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Lifetimes of C-2 in rotational levels of the B̃2Σ+u state in the gas phase
Volume9 1, number3
CllChllCAL
LIFETIMES OF CT IN ROTATIONAL LEVELS OF THE ii%‘,
Samuel LEUTWVLER, John P. MAIER and Liuboti Pir~sdalrsch Citcnrrsckes brstrfrrt der Umwsttat Reccwcd 14 June
10 September1982
PHYSICS LETTERS
STATE IN THE GAS PHASE
MISEV
Base/, Klrrrgelbergstmsse 80, CH4056
Bosel, Swinerland
1982
Llteumcs OTC; m rotational levels of the E’:i. d = 0, u’ = I states have been measured.Cl was produced from bromoacet) Icnc and r~regas merastablcs and the 6’~: - %kg transltion uas laser excited The hfetlmes are constant wthm a vlbrdttonal level,77 * 8 ns for u’ = 0 and 73 r 7 ns for v = I. The oscdlator s!rength&, = 0.044 5 0.004.
1. Introduction
2. Experimental
The fi ‘X: +, j;( “5; electronic transition of Cz was fist detected both III absorption and emission by
The 6 ‘Z’u - 2 2Xg excitation process of CT was observed with the apparatus used to record laser excitation spectra of open-shell organic cations which are generated by Penning ionisation [ 121. The CT species could be produced in sufficient concentration (>106 cmm3) by using helium or argon metastables interacting with bromo- (and also chloro-) acetylene gas. The pressure in the collision regron was 0.2-I Torr with argon, or l-3 Torr when using helium. The sample was mixed into the rare-gas stream ~5 cm above the laser excitation and fluorescence observatron regton. The excitation spectrum was recorded using a nitrogen laser pumped dye laser at 30 Hz and with 0.02 nm bandwidth. The signals from a photomultipher, monitoring the fluorescence, as well as those from the photodiodes measuring the intensity of the laser and the interference fringes from an external etalon used for frequency cahbration. were fed tnto a transient digttizer (Tektronix 7912 AD). The latter was coupled to a microcomputer (LSI 1l/23) which also controlled the stepping motor of the dye laser’s grating. The excitation spectrum is thus corrected for the laser intenvty and is relatively calibrated. Absolute calibration is achieved using atomic transitions which are excited by the laser, such as those of argon or tantalum, the source of which are the electrodes of the discharge lamp. After tuning the laser to the various rotational transitions, the decay curves were accumulated. For
Henberg and Lagerqvlst [I]. Unambiguous confiiation of their assignment was provrded by Lineberger and Patterson 13-Jwho followed this transition by
means of resonrutt two-photon detachment of electrons from a mass-selected CT beam. The only other spectroscopic study of CT in the gas phase, apart from the earlier mass spectroscopic measurements [3], was a shock-tube deterrmnatlon of the oscillator strength of the 6 ‘-‘;: - R ‘Zg transitron [4]. In rare-gas ma-
trices however Uus transrtion has been investigated several tunes [S-8]. There have also been theoretical treatments which were armed at locating the vartous electrotuc states of CT [9,10] or predictmg the transition moments [ 111. We have now been able to measure directly the lifetunes of CT m selected rotational levels of the ~‘X:,‘=Oa nd IJ’= 1 states. Thrs has been accomphshed by producing CT III tts ground state from raregas metastables and bromoacetylene. Specrfic rotattonal lransltrons of the G 5: - X 5; system were exerted by a pulsed, tunable dye laser. The decay characteristics are obtained by monitoring the wavelength-undispersed fluorescence. Thus from the measured hfetimes the oscillator strength for the O-O transition could be calculated and is found to be about three times larger than the value evaluated from the shock-tube data, 206
0 009-2614/82/0000-0000/$02.75
0 1982 North-Holland
Volume 91, number 3
CHChllCAL
PHYSICS LETTERS
10 Scptcmber 1981
selected bands these were also recorded at 0.2 and 3 Torr pressure and no significant differences UI the inferred lifetimes were seen. With laser intensities =106 W/cm2 saturation effects were also not apparent. The timescale of the translent digitizer system was checked using a time calibrator (Ortec 462) and the lifetimes were extracted from the decay curves by a feast-squares fit following subtraction of background. The overalltime resolutionof the system is =I ns which is given essentially by the laser pulse characteristics. FQ. 2. Decay curve for thek = 9 rotatronal level of the E’z& u’ = 0 state of CT plotted on a scmlloganthmlc scale with background subtracted.
