- Email: [email protected]
C2(d3Πg) formation in the arF-laser multiphoton absorption of vinyl chloride. Quenching by O2
CHEhllPAL PHYSICS LETTERS
Volome 107. number 6
15June1981
C,(d31-lg) FORMATION IN THE ArF-LASER MULTIPHOTON ABSORPTION OF VINYL CHLORIDE. QUENCHING BY 0, Marta CASTILLEJO, Juan M. FIGUERA and Margarita MARTiN Institute de Quhica
F&a
‘Rocasolano ‘: Serrano, I 19, Madrid-6, Spain
Received 8 February
1984; in final form 5 April 1984
C,(d %I - a3nu) fluorescence is reported following ArF-laser multiphoton absorption of vinyl cl. cr.de nnd 3 possible mechanism 5 or production of Cz(d 3~g) is discussed. The rate constant obtained for removal of C2(d 3n._,)by 02 is 2 (6.1 + 0.85) X IO-” molecule-’ cm3 s-I_
1_ Introduction
output energy of 50 mJ and 1 Hz repetition rate) is focused 5 cm behind the rear window of a fluorescence cell by a 50 cm focal-length specrrosil lens. The fluorescence excited after irradiation of samples of vinyl chloride or vinyl chloride and oxygen is observed at right angles to the laser beam. The fluorescence is either imaged on the entrance slit of a 0.5 m Jarrell-Ash monochromator and viewed by an EMI 9783B photomultiplier, or collected by a highintensity Applied Photophysics monochromator and detected by an RCA 1P28 photomultiplier. The photomultiplier signals are collected by a two-channel Datalab transient recorder (DL 922) and displayed on an oscilloscope (Telequipment DlOl 1). Laser energies are measured by a Gen-Tee ED 200 joulemeter. The samples of vinyl chloride have less than 55 of acetylene and ethylene, as measured by gas chromarography. Mhtures of vinyl chloride and 0, are premixed and stored for several hours prior to use. Pressures are measured wirh MKS Bararron type 22 IA capacitance manometers. (ma.ximum
UV multiphoton absorption of molecules is a useful tool to produce excited and ground-state photofragments. The kinetics of several radicals and excited species generated in this way have been studied
t1--41. ln the
case of UV multiphoton dissociation of molecules containing carbon-carbon bonds, a large number of the observed fragmentation channels preserve the C-C bond, and several electronic states of the C2 molecule have been observed either by detecting their fluorescence or by LIF techniques [Z--5]. While most of the available kinetic data on the C2 molecule are concerned with the a3Hu [2,3,6-IO] and the ground state [4,8-JO], very little information has been reported concerning the d 311-estate. In the present work we produce the d 3iIp state of C2 in the ArF-laser multiphoton absorption of vinyl chloride, and a Stem-Volmer analysis is carried out to obtain the rate constant for removal of C,(d 3Hg) in the presence of 02_
3. Results and discussion
2. Experimental
The output
of a home-made
ArF excimer laser *
3. I. Production
* This laser has been built following the design developed by Dr. C. Webb and coworkers at Clarendon Lab.; see, e.g. ref. [ll]. 0 009-2614/84/S
(North-Holland
03.00
0 Elsevier
Physics Publishing
Science
Division)
Publishers
of Cdd 3Hg)
Irradiation of 70 mTorr of vinyl chloride by the focused output of the ArF excimer laser gives rise to B.V.
561
Volume
107. number 6
CHEhlJCAL
PHYSICS
strong green fluorescence (visible to the naked eye). In the region from 525 to 460 nm, the low-resolution fluorescence spectrum (resolution ~0.8 nm) clearly shows the Au = 0 and Au = +I C, Swan bands, peaking around 516 and 470 run, respectively_ Fluorescence is observed at pressures of vinyl chloride of %‘I mTorr. Work is in progress to record higher-resolution spectra of the weaker emissions observed from 460 to 350 run. No other fluorescence is obtained below 350 nm down to 240 nm. The mechanism by which C,(d 311s) is formed from vinyl chloride is currently under investigation. C,(d 311s) emission has also been reported following multiphoton absorption of several polyatomic molecules [5,12,13] ; different mechanisms proceeding either by multiphoton absorption by the parent molecule or via secondary excitation of fragments have
been favoured in every case. In the case of vinyl chloride, three AIF photons are needed to produce C2(d 3Q,) via the spin-allowed dissociation channel H$=CHCl
+ C,(d 3”g) + 2H + HCl.
