for rR. The results are compared with the rather sparse previous data. Huie et al. [3] measured lifetimes of several vibronic states of BO, in the sub-Torr pressure range using LIF without ro-vibrational resolution_ For the (000) state of BO, A2II they reported 7R = 87.2 f 2.6 ns, in good agreement with the present resolved, collision-free study. Clyne and Heaven [2] determined the somewhat lower value 7-R = 182 = 20 ns for the (002) state of BOz A 211, at pressures down to 30 mTorr, using resolved LIF. McIntosh et al. [4] measured lifetimes from 03 to 1.8 mTorr in the 100, 110 and 200 levels, using a braod-band dye laser. By extrapolation to zero pressure, they reported values for iR between 133 5 14 ns and 141 + 11 ns. For the 110 level, this magnitude for rR agrees with that calculated from work on the Hanle effect [5] _However, in view of the relatively high pressures employed by McIntoch et al., it is possible that the error limits for rR given above were underestimated.
St~tSkN2~5X10-11cm3S-1,kAr~2X10-1' s-1 _ For 0, as bath gas, ICC, was estimated to be 0.9 f 0.3 X 10-l’ cm3 s-l, in fair agreement with recent data of McIntosh et al. for different vibronic states of BOz. Based on measurements in the mTorr range that involved a negligible extrapolation to zero pressure, the mean radiative lifetime rR of the A 'n3/2(000) state of B02 was determined to be 91 i- 4 ns (I u). It is noted that the fluorescence decay measurements at higher pressures (>lOO mTorr) sometimes showed a slowly decaying component in the tail of fast B02
cm3
References [l] J.WC. Johns, Can. J. Phys. 39 (1961) 1738_ [2] M&A. Clyne and h1.C. Heaven, Chem. Phys. 51 (1980) 299. [3] R.E. Huie, NJ-T. Long and BA. Thrush, Chem. Phys. Letters 55 (1978) 404. [4] S. hiclntosh, R.A. Beaudet and D.A. Do\+s, Chem. Phys.
Letters 78 (1981) 270. [S] D.R. Coulter, C.Y.R. Wu and D.A. Dews. Chem. Phys. Letters 60 (1978)
51.
99
STATE
LlJ Cheng-zai ~parfment of
27 January 1984
CHEMICAL PHYSICS LETTERS
Volume 104, number 1
DYNAMICS
Physics,
OF THE llBO,
FREE
RADICAL
Fudnn University. Shanghai, China
and Michael A.A. CLYNE* and Ian P. LLEWELLYN Deportment of Chemistry. Queen Mary College, Mile End Road, London El 4NS. UK Received 21 July 1983;
in fiil
form
3 November 1983
The excitation spectrum of the double-headed OO’O-00’0 band of B02(AZrIn-X 2~g) wasrecorded by LIF. Special attention was paid to determine the dependence of the radiative lifetime of (OO”O) A 2~ state with J and the quenching by bath gases N2. Ar, 02. The determinations of fluorescence decay were made in real time. The mean radiative lifetime 7, of the A2~3,2(0000) state of l1 BOz was determined to be 91 * 4 ns (1~).
2. Experimental
1. Introduction B02 is a linear symmetric triatomic radical, analogous to the isoelectronic molecule CO;. Its electronic spectra have been studied by a number of workers. The absorption spectrum of B02 has been analysed systematically by Johns [ 11, showing that the bands arise from two electronic transitions, B 2Zi-X 211g and A 2fIu-X211,. Both 211 states show the Renner xffect. In this work, resolved ro-vibrational states of excited B02 have been produced using pulses from a Doppler-limited scanning dye laser, by absorption in the A 211u-X2fI, transition_ The excitation spectrum of the double-headed OO”O-OO”O band was recorded (fig. 1) and assigned up to J = 5 7.5_ Determinations of fluorescence decay were made in real time. The mean radiative lifetime 7R of the A2113/2(OOO) state of B02 91 * 4 ns (10).
was determined
Experiments
performed
in both
a flow cell
(300-l 200 mTorr) and in a lowpressure chamber (C500 mTorr)_ The flow cell has been described previously [2] _Most of the experiments were carried out at low pressure in a cyliidrical stainless steel chamber. A flow tube was attached to the top of the chamber, and was used to generate BO, by the reaction of BC13 with O(3P) atoms from a microwave discharge in 02_ The BO, radicals were sampled through a pinhole 0.9 mm in diameter, and expanded into the fluorescence cell. Pressure was monitored with a monel capacitance manometer (M-KS. Baratron model 3 IO). A Q-switched frequency-doubled YAG laser was used for pumping a pressure-tuned dye laser. Coumarin 495 in ethanol was used, both in the oscillator and the amplifier, covering the 540-550 nm wavelength range. The laser spectral width was 1 pm with a 20 Hz repetition rate. For obtaining excitation spectra, the fluorescence signal was detected with a cut-off filter and a photomultiplier tube (520 photocathode, 2 ns rise time), and input to a boxcar averager (Brookdeal). Determinations of fluorescence decay were made in real time using a fast transient digitizer (Biomation 6500,2 ns/channel) and a minicomputer_
to be
* Deceased_
0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
were
at high pressure
B.V.
97
V&me
CHEMICAL PHYSICS LETTERS
104,number 1
wavelength
Laser
Fig. 1. Part of the laser excitation spectrum of BOz A *II~,~-X
3. Results and discussion 3.1. Laser excitation
Ln
and relaxation of 30,
A 21&
Typical logarithmic decay curves of fluorescence were shown in fig;2. Fluorescence decay rates showed 98
nn
‘13~~ at 1 pm kser bandwidth.