3. Results and discussion In the laser excitation spectra of the 6 22i + k 2Si transition of CT the O-O, l-0,2-0 and 1-I vibrational band systems were detected and rotationally resolved. Their Identification follows due&y by comparison with the high-resolution data given by Herzberg and Lagerqvlst [l]. In fig. 1, the excitation spectrum of the O-O vibrational transition is given, showing the well-resolved rotational structure. Lifetimes could be measured for rotational levels uptoN’=21inthev’=OlevelanduptoN’=13in the d = 1 level of the excited state of CT. A typical decay curve is reproduced in fig. 2. The uncertainty of the lifetime values is taken to be *loTo in view of the limited pressure range accessible (0.2-3 Torr). However, as a further check the lifetime of diacetylene cation in the x *flu O” level was recorded under the
czp
‘6ii,I,II
I
0 2 L 6 8
18500
I
I
x) 12
I
IS
I
I
I
lb
18
20
I-R 22
18600 i7(cni’)
Fig. I. Laser excltahan spectrum of the 6’~: .+ %2XdO-0 transmon of Cl recorded with 0.02 run bandwIdth.
same conditions and was found to be 74 f 4 ns III exact agreement with the hfetrmes measured under collision-free conditions using two drfferent techniques [13]. There is no significant vanation of the lifetimes among the measured rotational levels within a vrbrational level. Rotational levels of the u’ = 0 or U’= I states which are perturbed according to the Herzberg and Lagerqvlst analysis [I] could not be studied smce they are not sufficiently populated in the excitation process. The rotational temperature deduced from the relative band mtensitles in the spectrum shown (fig. 1) is ~320 K, whereas in the flash discharge work of Herzberg and Lagerqvist It is much higher, as rotational levels up to IV = 74 are apparent [ 11. The mean lifetime for the u’ = 0 level is 77 -+8 ns whereas for the u’ = 1 level it is 73 + 7 ns. Thus there may be a transition moment dependence on the internuclear distance but the variation could not be followed further since the decay curve statistics for the only other vibrational level reached, d = 2, were inadequate for a reliable bfetime evaluation. Consequently the oscillator strength is calculated only at the O-O origin, using the Franck-Condon factors tabulated in ref. [4], and is found to beSo = 0.044 f 0.004.Cathro and Made givef=l = 0.017 + 0.008 at the O-O ongin from their shock-tube data [41, which can be compared to the value of/,] = 0.060 + 0.006 evaluated from our measured lifetime using the same approximate equation as they used and to the theoretically cal. culated valueof0.0066 1141. Forcomparison, thesimilarly evaluated oscillator strengths dfel) of the ‘S-%
207
Volume 91. number 3
CHLLIICAL
PHYSICS LETTERS
transrdons, also at the O-O origin, for the rsoelectroruc N$, CN and CO+ are 0.045,0.040 and 0.014 respectively [4, IS] _The purely radiative lifetimes of CT in _” state in rare-gas matrices, estimated from the B ‘s+ the measured decay hfetrmes, are 34-40 ns for Xe, Kr and Ar, and 64 ns for a Ne matrix [S]. The last value is less than 20% shorter than the gas-phase lifetrme and merely reflects the bulk rndex of refraction effect (weak coupling lirrut). For the heavier rare gases the shortening 1ssignificant. As far as the mechanism of CT for-matron IS concerned, only a possible explanatron can be put forward. With chloro- or bromo-cyanoacetylene and dichloro- or dbromo-acetylene samples as precursors, no CT?was detected whereas the C2 signals were relatively intense. Cf could be produced from bromo- and chloro-acetylene but the CT concentration was higher with the former compound. The band mtensities of the CT B ‘Zi +, X *Cg and C2 b 32, fr a 3llu transitions were of comparable magnitude. Thus the formation of CT by three-body recombination of C? and electrons, as was the proposed mechamsm rn the shock-tube study [4], seems unhkely m the present case. On the other hand, an alternative route ISsuggested by analogy with the formation of stable negahve-ion fragments by dissociative electron attachment [ 161. Bromo- or chloroacetylene may capture a low-energy electron tnto the lowest unoccupied n* orbrtal and since the resultmg anion IS unstable, CT is formed in a dissociative process. Using known thermodynamic data and an esttmated heat of formation for the haloacetylene, electrons of %? eV would suffice for this process and this may well correspond to the lowest resonance energy for the formation of the bromo-acetylene aruon. A similar appearance energy for CT is expected with chloro- and rodo-acetylene as the precursor, whereas for the dihaloacetylenes and halocyanoacetylenes the necessary electron energies are hgher, ;=3-4 eV, and
IO
Scplcmber 1982
probably do not coincide with the excitation energy for the parent anion formation in its ground state.