(1)
The one-photon photochemistry of vinyl chloride around 193 nm is well known since vinyl chloride is the most common parent for the HCl photodissociation laser [ 141. Photoelimination of HCl is known to proceed via both a, Q!and (Y,p elimination; the or, a is estimated to account for only 8% of the total HCl production for photolysis at X > 150 run although it may be higher in the longer-wavelength photolysis region; (Y,OLelimination is assumed to primarily produce the intermediate vinylidencarbene [ 151. On the other hand, the a, 0 elimination could be viewed as giving the bent diradical HCCH (2) _lron
LETTERS
15 June 1984
in the sense that their ESR spectra are characteristic of two doublet states. However, in most systems, coupling between the odd electrons gives two distinct singlet and triplet states. Nevertheless, it has been pointed out [ 161 that, even in these cases, the oddelectron interaction is expected to be weak and larger perturbations, such as the chemical perturbation created by the approach of a reactant, can “mix” the two spin states of the diradical, leading to two doublet states. If these ideas are of application to our case, in the ArF-laser photodissociation of vinyl chloride, production of C,(d 311,J could arise via absorption of a second ArF photon by the strongly perturbed diradical (2) before the bending vibration involving the two H atoms (corresponding to the antisymmetrical bending v5 of acetylene [ 171) couples the two odd electrons into the well-separated triplet and singlet states. In that case, absorption by this short-lived “bifunctional” [16] diradical could give the otherwise forbidden products C2(d 311g) + Hz_ On the other hand, we fiid that the dependence of the CZ(d 3flg + a 3fl,) fluorescence on laser energy is linear over a range of laser energies from 3 to 14 ml. Vinyl chloride strongly absorbs radiation at 193 run and a rough calculation using the reported vinyl chloride absorption coefficients at 193 nm [ 14,181 shows that the first-photon absorption is most likely to be saturated at a vinyl chloride pressure of 70 mTorr and the laser energies used in our experiments. In that case, if the above mechanism of production of C2(d 311s) through the diradical(2) is assumed, then the linear dependence on laser energy of the fluorescence would reflect the competition between the intensitydependent absorption of a second photon by (2) and the fast unimolecular process which depletes the intermediate (2). Finally, we mention that C2(d 311g) has been reported as the most important fragment produced in the unimolecular multiphoton dissociation of acetylene excited by a sub-nanosecond laser pulse at 266 nm [ 121. In this case, excitation of the parent within the triplet manifold is not excluded but a mechanism involving a bent state of C2H is preferred. However, the authors propose *Aat a major dissociative route for production of C,H from C,H, should be operating other than that observed in the one-photon excitation at the same energy and also that a
Volume
107. number 6
CHEMICAL
PHYSICS
different channel for C, elimination than that observed in the work by McDonald et al. [S], where mainly carbon singlet states are formed, is operating. This suggests that a variety of C,H, states exhibiting different photochemical behaviour may be pumped either by multiphoton absorption of the parent or by other means as proposed in our work. 3.2. Removal of Cz(d311g) by O2 The C,(d 311p -+ a 3nu) fluorescence at 5 16 nm (u’ = 0 +‘l” = 0) and 5 12 nm (u’ = 1 + u” = 1) was