I
spectrum of BUz(A-X)
The high-resolution laser excitation spectrum of B02(A-X) from 540 to 550 mn was obtained by recording the undispersed fluorescence intensity as a function of laser waveleng~. Fig. 1 shows part of the laser excitation spectrum of B02(A-X). like spectrum was assigned as a double-headed band, namely, the (OO*O--OO”O) 211band of the BO (A 2IIu-X 211g) systerri. The two sub-bands, 211S,2- 3 II,,, and each have a strong P and R branch. 2q2-2nl/2 The branches diverge slowly since the values of the rotational constants B,v (0.3 106) in the upper state and B,.* (0.3292) in the lower state are very similar. The spectra were tialysed up to at least J = 57.5. It was apparent that the rotational structure shows a distinct staggering caused by the absence of alternate rotational levels, as would be expected for a linear symmetric molecule with a nucfeti spin of zero. Thus, odd and even J-value lines are dud to different A doublets and the spacing between these lines is not exactly equal. 3.2. &&ztion
27 January 1984
Time
MS
Fig. 2. Fluorescencedecay of BOz A *n following excitation of the (ODD) vibronic state. Total pressure of BO2 + Oz. 50 mTorr. Plot is a logarithmic decay curve.
single-exponential decays up to at least 5 lifetimes. For the A2113,2 sub-state (000) of BO,, no significant variation of lifetime could be found with rotational energy over the range 48.5 > J’> 6.5 (table 1). Since predissociation in the Iower part of the excited-state manifold is energetically ruled out, this result is not unexpected. The result also confms the I absence of perturbation in the A 211~,2(000) state. We note that the variation of Franck-Condon densities with J’ would be small over the range of rotational energies examined; consequently, little effect upon lifetime is expected.
Volume 104, number 1 Table 1 Lifetime determinations ues of J Line
CHEMICAL
fluorescence of BOz A2Q12
7 (ns)
with different val-
Correlation coefficient of log fit
87 f 2
0.974
82F2
0.965
p13.5
93 + 2
0388
h7.5
8552
0.990
p20.5
90+2
0.987
P6.5 P10.5
p24.5 P28.5 p30.5 p34.5 p42.5
PHYSICS LETTERS
9012 851r2 9222 89k2 10322
0.983 0.978 0.986 0.991 0.968
p45.s
99+2
0.962
P48.5
92+2
0.966
Detailed quenching rate determinations were not feasible, in view of the limited range of total pressures accessible; the emphasis of the present work was upon lifetime determinations under collision-free conditions. However, the variation of lifetime with quenching gas pressure was examined for 0, (O-760 m Torr), N2 (0400 m Torrj and Ar (O-220 m Torr). No significant quenching was found for N2 and Ar as bath gases, giving an upper limit for the rate con-
17 January decay,
probably
due to trace
1984
impurities
such as N02_ Correct weighted averaging of the decay curves and appropriate truncation minimized the contribution of the long-lived transient to the measured lifetimes of BO2. However, the more reliable BO2 data obtained below 10 mTorr was used to give the best estimate for rR. The results are compared with the rather sparse previous data. Huie et al. [3] measured lifetimes of several vibronic states of BO, in the sub-Torr pressure range using LIF without ro-vibrational resolution_ For the (000) state of BO, A2II they reported 7R = 87.2 f 2.6 ns, in good agreement with the present resolved, collision-free study. Clyne and Heaven [2] determined the somewhat lower value 7-R = 182 = 20 ns for the (002) state of BOz A 211, at pressures down to 30 mTorr, using resolved LIF. McIntosh et al. [4] measured lifetimes from 03 to 1.8 mTorr in the 100, 110 and 200 levels, using a braod-band dye laser. By extrapolation to zero pressure, they reported values for iR between 133 5 14 ns and 141 + 11 ns. For the 110 level, this magnitude for rR agrees with that calculated from work on the Hanle effect [5] _However, in view of the relatively high pressures employed by McIntoch et al., it is possible that the error limits for rR given above were underestimated.
St~tSkN2~5X10-11cm3S-1,kAr~2X10-1' s-1 _ For 0, as bath gas, ICC, was estimated to be 0.9 f 0.3 X 10-l’ cm3 s-l, in fair agreement with recent data of McIntosh et al. for different vibronic states of BOz. Based on measurements in the mTorr range that involved a negligible extrapolation to zero pressure, the mean radiative lifetime rR of the A 'n3/2(000) state of B02 was determined to be 91 i- 4 ns (I u). It is noted that the fluorescence decay measurements at higher pressures (>lOO mTorr) sometimes showed a slowly decaying component in the tail of fast B02
cm3
References [l] J.WC. Johns, Can. J. Phys. 39 (1961) 1738_ [2] M&A. Clyne and h1.C. Heaven, Chem. Phys. 51 (1980) 299. [3] R.E. Huie, NJ-T. Long and BA. Thrush, Chem. Phys. Letters 55 (1978) 404. [4] S. hiclntosh, R.A. Beaudet and D.A. Do\+s, Chem. Phys.
Letters 78 (1981) 270. [S] D.R. Coulter, C.Y.R. Wu and D.A. Dews. Chem. Phys. Letters 60 (1978)
51.
99