Acknowledgement This work is part of project 2.017-0.81 of the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung. CrbaGeigy SA, Sandoz SA and F. Hoffman&a Roche & Cie., Base1are also thanked for fmancial support.
References [I ] C. Herzberg
and A. Lagerqvtst, Can. .I. Phys. 46 (1968)
2363. [2] W.C. Lmeberger and T.A. Patterson, Chem. Phys. Letters 13 (l-972) 40; P.L. Jones, R.D. hlcad, B E Kohlcr, S D. Rower and W.C Lmebcrger, J Chem. Phys 73 (1980) 4419. [3] R. Locht and J. hlomtgny. Chem. Phys. Letters6 273, and references therem. [4] W.C. Cathro and J.C. Mackre,J. Tranr I2 (1973) 237.
(1970)
Chem.Sot. Faraday
[S] DE. Milligan and M E. Jacou, J. Chem. Phys. 51 (1969) 1952. 161 R.P. Frosch, J. Chem. Phys 54 (1971) 2660. 171 V.E. Bondybey and J.W. Ntbler, J. Chem. Phys. 56 (1972) 4719. IS] V.E. Bondybey and L.E. Brus, J. 2223.
Chem. Phys. 63
(1975)
191 J. Barsuhn. J. Phys. B7 (1974) LSS. P.W. Thulstrup and E.W. Thulstrup. Chem. Phys. Letters 26 (1974) 144. [ 111 H.E. Popkte and W.H. Henneker. J. Chem. Phys 55 (1971) 617.
[lo]
[ 121 J.P. hlaier and L. hlaev, Chem. Phys 5 1 (1980) 311. [ 131 hl. Alian, E Kloster-Jensen and J.P. M&r. Chem. Phys. 17 (1976) Il. J.P. hlater and F. Thommen, J. Chem. Phys. 73 (1980) 5616. [ 141 hl. Zeitz, S.D. Peycrrmhoff and R J. Buenkcr, Chcm. Phys Letters 64 (1979) 243.
[ 15 ] J.E. Hesser,J. Chem. Phys. 48 (1968) 25 18. [ 161 ht. Heni, E. lllenberger. H. Baumgartel and S. Siizer, Chem. Phyr Letters 87 (1982) 244.
208
CllChllCAL
LIFETIMES OF CT IN ROTATIONAL LEVELS OF THE ii%‘,
Samuel LEUTWVLER, John P. MAIER and Liuboti Pir~sdalrsch Citcnrrsckes brstrfrrt der Umwsttat Reccwcd 14 June
10 September1982
PHYSICS LETTERS
STATE IN THE GAS PHASE
MISEV
Base/, Klrrrgelbergstmsse 80, CH4056
Bosel, Swinerland
1982
Llteumcs OTC; m rotational levels of the E’:i. d = 0, u’ = I states have been measured.Cl was produced from bromoacet) Icnc and r~regas merastablcs and the 6’~: - %kg transltion uas laser excited The hfetlmes are constant wthm a vlbrdttonal level,77 * 8 ns for u’ = 0 and 73 r 7 ns for v = I. The oscdlator s!rength&, = 0.044 5 0.004.