measured at 70 mTorr of vinyl chloride and different pressures of added 0,. A Stem-Volmer plot is shown in fig. 1. From the slope of the straight line fitted to the experimental points of fig. 1, and assuming the C,(d 311g) radiative lif%rne to be 119 ns [S], a rate co&m of (6.1 f 0.85) X 1O-12 molecule-l cm3 s-1 is obtained. Rate coefficients for the analogous processes of the C,(a 3Tlu) and C,(X 1x3 states have been reported in the literature [2,4,6,8-IO] _Removal by O2 of the C,(a 3flu) state has been shown to proceed
LEI-I-ERS
15 June 1984
via fast intersystem crossing to the closely lying ground state [lo] _On the other hand, in the case of the C,(X lIZi> ground state, it has been pointed out that, despite the number of exceedingly exothermic channels available for reaction with O,, the rate constant is rather slow, requiring about 200 collisions_ The higher energy content of the C,(d 3Tla) state opens new highly exothemric spin-allowed channels for reaction with 0,; however, the removal rate constant obtained here is also slow, about twice as fast as for the ground state [4,9]. Further work is required to identify the pathways involved in the removal of C,(d 3lI,) by O2 and to establish the relative importance of possible reactive channels versus energy transfer. It has been suggested that most of the CO product of the C,(a 3TIu) + O2 reaction is formed in its ground and fit excited singlet (A Ill) state [8]. For the C>(d 3flg) state, a possible reaction channel, both thermodynamically and spin allowed, could lead to formation of the CO(A Ill) first excited singlet plus CO in several triplet states. We note that this channel resembles somewhat the photolysis of dioxetanes, where a singlet plus triplet ketones are obtained. We suggest that an investigation of the reaction channels of C,(d 3QJ plus 0, could provide useful information on the above energy-storing molecules.
Acknowledgement
We are indebted to Dr. C. Webb and co-workers for providing us with detailed information on their excimer laser designs. We also thank Dr. A. Costela for his help.
References
oc
100
[l] C. Nokes.G.
200
300 P(Torr)
Fig. 1. Stem-Volmer plot of vinyl chloride fluorescence versus pressure of 0,. Fo/Fhf is the ratio of the fluorescence from pure vinyl chloride (70 mTorr) to the fluorescence in the presence of 02. Full circles at 5 16 MI; open circles at 512 nm.
Gilbert and R.J. Donovan, Chem. Phys. Lerters 99 (1983) 491. [2] VBf. Donnelly and 1. Pasrernack.Chem. Phys. 39 (1979) 427. [3] L. Pas-ten-tack. W.M. Pittsand J.R.-McDonald, Chem.
Phvs. 57 (1981) 19. [4] L.&ten&k id J.R. hIcDonald, Chem. Phys. 43 (1979) 173. [S] J.R. McDonald, AP. Baronavski and Vhf. Donnelly, Chem. Phys. 33 (1978) 161.
563
Volume 107, number 6
CHEMICAL
[6] S.V. Fttseth. G. Hancock, J. Fournier and K. Meier. Chem; Phys. Letters 6 l(l979) 288. [7] H. Reisler. M. Mangir and C. Witlig. J. Chem. Phys. 71 (1979) 2109. [S] M.S. Man&, H. Reisler and C. Wittig, J. Chem. Phys. 73 (1980) 829. [ 91 H. Reisler, M. Mangir and C. Wittig. Chem. Phys. 47 (1980) 49. ]lO] H. Reisler, MS. Mangir and C. Wittig, J. Chem. Phys. 73 (1980) 2280. ill] AJ. Kearsley. AJ. Andrews and C.E. Webb, Opt. Commun. 31(1979) 181.
564
PHYSICS LETTERS
ISJune
1984
1121 BB. Craig. W.L. Faust, LS. Goldberg and R.C. Weiss, 1. Chem. Phys. 76 (1982) 5014. 113) N. Nishi. H. Shinohara and 1. Hanazaki. J. Chem. Phys. 77 (1982) 246. [ 141 MJ. Berry, J. Chem. Phys. 61(1974) 3114. (151 C. Reiser and J.I. Steinfeld. J. Phys. Chem. 84 (1980) 680. 1161 L. Salem and C. Rowland, Angew. Chem. 11 (1972) 92. [ 171 G. Heraberg, Molecular spectra and molecular structure. II. Infrared and Raman spectra of polyatomic molecules (Van Nostrand, Princeton, 1945). [ 181 S.P. Sood and K. Watanabe, J. Chem. Phys. 45 (1966) 2913.