1. Introduction
2. Experimental
The fi ‘X: +, j;( “5; electronic transition of Cz was fist detected both III absorption and emission by
The 6 ‘Z’u - 2 2Xg excitation process of CT was observed with the apparatus used to record laser excitation spectra of open-shell organic cations which are generated by Penning ionisation [ 121. The CT species could be produced in sufficient concentration (>106 cmm3) by using helium or argon metastables interacting with bromo- (and also chloro-) acetylene gas. The pressure in the collision regron was 0.2-I Torr with argon, or l-3 Torr when using helium. The sample was mixed into the rare-gas stream ~5 cm above the laser excitation and fluorescence observatron regton. The excitation spectrum was recorded using a nitrogen laser pumped dye laser at 30 Hz and with 0.02 nm bandwidth. The signals from a photomultipher, monitoring the fluorescence, as well as those from the photodiodes measuring the intensity of the laser and the interference fringes from an external etalon used for frequency cahbration. were fed tnto a transient digttizer (Tektronix 7912 AD). The latter was coupled to a microcomputer (LSI 1l/23) which also controlled the stepping motor of the dye laser’s grating. The excitation spectrum is thus corrected for the laser intenvty and is relatively calibrated. Absolute calibration is achieved using atomic transitions which are excited by the laser, such as those of argon or tantalum, the source of which are the electrodes of the discharge lamp. After tuning the laser to the various rotational transitions, the decay curves were accumulated. For
Henberg and Lagerqvlst [I]. Unambiguous confiiation of their assignment was provrded by Lineberger and Patterson 13-Jwho followed this transition by
means of resonrutt two-photon detachment of electrons from a mass-selected CT beam. The only other spectroscopic study of CT in the gas phase, apart from the earlier mass spectroscopic measurements [3], was a shock-tube deterrmnatlon of the oscillator strength of the 6 ‘-‘;: - R ‘Zg transitron [4]. In rare-gas ma-
trices however Uus transrtion has been investigated several tunes [S-8]. There have also been theoretical treatments which were armed at locating the vartous electrotuc states of CT [9,10] or predictmg the transition moments [ 111. We have now been able to measure directly the lifetunes of CT m selected rotational levels of the ~‘X:,‘=Oa nd IJ’= 1 states. Thrs has been accomphshed by producing CT III tts ground state from raregas metastables and bromoacetylene. Specrfic rotattonal lransltrons of the G 5: - X 5; system were exerted by a pulsed, tunable dye laser. The decay characteristics are obtained by monitoring the wavelength-undispersed fluorescence. Thus from the measured hfetimes the oscillator strength for the O-O transition could be calculated and is found to be about three times larger than the value evaluated from the shock-tube data, 206
0 009-2614/82/0000-0000/$02.75
0 1982 North-Holland
Volume 91, number 3
CHChllCAL
PHYSICS LETTERS
10 Scptcmber 1981
selected bands these were also recorded at 0.2 and 3 Torr pressure and no significant differences UI the inferred lifetimes were seen. With laser intensities =106 W/cm2 saturation effects were also not apparent. The timescale of the translent digitizer system was checked using a time calibrator (Ortec 462) and the lifetimes were extracted from the decay curves by a feast-squares fit following subtraction of background. The overalltime resolutionof the system is =I ns which is given essentially by the laser pulse characteristics. FQ. 2. Decay curve for thek = 9 rotatronal level of the E’z& u’ = 0 state of CT plotted on a scmlloganthmlc scale with background subtracted.
3. Results and discussion In the laser excitation spectra of the 6 22i + k 2Si transition of CT the O-O, l-0,2-0 and 1-I vibrational band systems were detected and rotationally resolved. Their Identification follows due&y by comparison with the high-resolution data given by Herzberg and Lagerqvlst [l]. In fig. 1, the excitation spectrum of the O-O vibrational transition is given, showing the well-resolved rotational structure. Lifetimes could be measured for rotational levels uptoN’=21inthev’=OlevelanduptoN’=13in the d = 1 level of the excited state of CT. A typical decay curve is reproduced in fig. 2. The uncertainty of the lifetime values is taken to be *loTo in view of the limited pressure range accessible (0.2-3 Torr). However, as a further check the lifetime of diacetylene cation in the x *flu O” level was recorded under the
czp
‘6ii,I,II
I
0 2 L 6 8
18500
I
I
x) 12
I
IS
I
I
I
lb
18
20
I-R 22
18600 i7(cni’)
Fig. I. Laser excltahan spectrum of the 6’~: .+ %2XdO-0 transmon of Cl recorded with 0.02 run bandwIdth.
same conditions and was found to be 74 f 4 ns III exact agreement with the hfetrmes measured under collision-free conditions using two drfferent techniques [13]. There is no significant vanation of the lifetimes among the measured rotational levels within a vrbrational level. Rotational levels of the u’ = 0 or U’= I states which are perturbed according to the Herzberg and Lagerqvlst analysis [I] could not be studied smce they are not sufficiently populated in the excitation process. The rotational temperature deduced from the relative band mtensitles in the spectrum shown (fig. 1) is ~320 K, whereas in the flash discharge work of Herzberg and Lagerqvist It is much higher, as rotational levels up to IV = 74 are apparent [ 11. The mean lifetime for the u’ = 0 level is 77 -+8 ns whereas for the u’ = 1 level it is 73 + 7 ns. Thus there may be a transition moment dependence on the internuclear distance but the variation could not be followed further since the decay curve statistics for the only other vibrational level reached, d = 2, were inadequate for a reliable bfetime evaluation. Consequently the oscillator strength is calculated only at the O-O origin, using the Franck-Condon factors tabulated in ref. [4], and is found to beSo = 0.044 f 0.004.Cathro and Made givef=l = 0.017 + 0.008 at the O-O ongin from their shock-tube data [41, which can be compared to the value of/,] = 0.060 + 0.006 evaluated from our measured lifetime using the same approximate equation as they used and to the theoretically cal. culated valueof0.0066 1141. Forcomparison, thesimilarly evaluated oscillator strengths dfel) of the ‘S-%
207
Volume 91. number 3
CHLLIICAL
PHYSICS LETTERS
transrdons, also at the O-O origin, for the rsoelectroruc N$, CN and CO+ are 0.045,0.040 and 0.014 respectively [4, IS] _The purely radiative lifetimes of CT in _” state in rare-gas matrices, estimated from the B ‘s+ the measured decay hfetrmes, are 34-40 ns for Xe, Kr and Ar, and 64 ns for a Ne matrix [S]. The last value is less than 20% shorter than the gas-phase lifetrme and merely reflects the bulk rndex of refraction effect (weak coupling lirrut). For the heavier rare gases the shortening 1ssignificant. As far as the mechanism of CT for-matron IS concerned, only a possible explanatron can be put forward. With chloro- or bromo-cyanoacetylene and dichloro- or dbromo-acetylene samples as precursors, no CT?was detected whereas the C2 signals were relatively intense. Cf could be produced from bromo- and chloro-acetylene but the CT concentration was higher with the former compound. The band mtensities of the CT B ‘Zi +, X *Cg and C2 b 32, fr a 3llu transitions were of comparable magnitude. Thus the formation of CT by three-body recombination of C? and electrons, as was the proposed mechamsm rn the shock-tube study [4], seems unhkely m the present case. On the other hand, an alternative route ISsuggested by analogy with the formation of stable negahve-ion fragments by dissociative electron attachment [ 161. Bromo- or chloroacetylene may capture a low-energy electron tnto the lowest unoccupied n* orbrtal and since the resultmg anion IS unstable, CT is formed in a dissociative process. Using known thermodynamic data and an esttmated heat of formation for the haloacetylene, electrons of %? eV would suffice for this process and this may well correspond to the lowest resonance energy for the formation of the bromo-acetylene aruon. A similar appearance energy for CT is expected with chloro- and rodo-acetylene as the precursor, whereas for the dihaloacetylenes and halocyanoacetylenes the necessary electron energies are hgher, ;=3-4 eV, and
IO
Scplcmber 1982
probably do not coincide with the excitation energy for the parent anion formation in its ground state.
Acknowledgement This work is part of project 2.017-0.81 of the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung. CrbaGeigy SA, Sandoz SA and F. Hoffman&a Roche & Cie., Base1are also thanked for fmancial support.
References [I ] C. Herzberg
and A. Lagerqvtst, Can. .I. Phys. 46 (1968)
2363. [2] W.C. Lmeberger and T.A. Patterson, Chem. Phys. Letters 13 (l-972) 40; P.L. Jones, R.D. hlcad, B E Kohlcr, S D. Rower and W.C Lmebcrger, J Chem. Phys 73 (1980) 4419. [3] R. Locht and J. hlomtgny. Chem. Phys. Letters6 273, and references therem. [4] W.C. Cathro and J.C. Mackre,J. Tranr I2 (1973) 237.
(1970)
Chem.Sot. Faraday
[S] DE. Milligan and M E. Jacou, J. Chem. Phys. 51 (1969) 1952. 161 R.P. Frosch, J. Chem. Phys 54 (1971) 2660. 171 V.E. Bondybey and J.W. Ntbler, J. Chem. Phys. 56 (1972) 4719. IS] V.E. Bondybey and L.E. Brus, J. 2223.
Chem. Phys. 63
(1975)
191 J. Barsuhn. J. Phys. B7 (1974) LSS. P.W. Thulstrup and E.W. Thulstrup. Chem. Phys. Letters 26 (1974) 144. [ 111 H.E. Popkte and W.H. Henneker. J. Chem. Phys 55 (1971) 617.
[lo]
[ 121 J.P. hlaier and L. hlaev, Chem. Phys 5 1 (1980) 311. [ 131 hl. Alian, E Kloster-Jensen and J.P. M&r. Chem. Phys. 17 (1976) Il. J.P. hlater and F. Thommen, J. Chem. Phys. 73 (1980) 5616. [ 141 hl. Zeitz, S.D. Peycrrmhoff and R J. Buenkcr, Chcm. Phys Letters 64 (1979) 243.
[ 15 ] J.E. Hesser,J. Chem. Phys. 48 (1968) 25 18. [ 161 ht. Heni, E. lllenberger. H. Baumgartel and S. Siizer, Chem. Phyr Letters 87 (1982) 244.
208