Volome 107. number 6
15June1981
C,(d31-lg) FORMATION IN THE ArF-LASER MULTIPHOTON ABSORPTION OF VINYL CHLORIDE. QUENCHING BY 0, Marta CASTILLEJO, Juan M. FIGUERA and Margarita MARTiN Institute de Quhica
F&a
‘Rocasolano ‘: Serrano, I 19, Madrid-6, Spain
Received 8 February
1984; in final form 5 April 1984
C,(d %I - a3nu) fluorescence is reported following ArF-laser multiphoton absorption of vinyl cl. cr.de nnd 3 possible mechanism 5 or production of Cz(d 3~g) is discussed. The rate constant obtained for removal of C2(d 3n._,)by 02 is 2 (6.1 + 0.85) X IO-” molecule-’ cm3 s-I_
1_ Introduction
output energy of 50 mJ and 1 Hz repetition rate) is focused 5 cm behind the rear window of a fluorescence cell by a 50 cm focal-length specrrosil lens. The fluorescence excited after irradiation of samples of vinyl chloride or vinyl chloride and oxygen is observed at right angles to the laser beam. The fluorescence is either imaged on the entrance slit of a 0.5 m Jarrell-Ash monochromator and viewed by an EMI 9783B photomultiplier, or collected by a highintensity Applied Photophysics monochromator and detected by an RCA 1P28 photomultiplier. The photomultiplier signals are collected by a two-channel Datalab transient recorder (DL 922) and displayed on an oscilloscope (Telequipment DlOl 1). Laser energies are measured by a Gen-Tee ED 200 joulemeter. The samples of vinyl chloride have less than 55 of acetylene and ethylene, as measured by gas chromarography. Mhtures of vinyl chloride and 0, are premixed and stored for several hours prior to use. Pressures are measured wirh MKS Bararron type 22 IA capacitance manometers. (ma.ximum
UV multiphoton absorption of molecules is a useful tool to produce excited and ground-state photofragments. The kinetics of several radicals and excited species generated in this way have been studied
t1--41. ln the
case of UV multiphoton dissociation of molecules containing carbon-carbon bonds, a large number of the observed fragmentation channels preserve the C-C bond, and several electronic states of the C2 molecule have been observed either by detecting their fluorescence or by LIF techniques [Z--5]. While most of the available kinetic data on the C2 molecule are concerned with the a3Hu [2,3,6-IO] and the ground state [4,8-JO], very little information has been reported concerning the d 311-estate. In the present work we produce the d 3iIp state of C2 in the ArF-laser multiphoton absorption of vinyl chloride, and a Stem-Volmer analysis is carried out to obtain the rate constant for removal of C,(d 3Hg) in the presence of 02_
3. Results and discussion
2. Experimental
The output
of a home-made
ArF excimer laser *
3. I. Production
* This laser has been built following the design developed by Dr. C. Webb and coworkers at Clarendon Lab.; see, e.g. ref. [ll]. 0 009-2614/84/S
(North-Holland
03.00
0 Elsevier
Physics Publishing
Science
Division)
Publishers
of Cdd 3Hg)
Irradiation of 70 mTorr of vinyl chloride by the focused output of the ArF excimer laser gives rise to B.V.
561
Volume
107. number 6
CHEhlJCAL
PHYSICS
strong green fluorescence (visible to the naked eye). In the region from 525 to 460 nm, the low-resolution fluorescence spectrum (resolution ~0.8 nm) clearly shows the Au = 0 and Au = +I C, Swan bands, peaking around 516 and 470 run, respectively_ Fluorescence is observed at pressures of vinyl chloride of %‘I mTorr. Work is in progress to record higher-resolution spectra of the weaker emissions observed from 460 to 350 run. No other fluorescence is obtained below 350 nm down to 240 nm. The mechanism by which C,(d 311s) is formed from vinyl chloride is currently under investigation. C,(d 311s) emission has also been reported following multiphoton absorption of several polyatomic molecules [5,12,13] ; different mechanisms proceeding either by multiphoton absorption by the parent molecule or via secondary excitation of fragments have
been favoured in every case. In the case of vinyl chloride, three AIF photons are needed to produce C2(d 3Q,) via the spin-allowed dissociation channel H$=CHCl
+ C,(d 3”g) + 2H + HCl.
(1)
The one-photon photochemistry of vinyl chloride around 193 nm is well known since vinyl chloride is the most common parent for the HCl photodissociation laser [ 141. Photoelimination of HCl is known to proceed via both a, Q!and (Y,p elimination; the or, a is estimated to account for only 8% of the total HCl production for photolysis at X > 150 run although it may be higher in the longer-wavelength photolysis region; (Y,OLelimination is assumed to primarily produce the intermediate vinylidencarbene [ 151. On the other hand, the a, 0 elimination could be viewed as giving the bent diradical HCCH (2) _lron
LETTERS
15 June 1984
in the sense that their ESR spectra are characteristic of two doublet states. However, in most systems, coupling between the odd electrons gives two distinct singlet and triplet states. Nevertheless, it has been pointed out [ 161 that, even in these cases, the oddelectron interaction is expected to be weak and larger perturbations, such as the chemical perturbation created by the approach of a reactant, can “mix” the two spin states of the diradical, leading to two doublet states. If these ideas are of application to our case, in the ArF-laser photodissociation of vinyl chloride, production of C,(d 311,J could arise via absorption of a second ArF photon by the strongly perturbed diradical (2) before the bending vibration involving the two H atoms (corresponding to the antisymmetrical bending v5 of acetylene [ 171) couples the two odd electrons into the well-separated triplet and singlet states. In that case, absorption by this short-lived “bifunctional” [16] diradical could give the otherwise forbidden products C2(d 311g) + Hz_ On the other hand, we fiid that the dependence of the CZ(d 3flg + a 3fl,) fluorescence on laser energy is linear over a range of laser energies from 3 to 14 ml. Vinyl chloride strongly absorbs radiation at 193 run and a rough calculation using the reported vinyl chloride absorption coefficients at 193 nm [ 14,181 shows that the first-photon absorption is most likely to be saturated at a vinyl chloride pressure of 70 mTorr and the laser energies used in our experiments. In that case, if the above mechanism of production of C2(d 311s) through the diradical(2) is assumed, then the linear dependence on laser energy of the fluorescence would reflect the competition between the intensitydependent absorption of a second photon by (2) and the fast unimolecular process which depletes the intermediate (2). Finally, we mention that C2(d 311g) has been reported as the most important fragment produced in the unimolecular multiphoton dissociation of acetylene excited by a sub-nanosecond laser pulse at 266 nm [ 121. In this case, excitation of the parent within the triplet manifold is not excluded but a mechanism involving a bent state of C2H is preferred. However, the authors propose *Aat a major dissociative route for production of C,H from C,H, should be operating other than that observed in the one-photon excitation at the same energy and also that a
Volume
107. number 6
CHEMICAL
PHYSICS
different channel for C, elimination than that observed in the work by McDonald et al. [S], where mainly carbon singlet states are formed, is operating. This suggests that a variety of C,H, states exhibiting different photochemical behaviour may be pumped either by multiphoton absorption of the parent or by other means as proposed in our work. 3.2. Removal of Cz(d311g) by O2 The C,(d 311p -+ a 3nu) fluorescence at 5 16 nm (u’ = 0 +‘l” = 0) and 5 12 nm (u’ = 1 + u” = 1) was
measured at 70 mTorr of vinyl chloride and different pressures of added 0,. A Stem-Volmer plot is shown in fig. 1. From the slope of the straight line fitted to the experimental points of fig. 1, and assuming the C,(d 311g) radiative lif%rne to be 119 ns [S], a rate co&m of (6.1 f 0.85) X 1O-12 molecule-l cm3 s-1 is obtained. Rate coefficients for the analogous processes of the C,(a 3Tlu) and C,(X 1x3 states have been reported in the literature [2,4,6,8-IO] _Removal by O2 of the C,(a 3flu) state has been shown to proceed
LEI-I-ERS
15 June 1984
via fast intersystem crossing to the closely lying ground state [lo] _On the other hand, in the case of the C,(X lIZi> ground state, it has been pointed out that, despite the number of exceedingly exothermic channels available for reaction with O,, the rate constant is rather slow, requiring about 200 collisions_ The higher energy content of the C,(d 3Tla) state opens new highly exothemric spin-allowed channels for reaction with 0,; however, the removal rate constant obtained here is also slow, about twice as fast as for the ground state [4,9]. Further work is required to identify the pathways involved in the removal of C,(d 3lI,) by O2 and to establish the relative importance of possible reactive channels versus energy transfer. It has been suggested that most of the CO product of the C,(a 3TIu) + O2 reaction is formed in its ground and fit excited singlet (A Ill) state [8]. For the C>(d 3flg) state, a possible reaction channel, both thermodynamically and spin allowed, could lead to formation of the CO(A Ill) first excited singlet plus CO in several triplet states. We note that this channel resembles somewhat the photolysis of dioxetanes, where a singlet plus triplet ketones are obtained. We suggest that an investigation of the reaction channels of C,(d 3QJ plus 0, could provide useful information on the above energy-storing molecules.
Acknowledgement
We are indebted to Dr. C. Webb and co-workers for providing us with detailed information on their excimer laser designs. We also thank Dr. A. Costela for his help.
References
oc
100
[l] C. Nokes.G.
200
300 P(Torr)
Fig. 1. Stem-Volmer plot of vinyl chloride fluorescence versus pressure of 0,. Fo/Fhf is the ratio of the fluorescence from pure vinyl chloride (70 mTorr) to the fluorescence in the presence of 02. Full circles at 5 16 MI; open circles at 512 nm.
Gilbert and R.J. Donovan, Chem. Phys. Lerters 99 (1983) 491. [2] VBf. Donnelly and 1. Pasrernack.Chem. Phys. 39 (1979) 427. [3] L. Pas-ten-tack. W.M. Pittsand J.R.-McDonald, Chem.
Phvs. 57 (1981) 19. [4] L.&ten&k id J.R. hIcDonald, Chem. Phys. 43 (1979) 173. [S] J.R. McDonald, AP. Baronavski and Vhf. Donnelly, Chem. Phys. 33 (1978) 161.
563
Volume 107, number 6
CHEMICAL
[6] S.V. Fttseth. G. Hancock, J. Fournier and K. Meier. Chem; Phys. Letters 6 l(l979) 288. [7] H. Reisler. M. Mangir and C. Witlig. J. Chem. Phys. 71 (1979) 2109. [S] M.S. Man&, H. Reisler and C. Wittig, J. Chem. Phys. 73 (1980) 829. [ 91 H. Reisler, M. Mangir and C. Wittig. Chem. Phys. 47 (1980) 49. ]lO] H. Reisler, MS. Mangir and C. Wittig, J. Chem. Phys. 73 (1980) 2280. ill] AJ. Kearsley. AJ. Andrews and C.E. Webb, Opt. Commun. 31(1979) 181.
564
PHYSICS LETTERS
ISJune
1984
1121 BB. Craig. W.L. Faust, LS. Goldberg and R.C. Weiss, 1. Chem. Phys. 76 (1982) 5014. 113) N. Nishi. H. Shinohara and 1. Hanazaki. J. Chem. Phys. 77 (1982) 246. [ 141 MJ. Berry, J. Chem. Phys. 61(1974) 3114. (151 C. Reiser and J.I. Steinfeld. J. Phys. Chem. 84 (1980) 680. 1161 L. Salem and C. Rowland, Angew. Chem. 11 (1972) 92. [ 171 G. Heraberg, Molecular spectra and molecular structure. II. Infrared and Raman spectra of polyatomic molecules (Van Nostrand, Princeton, 1945). [ 181 S.P. Sood and K. Watanabe, J. Chem. Phys. 45 (1966) 2913.